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Robert S. Houston 
Richard P. Hallion 
Ronald G. Boston 

from
The Hypersonic Revolution
Case Studies in the History of Hypersonic Technology
Air Force History and Museums Program
1998 

 

Editor's Introduction

 

The first call for an X-15-class research vehicle came from Robert J. Woods, a colleague of Walter Dornberger at Bell, during a meeting of the prestigious NACA Committee on Aerodynamics on October 4, 1951. He reiterated his support for such a vehicle during subsequent meetings and, as a result, the NACA committee passed a motion on June 24, 1952 that charged the agency to expand its research aircraft program to include studying the problems of manned and unmanned flight at altitudes between 12 and 50 miles, and velocities of Mach 4 to Mach 10, as well as devoting "a modest effort" to study exoatmospheric flight from Mach 10 to escape velocity. The major NACA field centers exchanged various paper plane proposals. NACA engineers L. Robert Carman and Hubert Drake of the High-Speed Flight Station drew up configurations for Mach 3+ launch aircraft carrying small hypersonic research aircraft including, in August 1953, a five-phase proposal culminating in the design of an orbital air-launched hypersonic boost-glide winged vehicle. The NACA shelved this bold proposal as too futuristic, which it was; its advocacy of a "two-stage to orbit" research vehicle was one of the earliest of the "piggyback" concepts predating the current Space Shuttle. The NACA, like other federal and private organizations, favored a more modest approach. In October 1953, the Air Force's Scientific Advisory Board recommended development of a Mach 5-7 research aircraft, and at the same time, the Office of Naval Research had funded the Douglas Aircraft Corporation to study the feasibility of a Mach 7+ rocket-propelled research airplane, informally referred to as the D-558-3 ¹.

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Figure 1 AN EARLY AMERICAN AIRCRAFT/ORBITER PROPOSAL: THE DRAKE-CARMAN COMPOSITE RESEARCH AIRCRAFT PROPOSAL OF 1953.

During 1954, the NACA, in partnership with the Air Force and Navy, further explored the hypersonic aircraft concept. The agency's Langley laboratory (later NASA's Langley Research Center) had formed a hypersonic study team comprised of chairman John V. Becker, Maxime Faget, Thomas Toll, N. F. Dow, and J. B. Whitten, and this group subsequently evolved a baseline design that closely resembled the ultimate X-15 configuration. Their conception incorporated Inconel alloy heat-sink construction, had a cruciform tail configuration, a wedge vertical fin for increased directional stability, and similar weights and specifications as the final aircraft. In December 1954, the NACA, Air Force, and Navy agreed to undertake joint development of the proposed hypersonic research aircraft, and in January 1955 it received the designation X-15. That same month, the Air Force (which administered the design and construction phases of the project) held the first briefings for potential contractors. This culminated in a competition between North American, Bell, Douglas, and Republic, which North American won on September 30, 1955. The Bell entry, which featured a novel form of "double-wall" construction, reflected the firm's obsession with Sänger-like boost-gliders (indeed, in April 1952, Bell's Dornberger had journeyed to France in a vain attempt to convince Sänger and his wife to join the company), and had no real hope of winning. The subsequent technical development of the North American X-15 went smoothly, with the exception of its rocket powerplant, which generated great concern before it, too, reached fruition ².

The X-15, "Round Two" in the parlance of the NACA, had many features that separated it from the previous rocket research aircraft and placed it at an intermediate level between the purely supersonic aircraft (such as the X-1) and the purely winged reentry vehicles (like the proposed "Round Three" Dyna-Soar and the eventual Space Shuttle). For example, it incorporated a reaction control system of hydrogen peroxide rocket thrusters for keeping the aircraft under control at high altitudes; the pilot wore a full pressure pilot protection suit (the Clark MC-2) having provisions for physiological monitoring. It was the first flight vehicle to blend the application of hypersonic aerodynamic theory to an actual aircraft. It incorporated high temperature seals and lubricants, and had a "Q-ball" flow direction sensor capable of operating with stagnation air temperatures of 3500° F. The pilot relied on inertial flight data systems developed especially for operation under space-like conditions. The X-15's Inconel structure was the first reusable super-alloy structure capable of withstanding the temperatures and thermal gradients of hypersonic reentry. Subsequently, during its flight program, the X-15 spawned development and application of a refurbishable ablative heat protection system (the Martin MA-25S) ³.

The X-15 spanned 22 ft. 4 in., and had a length of 50 ft. 9 in. It utilized a Thiokol (Reaction Motors Division) XLR-99 throttleable rocket engine, burning a mixture of anhydrous ammonia and liquid oxygen. (Delays in the development of this engine forced North American to install two XLR-11 engines in the X-15s during 1959, before beginning the research program, for purposes of checking out the aircraft and its systems; the first XLR-99 flight did not come until November 15, 1960). The three X-15 aircraft quickly established a number of speed and altitude marks, which often obscured the less glamorous but occasionally more important work they accomplished in mapping out the frontiers of' hypersonic flight. By the end of 1961, the X-15 had achieved its Mach 6 design speed, and had reached altitudes in excess of 200,000 feet. On August 22, 1963, NASA research pilot Joseph Walker reached 354,200 feet in the third X-15 aircraft, still a record for winged vehicles. X-15 testing revealed a number of interesting conditions about hypersonic flight, including the discovery that hypersonic boundary layer flow is turbulent and not laminar, that turbulent heating rates were lower than predicted by theory, that supersonic skin friction was likewise lower than predicted, that local surface irregularities generated hot spots (in one notable case, aerodynamic heating caused buckling of the wing skin behirid leading edge heat expansion slots), and that the cruciform tail configuration created a serious adverse roll problem at high angles of attack during atmospheric reentry (NASA cured this by removing the jettisonable lower half of the craft's ventral fin). The flights demonstrated that a pilot could successfully transition from aerodynamic to reaction controls and back again, function in a weightless environment (which became an academic question after Vostok and Mercury), control a rocket-boosted vehicle during atmospheric exit, and use energy management techniques to make a hypersonic/supersonic reentry and glide approach to a precision landing. The X-15 eventually made reentries at angles of attack up to 26 deg. and at flightpath angles as low as -38 degrees at Mach 6 flight speeds ⁴.

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Figure 2 THE X-15 ROCKET RESEARCH AIRPLANE

As with the previous "Round one" rocket research airplanes, the X-15 was air launched, being dropped from a modified Boeing B-52 jet bomber. The flights were made over a specially instrumented 485-mile-long 50-mile-wide flight test corridor stretching from Nevada to Edwards Air Force Base in California. Following a landing accident with the second X-15, the Air Force and NASA authorized the manufacturer to modify it as a special testbed for NASA's planned Hypersonic Ramjet Experiment. North American lengthened the aircraft, making numerous modifications to it, and added provisions for two large jettisonable external tanks. Thus equipped, the aircraft, designated the X-15A-2, was capable of Mach 7 flight speeds, if equipped with a pr3per thermal protection system. NASA finally selected Martin to develop a suitable ablator, and that company derived the MA-25S, an ablator mix consisting of a resin base, a catalyst, and a glass bead powder. Hopes that such ablators could enable designers to build refurbishable spacecraft that could be stripped and recoated after each flight proved ill-founded, however. On October 3, 1967, the X-15A-2 attained Mach 6.72 (over 4,520 mph), while piloted by Air Force Maj. William J. Knight. Unfortunately, the plane landed in extremely worn condition -- a dummy ramjet had separated off the craft, in fact -- and the ablator would have required massive cleanup efforts prior to reapplication. North American repaired the craft and returned it to NASA, but it never flew again. The third X-15 made a number of notable high-altitude flights above 50 miles. Unfortunately, this aircraft was lost, together with pilot Michael J. Adams, on November 15, 1967. The first X-15 completed its last flight, the 199th flight for the type, on October 24, 1968 ⁵.

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Figure 3 THE MODIFIED X-15A-2 ROCKET RESEARCH AIRCRAFT

Following awarding of the X-15 development contract, North American had considered a so-called "X-15B" orbital spacecraft (even before Sputnik), to be launched by two Navaho boosters and possibly carry a two-astronaut crew. After Sputnik, it went through a cycle of shelving and revival until finally overcome by the ballistic blunt-body spacecraft approach as taken by the McDonnell Mercury vehicle. The X-15 series itself, however, did perform a number of "Shuttle" like missions, for after 1962, the X-15 program switched concentration from hypersonic aerodynamics to using the vehicle as a testbed carrying a wide range of applications and experiments, such as insulation intended for the Saturn booster, and navigation instruments under development for Apollo. By 1964, fully 65 percent of all data returned from the X-15 related to follow-on programs, and this figure continued rising until the conclusion of the program in December 1968. NASA even briefly considered using the X-15 as a launcher for Scout rockets carrying small satellite payloads, the B-52/X-15/Scout becoming, in effect, one large booster, but after examining the idea, NASA rejected it on grounds of safety, cost, and practicality. Fittingly, in December 1968, the Deutsche Gesellschaft fUr Raketentechnik und Raumfahrt awarded John Becker and the X-15 team with the Eugen Sänger Medal, created to honor individuals and groups who have-made special contributions to the field of recoverable spacecraft ⁶.

The following case study of the X-15 was prepared by the late Robert S. Houston of the then-Historical Branch, Office of Information Services, Wright Air Development Center, Wright-Patterson AFB, Ohio, in 1959. It has been expanded and updated by the editor to treat the X-15's flight test program and research legacy as well, with much of this supplementary material drawing upon the editor's On the Frontier: Flight Research at Dryden, 1946-1981 (Washington, D.C.: NASA, 1984), and then-Captain Ronald G. Boston's "Outline of the X-15's Contributions to Aerospace Technology", prepared in support of the National Hypersonic Flight Research Facility effort in 1977. At the time, Captain Boston was an instructor in the Department of History, Air Force Academy, Colorado Springs, Colorado.

 

 

Section I  Genesis Of A Research Air Plane

 

During the spring of 1952, the Committee on Aerodynamics of the National Advisory Committee for Aeronautics (NACA) recommended that several NACA laboratories begin studies of problems likely to be encountered in spaceflight and examine methods of exploring such problems. The NACA Executive Committee, which endorsed the recommendation, directed consideration of laboratory techniques, missiles, and manned aircraft.

Work along these lines progressed quietly for the next two years. Then, in February 1954, the NACA stepped up the pace, undertaking a more specific study to determine the extent to which an advanced research aircraft could contribute to the solution of problems earlier identified. Technical areas of concern at that time included high temperature structures, hypersonic aerodynamics, stability and control, and pilotage. An important requirement, specified at the outset of the work, was that "a period of only about three years be allowed for design and construction in order to provide the maximum possible lead time for application of the research results." Such a requirement precluded the development of new materials, new construction techniques, or improved launching practices. As one official subsequently observed, "it was obviously impossible that the proposed aircraft be in any sense an optimum hypersonic configuration."

NACA design engineers decided early that a relatively conventional airframe was essential to the resolution of low speed launch and landing difficulties. High speed requirements prompted the choice of a thick wedge tail to provide directional stability and a ventral tail to improve control at high angles of attack (where the upper vertical tail surface was immersed in low pressure flow fields generated by the wing and fuselage). Artificial damping seemed essential because of persistent uncertainties about the aerodynamic environment at extreme speeds and altitudes. Static stability for all flight conditions and the employment of hydrogen peroxide rockets for high altitude attitude control also became objectives of the tentative design. NACA materials experts decided that Inconel X offered the best heat sink structure and that heating problems in general would impose the use of a blunt wing leading edge. Assuming that air launch in the fashion of the X-1 and X-2 aircraft would be necessary, NACA established aircraft size as the largest that could conveniently be handled by B-36 or B-50 carriers. A maximum velocity of 6,800 feet per second, an altitude potential of 400,000 feet, and a gross weight of 30,000 pounds (18,000 pounds of fuel) completed the general proposal ¹.

The studies that had prompted these recommendations of early 1954 were independently produced by the three NACA laboratory stations (Langley, Ames, and High Speed Flight Station). They induced NACA to adopt the official policy that a manned research airplane was essential for study of the problems earlier defined, that the construction of such an aircraft was wholly feasible, and that quick action should be taken to pursue the general objective. In June of 1954, therefore, the NACA contacted the Air Force and the Navy, asking that a special joint meeting be held to consider the need for a new research aircraft.

Wright Air Development Center (WADC), then having cognizance over system development, provided technical representation for the Air Force at the meeting - held in Washington on 9 July. Headquarters of the Air Research and Development Command (ARDC) and Headquarters, United States Air Force (USAF), sent policy representatives. In the course of the meeting it became apparent that neither the Air Force nor the Navy had been indifferent to the problems which had prompted NACA interest. The Air Force's Scientific Advisory Board had been urging the construction of a "super X-2" while the Navy's Bureau of Aeronautics had contracted for a feasibility study of a manned aircraft capable of reaching an altitude of 1,000,000 feet. The NACA proposal fell roughly between these extremes, being considerably less ambitious than the Navy program and substantially more advanced than the Air Force objective of the moment ².

Both Navy and Air Force representatives viewed the NACA proposal with favor, though each had some reservations. At the close of the meeting, however, there was agreement that both services would study further the justification and objectives of the NACA program, and that NACA would take the initiative in securing project approval from the Department of Defense ³.

Three weeks later, on 29 July, Headquarters ARDC instructed WADC to submit technical comments on the proposal and to make time and cost estimates ⁴. Almost immediately, the WADC Power Plant Laboratory identified the principal shortcoming of the original "study" - the apparent lack of a suitable rocket engine. Initially and tentatively, NACA had suggested employing a modified Hermes A-1 power plant; the Power Plant Laboratory early in August pointed out that "no current rocket engines" entirely satisfied the NACA requirements, and urgently emphasized that the Hermes engine was not designed to be operated in close proximity to humans - that it usually was fired only when shielded by concrete walls. Other major objections to the Hermes engine lay in its relatively low level of development, in its limited design life (intended for missile use, it was not required to operate successfully more than once), and in the apparent difficulty of incorporating thrust variation provisions.

In the stead of the Hermes power plant, the laboratory suggested consideration of several engines originally designed for use in manned aircraft. Hesitating to make any positive recommendations in the absence of more specific data on the aircraft, however, WADC recommended only that the selection of an engine be postponed until propulsion requirements could be more adequately defined ⁵. WADC technical personnel who visited Langley on 9 August drew a firm distinction between engines intended for piloted aircraft and those designed for missiles; NACA immediately recognized the problem, but concluded that although program costs would go up, feasibility estimates would not be affected ⁶.

WADC's official reaction to the NACA proposal went to headquarters ARDC on August 13 ^a. The director of laboratories (Colonel V. R. Haugen) reported "unanimous" agreement among WADC participants that the proposal was technically feasible; excepting the engine situation, there was no occasion for adverse comment from WADC technical sources on the NACA-proposed solutions to major problems.

In one respect, however, the official letter from WADC to ARDC did not reflect unanimity of opinion. The comment forwarded by Colonel Haugen contained a cost estimate of $12,000,000 "distributed over three to four fiscal years" for two research aircraft, modification of a suitable carrier, and necessary government-furnished equipment ⁷. Mr. R. L. Schulz, technical director for aircraft in the WADC Directorate of Weapon Systems Operations, commented informally that although his directorate had concurred in the letter, the concurrence included a reservation about the estimated cost which the Fighter Aircraft Division reportedly furnished. Said Mr. Schulz, prophetically: "Remember the X-3, the X-5, [and] the X-2 overran 200%. This project won't get started for $12,000,000. ⁸"

On 13 September, Major General F. B. Wood, ARDC's Deputy Commander for Technical Operations, forwarded to Air Force headquarters an endorsement of the NACA position and its WADC support. Specifically, General Wood recommended that the Air Force "initiate a project to design, construct, and operate a new research aircraft similar to that suggested by NACA without delay." The aircraft, emphasized ARDC, should be considered a pure research vehicle and should not be programmed as a weapon system prototype. The research command estimated that about three and one-half years would be consumed in the design and fabrication process and forwarded WADC's cost estimate, broken down into specifics, without change. (Estimated costs included: $1,500,000 for design work; $9,500,000 for construction and development, including flight test demonstration; $650,000 for government furnished equipment, including engines, $300,000 for design studies and specifications; and $250,000 for modification of a carrier aircraft.) ARDC further suggested a preliminary design competition, assignment of "sole executive responsibility" to the Air Force, and eventual transfer of the resulting aircraft to NACA following a limited Air Force flight demonstration program ⁹.

Brigadier General B. S. Kelsey, Deputy Director of Research and Development in the office of the USAF Deputy Chief of Staff, Development, on 4 October 1954 expressed general agreement with the ARDC position, noting however that the Department of Defense had decided that the project would be a joint Navy-NACA-USAF effort managed by the Air Force and guided by a joint steering committee. A 1-B priority, $300,000 in FY55 funds, and directions to support the undertaking accompanied this explanation. Air Force headquarters further pointed out the necessity for funding a special flight test range as part of the project ¹⁰.

Formalization of the arrangements thus proposed required nearly eight weeks. On 5 October, the NACA Committee on Aerodynamics formally endorsed the proposal to build a Mach 7 research airplane to explore the fringes of space. ¹¹ On 22 October a meeting of Navy, NACA, and Air Force representatives at Wright Field agreed on methods of originating and coordinating design requirements for an eventual competition. Additionally, the conferees settled on four development engines from which a power plant could be chosen by any interested airframe contractor. ¹² Early in November the two services and NACA reached a general agreement on future operating procedures; a formal memorandum of understanding emerged from the office of Mr. Trevor Gardner (Special Assistant for Research and Development to the Secretary of the Air Force), and was forwarded for the signatures of the Assistant Secretary of the Navy for Air (Mr. J. H. Smith Jr.) and the Director of the NACA (Dr. H. L. Dryden). The process was effectively complete by 23 December. ¹³

The memorandum of understanding, which set a general pattern for the future management of the project, assigned technical direction of the program to the director, NACA, "with the advice and assistance of a Research Airplane Committee" .that included Navy and Air Force representatives. (General Kelsey became the Air Force member and Rear Admiral R. S. Hatcher the Navy member.) The Navy and the Air Force were to finance the undertaking and the Air Force was to administer its design and construction phases. The preliminary NACA design was to be the basis for solicited proposals for a design and construction contract. Upon acceptance of the airplane from the contractor, it was to become NACA property. The memorandum concluded with the statement: "Accomplishment of this project is a matter of national urgency." ¹⁴ Accompanying the memorandum, as a matter of course, was a secretarial-level Air Force concurrence in the establishment of a joint project to build the proposed research airplane. ¹⁵

In the meantime, notwithstanding the absence of formal agreements or procedure, Wright Field had been making arrangements for a design competition. By 15 November, individual laboratories had compiled specification data for inclusion in a letter of invitation to prospective contractors. Coordination with NACA and Navy organizations presented no great difficulty; by 30 November headquarters ARDC had approved plans to prepare official copies of competition data and had advised Wright Field that in about two weeks the Office of the Secretary of Defense probably would authorize distribution of the material. ¹⁶ Air Force headquarters scheduled a 13 December briefing for the Secretary of Defense and approved certain changes in the draft requirements. (USAF specified that air-launch was required, that a prone-pilot provision would not be acceptable, that unconventional design approaches would be sought, that instrumentation space was to be increased, that non-NACA facilities would be used for flight tests, and that references to costs in excess of $5,000,000 and to 1956 engine availability were to be eliminated from the invitation to bid.) ¹⁷

Advance notice of the forthcoming competition was informally given to prospective contractors early in December. In the last week of December, headquarters ARDC directed that the letter of invitation be dispatched as soon as the center received an official teletype authorizing such action. As prescribed by existing regulations, the letter was to be circulated by the Air Materiel Command (AMC), although that organization declined responsibility for selecting the recipients and held to the policy that the competition was exclusively an ARDC affair. ¹⁸

On 29 December the action teletype from Air Force headquarters arrived. ¹⁹ Rubber stamp dates completed the preparation process, and on 30 December AMC, over the signature of Colonel C. F. Damberg, Chief, Aircraft Division, sent invitation-to-bid letters to 12 prospective contractors (Bell, Boeing, Chance-Vought, Convair, Douglas, Grumman, Lockheed, Martin, McDonnell, North American, Northrop, and Republic). The document asked that interested concerns notify Wright Field by 10 January 1955 and plan to attend a special briefing on 18 January.

Attached to the letter were a general preliminary outline specification, an abstract of the NACA preliminary study, a discussion of power plant requirements and development levels, a list of data requirements, and a cost outline statement. Each bidder was required to satisfy various requirements thus set forth, except in the case of the NACA abstract which was presented as "representative of possible solutions." ²⁰

Grumman, Lockheed, and Martin expressed slight interest in the competition and did not appear at the 18 January briefing. Subsequently, between that date and the 9 May deadline for the submission of proposals, Boeing, Chance-Vought, Convair, Grumman, McDonnell, and Northrop informed AMC that they would not participate. This left Bell, Douglas, North American and Republic as competitors.

Activity in the interim was varied. The contractors concentrated on the assembly of attractive proposals. In the course of this effort they had frequent recourse to the advisory services of both WADC and NACA. Concurrently, project officers (in the New Developments Office, Fighter Aircraft Division, Directorate of Weapon Systems Operations, which had been assigned full responsibility for the balance of the competition) attempted to refine an evaluation procedure acceptable to all concerned and sent supplemental data to the participating contractors. Of these tasks, the evaluation procedure loomed larger. Headquarters ARDC in early February emphasized the extreme importance of resolving all possible differences of opinion on the conduct of the technical evaluation; to this end, ARDC instructed that the ultimate recommendation reflect the opinion of NACA as well as that of WADC. Plans had been laid for submitting the evaluation rules to the Joint Steering Committee for approval. ²¹

Supplemental instructions to contractors reemphasized the urgency of the two and one-half year development period of the X-15. ^b The project office also relaxed very slightly the rigid limitations on engine selection, instructing competitors that "if ... an engine not on the approved list offers sufficient advantage, the airframe company may, together with the engine manufacturer, present justification for approval to the WSPO (Weapon System Project Office)." ²²

The Power Plant Laboratory had originally listed the XLR81 and the XLR73, the XLR10 (and its variants - a compound XLR10 and a modification of the XLR30), and the NA-5400 (a North American engine in early development, still lacking a military designation) as engines that airframe competitors could use in their designs. Early in January, the laboratory had become concerned that the builders of engines other than those listed might protest the exclusion of their products. Consequently there emerged from the Liquid Rocket Section of the laboratory an explanation and justification of the engine selection process. It appeared that the engineers had confidence in the ability of the XLR81 and XLR73 to meet airplane requirements, had doubts about the suitability of the XLR25 (a Curtiss-Wright product), and held the thrust potential of the XLR8 and XLR11 (similar engines) in low repute. This for practical purposes exhausted the fund of Air Force-developed engines suitable for manned aircraft. Navy consultants had introduced the other two engines defined as acceptable in terms of the competition. ²³

At about the time the industry briefing was held, the project office began seriously to consider sending copies of the bid invitation to "appropriate engine contractors." The Power Plant Laboratory discouraged unlimited distribution because of the possible compromise of proprietary data, but suggested that limited information be circulated and that inquiring contractors be informed what the Air Force had said about their own engines. ²⁴ A course similar to this eventually was adopted; on 4 February each of the prospective engine contractors earlier identified (Reaction Motors, General Electric, North American, and Aerojet) was asked to submit a suitable engine development proposal. ²⁵ Even earlier, certain of the engine contractors had been contacted for specific information about the engines originally listed as suitable for the X-15 program. ²⁶ This information, relating to design and performance details, was distributed to all four prospective airframe contractors. ²⁷ Data on the North American NA-5400 was scant, and the Reaction Motors XLR10 received a "not recommended" classification (at the suggestion of the engine contractor himself).

Progress in the completion of evaluation arrangements was less rapid than had originally been anticipated. A 1 March deadline established by ARDC early in February was later extended to 1 April, and the material itself did not leave Wright Field until 11 April. ²⁸ Nevertheless, by that time the evaluation rules had been fully coordinated within WADC and with NACA.

The burden of the evaluation process fell on the project office, the WADC laboratories, and NACA - in that order. AMC and the Navy were to play subordinate - though still significant - roles. Four evaluation areas were specified: performance, technical design, development capability, and cost. ²⁹

Headquarters ARDC forwarded the WADC evaluation plan to Air Force headquarters for approval and then advised WADC that the Research Airplane Committee planned to meet at Wright Field on 17 May to examine the submitted designs and to review evaluation arrangements. ARDC also suggested that commitments be obtained from the various engine contractors as early as possible so that the engine program would not adversely affect the selection of a winning airframe design. ³⁰

On the appointed day, 9 May 1955, Bell, Douglas, North American, and Republic submitted their proposals to the project office. Two days later the technical data went to the several laboratories with a request that evaluation results be reported by 22 June. On 17 May the bidders made separate presentations to the Research Aircraft Committee and to a group of senior officials from WADC, ARDC, headquarters USAF, NACA, and the Air Force Flight Test Center. Later that day the Research Aircraft Committee confirmed previous arrangements for the evaluation procedure. Subsequently, both the Bureau of Aeronautics (Navy) and NACA independently accepted the resultant evaluation plan. Bureau of Aeronautics took pains. to insure that Navy and NACA consultants participated in the joint evaluation. ³¹ Later arrangements insured that engine evaluations, also coordinated with the Navy and the NACA, would be available by 12 July. ³²

The final evaluation meeting, to consider the results of earlier examinations and comments, was scheduled for Wright Field on 25 July. In the interim, there was established a free interchange of preliminary opinion between Bureau of Aeronautics, NACA, and WADC laboratory and project office elements. ³³ Notwithstanding this advance coordination, the evaluation results were delayed, first by the interference of higher priority work at WADC, later by a need for formal coordination with Bureau of Aeronautics. ³⁴

By 5 August, the various portions of the evaluation had been completed and the evaluation report had identified North American's proposal as having considerably greater merit than any of the others. ^c On 12 August the Research Aircraft Committee accepted the findings. Preliminary moves to Confirm this decision and to award a design contract to North American hit a sudden snag, however, when on 23 August North American's local representative verbally notified the Fighter Aircraft Division that the firm was withdrawing its proposal because of the press of other work. ³⁵ The immediate reaction of Wright Field was to inform everybody concerned that the evaluation results would have to be reexamined. (No contractors had yet been notified of the outcome.) On 30 August, the contractor officially and in writing confirmed his earlier announcement, citing inability to perform the work in the time allotted and recent awards in interceptor and fighter-bomber competitions plus a heavy F-107A workload as the motives. Within a week the project office (and the directorate) had decided that North American should be asked to reconsider the decision. But there was agreement that if the company held firm, Douglas would probably be ruled the competition winner, although the Douglas design (which employed magnesium instead of Inconel X) would require considerable modification before it satisfied NACA and USAF requirements. ³⁶

During the middle weeks of September, both NACA and Air Force officials discussed with North American possible continuance of the contractor's X-15 activity. Dr. Dryden of NACA and Brigadier General H. M. Estes of the newly formed Directorate of Systems Management ^d had prominent roles in these negotiations. A presentation of the X-15 program at the Department of Defense level, on 14 September, induced a recommendation that the program be approved. Concurrently, however, two changes in philosophy appeared. First, the Army representative at the conference said flatly that the Army would oppose the project if it required special Department of Defense funds. This stand prompted an attempt to reduce program costs below earlier estimates. At the same time, it began to appear inevitable that the program would take more than the 30 months originally projected. On this basis, it seemed that North American might still be considered a competitor. The contractor's reluctance to proceed was frankly based on the thesis that the company could not devote sufficient effort to the X-15 project to permit its completion within the span of time initially provided. ³⁷

On 20 and 21 September, contacts with Air Force headquarters confirmed earlier information that the Department of Defense had approved the project and North American's selection. But before any formal contract negotiations could be authorized, said the Department of Defense, a reduction in annual budget requirements would be necessary.

As these instructions reached Wright Field, General Estes was conferring with Mr. J. L. Atwood, North American's president. Mr. Atwood told the general that his company would reconsider its decision on the X-15 if the program were extended by eight months (to 38 months). Two days later, on 23 September, this offer was made officially. North American emphasized, however, that a program extension was essential to the company's accepting a contract. ³⁸

On 27 September, Air Force headquarters agreed to this condition and canceled earlier instructions to negotiate a reduction in the contractor's fee. Information on the decision reached the center on 28 September; on the last day of that month, letters went to North American and to the unsuccessful bidders, officially advising them of the outcome of the competition. ³⁹

Price negotiations followed. Wright Field project officers took the results of preliminary contact with North American (and with Reaction Motors, the prospective engine contractor) to a Pentagon meeting of 11 October. By that time the contractor's estimate of project cost had been reduced from $56,000,000 to $45,000,000 and the maximum annual funds requirement from $26,000,000 to $15,000,000. The USAF Directorate of Research and Development made a presentation of these figures to the Department of Defense Coordinating Committee on Piloted Aircraft on 19 October. The result was a committee decision to support the project. Shortly thereafter, the Department of Defense released the funds needed for the start of work. More meetings between NACA, project office, and North American personnel were held on 27-28 October, largely to define changes to the aircraft configuration originally submitted by the contractor. On 7 November, the AMC Directorate of Procurement and Production took the first steps toward issuance of a letter contract, by 9 November the principal clauses of that document had been composed, on 15 November it received the approval of the procurement directorate, on 18 November it was sent to North American, and on 8 December the contractor returned an executed copy. ⁴⁰ At that point, about $2,600,000 was available to fund initial activity; a total contract cost of $39,000,000 was foreseen for design, development, three X-15 aircraft, and a flight demonstration program. ⁴¹

On 1 December 1955, a series of actions designed to produce an engine contract began. ⁴² A letter contract with Reaction Motors became effective on 14 February 1956. Its initial allocation of funds totaled $3,000,000, with an eventual expenditure of about $6,000,000 foreseen as necessary for the delivery of the first flight engine. ⁴³

A definitive contract for North American was completed on 11 June 1956, superseding the letter contract and two intervening amendments. To that time, $5,315,000 had been committed to North American, in three increments, under the letter contract. (Essentially, North American had been given $2,715,000 more than the initial allocations.) The definitive contract of June contemplated the eventual expenditure of $40,263,709 plus a fee of $2,617,075. For this sum, the government was to receive three X-15 research aircraft and other specified items: a high speed and a low speed wind tunnel model program, a free-spin model, a full-size mockup, propulsion system tests and stands, flight tests, modification of a B-36 carrier, a flight handbook, a maintenance handbook, technical data, periodic reports of several types, ground handling dollies, spare parts ($100,000), and ground support equipment ($200,000). Exclusive of contract costs were fuel and oil, special test site facilities, and expenses incident to operation of a B-36 carrier. Delivery date for the aircraft and support equipment was to be 31 October 1958. ⁴⁴

A final contract for the engine, the prime unit of government furnished equipment, was effective on 7 September 1956. Superseding the letter contract of February, it covered the expenditure of $10,160,030 plus a fee of $614,000. ^e For this sum, Reaction Motors agreed to deliver one engine, a mockup, reports, drawings, and tools. The engine described in the final contract was to have a maximum thrust of 50,000 pounds, to include provisions allowing for inflight thrust variation between 30 to 100 percent of maximum output, to be capable of 90 seconds operation at full thrust and 4 minutes 9 seconds at 30 percent thrust, to weigh 618 pounds (without fuel), and to have a specific impulse of 241 (pounds of 45 thrust per pound of fuel per second). ⁴⁵

 

 

Section II  Designing For Mach 6

 

Although the invitation-to-bid letter circulated to prospective contractors by the Air Materiel Command had specifically excluded the NACA Preliminary Study as a requirement, North American's winning proposal bore an unsurprising resemblance to the design envisioned by that study. A comparison of the suggested configuration contained in the NACA study and the North American configuration presented to the first industry conference in October 1956 revealed that the span of the X-15 had been reduced from the 27.4 feet of the suggested configuration to only 22 feet and that the North American fuselage had grown from the suggested 47.5 foot overall-length to 49 feet. The North American design contained 'the split tail surfaces, the wing and tail flaps, the leading-edge sweep for both wing and tail surfaces, and the skid-type landing gear which had been suggested by the preliminary study. The all-movable tail of the 1956 configuration still retained the thick wedge airfoil envisioned by NACA and the horizontal tail surfaces incorporated the cathedral (downward slope or negative dihedral) which had also been a feature of the NACA suggestions. The major differences in external configuration between the study proposal and the design which North American presented consisted of an elimination of ailerons and of separate stabilizers and elevators. North American eliminated the ailerons and elevators by utilizing all-movable horizontal tail surfaces that could be operated differentially so as to provide roll as well as pitch control (the "rolling tail"). North American had gained considerable experience with all-movable controls through using them on the F-107 fighter design, and in this instance use of the differentially operated surfaces permitted simplification of wing construction and elimination of the protuberances that would have been necessary if aileron controls had been incorporated in the thin airfoil sections of the X-15's wings. Such protuberances would have disturbed the airflow and created another heating problem. One other significant difference between the configuration of the NACA design and that of the X-15 stemmed from North American's incorporation of the propellant tanks in the fuselage structure and the use of tunnels on both sides of the fuselage to accommodate the propellant lines and engine controls that ordinarily would have been contained within the fuselage. North American followed the NACA suggestions by selecting Inconel alloy as the major structural material and in the design of a multi spar wing with extensive use of corrugated webs. ¹

The original North American proposal gave rise to several questions which in turn, on 24-25 October 1955, prompted a meeting attended by NACA and WADC personnel at Wright-Patterson Air Force Base. The purpose was to consider necessary changes in North American's preliminary design. The meeting formulated a list of questions and comments to serve as the basis of discussions with the contractor. Subsequent meetings of the WADC-NACA group with North American's engineers were held at the contractor's Inglewood plant on 28-29 October and 14-15 November. The items considered at the October and November meetings included North American's use of fuselage tunnels and the rolling tail. The government agencies expressed concern that the tunnels might create undesirable vortices that would interfere with the vertical tail, and suggested that the tunnels be kept as short as possible in the area ahead of the wing. North American agreed to make the investigation of the tunnels' effects a subject of an early inquiry in the model testing program. The contractor also agreed that the "rolling tail" should be proved or disproved as quickly as possible. ^a

NACA computations indicated that the minimum design dynamic pressure should be 2,100 pounds per square foot and that 2,500 pounds per square foot would be desirable, while North American's design had proposed a design dynamic pressure of only 1,500 pounds per square foot. A structural weight increase of slightly over 100 pounds would enable the design to withstand the 2,500 pound pressure; conferees agreed that the weight increase was justified and that North American should alter the design to meet the 2,500 pound per square foot requirement. On the other hand, a government request that the design be altered to increase the design load factor from 5.25g to 7.33g at a 30-percent fuel-remaining condition involved a weight increase of another 135 pounds which the agencies and North American agreed might better be used to raise the design dynamic pressure. North American also agreed to raise the 35 feet per second negative gust velocity of the design to the 55 feet per second considered desirable by the government representatives.

In addition to the discussions on structural criteria, considerable attention was devoted to the proposed structural materials. At the time of the meeting, neither the WADC-NACA representatives nor the North American engineers seemed to have any detailed information that would permit a final decision on the materials to be used in such critical structures as the leading edge of the wing and the dive brakes. Such diverse materials as plastic, titanium carbide, copper, and cermets (ceramic metallics) were considered for the leading edges; the only definite conclusion was that North American would investigate the relative advantages of several proposed materials. It was agreed to retain the design features which would enable the leading edge to be easily detached and replaced. The NACA-WADC team pointed out that the assumption of laminar flow in heating calculations was unrealistic and North American agreed to build in accordance with the results obtained from calculations based on both laminar and turbulent flow. It was also agreed that 0.020-inch titanium alloy was a more desirable material for the internal structure of the wings and horizontal and vertical stabilizers than the 24ST aluminum that had been proposed, even though the use of titanium produced a weight increase of approximately seven pounds. Another weight increase of 13 pounds was approved in order to allow the substitution of an Inconel-X sandwich construction in place of the stainless steel dive brakes proposed by North American, and to allow for additional dive brake hinges. Other structural problems discussed included a change from titanium to Inconel-X for the oxygen tank because of the low-impact strength of titanium at low temperatures and the need to include a pressure system for stabilizing the propellant tanks. Pressurization of the propellant tanks had been considered undesirable and in the original design had not been provided. The decision to increase the design dynamic pressure from the original 1,500 to 2,500 pounds, together with North American's previous decision to utilize the tanks as structural components, made it necessary to accept pressurization or a large increase in structural weight. The decision was for pressurization of the tanks.

The WADC-NACA group and the North American engineers were in agreement that provision would have to be made for correcting any thrust misalignment and that further investigation would be needed to determine how such misalignment could be corrected and the amount of misalignment that would not be amenable to corrective shimming. The fact that the proposed design would probably be sensitive to roll-yaw coupling was also discussed and the acceptable limits were agreed upon.

In the area of control systems, the WADC-NACA group pointed out to the North American engineers that a rate damping system in pitch and yaw and possibly in roll would probably be necessary. North American estimated that the damping system would increase the weight of the design by approximately 125 pounds. A decision as to whether duplication of the damping system would be necessary was postponed until NACA's Ames Laboratory could be consulted. Conferees also decided that no damping system would be needed in the space control system. It was tentatively agreed that the pilot's controls should consist of a conventional center stick but that the aerodynamic controls should also be operable from a side controller on the right console and that the space controls would be operated by a second side-controller on the left console. The space control system was the subject of further discussion that ended with North American's agreement to duplicate the entire system and to provide three and one-half times the hydrogen peroxide' initially specified. The company also agreed to study the system with a view to minimizing fire hazards, shortening the peroxide lines, and relocating the peroxide supply nearer to the center of the airplane. Separate sources of peroxide would be provided for the reaction controls and the auxiliary power units. Engineers estimated that such changes in the reaction control system would result in a weight increase of about 117 pounds.

At the time of the meeting it was thought that WADC already had a satisfactory stable platform and it was agreed that this platform would be provided as government furnished equipment. NACA promised to provide a nose (then in the development stage) that would contain flight-path indication equipment.

Power-plant discussions were limited because the engine was still subject to extensive development and detailed information was nonexistent. The conference group decided, however, to increase the amount of helium provided for pressurization of the liquid oxygen tank, to study the possible relocation of the helium supply to some area other than inside the oxygen tank, and to redesign the tank transfer tube inlets and the top-off system. Pressure systems were to be protected with relief valves and frangible disks or with duplicate relief valves. The number of engine restarts was to be raised from three to at least five, shut off valves were to be provided in the main propellant lines, and provision was to be made for selective jettisoning of the propellants. Peroxide tanks were to be compartmented and separated, particularly from the engine compartment; main propellant vents were also to be separated and located at the rear end of the jettison lines. Blow-off doors were to be put around the engine compartment and it was agreed to omit a thrust measuring system because of the additional complication such a system would entail.

Final decisions on the exact nature of the auxiliary power plants were delayed to permit further study but there was general agreement that two auxiliary power units should be provided and that they should include completely separate systems compartmented by fire walls.

The discussion between the government and company engineers covered several additional fields including ground check-out equipment, tankage, crew provisions, landing gear, ground equipment, the electrical system, and fire detection and extinguishing. With the exception of the crew provisions, these items were rather briefly considered and included such decisions as the use of nitrogen as the fire extinguishing agent both for ground and air use, the recalculation of the tankage requirements for liquid oxygen, the possibility of providing a jettisonable ventral fin, the various types of ground servicing equipment that would be necessary, the need for providing adequate electrical power for restarts, and.making the electrical components explosion safe.

The discussions on crew provision were more detailed. North American agreed to design an ejection seat system and to make a study justifying the selection of a seat in preference to a capsule system. North American was also to provide suitable head and limb restraints for the accelerations to be encountered and to provide a means for external depressurization and canopy removal independent of the internal canopy jettison system. Transparent cockpit materials were to be studied (transparent plastics, like plexiglass were considered unsatisfactory) and deviations from standard cockpit dimensions were authorized. A gaseous oxygen system replaced the originally proposed liquid system and provisions were included for ram air ventilation below 20,000 feet. Nitrogen was to be used for cockpit pressurization.

The meetings came to a close with a presentation by Douglas engineers of some of the ideas contained in that company's X-15 design and a presentation by North American of its own rocket engine proposals for the X-15. The North American engine would have used oxygen and JP-4 or gasoline as the propellants. North American also presented the results of performance calculations based on the changes that had been discussed and determined upon at the meeting. ²

By January of 1956, North American's design had progressed rapidly enough to require decisions on several questions that had not been discussed or on which no final decisions had been reached at the October-November meetings of the previous year. An NACA group visiting North American on January 18 was asked to provide additional information and guidance on a plan to use a removable instrument rack for the main instrument compartment. Some instruments were to be mounted permanently in the fuselage tunnels, but North American felt a removable rack would provide ready access to the instruments and allow the removal of the instruments during ground operations. This latter feature was considered desirable in order to reduce the exposure of the instruments to ammonia fumes. North American also requested drawings of NACA research instruments and a statement as to which instruments would need to be shock mounted so that the company could complete its instrumentation plans. The company had also reached a stage in the design that required definite decisions on type and gauge of the wire to be used for thermocouples. North American also advised the NACA representatives of plans to use a modified ARC-48 radio communicator with four channels.

The subject of a stable platform came up ^b and, contrary to the statements made at the October-November meetings of the previous year (that Wright Field had a stable platform and would furnish it to the X-15 project), the NACA group was advised that no decision had been made as to who would furnish the platform. The company asked for further information on instrument duty cycles because without such information, engineers were having difficulty in determining auxiliary power plant loads and heating and cooling requirements.

The NACA representatives agreed with a North American suggestion that the ammonia tank vents could be closed on the ground after filling, thus permitting the pressure to stabilize at the vapor pressure of ammonia. As this procedure prevented boil off of ammonia, it eliminated the necessity for an ammonia top-off system.

Preliminary sketches of the aerodynamic side-controller were shown to the NACA group and as the sketches looked promising, plans were made to have NACA's Langley Laboratory evaluate the system envisaged by North American.

Other topics discussed at this time included the design of the dive brakes and a landing study conducted by North American. Full extension of the dive brakes at pressures of 2,500 pounds per square foot would have created excessive longitudinal accelerations and the brakes were therefore designed to open only to a point where the pressure on them would be 1,500 pounds per square foot. The brakes would then open progressively, maintaining a constant pressure at the 1,500 pound level until the full open position was reached.³

During the spring and summer of 1956, several scale models were exposed to rather intensive wind tunnel tests. A i/50-scale-model was tested in the 11-inch hypersonic and the 9-inch blowdown tunnels at Langley, and another in a North American tunnel. A i/15-scale model was also tested at Langley and a rotary-derivative model was prepared for test at the Ames Laboratory. North American gave thought to a plan to mount a small model on the nose of a rocket in order to obtain heat-transfer data under flight conditions. Langley, not fully approving of North American's plans, undertook the study of possible alternatives. The various wind tunnel programs included investigations of the speed brakes, horizontal tails without dihedral, several possible locations for the horizontal tail, modifications of the vertical tail, the fuselage side fairings, and control effectiveness. Another subject in which there was considerable interest was that of determining the cross-section radii for the leading-edges of the various surfaces. A free-flight model tested at Langley indicated that the X-15 would have satisfactory handling characteristics. (The NACA studies confirmed the desirability of control system dampers, while during the same period, North American arrived at the conclusion that the airplane could be flown safely without them.)

At the conclusion of a meeting of NACA, WADC, Navy, and North American representatives held at WADC on 2-3 May for the purpose of settling upon specifications, the subject of escape was taken up once more. WADC personnel apparently were not convinced that the ejection seat previously decided upon was going to be adequate, They pointed out that Air Force policy required an enclosed system in all new airplanes and that a change to some form of capsule would not only be in accordance with this policy, but would provide research data on such escape systems. Those opposed to the WADC view objected to any change on the grounds that it would disrupt time schedules, increase weight, and that there was still considerable ignorance about capsule design. The group that opposed the change felt that the safety features of the X-15's structure made the ejection seat acceptable. As a result of this meeting, North American was asked to document the arguments justifying the use of the ejection seat.

A meeting, held at Langley on 24 May and attended by WADC personnel as well as NACA, North American, and Eclipse-Pioneer representatives, explored the possibilities for obtaining a suitable stable platform for the X-15. It appeared that such a platform could be ready in 24 months and that 40 pounds of the estimated total weight of 65 pounds could be charged to research instrumentation rather than to the aircraft itself. ⁴

By June, NACA had completed the preliminary design for the spherical nose cone and had undertaken the construction of a heat transfer model. They were in the process of preparing detailed specifications for the award of a contract for the cone and its drive mechanism. ⁵

June was also the month in which formal assignment of Air Force serial numbers was made. The numbers were 56-6670 through 56-6672. Originally furnished by telephone on 28 May, these numbers were officially confirmed by the acting chief of the Contract Reporting and Bailment Branch on 15 June. ⁶

By July, NACA felt that sufficient progress had been made on the design problems presented by the X-15 to make an industry conference on the project worthwhile. Dr. Dryden, the director of NACA, invited WADC to participate in such a conference and asked that WADC review any material that might be suitable for presentation at the proposed conference. Dr. Dryden also asked that such material be summarized prior to August 8, as that date had been selected for a preliminary meeting of NACA, WADC, Navy, and North American representatives. The participants in the August meeting were to review the summarized material, decide whether the material was of sufficient interest to warrant an industry-wide meeting, and if the material did prove interesting, to make definite plans for a program to be conducted in October at one of the NACA's own facilities. ⁷

The material did prove interesting and the proposed conference was held at Langley Field, Virginia, on 25-26 October. Eighteen technical papers were presented to an audience of 313 individuals. Approximately ten percent of those attending the conference were representatives of various Air Force activities, and over half of these were WADC personnel. In view of the part which the Air Force had played in evaluating the original design and in the preliminary financing and procurement activities, it was surprising that there was absolutely no Air Force participation in the presentations. The majority of the twenty-seven authors who contributed papers were drawn from the NACA (16), while the remaining papers were authored by employees of the airframe (9) and engine (2) contractors.

It was evident from the papers presented at the industry conference that a considerable amount of valuable data had already been gathered but that a number of areas still awaited exploration. The airframe design differed from that originally envisaged by NACA and departed significantly from the design originally submitted by North American. The major external difference was a result of the need for additional directional stability at high angles of attack. This increased stability was provided by the addition of a ventral tail. One of the papers summarized the aerodynamic characteristics that had been obtained by tests in eight different wind tunnel facilities. ^c These tests had been made at Mach numbers ranging from less than 0.1 to about 6.9. The wind tunnel investigations were concerned with such problems as the effects of speed-brake deflection on drag, the lift-drag relationship of the entire aircraft, of individual components such as the wings and fairings, and of combinations of individual components. One of the interesting products was a finding that almost half of the total lift at high Mach numbers would be derived from the body-side fairing portion of the airplane. Another result was the confirmation of NACA's prediction that the original side fairings would cause longitudinal instability. (For subsequent testing the fairings had been shortened in the area ahead of the wing.) Still other wind tunnel tests had been conducted in an effort to establish the effect of the vertical and horizontal tail surfaces on longitudinal, directional, and lateral stability. Results of the wind tunnel tests were used to calculate the response characteristics of a configuration without dampers in order to determine if the aircraft would be flyable if the dampers should fail. Results indicated considerable instability and further investigations of alternate tail and rudder configurations were undertaken.

Other papers presented at the industry conference dealt with research into the effect of the aircraft's aerodynamic characteristics on the pilot's control. pilot-controlled simulation flights for the exit and entry phases had been conducted; researchers reported that the pilots had found the early configurations unflyable without damping, and that even with dampers the airplane possessed Only minimum stability for portions of the programmed flight plan. A program utilizing a free-flying model had proved low-speed stability and control to be adequate.

As some aerodynamicists had questioned North American's substitution of a differentially-operated horizontal tail for aileron control, the free-flying model had also been used to investigate that feature. The results indicated that such a tail provided the necessary lateral control.

Three of the papers presented at the conference dealt with aerodynamic heating. The first of these was a summary of the experience gained with the Bell X-1B. and X-2 aircraft. The information was incomplete and not fully applicable to the X-15, but it did provide a basis for comparison with the results of the wind tunnel and analytical studies. The second paper contained information derived from wind-tunnel tests on various bodies similar to those employed in the X-15. The third paper dealt with the results of the structural temperature estimates that had been arrived at analytically. It was apparent from the contents of the papers on aerodynamic heating that the engineers compiling them were confronted by a paradox. In order to attain an adequate and reasonably safe research vehicle, they had to foresee and compensate for the very aerodynamic heating problems that were to be explored by the completed aircraft.

In addition to the papers on the theoretical aspects of aerodynamic heating, a report was made on the structural design that had been accomplished at the time of the conference. The paper dealt with the wing, fuselage, and empennage. As critical loads would be encountered during the accelerations at launch weight and during reentry into the atmosphere, and as maximum temperatures would be encountered only during the second of these two phases, the paper was largely confined to the results of the investigations of the load-temperature relationships that were anticipated for the reentry phase. The selection of Inconel-X sheet as the covering for the multi-spar box-beam wing was justified on the basis of the strength and favorable creep characteristics of that material at 1,200 degrees Fahrenheit. A milled bar of Inconel X was to be utilized for the leading edge, as it was intended that that portion of the wing act as a heat sink. The internal structure of the wing was to be of titanium-alloy sheet and extrusions. The front and rear spars were to be flat web channel sections with the intermediate spars and ribs of corrugated webs of the same material. For purposes of the tests the maximum temperature differences between the upper and lower wing surfaces had been estimated to be 400 degrees Fahrenheit and that between the skin and the center of the spar as 960 degrees. Laboratory tests indicated that such differences could be tolerated without any adverse effects on the structure. Other tests had proved that thermal stresses for the Inconel-titanium structure were less than those encountered in similar structures constructed entirely on Inconel. Full scale tests had been made to determine the effects of temperature on the buckling and ultimate strength of a box beam, the amount of the deformations at varying loads, temperatures and temperature differences, to ascertain creep effects due to repeated loads and heating, to evaluate structural attachments and the effect of large temperature differences on the bending stresses of the spars. Simply heating the test structure produced no surface buckles. Compression buckles had appeared when ultimate loads were applied at normal temperatures but the buckles disappeared with the removal of the load. Tests at higher temperatures and involving large temperature differences had finally led to the failure of the test box, but it seemed safe to conclude that "thermal stresses had very little effect on the ultimate strength of the box."

Tests similar to those conducted on the wing structure had also been performed on the horizontal stabilizer. The planned stabilizer structure differed from the wing in that it incorporated a stainless steel spar about halfway between the leading and trailing edges, and an Inconel spar three and one-half inches from the leading edge. The remainder of the internal structure was to be similar to that of the wing in that it incorporated titanium components. The stabilizer skin was similar to that of the wing in being of Inconel-X sheet. Tests of the stabilizer had indicated that a design which would prevent all skin buckling would be inordinately heavy, so engineers decided to tolerate temporary buckles.. The proposed stabilizer had flutter characteristics that were within acceptable limits.

Brief summaries of the vertical tail and speed brake structures were also presented but as these components ultimately underwent extensive modifications, the items described had little relation to the final design.

The fuselage was to be of Inconel X. A semi-monocoque structure of titanium ribs and an inner aluminum skin were to be employed in the area ahead of the propellant tanks, and that section was to be insulated with spun glass. In the area of the propellant tanks, the circular fuselage was to be of full monocoque construction. One speaker pointed out that a full monocoque design would utilize only slightly thicker skins than a semi-monocoque design, would possess adequate heat sink properties, would reduce stresses caused by temperature differences by placing all of the material at the surface, and that the resulting structure would be ideal for use as a pressure tank. The design eliminated skin buckling and bulging, provided stiffness, had a uniformity that reduced fatigue and creep problems, and was simple to fabricate. The thickness of the monocoque walls would also make sealing easier and leaks less likely.

Fuselage problems which had not been resolved at the time of the industry conference included the reduction in buckling strength that was anticipated in the areas where the cooler internal rings of the tank bulkheads and wing support frames restrained the heated outer shell. It was known that this restraint would induce compression stresses in the shell and thereby reduce buckling strength. Another problem arose because of the side tunnels incorporated in the design. As the tunnels would protect the side portions of the circular shell from aerodynamic heating, the sides would not expand as rapidly as the areas exposed to the air and another undesirable compressive stress had to be anticipated. It was thought that beading the skin of the areas protected by the tunnels would provide a satisfactory solution but beading introduced further complications by reducing the structure's ability to carry pressure loads.

Structural design in the case of the X-15 definitely involved the propellant tanks. Each of the two main tanks was to be divided into three compartments by torus (curved) bulkheads; the two compartments furthest from the aircraft center of gravity were to be subdivided by slosh baffles. Plumbing was to be installed in a single compartment, the compartment sealed by a bulkhead, and the process repeated until all the compartments were completed. The tank ends were to be semi-torus in shape to keep them as flat as possible, to reduce weight, and to permit thermal expansion of the tank shell. This entire structure was to be of welded Inconel X. At the time of the industry conference a full-size test specimen was under construction for the purpose of testing tank pressures, external loads, temperature environments and leakage rates. A wing support frame and a section of the fuselage tunnel were to be included in the test structure in the hope that the experimental section would provide valuable static test data prior to the completion of an actual fuselage for the X-15.

Because the X-15 was expected to produce large accelerations, it seemed best to develop a side controller that would allow the pilot's arm to be restrained by an armrest without depriving him of full control over the aircraft. At the time of the industry conference in 1956, the design for the X-15 side controller had not been definitely established but a summary of the previous experience with such controllers was available. Experimental controllers had been installed on a Grumman F9F-2, a Lockheed TV-2, a Convair F-102, and on a simulator. The pilots who had tried side controllers had reported no difficulty in maneuvering, but they generally felt that greater efforts would have to be made to eliminate backlash and to control friction forces; they had also urged that efforts be made to give the side controllers a more "natural" feel.

Another problem which had not been thoroughly explored at the time of the 1956 conference concerned the proposed reaction controls that would be necessary for the X-15 as dynamic pressures decreased to the point where the aerodynamic controls would no longer be effective. Analog-computer and ground-simulator studies were then under way in an effort to determine the best relationship between the control thrust and the pilot's movement of the control stick. Attempts were also being made to determine the amount of fuel that would be required for the control rockets. No significant problems were uncovered during these early investigations, but it was clear that the pilot would have to give almost constant attention to such a control system and that pilots who were to use this form of control should be given extensive practice on simulators before being allowed to attempt actual flight.

As in the case of the other papers presented to the 1956 industry conference, the report on ground and aircraft instrumentation was very tentative in nature. Nevertheless, plans were already well along for the establishment of ground tracking stations to assist the pilot with data and advice, to record accurate measurements, and to provide navigational assistance to both the X-15 and its mother aircraft. Such a range would also prove valuable for search in case of emergency. This ground range was to be established along a line extending from Wendover AFB, Utah, to Edwards AFB, California, and was to have installations at Ely and Beatty in Nevada as well as at Edwards. The range was to be equipped so as to determine velocity, range, elevation, and azimuth with radar. Engine and aerodynamic data were to be transmitted from the X-15 by telemetering and voice radio. Each ground station was to overlap the next and all were to be interconnected so that timing signals, voice communication, and radar data would be available to all. The timing signals were to originate at Edwards. Provision was to be made for recording the acquired data on tape and film; some was to be directly displayed. Design and fabrication of this complex had been undertaken by the Electronic Engineering Company of Los Angeles. Project planners estimated that the range would be ready for operation by 1958.

In the X-15 itself, provision was being made for a pressure recorder in the nose, a main instrument compartment directly behind the pilot, and for accelerometers and other small sensing devices in a center-of-gravity compartment.

Some of the anticipated difficulties in the field of instrumentation arose because available Strain gauges were not considered satisfactory at the expected high temperatures and because of difficulties in recording the output of thermocouples. Large structural deformations of wings and empennage were to be recorded by cameras in special camera compartments.

Another instrumentation problem arose because the sensing of static pressure, ordinarily difficult at high Mach numbers, was compounded in the case of the X-15 by heating that would be too great for any conventional probe and by the low pressure at the high altitudes to be explored. Project personnel hoped that a stable-platform-integrating-accelerometer system could be developed to provide velocity, altitude, pitch, yaw, and roll angle information. Available accelerometer systems were limited to two axes and were too large and heavy for X-15 use, but it appeared that a three-axis platform within the space and weight limitations of the X-15 could be developed, and at the time of the meeting in 1956, manufacturer's proposals for such a system were being considered.

An unsolved problem was that of recording outside temperatures. The only solution appeared to be the use of radiosondes, but that was not completely satisfactory as such devices were limited to altitudes of about 100,000 feet, far less than the altitude to be attained by the X-15.

Still another instrumentation difficulty was created by the desirability of presenting the pilot with angle-of-attack and side slip information, especially for the critical exit and reentry periods. Any device to furnish this information would have to be located ahead of the aircraft's own flow disturbances, would have to be structurally sound at elevated temperatures, would have to be accurate at low pressures, and would have to cause a minimum of flow disturbance so as not to interfere with the heat transfer studies that were to be conducted in the forward area of the fuselage. These requirements had led to the development of a null balance sensing device, preliminary work had resulted in the design of a six-inch Inconel sphere capable of withstanding 1,200 degree temperatures. The sphere, to be placed in the nose of the X-15, was to be gimbaled and servo-driven in two planes. It was to have five openings: a total-head port opening directly forward and two pairs of angle-sensing ports in the pitch and yaw planes, located at an angle of 30 to 40 degrees from the central port. (Pitch and yaw of an aircraft could be sensed as pressure differences and these differences converted into signals that would cause the servos to realign the sphere in the relative wind.) As a null-balance device had no source of static pressure, it was not suitable for furnishing indicated airspeed, so some alternate pitot-static system would be necessary to provide the airspeed information required for landing the X-15 safely.

The report on crew provisions and escape presented to the 1956 industry conference dealt with escape, cockpit environment, pilot's working area, flight accelerations, landing, and landing gear stability.

Two main criteria had governed the selection of an escape system for the X-15, and these two criteria were not necessarily complementary. The first requirement had been that the system be the most suitable that could be designed while remaining compatible with the airplane. The second requirement had been that no system would be selected that would delay the development of the X-15 or leave the pilot without any method of escape when the time arrived for flight testing the completed vehicle. The four possible escape systems that were considered included cockpit capsules, nose capsules, a canopy shielded seat, and a stable-seat, pressure-suit combination. An analysis of the expected flight hazards had indicated that because of the fuel exhaustion and low aerodynamic loads, the accident potential at peak speeds and altitudes was only about two percent of the total accident potential.

The final decision for a stable-seat, pressure-suit combination was made because most of the potential accidents could be expected to occur at speeds of Mach 4 or less, because system reliability always decreased with system complexity, and finally, because it was the system that imposed the smallest weight and size penalties upon the aircraft. The selected system would not function successfully at altitudes above 120,000 feet and speeds in excess of Mach 4, but designers held that the aircraft itself would be its own best escape capsule in the areas where the seat-suit combination was inadequate.

The preliminary ejection seat design utilized a rocket-type ejection gun. One proposed version was fin-stabilized and another incorporated a skip-flow generator. ^d A preliminary decision had been made to use the skip-flow type. The seat also incorporated restraining devices for the pilot's extremities.

An emergency oxygen system was to be capable of providing suit pressurization and a breathing supply for a period of twenty minutes. The pressure suit was to be similar to those already in development for high performance military aircraft. Such a suit was considered adequate for protection against the ozone hazard and it had been decided that there was no necessity for concern with exposure to cosmic rays. Concern was expressed, however, for the problem of rapid pressure changes during the various stages of the ejection sequence. Researchers concluded that careful consideration would have to be given to the possible pressure surges within the helmet and their potential for damaging the pilot's ears and lungs. It had already been determined that the proposed suit materials could withstand the maximum pressure and temperatures to which they would be subjected within the operational limits of the escape system as a whole.

The plans for the cockpit environment of the X-15 were based on the use of nitrogen. Cockpit and instrument cooling, pressurization, suit ventilation, windshield defogging, and fire protection were all to be provided from a liquid nitrogen supply. Vaporization of the liquid nitrogen would keep the pilot's environment within comfortable limits at all times. An interesting aspect of the cooling problem was an estimate that only 1.5 percent of the system's capacity would be applied to the pilot; the remaining 98.5 percent was required for the equipment. Cockpit temperatures were to be limited to no more than 150 degrees, the maximum limit for some of the equipment. The pilot would not be subjected to that temperature, however, as the pressure suit ventilation would enable him to select a comfortable temperature level for himself. Cockpit pressure was to be maintained at the 35,000 foot level and as the pressure suit was also designed to operate at the same level if cabin pressure should fail, there would be no pressure variations during the exploratory phases of the X-15's flights and the pilot would have adequate protection against explosive decompression. Provision was to be made for the pilot to clear the cockpit area of its nitrogen atmosphere by the use of ram air pressure.

The various switches and controls were to be selected and placed to minimize pilot movements. Instrument, warning light, and control location had been determined by analysis of the pilot's duties and instruments were to be arranged in a manner that would permit a maximum of attention to be directed toward one area at a time. Visibility from the cockpit would be excellent, but some questions remained unanswered as to the vision-degrading effects of heat distortion from the hot windshield. Key to detailed layout of the cockpit was the planned use of side controllers and the possible elimination of the center stick. Scott Crossfield, a former NACA test pilot who was to make the initial X-15 flights as a North American pilot, commented that the decision to abandon the center stick would rest on the results of further tests and the necessity to "break with tradition." (He may have reflected that the world's first military airplane was guided with side controllers and that it had no center stick.)

The effects of flight accelerations upon the pilot's physiological condition and upon his ability to avoid inadvertent control movements had not been completely explored, but it was recognized that high accelerations could pose medical and restraint difficulties. In addition to the accelerations that would be encountered during the exit and reentry phases of the X-15's flights, a very high acceleration of short duration would be produced during the landings. This latter acceleration was a result of the location of the main skids at the rear of the aircraft. Once the skids touched down, the entire aircraft would act as if it were hinged at the skid attachment points and the nose section would slam downward. Reproduction of this landing acceleration on simulators showed that because of the short duration, no real problem existed. There were however, numerous complaints about the severity of the jolts.

The 1956 industry conference heard two papers on the proposed engine and propulsion system for the X-15. The first of these dealt only with the engine, the second with the installation of the engine and its associated systems in the aircraft. At the time of the conference the proposed XLR99-RM-1 engine was scheduled to have a variable thrust of from 19,200 to 57,200 pounds at 40,000 feet. It was to employ anhydrous ammonia, liquid oxygen and a 90-percent hydrogen peroxide solution as propellants, was to have a dry weight of 618 pounds, and a wet weight of 748 pounds. Specific impulse was to vary from a minimum of 256 seconds to a maximum of 276 seconds. The proposed engine was to fit into a space with a length of 71.7 inches and a diameter of 43.2 inches. A single thrust chamber was to be supplied by a turbopump with the turbopump's exhaust being recovered in the thrust chamber. A two-stage impulse turbine was to drive a dual inlet fuel pump and a single inlet oxidizer pump. Thrust control was by regulation of the turbopump speed, the regulation to be accomplished by a pilot-controlled governor. ^e

In the design stages of the XLR99's development, Reaction Motors was concerned with the engine's safety and reliability in terms of the requirement to produce an engine that could be throttled and that would meet the established specifications. (Another factor of some importance was the requirement that the engine should be capable of being restarted.)

The decision to control the engine's thrust by regulation of the turbopump's speed was made because the other possibilities (regulation by measurement of the pressure in the thrust chamber or of the pressure of the discharge) would cause the turbopump to speed up as pressure dropped. As the most likely cause of pressure drop would be cavitation in the propellant system, an increase in turbopump speed would aggravate rather than correct the situation. Reaction Motors had also decided that varying the injection area was too complicated a method for attaining a variable thrust engine and had chosen to vary the injection pressure instead.

The regenerative cooling of the thrust chamber created another problem for the designers as the varying fuel flow of a throttleable engine meant that the system's cooling capacity would also vary and that adequate cooling throughout the engine's operating range would produce excess cooling under some conditions. Engine compartment temperatures also had to be given more consideration than in previous rocket engine designs because of the higher radiant heat transfer from the structure of the X-15.

The restart requirements for the XLR99 introduced some additional complications, particularly in regard to safety provisions. At the time of the conference, a two-stage ignition system was planned; the effort to produce a fail-safe design for the ignition system and the engine itself necessitated a purge system, inert gas bleed for both stages of the ignition and thrust chamber, and the duplication of numerous system components. On the other hand, the fact that both fuel and oxidizer were volatile reduced the hazard of an unsafe accumulation of propellants in the system.

Reaction Motor's spokesman at the conference of 1956 concluded that the development of the XLR99 was going to be a difficult task. Subsequent events were certainly to prove the validity of that assumption.

A second paper dealt with engine and accessory installation, the location of the propellant system components, and the engine controls and instruments. The main propellant tanks were to contain the liquid oxygen (LOX), ammonia, and the hydrogen peroxide. The oxygen tank, with a capacity of approximately 1,000 gallons, was to be located just ahead of the aircraft's center of gravity; the ammonia tank, with a capacity of approximately 1,400 gallons, just aft of the same point. A center core tube within the oxygen tank would provide a location for a supply of helium under a pressure of 3,600 pounds per square inch. Helium was to be utilized for the pressurization of both the oxygen and ammonia tanks. A 75-gallon hydrogen peroxide tank behind the ammonia tank was to provide the monopropellant for the engine's turbopump. An additional supply of helium was to be utilized for pressurizing the monopropellant tank. The LOX and ammonia tanks were designed with triple compartments arranged to permit both propellants to be forced toward the center of gravity as they were expelled, either during normal operations or jettisoning. The transfer tubes between compartments demanded considerable study because the high accelerations of the X-15 would tend to force the contents of the tanks toward one end or the other. The compartmental divisions were further complicated by the necessity for efficient fueling and the need to keep the quantity of propellants remaining after burnout or jettisoning at the lowest possible figure. As the acceleration, efficient fueling, and maximum evacuation called for features not entirely compatible, compromises were necessary. Fortunately, no insoluble problems arose during early tests.

Provision was also made for top-off of the LOX tank from a supply carried aloft by the mother aircraft. Top-off from the mother airplane was considered to be beneficial in two ways. The LOX supply in the mother ship could be kept cooler than the oxygen already aboard the X-15, and the added LOX would permit cooling of the X-15's own supply by boil-off, without reduction of the quantity available for flight. The ammonia tank was not to be provided with a top-off arrangement, as the slight increase in fuel temperature during carried flight was not considered significant enough to justify the complications such a system would have entailed.

A suitable material had not yet been selected for the tank that was to contain the high-pressure, low-temperature helium supply for propellant tank pressurization. The entire propellant system presented problems difficult to foresee, primarily because of the large variations of temperature and pressure that would occur during a single flight of the X-15.

Because engine vibration characteristics were unknown, the engine mount was designed to be rigid without any special effort at vibration shielding. The engine-mount truss was to join the thrust chamber at several points and was to be attached to the fuselage by three fittings designed so that the top attachment provided the main pivot point. The two lower fittings were to be adjustable to allow accurate alignment of the engine's thrust vector.

Three large removable doors were to provide access to the engine area and to permit observation of the engine by closed circuit television cameras during ground testing of the engine. The entire engine compartment was designed to explode open at a pressure lower than that which the forward structure was capable of withstanding, thus providing relief in case of an engine explosion. As engine compartment temperatures were not expected to be a problem, no insulation was being planned.

In 1956, the cockpit engine instruments had not been finally selected and the preliminary choices were to be altered as the engine and the propellant system were developed. A throttle with an engine prime switch was to be located on the left console, with the tank pressurization switches and jettisoning controls in the immediate vicinity of the throttle. Electric switches were to be provided for engine arming, for fire extinguisher control, and for master control. Space was allotted for several indicators to furnish the pilot with pressure information on the propellant and engine systems. A place had also been reserved for six lights to indicate various engine malfunctions. It had also been decided that the pilot would need an instrument (a totalizing impulse indicator) capable of showing the total thrust remaining at any given instant during powered flight.

The final paper presented to the 1956 industry conference was a summary of the preceding papers and of the major problems that existed at that time. The author considered flutter to be an unsolved problem, primarily because of a lack of basic data on aero-thermal-elastic relationships and because little experimental data was available on flutter at the hypersonic Mach numbers that would be reached by the X-15. He pointed out that available data on high speed flutter had been derived from experiments conducted at Mach 3 or less, and that not all of the data obtained at those speeds were applicable to the problems faced by the designers of the X-15. He felt that the solution of the problem was full-scale robot testing of X-15 components. Another difficulty was the newness of Inconel-X as a structural material and the necessity of experimenting with fabrication techniques that would permit its use as the primary structural material for the X-15. Problems were also expected to arise in connection with sealing materials, most of which were known to react unfavorably when subjected to high temperature conditions. Preliminary wind-tunnel tests had also indicated that the original configuration of the X-15 did not have adequate stability and that modification and further testing would be essential.

The closing portion of the final paper dealt briefly with North American's schedule for drawings, jig construction, and fabrication of the aircraft. ⁸

That the design features of the X-15 presented to the industry conference in October 1956 were only tentative was made apparent by the results of a development engineering inspection held at North American's Inglewood plant on 12-13 December 1956. This inspection of a full-scale mockup was intended to reveal unsatisfactory design features before fabrication of the aircraft got under way. Thirty-four of the forty-nine individuals who participated in the inspection were representatives of the Air Force, and twenty-two of them were from Wright Air Development Center. The important role of the Air Force in the determination of the X-15's design was evident from the composition of the committee chosen to review the alteration requests. Major E. C. Freeman, of the Air Research and Development Command, served as committee chairman, Mr. F. Orazio of Wright Air Development Center and Lieutenant Colonel K. C. Lindell of Air Force headquarters were committee members, and Captain C. E. McCollough Jr. of the Air Research and Development Command and Captain I. C. Kincheloe of the Air Force Flight Test Center served as advisors. The Navy and NACA each provided a single committee member; three additional advisors were drawn from the staff of the NACA.

The inspection committee considered 84 requests for alterations, decided to reject 12, and placed 22 in a category requiring further study. The change requests covered a variety of features, including the controls, electrical and hydraulic systems, the escape system, and the power plant. Some of the accepted changes were the addition of longitudinal trim indications from the stick position and trim switches, relocation of the battery switch, removal of landing gear warning lights, rearrangement and redesign of warning lights, and improved marking for several instruments and controls. Other accepted recommendations concerned improved wiring for the fire detection system, improved insulation of sensitive electrical equipment, inclusion of an overheat warning system for hydrogen peroxide compartments, and the relocation of some of the electrical wiring in order to protect it from hydraulic fluids and to reduce the possibility of damage during the installation and removal of equipment. Inspection personnel also requested that the escape system be provided with better markings, that safety pins be identified by streamers, and that a dependable linkage be installed between the canopy and seat catapult initiator. Still other approved changes concerned such items as a lock for the LOX filler cover, and improvement of the hydraulic system by the substitution of some components and by better installation and marking. The landing gear was the subject of a number of suggestions, including the elimination of cadmium plating on certain heat treated steels employed in the gear, provision for inspection panels, the use of new tires on each of the early flights, and for additional design and testing of all components of the skids and nose gear.

The requested changes in the propulsion system were concerned with the improvement of the hydrogen peroxide system by the inclusion of better leak protection methods, by better support for the tanks, and relocation of shut-off valves. Inspection personnel also recommended attention be given to keeping engine components in locations where they could be easily inspected and maintained, that adequate drainage and ventilation be provided for the engine compartment, that North American provide engine mounts to Reaction Motors in order to simplify engine handling and installation, and that engine mount bolts be safetied. Improvements were also asked if the design of the jettisoning system and in the identification and marking of the propellant system's components.

Some of the most interesting of the proposed changes were rejected by the committee. For instance, the suggestions that the aerodynamic and reaction controller motions be made similar, that the reaction controls be made operable by the same controller utilized for the aerodynamic controls, or that a third controller combining the functions of the aerodynamic and reaction controllers be added to the right console, were all rejected on the grounds that actual flight experience was needed with the controllers already selected before a decision could be made on worthwhile improvements or combinations. As two of the three suggestions on the controllers came from potential pilots of the X-15 (J. A. Walker of the NACA and Captain Kincheloe), it would appear that the planned controllers were not all that might have been desired. A warning light for the canopy lock was also rejected, as was the suggestion that the pilot be provided with easier entrance and exit by extension of the canopy's travel - both on the grounds that the existing provisions were adequate. Simplification of the hydraulic system on the first airplane was ruled out on the basis that there was nothing that could be spared. A request that the pilot be provided with continuous information on the nose-wheel door position (because loss of the door could produce severe structural damage) was rejected because the committee felt that the previously approved suggestion for gear-up inspection panels would make such information unnecessary. A suggested study of the ignition and fire hazard potential of the various mixtures that might accumulate in the engine compartment was held to be unnecessary in light of the ventilation provisions for that compartment. Joining the auxiliary power plant exhausts in a single manifold to avoid out-of-trim moments if one auxiliary power plant should fail was not felt to be necessary. A suggested addition of check valves in the hydrogen peroxide system was considered to have been adequately taken care of by previously accepted suggestions. A request for entirely separate systems for each auxiliary power unit and for the ballistic controls was supported by the argument that separate systems had been requested earlier. (Such separate systems had been accepted during the meetings held at the North American plant in the fall of 1955.) In spite of the earlier plans for such separate systems, the committee held that with the addition of shut off valves, the system would be adequate as installed.

An even more surprising rejection of a requested change occurred in regard to changeable leading edges. An NACA representative (Harry J. Goett of Ames Aeronautical Laboratory) asked that the lower flange of the front spar be widened and that the ballistic roll controls be moved to the rear of the same spar. He justified these requests on the grounds that the research goals for the X-15 included investigations to determine the best materials, profiles, and cooling methods for various leading edges; that interchangeable leading edges had been a part of the original proposals; and that North American had originally agreed to make the leading edge detachable. In spite of Mr. Goett's apparently logical arguments, the committee decided his request could not be honored. The reasons for their rejection of the request were that North American had already determined to use a solid plate for the lower wing surface and that the required changes would impose a three-pound weight penalty. It seemed to at least one participant that the negative decision on interchangeable leading edges marked the abandonment of a feature that would have considerably enhanced the research value of the X-15.

That a number of design features still were unsettled as late as the mockup inspection of 13 December 1956, was indicated by the 22 change requests placed in a category requiring further study. Some of these deferred requests were concerned with the B-36 carrier aircraft, which was eventually eliminated; other change requests required feasibility studies, however, and some needed further study as to desirability. The deferred requests included such suggestions as the complete elimination of the center control stick, a study of antenna locations to insure there would be no adverse effects on directional stability, the installation of an engine ignition gauge, a LOX top-off indicator, and improvements in the rigidity and alignment of the accelerometer mounts. Doubts were expressed about the adequacy of a single antenna for transmitting and receiving radar signals and further studies were promised. A request for hydraulic pressure indication when both generators were out was also deferred until it could be determined if such indication was feasible. Three deferred requests on the escape system involved the continuing development of seat and pressure suit by further sled and tunnel tests, a study to determine the desirability of a spoiler plate to be located ahead of the cockpit and operated in the canopy ejection sequence, and the selection of an improved location for the canopy's emergency release handle.

Other requests which the committee decided to be worth further study included the replacement of machine screws by quick fasteners for some of the fuselage access panels, vibration testing of propellant lines, relocation of components of the helium system to minimize the possibility of leaks, and the use of expulsion bags in the hydrogen peroxide tanks. Further study was to be conducted to determine the best type of bag or diaphragm for hydrogen peroxide expulsion, the adequacy of the helium tank mounts, the ability of the propellant lines to withstand stresses imposed by engine misalignment, and the feasibility of starting the engine ignition system prior to launch. A request to approve the shift of all controls and switches to locations where they could be easily reached from the pilot's normal seated position, (even when the pilot was of small stature), received the "further study" classification, but in this case the group also authorized such changes as appeared necessary. ⁹

After the completion of the development engineering inspection, the X-15 airframe design changed only in relatively minor details. North American essentially built the X-15 described at the industry conference in October and inspected in mockup in December. (Continued wind tunnel testing resulted in some external modifications, particularly of the vertical tail, and some weight changes occurred as plans became more definite.) But while work on the airframe progressed smoothly, with few unexpected problems, the project as a whole did encounter difficulties, some of them serious enough to threaten long delays. In fact, North American's rapid preparation of drawings and production planning served to highlight the lack of progress on some of the components and sub-systems that were essential to the success of the program.
 

 

Section III  The Propulsion Story

 

Those concerned with the success of the X-15 had to monitor the development of the proposed XLR99 rocket engine, the auxiliary power plants, an inertial system, a tracking range, a pressure suit, and an ejection seat. They had to make arrangements for support and mother aircraft, for ground equipment, for the selection of pilots, and for the development of simulators for pilot training. It was necessary to secure time on centrifuges, in wind tunnels and on sled tracks. The NACA ”Q-ball” nose had to be developed, studies made of the compatibility of the X-15 and the mother aircraft, other studies on the possibility of extending the X-15 program beyond the goals originally contemplated and on the potential of the X-15 as a trainer in other space programs. In addition to such tasks, funds to cover ever increasing costs had to be secured if the project were to have any chance of ultimate success, and at certain stages, the effects of possibly harmful publicity had to be considered. With such multiplicity of tasks, it could be expected that difficulties would be encountered and several serious problems did arise. Probably the most serious difficulties, and certainly those which gave rise to the greatest concern, arose during the development of the XLR99 engine.

A suitable engine for the X-15 had been somewhat of a problem from the earliest stages of the project, when the WADC Power Plant Laboratory had pointed out that the lack of an acceptable rocket engine was the major shortcoming of the NACA's original proposal. While the Power Plant Laboratory felt that the Hermes A-1 engine selected by the NACA planners was not capable of being developed into a safe engine for a manned vehicle, no very practical alternative was immediately available. The laboratory did suggest several engines ”more suitable” for manned aircraft, but essentially WADC urged further study before the final selection of a specific engine. In October 1954, the representatives of the Air Force, Navy, and NACA, who were planning the X-15 competition, selected four engines as possible X-15 power plants. They did not forbid proposals to use engines other than those named, but a bidder who desired to utilize another engine was faced with the additional complication of joining with the manufacturer of the proposed engine to produce a justification for the selection of a non-listed item. The justification was to be presented to the Weapon System Project Office and that office could approve or disapprove the use of the engine. ¹

The first really concrete descriptions of the proposed X-15 engines appeared in correspondence of 15 November 1954. Mr. T. J. Keating, chief of the Non-Rotating Engine Branch, Power Plant Laboratory, wrote Mr. J. B. Trenholm, of the systems directorate, as a result of a conference of 22 October 1954. The conference, held at the Directorate of Laboratories and attended by representatives of the Navy, Air Force, and the NACA, had been for the purpose of planning the procurement procedures to be followed in selecting a contractor for the X-15. Those attending the conference evidently felt that they did not have adequate information on rocket engines; Mr. Keating's note constituted a summary of the status of rocket engines under development at that time, particularly those that seemingly could, with further development, be made into suitable power plants for the X-15.

The Power Plant Laboratory did not believe that any available engine was entirely suitable for the X-15 and held that no matter what engine was accepted, a considerable amount of development work could be anticipated. Most of the possible engines were either too small or would need too long a development period. In spite of these reservations, the laboratory listed a number of engines worth considering and drew up a statement of the requirements for an engine that would be suitable for the proposed X-15 design. The laboratory also made clear its stand that the government should ”… accept responsibility for development of the selected engine and … provide this engine to the airplane contractor as Government Furnished Equipment.” ²

The primary requirement for .an X-15 engine, as outlined by the Power Plant Laboratory in 1954, was that it be capable of operating safely under all conditions. Service life would not have to be as long as for a production engine, but engineers hoped that the selected engine would not depart too far from production standards. The same attitude was taken toward reliability, that is, the engine need not be as reliable as a production article, but it should approach such reliability as nearly as possible. There could be no altitude limitations for starting or operating the engine, and the power plant would have to be entirely safe during start, operation or shutdown, no matter what the altitude. The engine was also to be capable of safe operation under the highest ”g” conditions to be encountered during the operation of the X-15.

The Power Plant Laboratory did not try to define the exact thrust values to be attained by the selected engine, holding that such a determination would have to await a more complete definition of the aircraft itself. However, the laboratory did make it quite clear that a variable thrust engine capable of repeated restarts was essential. Again, laboratory specialists did not try to set the range of variability or the number of restarts, preferring to wait until more was known about the X-15 design itself.

The laboratory also warned that none of the engines tentatively selected was entirely satisfactory for the proposed program; the list was composed of engines with the best possibilities for development into a suitable power plant for the X-15. To assist in evaluation, the Power Plant Laboratory prepared a summary of the current status of each engine, and forwarded an estimate of necessary changes to development objectives and development schedules in order to produce an adequate engine within the time limit imposed by the X-15 program.

The engine ultimately selected was not one of the four originally presented as possibilities by the Power Plant Laboratory. The original list included the Bell XLR81, the Aerojet General XLR73, North American's NA-5400 and Reaction Motors' XLR10. The ultimate selection was foreshadowed, however, in discussions of Reaction Motors' XLR10, during which attention was drawn to what was termed ”… a larger version of Viking engine (XLR30).” ³ In the light of subsequent events, it was interesting to note that the laboratory thought the XLR30 could be developed into a suitable X-15 engine for ”less than $5,000,000” and with ”approximately two years' work.” ^{4 a}

(The AMC letter of 30 December 1954, which invited selected members of the aircraft industry to participate in the development of a new research aircraft, incorporated the Power Plant Laboratory's recommendations of November, in their entirety.) ⁵

During the month of January, additional interest was shown in the XLR30 engine as a possible X-15 power plant; on 25 January 1955, AMC asked Reaction Motors for additional details on that company's engines. ⁶Reaction Motors replied on 3 February 1955 by elaborating on the details of both the XLR10 and the XLR30. The firm recommended four possible combinations as being suitable for the X-15 program: an XLR30 using liquid oxygen and anhydrous ammonia, an XLR30 using liquid oxygen and a hydrocarbon fuel, an XLR10 using liquid oxygen and ethanol, and an engine to be composed of two XLR10 chambers fed by a single XLR30 turbopump. All four versions utilized hydrogen peroxide for turbopump drive. Evidently Reaction Motors already had an idea of what the airframe contractors were planning, for the company frankly stated doubt that one XLR10 was ”… adequate to perform the objectives of this type of aircraft.” Reaction Motors also recommended against an attempt to make the XLR30 operable with hydrocarbon fuel, largely because the company felt this version of the engine would require a longer development period than would the version utilizing anhydrous ammonia. Again, Reaction Motors preferred the XLR30 over the proposed combination of XLR10 thrust chambers with an XLR30 turbopump. This last choice was made because, at relatively the same cost, the single chamber XLR30 would result in a simpler and more reliable engine. Reaction Motors also pointed out that the volatility of anhydrous ammonia would make for safer restarts, that the XLR30 would need fewer parts, it would be simpler to install than the configuration utilizing two XLR10 chambers.

In summing up arguments for the XLR30 utilizing ammonia as a fuel, Reaction Motors stated that the cost of such an engine would be as low as any of the configurations, that it would be simpler and more reliable, and that its weight would be only 420 pounds compared to 815 pounds for the double-chambered engine. The company also estimated that the XLR30 could be throttled to 30 percent of full thrust, permitting a variation between 17,000 and 57,000 pounds of thrust at 40,000 feet. A specific impulse of 278 seconds seemed possible at full rated thrust. The compact size of the XLR30 (installation space was to be 30 inches in diameter and only 70 inches in length) was given as an additional reason for preferring that engine over the larger XLR10-XLR30 combination. ⁷

While Reaction Motors was clearly interested in promoting the anhydrous ammonia version of the XLR30, the Air Force still favored the XLR10. On 4 February 1955, AMC asked Reaction Motors for still more detailed information on the XLR10. ⁸ On the same date a conference between Reaction Motors and the Air Force decided that all data submitted for the proposed X-15 engine would be for the XLR30 rather than for the XLR10. The Reaction Motors' representatives indicated that the XLR10 would need considerable development if it was to be made into a safe engine at all flight attitudes. They contended that since both engines required further development, the XLR30 was the better choice because it would ultimately be a superior engine. Reaction Motors' opinion prevailed, and on 24 February, the company was advised that it ”… should make all further estimates on the basis of the XLR30's development.” ⁹

The engine information submitted by Bell, Aerojet, and Reaction Motors was forwarded to the prospective airframe bidders on 18 March 1955. ¹⁰

On 22 March the project office forwarded its comments on the data furnished three days earlier. Among the comments was the statement that the Bell and Aerojet engines would probably have to be used in multiples if the thrust requirements of the X-15 were to be met. Prospective contractors were also advised that the engine that was eventually to be furnished would be capable of safe operation, whether or not fuel and oxidizer exhaustion was signaled to the pilot. The fact that the developed engine was not being considered as a production item was made clear, and the airframe manufacturers were told that the operating time of the engine should only be limited by the amount of the propellants available. The prospective contractors were optimistically told they could expect the selected engine to be ready for flight test use within 30 months after the airframe contract was signed, but they were also warned that there probably would be some change in weights as a result of the development effort. ¹¹

On 26 April 1955, WADC received approval from Headquarters ARDC for a plan to require detail configurations of the engines involved in the X-15 program. Command headquarters requested that "the engine program be subjected to a final critical review apart from, but concurrent with the evaluation of the airframe proposals." WADC was advised to get a firm commitment from each of the engine contractors and to include the results of the engine evaluations in support of the recommendations on the X-15 itself. ¹²

On 20 June 1955, the Directorate of Weapon Systems Operations asked the Power Plant Laboratory for an evaluation of the proposed engines for the X-15. The laboratory was advised that the evaluation was to be conducted in cooperation with the Navy and NACA, and that the results were needed by 12 July. ¹³ Results of the requested evaluation were forwarded to the project office in mid-July. Evaluations were based in part on briefings presented by the contractors on 14 June and on the outcome of evaluation meetings held on 15–16 June and 6–7 July .

The evaluation group reported that none of the proposed power plants had sufficient superiority over the others to justify changing the engine selected by the contractor with the best X-15 design. As none of the X-15 proposals included the Aerojet engine as a first choice, that company's XLR73 was eliminated from final consideration. ^b The evaluators felt that the Bell engine was more likely to be developed within the time limits of the project but that its superiority in this respect was so small as not to dictate its choice over the engine proposed by Reaction Motors. At the time of the evaluation, the cost of the Bell engine was estimated at $3,614,088 while a figure of $2,699,803 was given for the engine proposed by Reaction Motors.

In comparing the relative merits of the Bell and Reaction Motors' proposals, the Power Plant Laboratory pointed out that the internal fuel and gas generator systems of the Bell design each utilized two fuels interchangeably and that this feature made for complicated valving and fuel flow systems. It seemed probable that the separate starting system for meeting the repeated start requirements of the X-15 engine would create some problems of safety and reliability. The Reaction Motors' engine, while more orthodox than the Bell, had been little tested. The laboratory correctly predicted that difficulties would be encountered in attempting to achieve an acceptable service life and the required degree of reliability. In considering the safety of the two designs, the laboratory reported that Bell had more experience than had Reaction Motors, but that both designs would need additional development before either could be considered a safe engine for a manned aircraft. The laboratory was also correct in predicting that the thrust chamber cooling of the Reaction Motors' design might present some difficulty.

The Power Plant Laboratory judged the designs on the basis of their feasibility, safety, reliability, performance characteristics, weight, installation requirements, the magnitude of development problems, the capability of the contractor, and the applicability of the engine for the proposed missions of the X-15. The laboratory's report pointed out that the airframe designers would undoubtedly take other factors into account, factors such as the nature of the propellants and their weight, the number of controls required, and the merits of multiple versus single engine installations.

An additional factor which, in the view of the Power Plant Laboratory, had not been given adequate consideration by the airframe contractors or by the evaluation rules, was that of minimum thrust. The laboratory stated that if a requirement existed for operation of the engine at less than 50 percent of the rated thrust, such a requirement would have an important bearing on engine selection. It was intimated that the laboratory's evaluation would have been different if one of the design objectives had specified an engine capable of operating at half or less than half of the rated thrust.

The Power Plant Laboratory's evaluation, while making no major distinction between Reaction Motors' proposals and those of Bell, left the definite impression that the Bell design was favored. Nowhere was this more clearly apparent than in the laboratory's statements that the ”… Bell engine would have potential tactical application for piloted aircraft use whereas no applications of the RMI engine are foreseen,” and ”in the event that the XLR73 development does not meet its objectives, the Bell engine would serve as a 'backup' in the Air Force inventory.”

Looking forward to the actual selection of one of the two proposals under consideration, the laboratory made recommendations on the course of development that should be followed. In the case of the Bell engine, evaluators suggested that hydrogen peroxide be considered for the turbine drive, and that an effort be made to simplify starting and to reduce the development effort by substituting unsymmetrical dimethyl hydrazine for JP-X. If the Reaction Motors' design emerged as the final selection, it would meet laboratory recommendations that the throttling range be restricted in order to reduce the development effort, that consideration be given to converting the engine from ammonia to JP-4 to reduce corrosion and handling problems, and finally, that an NACA suggestion to use an interim ”off-the-shelf” engine for initial flight testing be adopted. ¹⁴

Apparently little attention was paid to these recommendations as the throttling range was not reduced, ammonia was retained as a fuel, and no consideration was given to the use of an interim engine until development difficulties compelled the selection of such an engine in early 1958.

After North American had been selected as the winner of the X-15 competition, plans were instituted to procure the modified XLR30 engine that had been incorporated in the winning design. Late in October, Reaction Motors was notified that North American had won the X-15 competition and that the winner had based his proposals upon the XLR30 engine. ¹⁵

On 1 December 1955, the New Developments Office of Fighter Aircraft Division, Directorate of Systems Management, asked the Power Plant Laboratory to initiate a purchase request that would provide $1,000,000 for a proposed letter contract with Reaction Motors. ¹⁶ On 8 December the Air Materiel Command asked Reaction Motors to submit a proposal that would permit the Air Force to prepare a contract covering the development of an engine for the X-15. AMC suggested that the proposal contain visual presentations of a proposed development program, a chart of important milestones, and various cost estimates. The materiel organization also asked Reaction Motors to provide information on the amount of testing anticipated and contractor capabilities for conducting the required tests. ¹⁷ The letter requesting a proposal from Reaction Motors included a preliminary informal work statement and list of the minimum requirements for the modified XLR30 engine. The content of the attachments differed only slightly from the requirements eventually incorporated in the formal contract. ¹⁸

While preliminary steps were being taken to procure the required engine, the Power Plant Laboratory raised a further question as to the desirability of the engine selected. The NACA had, as a result of preliminary discussions with Reaction Motors, expressed concern that the ammonia fuel might have an adverse effect on planned instrumentation and had asked that the possibility of converting to another fuel be given further study. The Power Plant Laboratory, already convinced that the contractor's estimate of a two-year development period was much too optimistic, viewed any change with disfavor. The laboratory had been reluctant to accept a two-and-one-half year estimate during the original evaluation of the proposed X-15 engines, holding that a three year period was probably more realistic. Propulsion engineers estimated that a change in fuels would extend the development period to four years, and the laboratory held that such an extension would make the original evaluation invalid. If a four-year development period was to become acceptable, the laboratory recommended a reevaluation that would permit reconsideration of engines that had considerable potential but which had been eliminated from the original evaluation because their development period had been estimated at more than two and one-half years. ¹⁹

In late December, the Power Plant Laboratory advised the project office that whatever procurement procedure was followed in securing an engine for the X-15, certain features should be insisted upon. Among the features that the laboratory felt to be important were the retention of Reaction Motors as the engine contractor, a requirement that North American could not change the engine selection without prior approval of the project office and the Power Plant Laboratory, and a provision for close coordination and direct contact between the laboratory and Reaction Motors, no matter what contractual procedure was utilized. ²⁰

The end of 1955 was also marked by a skirmish over the assignment of cognizance for the development of the engine. The skirmish began with a letter to Air Force headquarters for Rear Admiral W. A. Schoech, assistant chief for research and development in the Bureau of Aeronautics. Admiral Schoech contended that since the XLR30-RM-2 rocket engine was the basis for the X-15 power plant, and the Bureau of Aeronautics had already devoted about three years to the development of that engine, it would be logical to assign the responsibility for further development to the Navy. The admiral felt that retention of the program by the bureau would expedite development, especially as the Navy could direct the development toward an X-15 engine by making specification changes rather than by negotiating a new contract. Other arguments advanced for bureau retention of the project included the close and satisfactory working relationships between the bureau and Reaction Motors and the ability of the Navy to make the facilities at Lake Denmark available for the program. The Navy's extensive experience with hydrogen peroxide was also put forth as a justification for continuing the program under the Bureau of Aeronautics. ²¹

Air Force headquarters sent the admiral's letter to the commander of the Air Research and Development Command on 9 December asking for resolution and comment by 3 January. ²² On 29 December a teletype conference was held between ARDC headquarters and personnel from ARDC Detachment One at Wright Field. The Navy's bid for responsibility for the development of the engine had apparently been forwarded to Detachment One and the Power Plant Laboratory for comment, as the conference was devoted to refutation of the arguments advanced by the Bureau of Aeronautics for retention of the engine program. ²³

ARDC headquarters summarized the arguments of Detachment One and the Power Plant Laboratory and forwarded the summary to Air Force headquarters on 3 January 1956. The Navy's bid for control of the engine development was rejected on the grounds that the management responsibility should be vested in a single agency, that conflict of interest might generate delay, and that the Bureau of Aeronautics was underestimating the time and effort that would be needed to make the XLR30 a satisfactory engine for manned flight.

The arguments for Air Force retention of control were based on the fact that the Power Plant Laboratory was acquainted with the status of Reaction Motors' developments, that it had experience with several similar projects for the development of rocket engines for manned aircraft, and that experienced personnel were available to monitor the program. The ARDC letter also pointed out that past experience had shown that more problems could be expected in the assembly of components into an operating engine and adaptation of the engine to the airframe than in the development of components-and the Navy's experience with the XLR30 had been largely with component development. As the original Bureau of Aeronautics letter had raised the problem of the availability of test facilities, ARDC noted that the Air Force was already using Reaction Motors' facilities and could expect that those facilities would be made available for the XLR30 program. Admiral Schoech's letter had also stated that plans called for the use of an XLR8-RM-8 as an interim engine and that the Bureau of Aeronautics' knowledge of this engine was an additional reason for assigning engine development to the Navy. This last contention was denied with the flat statement that there were no plans for the use of an interim engine. ^{24 c}

Apparently the Bureau of Aeronautics accepted the Air Force's decision that engine development was to remain an Air Force responsibility for there was no evidence of additional correspondence on the subject.

The Navy's bid for cognizance over engine development may have served to hasten the procurement procedures; Reaction Motors was furnished with a final work statement and the performance requirements for the engine on 4 January 1956, the day after ARDC's comments on the Bureau of Aeronautics' letter went forward to Air Force headquarters. ²⁵ The Power Plant Laboratory received Reaction Motors' technical proposal on 24 January and the company's cost proposals on 8 February. ²⁶

The cover letter which accompanied the various reports and cost breakdowns from Reaction Motors promised delivery of the first complete system ”… within thirty (30) months after we are authorized to proceed.” ²⁷ The same letter marked the abandonment of the XLR30 designation that had been used for convenience in previous discussions of the proposed X-15 engine. Reaction Motors, recognizing that the developed engine was going to have numerous differences from the XLR30, gave the new design a ”company designation,” TR-139. (On 21 February the Power Plant Laboratory formally requested assignment of an XLR99-RM-1 designation.) ^d Reaction Motors also estimated that the entire cost of the program would total $10,480,718, stated that the company would prefer that the fee be determined by later negotiation, and noted that preliminary design and liaison work had begun on 1 January in anticipation of a contract award. ²⁸

Evidently the rate at which the procurement negotiations were proceeding was unsatisfactory to the NACA, for on February 15 Brigadier General V. R. Haugen, then the WADC deputy commander for development, felt it necessary to reassure the NACA that he had investigated the apparent delay in awarding the engine contract and had determined that the procurement procedures were moving at an acceptable pace.

General Haugen pointed out that nearly one month of the time that had elapsed since procurement was authorized on 27 October 1955 had been consumed by a study of the NACA suggestion for changing from ammonia to another fuel. The general estimated that a letter contract would be issued no later than 1 March. ²⁹ (As a matter of fact, his letter was dated one day after the date of the letter contract.)³⁰

While the procurement difficulties were relatively minor, and in retrospect seemed to have consumed a relatively small portion of the time eventually devoted to the new engine, it was not long before other and serious questions were being posed.

Less than two months after General Haugen's letter to the NACA, that organization was criticizing Reaction Motors' conduct of the program. Mr. John L. Sloop, of the NACA's Lewis Laboratory, visited the company's facilities on 11 April 1956, and his report of the visit contained a list of the anticipated development problems. The problems included the provision of an adequate ignition system for the ammonia fuel, achievement of safety under all conditions, assurance that the design would be capable of meeting the severe environmental temperatures to be encountered, attainment of the performance requirements, and the development of a throttling system that would give combustion and cooling stability throughout the throttled range.

Mr. Sloop reported that Reaction Motors had assigned about a dozen engineers to the project and that they were receiving support from some 28 other staff members. He also included a summary of the company's development schedule which showed integration of a complete engine was to start in May 1957. Neither of these items drew approval from the NACA spokesman, who thought Reaction Motors' effort inadequate. Mr. Sloop also questioned the validity of the company's estimate that essential test stands would be ready in late 1956; NACA felt this date to be optimistic by a year. Sloop also suggested that Reaction Motors place considerably more effort on the development of the engine, that the company was pursuing too many different goals without adequate basic information, and that a company proposal to study ”spaghetti tube” bundle fabrication ^e had small potential value in view of the fact that they had already been studying the problem for about five years. ³¹

The first indication of Air Force concern with Reaction Motors' progress appeared in a letter from Mr. H. P. Barfield, assistant chief of the Non-Rotating Engine Branch of the Power Plant Laboratory on 1 August 1956. Barfield inquired as to why the tests of the thrust chamber, programmed for April in Reaction Motors' original proposals, had not yet taken place. ³²

Reaction Motors explained that the delay was the result of using the company's facilities for work on other Air Force projects, such use extending beyond the period originally contemplated. The company also admitted having subordinated the preparation of hardware to a program of engine design studies. It was the company's opinion that the preliminary design studies were of more importance in the maintenance of the schedule than were the thrust chamber tests. The delay in testing was also attributed to the modification of the two available test chambers, modifications intended to extend the chambers' utility for test purposes. Pump failures that had required three teardowns were also offered as justification for the company's failure to meet its planned development schedule. ³³

By 1 February 1957, North American was also becoming perturbed at the lack of progress in engine development. R. H. Rice, vice president and general manager of North American, estimated that the engine was already four months behind schedule. He also held that the engine's weight was growing while its specific impulse was deteriorating. North American, in an attempt to accelerate the development of the engine, asked Major General H. M. Estes Jr., ARDC's assistant deputy commander for weapon systems, to cooperate in securing ”… additional effort on the part of Reaction Motors, Inc.” ³⁴

North American's request for cooperation initiated a flurry of activity that included meetings between Air Force and Reaction Motors on 12 and 18 February and a meeting of personnel from those organizations with representatives of North American and the NACA on 19 February. The meeting confirmed North American's fears that the engine program was four months behind schedule and that engine weight was increasing. The deterioration of performance appeared to be less serious than North American had anticipated. General~Estes, in his reply to Mr. Rice's letter of 1 February , advised that ”… every effort will be expended to prevent further engine schedule slippages.” ³⁵

Although General Estes' letter appeared to be reassuring, the NACA report of the February meetings was not optimistic. Hartley A. Soule , NACA's research airplane projects leader, reported that the meeting of 19 February had resulted in a decision to accept the four months' delay in delivery, but that Reaction Motors had agreed to deliver two operable engines instead of one by 1 September 1958. The decrease in specific impulse (from 241 to 236 seconds) was also accepted. The weight had increased from 588 to 618 pounds. Mr. Soule pointed out that no thrust chamber runs had been made and expressed doubt that the new schedule could be achieved. It was his opinion that the Power Plant Laboratory might be forced to accept delivery of a lower performance ”first phase” engine if the proposed flight schedule for the X-15 were to be maintained. He also noted that additional engine progress meetings were to be held in June and September, and that the NACA had promised Reaction Motors its assistance in a program to increase performance by redesigning the exhaust nozzle for higher altitudes. ³⁶

Additional assistance was to be provided by WADC's Power Plant Laboratory. Reaction Motors had been concentrating on a ”spaghetti” type fuel injector which consisted of bundled metal tubing. Captain K. E. Weiss, the Power Plant Laboratory's XLR99 project engineer, designed a number of ”spud” injectors that utilized small perforated disks. Several of Captain Weiss' designs were built in Wright Field machine shops and run through firing tests during the first part of 1958. By March, one of the designs had proved so promising that Reaction Motors considered adapting it to the XLR99 engine. The company, however, had had some success with its own ”spud” designs, and eventually it utilized its own design in.preference to the laboratory-developed injector. ³⁷

(On 29 March 1957, Captain Weiss – then a lieutenant – had submitted a management report that indicated an increase in engine costs to a new total of $14,000,000 – plus fee!) ³⁸

Unfortunately, Mr. Soule's premonition that the revised schedule and performance specifications established in February were unrealistic proved entirely correct. On 10 July 1957, Reaction Motors advised Wright Air Development Center that an engine satisfying the February specifications could not be developed unless the government agreed to a nine-month schedule extension and an increase in cost from $15,000,000 to $21,800,000. At the same time, Reaction Motors offered to provide an engine of the specified performance within the established time limits if permitted to increase the weight from 618 pounds to 836 pounds. The company estimated that this overweight engine could be provided for $17,100,000. Representatives of North American, Reaction Motors, and of all the government agencies involved in the X-15 program met at WADC on 29 July to consider the effects of an overweight engine on the performance of the X-15. The deterioration of performance was generally considered to be a lesser evil than the increased cost and additional delay that would be incurred by insistence upon a ”specification” engine.

Those who hoped that the over-all performance of the X-15 could be maintained were somewhat encouraged by Reaction Motors' report that the turbopump was more efficient than anticipated and that this would allow a reduction of 197 pounds in the weight of the hydrogen peroxide necessary to its operation. The decrease in the amount of required hydrogen peroxide, the possibility that North American might remain under specified airframe weight and reduce ballast requirements, together with the increase in launch speeds and altitudes provided by the substitution of a B-52 for the B-36 carrier, offered some hope that the original goals might still be achieved. At the time of the July meeting, Reaction Motors was still experiencing difficulties with the thrust chamber and the injector assemblies. The chief problem was the burnout of the oxidizer tubes of the ”spaghetti” type injector at low thrust levels. NACA and the Air Force advised the company to continue the development of the injectors and agreed to consider relaxing the minimum thrust requirements if the difficulties continued. The possibility of switching to a spud injector was also discussed, but a final decision on such a change was deferred. ³⁹

Despite the relaxation of the weight requirements, the engine program failed to proceed at a satisfactory pace. On 11 December, during a meeting at the propulsion Laboratory, ^f Reaction Motors reported a new six-month slippage in the schedule. At that point, the company attributed its continued difficulties to a malfunction which destroyed the first development engine, to a series of pump failures, and to inability to produce an injector that would meet both performance and durability requirements. The failures were compounded because pump shortages had delayed the injector tests.

The threat to the entire X-15 program posed by these new delays was a matter of serious concern. Major General S. T. Wray, Wright Air Development Center's commander, working with General Haugen, then ARDC's director of systems management, decided to have the Directorate of Laboratories explore the technical and managerial problems involved. As a result, on 7 January 1958, Reaction Motors was asked to furnish a detailed schedule and to propose means for solving the difficulties. The new schedule, which reached WADC in mid-January, indicated that the program would be delayed another five and one-half months and that costs would rise to $34,400,000 – almost double the cost estimate of the previous July. ⁴⁰

On 28 January 1958, General Haugen and General Wray, accompanied by Propulsion Laboratory and X-15 project office personnel, visited Reaction Motors to discuss the lack of progress on the XLR99 and to determine what steps the company was taking to improve its performance. General Haugen emphasized the importance of the X-15 project and commented upon Reaction Motors' record up to that time. Evidently the comments were rather forceful, as a company spokesman felt compelled to admit to ”past deficiencies.” Nevertheless, Reaction Motors asserted that its latest proposals were firm and expressed complete confidence in the company's ability to meet the revised schedules. ⁴¹

The Propulsion Laboratory and the project office, after evaluating Reaction Motors' program, reported their recommendations to General Haugen on 17 February, and to Lieutenant General S. E. Anderson, ARDC commander, Major General R. P. Swofford Jr., director of.research and development in Air Force headquarters, and General Wray on 21 February. The recommendations included the continuation of Reaction Motors' program, the use of an XLR11 rocket engine for initial X-15 flights, the approval of overtime, the assignment of a top Defense Department priority (DX rating) to the project, increased effort by Reaction Motors, the establishment of a technical advisory group, and the start of a backup engine development program. The use of the XLR11 engine and an increase in effort by Reaction Motors were approved. Additional funds to cover the increased effort were also approved, as was the establishment of an advisory group. The top priority was denied (although the request eventually led to an improved priority) ⁴² and it was decided to postpone a decision on the possibility of using an alternate engine. ^g

The most immediate result of the recommendations was the establishment of the technical advisory group which first met at Reaction Motors' plant on 24 February 1958. The group consisted of representatives from the NACA, the Bureau of Aeronautics, ARDC's weapon system group, and WADC. It was immediately apparent that the injectors and the thrust chamber presented the greatest development difficulties and that these were the areas in which the advisory group could render the greatest assistance to Reaction Motors. ⁴³

That measures taken as a result of the February meetings were not completely satisfactory to all of the parties concerned was quite evident. A summary of the NASA-ARDC position dated 20 February 1958 and retained in the files of the X-15 project office stated that there was ”… only a remote possibility of getting any engine for the 1960 flight period.” The same document contained an estimate that the value of the X-15 equipped with the XLR11 engines would ”… diminish to almost zero by start of 1960 flight period.” The frustration produced by the engine situation at that time was evidenced by another statement – that even at this late date North American and Aerojet were better prospects to complete satisfactory engine developments before Reaction Motors, but not before the 1961 flight period. ⁴⁴

Despite severe criticism of the contractor, continued development of Reaction Motors' engine offered the only practical source, so project monitors decided that the contract should be continued. ⁴⁵

Of the three types of assistance offered to Reaction Motors (a government technical supervisory group, a government advisory group, and participation by other rocket engine contractors), the multi-contractor effort appeared to promise the greatest success. A government supervisory group was ruled out because of a lack of manpower. An advisory group was thought desirable for purposes of keeping Reaction Motors' progress under close surveillance, but fear was expressed that such a group would not be capable of providing the desired improvement in the company's efforts. The assistance of other engine contractors seemed to promise the greatest benefits, so the February conferees recommended that the possibility of obtaining such assistance be explored. ⁴⁶

During March, the Air Force opened negotiations with the Rocketdyne Division of North American Aviation in an effort to secure alternate injectors and an alternate thrust chamber. ⁴⁷ North American was reluctant to undertake the development and it was not until General Wray and General Haugen arranged a personal conference wit.h North American's vice president, Mr. Lee Atwood, that Rocketdyne agreed to render general assistance to Reaction Motors and to undertake the development of injectors and a thrust chamber that could serve as alternates for the items that were giving Reaction Motors so much difficulty. ⁴⁸

Once North American's reluctance had been overcome, Rocketdyne immediately began tests of an S-4 injector and chamber from an XLR105-NA-1 (Atlas sustainer) engine, in an effort to adapt them to the Reaction Motors' design. ⁴⁹

In addition to the numerous meetings held in February and March, and the important decisions emanating from them, an additional factor of some importance influenced the development of the XLR99, and while this factor apparently did not materially alter the course of events, it could not help but add to the confusion that already existed. The additional factor was the absorption of Reaction motors by the Thiokol Chemical Corporation. Negotiations for the proposed combination were conducted throughout the early part of 1958. The anticipated reorganization and pruning undoubtedly created a state of mind that was not conducive to the best efforts of Reaction Motors' management. ⁵⁰

The absorption of Reaction Motors by Thiokol was not completed until 17 April 1958, when stockholders of Reaction Motors approved the merger. Reaction Motors was subsequently renamed and became the Reaction Motors Division of the Thiokol Chemical Corporation. ⁵¹

The decision to turn to Rocketdyne for assistance apparently spurred Reaction motors' efforts toward the development of a ”backup” design, for by the end of April, the Air Force felt it necessary to point out that the funds available were not sufficient to permit the development of both a Rocketdyne and a Reaction Motor design. Reaction motors was urged to subcontract with Rocketdyne for further developments of the XLR105 chamber. The president of Reaction Motors agreed to a study to determine whether his own company's approach or Rocketdyne's offered the most promise. The results of the study were presented at a meeting held on 27 May at WADC. Reaction Motors, Rocketdyne, and NASA representatives, as well as Air Force personnel, attended the meeting and reviewed the alternate proposals. It appeared that Reaction Motors' alternate design (a concentric shell thrust chamber) would not solve the problem of chamber burnout, and that the design could not be translated into hardware in time to meet the schedules for the X-15 engine. As Reaction Motors' proposals were considered unsuitable, it was decided that the company should not pursue the concentric shell chamber further. On the other hand, Rocketdyne's proposals seemed to offer some hope of success, so the conferees agreed to continue the development of that firm's design. ⁵²

The decisions reached at the 27 May 1958 meeting were officially transmitted to Reaction Motors two days later. The letter specifically instructed Reaction Motors to subcontract for the development of the Rocketdyne designs. The same letter warned Reaction Motors that a ”… demonstration of thrust chamber performance and satisfactory progress in all other areas must be apparent by mid-July.” ⁵³

Complying with instructions, Reaction Motors provided $500,000 to fund Rocketdyne's program from 28 May until mid-July; the firm also made arrangements for continued development after that date. Rocketdyne's chamber development cost estimate was $1,746,756, with an additional $811,244 for the delivery of 14 research and development chambers and a further $657,300 for 14 flight chambers. ⁵⁴

While the increased efforts by Reaction Motors appeared to be having some favorable effect on the progress of the XLR99, and Rocketdyne's supplementary efforts got off to a promising start, the Air Force was not convinced that everything was proceeding as rapidly as possible. The Propulsion Laboratory, in an effort to stimulate Reaction Motors to even greater efforts, undertook the preparation of two letters. The first, dated 17 June 1958, was from General Wray to General Anderson. Its tenor was not obscure:

For sometime General Haugen and I have been concerned by the poor progress made by Reaction Motors Division on the development of the XLR99 rocket engine for the X-15 airplane program.
This engine was one that had been recommended … on the strength of a supposed advanced state of development of the LR30 rocket engine …
… in spite of this state of development, Reaction Motors Division has experienced continual schedule slippage and financial overruns …
It is by their own admission as well as the conclusions of our project engineers a fact that Reaction Motors Division has used poor judgment and management during the early stages of the engine development program.
Inability to meet performance and original Preliminary Flight Rating Test initiation date, which was a contractor deficiency, has resulted in submission of supplemental proposals. This by acceptance or rejection has placed the Air Force in the undesirable position of making program decisions which we would have preferred the contractor, through better management, to have made at a much earlier date.

General Wray also advised that a decision as to whether Rocketdyne's or Reaction Motors' chamber and injector designs should be continued was scheduled for July. ⁵⁵

The second letter, prepared by the Propulsion Laboratory, was enclosed with the first and was directed to Mr. J. W. Crosby, president of the Thiokol Chemical Corporation. General Wray felt that this second letter would have a greater impact if it went forward over General Anderson's signature. General Anderson's staff shortened the four-page draft letter to two pages which, suitably signed, went to Mr. Crosby on 27 June. The general tone of the revision was somewhat milder than the original, but the statements that ”… the results of the next few weeks … development effort will be extremely crucial in determining the direction of this engine procurement” and ”I recognize the possible impact which pending Air Force decisions may have on Reaction Motors Division” could have left little doubt as to its meaning. ⁵⁶ The Air Force had quite lost patience with Reaction Motors. The implication of contract cancellation was not difficult to derive.

Mr. Crosby replied to General Anderson's letter on 3 July 1958 with the admission that a ”… decision to take this development work away from Reaction Motor(s) Division would have a serious effect on the organization.” He defended Reaction Motors' conduct of the program by emphasizing that the safety and reliability requirements of an engine intended for a manned aircraft had created unusually difficult development problems. He also offered to arrange a presentation of the current status of the XLR99 program for General Anderson. ⁵⁷

As it happened, progress at Reaction Motors began to improve while General Anderson and Mr. Crosby were exchanging letters and, as a consequence, no presentation was made. In a letter of 1 August 1958, General Anderson thanked Mr. Crosby for his reply of 3 July and declined the offered presentation. ⁵⁸

The threat that the engine delays would seriously impair the value of the X-15 program had generated a whole series of actions during the first half of 1958: personal visits by general officers to the contractor's plant, numerous conferences between the contractor and representatives of the government agencies involved in the program, increased support from the WADC Propulsion Laboratory and NASA, an increase in funds, an increase in effort within Reaction Motors' plant, the composition of letters containing severe censure of the company's conduct of the program, and the introduction of another contractor (Rocketdyne). Whether any of these actions, or even the threat of XLR99 cancellation implied in General Anderson's letter of June, had any real effect on the program was difficult to determine. An emergency situation had been encountered, emergency remedies were used, and by midsummer improvements began to be noted.

Reaction Motors accumulated more engine test time in the first two weeks of July than during the entire program prior to that date. Performance was somewhat low but was high enough to offer reasonable encouragement that the specification performance could be met. ⁵⁹ By 7 August 1958, performance had been raised to within two and one-half 'percent of specifications. By August, it was also apparent that the Rocketdyne proposal, rather enthusiastically endorsed by the North American project group was rather optimistic. By that time, Reaction Motors' subcontract with Rocketdyne had cost $3,125,000, which the Propulsion Laboratory felt was ”particularly unreasonable since the Rocketdyne program was initiated on the basis that little development effort would be required.” The Rocketdyne chamber had failed to start on two attempts and a review of Rocketdyne's progress indicated a six to twelve month delay in the delivery schedule. The improvement at Reaction Motors and the lack of success at Rocketdyne led the Propulsion Laboratory in August 1958 to ask for the termination of the Rocketdyne program ”as soon as possible.” ⁶⁰

North American and Rocketdyne officials were notified of the Air Force's intention to terminate the backup program during a visit of Generals Wray and Haugen to the contractor's Inglewood plant. Major Arthur Murray, the X-15 project officer, took the opportunity of the visit to express his opinion that Rocketdyne's failure to achieve a suitable backup chamber within the sixty days and for the few hundred thousand dollars originally contemplated came because ”… no amount of optimism or salesmanship” could change the total effort required to develop advanced equipment. ⁶¹

On 15 August 1958, another management meeting was held at Reaction Motors. Among those in attendance were General Haugen, Brigadier General W. A. Davis of the Air Materiel Command, Mr. Soule of NASA, and representatives from Air Force headquarters, ARDC and WADC. After evaluating the status of the XLR99 and of Rocketdyne's thrust chamber, the participants decided that the engine design should be frozen immediately and that it should incorporate Reaction Motors' chamber for purposes of the Preliminary Flight Rating Test. The Air Force and NASA urged Reaction Motors to continue its efforts to reach specification performance by minor changes of the injector design, but not at the expense of reliability or of further delaying the development schedule. A final decision on the continuation of the Rocketdyne program was postponed until October. ⁶²

During September, the progress of Reaction Motors continued to be encouraging as engine and injectors were subjected to increased testing. The Rocketdyne program continued to lag, primarily because of difficulties in mating Reaction Motors' ignition system to the Rocketdyne chamber. By the end of the month, the X-15 project office was convinced that the Rocketdyne program was not going to be a success, calling it an ”expensive and apparently fruitless” effort. ⁶³

On 7 October, the Technical Advisory Group met at Reaction Motors Division for a review of Reaction Motors' and Rocketdyne's progress. The review convinced the group that, while Rocketdyne's program might eventually lead to a higher performance engine, Reaction Motors' program would provide an acceptable engine at an earlier date. As a result of the group's recommendations and subsequent discussions at WADC, the Propulsion Laboratory recommended (on 10 October 1958) termination of Rocketdyne's development program. Headquarters of WADC and the X-15 project office agreed to the termination shortly thereafter. ⁶⁴

Engine progress continued to be reasonably satisfactory during the remainder of 1958. A destructive failure that occurred on 24 October was traced to components that had already been recognized as inadequate and that were in the process of being redesigned. The failure, therefore, was not considered of major importance. ⁶⁵

By the end of November, the X-15 project office could report that an engineering inspection on 18 November and a Technical Advisory Group meeting the same day had revealed promising progress. ⁶⁶

Although the emergency actions of 1958 appeared to have produced a considerable improvement in the engine development program, all of the difficulties had not been resolved. At a Technical Advisory Group meeting of 20 January 1959, it became apparent that there were still some minor system leakage problems; that injector tests were still producing failures, particularly under low thrust and idle conditions; and that excessive heating was being encountered during idle. On 23 January, a fuel manifold failed because of excessive vibration. Reaction Motors also reported encountering difficulties in obtaining satisfactory delivery from its suppliers. ⁶⁷

On 12–13 February, the project office and the Propulsion Laboratory made two presentations at WADC, one to brief General Wray on the current status of the XLR99 program, and the second to a number of the contractor's management personnel-including the president of Reaction Motors Division. The purpose of the second presentation was to reemphasize the ”Air Force's concern over the problems and delays which have been encountered.” One result of the presentations was a decision to send several of WADC's technical personnel to the contractor's plant to investigate instrumentation, vibration, materials, and fluid flow. The Air Force hoped that the investigation of these problem areas would assist the contractor in overcoming the difficulties being encountered. The new group made its initial visit during the last week of February and the first week of March. ⁶⁸

A long-sought goal was finally reached on 18 April 1959 with completion of acceptance tests of the first Preliminary Flight Rating Test (PFRT) engine. The flight rating program began at once. ⁶⁹

At the end of April, representatives of ARDC, WADC, AMC and Reaction' Motors met at the contractor's plant to decide on a ”realistic” schedule for the remainder of the program. They agreed that the performance flight rating test should be completed by 1 September 1959. The first engine equipped with the final flight-type injector was to be ready for running in the latter part of May, the first ground test engine was to be delivered to Edwards Air Force Base by the end of May, and the first flight engine was to be delivered by the end of July. The conferees also decided that an additional engine should be subjected to the flight rating program in order to test a 30-second idle feature which had not been included in the original test engine. ⁷⁰

At the time these decisions emerged, it was quite impossible to determine whether the ”realistic” schedule could actually be achieved, but delays in the overall X-15 test program, imposed by other factors, had reduced the air of urgency which surrounded the engine program throughout 1958. Some of the factors contributing to the less-than-perfect record of engine development were obvious.

Others were relatively obscure. Early in 1959 the X-15 project officer, Major Murray, summarized some of the development difficulties, and their causes. He stated quite bluntly that prior to the stimulus provided by the Russian satellite achievements of late 1957, there had been inadequate support for programs that did not lead directly to weapons. In his view, this lack of R&D support was not the result of the policies of individuals or even of commands, but was an inherent Air Force-wide phenomenon that was only overcome by the existence of a few ”crusaders” at all levels and by the intensive efforts of those directly concerned with the individual projects. Major Murray considered that the original development schedule had been tight, that the funds had only been marginally sufficient at some stages in the program, and that personnel shortages, particularly a propulsion-expert vacancy within the project office, had all contributed to the contractor's repeated failures to meet the proposed development schedules. He also' pointed out that the entire project was in advance of the state-of-the-art and that there was a tendency on the part of scientists engaged in such projects to postpone any commitment to a final design because of recurrent hopes of finding something just a little bit better. ⁷¹ (This latter problem – leading to the cynical expression ”… best is the enemy of better” – one that still afflicts both the scientific and technical communities).
 

 

Section IV  On-Board System

 

It might seem that a kind fate, after imposing the burdens involved in the development of the XLR99, would have permitted other phases of the X-15 program to proceed without hindrance. None of the other phases did present the problems posed by the engine, but difficulties continued to occur.

In early 1958, at the very height of the furor over the problems connected with the XLR99, a note of warning sounded for the auxiliary power unit (APU). North American had subcontracted the development of this important piece of equipment to the General Electric Company. On 26 March 1958 and again on 11 April 1958, General Electric notified North American of inability to meet the original specifications in the time available, and requested approval of new specifications. North American, with the concurrence of the X-15 project office, agreed to modify the requirements. The major changes involved an increase in weight from 40 to 48 pounds, an increase in start time from five to seven seconds, and a revision of the specific fuel consumption curves. In an effort to keep the fuel requirement as low as possible, North American asked General Electric to investigate the possibility of creating a derated auxiliary power unit. North American advised the project office that such a unit would reduce fuel consumption from 101 pounds per mission to 96 pounds and that the unit would still be capable of meeting the expected loads. General Electric was also fearful that an excessive amount of nitrogen might be necessary to overcome difficulties in cooling the upper turbine bearing of the power unit. ¹

In late March and early May, Propulsion Laboratory and weapon system project office representatives visited the General Electric facilities at Malta, New York, and Lynn, Massachusetts, to review the auxiliary power unit development. The group found that testing was proceeding at a satisfactory pace, that the heating of the upper turbine bearing had been reduced by a change in one of the unit's seals, but that little progress had been made in reducing the amount of nitrogen required for cooling the unit. Nitrogen flow was running as much as 80 percent over that required by specifications. Although no tests had yet been performed, General Electric reported progress in the development of the derated unit requested by North American. ²

Actual tests of the derated unit began shortly after, and proved very encouraging. Bearing temperatures were held to about 300 degrees Fahrenheit with a nitrogen flow of only 12 pounds per hour. The predicted fuel economy of the derated unit was also established. ³ General Electric continued to alter the design in order to improve the cooling characteristics. A new inlet which permitted the introduction of the cooling nitrogen directly into the most troublesome bearing area alleviated the problem and reduced the nitrogen flow. Improvement was so substantial that them first production units were scheduled for shipment to North American in June. ⁴

Some difficulties continued to arise in the course of the testing which was carried on in the summer of 1958, but most of them proved amenable to correction or were traced to malfunctions of the testing equipment rather than to the unit itself. Starting times at low temperatures remained excessive, prompting investigations of the advisability of increasing the wattage of the hydrogen peroxide heater and of adding a wetting agent. ^{5 a} The heater capacity was increased but the decision on the use of a wetting agent was postponed. ⁶

By the end of the summer of 1958, the auxiliary power unit seemed to have reached a more satisfactory state of development, an alternate machining method had effectively corrected undesirable stresses that had caused a turbine shaft failure, and satisfactory production units were ready for shipment. ⁷ The records of the project office thereafter failed to reveal any further concern with the auxiliary power unit until after the first captive flight in 1959. Those flights, unfortunately, did reveal some additional bearing problems and actually produced bearing failures. But investigation showed that the inflight failures had occurred because captive testing subjected the units to an abnormal operational sequence that would not be encountered during glide and powered flight. ⁸

During the course of the X-15 program, project personnel from time to time had some concern for the development of an escape system and a pressure suit. Of the many accessory tasks included in the X-15 program, these caused the most concern, probably because they seemed to offer the greatest threat to the total development schedule.

Although full-pressure suits had been studied during World War II, attempts to fabricate a practical garment had met with failure. The Air Force took renewed interest in pressure suits in 1954, for by then it had become obvious that the increasing performance of aircraft was going to necessitate such a garment. The first result of the renewed interest was the creation of a suit that was heavy, bulky and unwieldy. The garment had only limited mobility and various joints created painful pressure points. It was not until 1955 that the David Clark Company, utilizing a distorted-angle fabric, succeeded in producing a garment that held some promise of ultimate success. ⁹

As the Aero Medical Laboratory had met with only partial success in the design of a full-pressure suit at the time of the X-15 evaluation, there followed a certain amount of indecision as to the type of garment to be selected for the X-15 program. North American evidently had more confidence in the potential of full pressure suits than did the Air Force; in any event, the Plans Office of the Directorate of Research advised the Aero Medical Laboratory on 23 August 1955 that ”… the possibility and problems of utilizing a full-pressure suit” required further study. The same office felt that North American would require guidance in the field of pressure suits and that the Aero Medical Laboratory should determine whether a partial-pressure suit would be adequate for an aircraft with the proposed performance of the X-15. ¹⁰

Despite the reluctance of the Air Force to commit all effort to a full-pressure suit, North American's detail specifications of 2 March 1956 called for just such a garment – to be furnished by the contractor. ¹¹ The company continued to proceed as though the matter had been entirely settled, issuing an equipment specification for an omni-environmental, full-pressure suit on 8 April 1956. ¹² That the matter was not entirely settled, however, was evidenced by the fact that on 4 May 1956 the Aero Medical Laboratory advised the project office to forward details of partial-pressure suit equipment to North American for ”… engineering of installation of subject provisions in the X-15 aircraft.” ¹³

A positive step toward Air Force acceptance of a full pressure suit occurred during a conference held at North American's plant on 20–22 June 1956. A full-pressure suit developed by the Navy demonstrated during an inspection of the preliminary cockpit mockup, and although the Navy suit still had a number of deficiencies, the project office concluded that ”… the state-of-the-art on full pressure suits should permit the development of such a suit satisfactory for use in the X-15.” ¹⁴

On 12 July 1956, during a conference on the personal equipment of the X-15, representatives of the Aero Medical Laboratory reviewed the status of the laboratory's pressure suit development and indicated that the laboratory was willing to make any modifications necessitated by the requirements of the X-15 program. Mr. Crossfield, representing North American at the conference, had previously advocated a full-pressure suit and had taken the position that such a suit should be procured by his company rather than by the government. The reconciliation of divergent viewpoints at this pressure-suit conference influenced all subsequent government-contractor relationships, as the project officer was frequently faced with the necessity of reconciling the conflicting design philosophies of divergent personalities. The Aero Medical Laboratory's presentation at the conference convinced Crossfield that the laboratory could provide an adequate suit for the X-15 program. He insisted that the garment be designed specifically for the X-15 and that every effort be made to meet the laboratory's estimate schedule which called for an operational suit in the latter half of 1957. North American decided to take full advantage of the Aero Medical Laboratory's full-pressure suit, the laboratory agreed to work in close conjunction with the company in order to insure the suit would be suitable for the X-15, and the X-15 office accepted responsibility for providing funds to assist the laboratory's development program. Crossfield conceded that he could not commit North American to the change from a contractor furnished item to a government-furnished suit, but he added that he was in personal agreement with such a change and would so advise his company. ^{15 b}

While the conference of 12 July settled the question of a full pressure garment as opposed to a partial-pressure suit and committed the Aero Medical Laboratory to the task of developing and supplying such a suit, the decision was not for realized for several months. North American's engineering change proposal which called for a government-furnished full pressure suit in lieu of contractor-furnished equipment was not issued until October 4, 1956, and it was not until 16 January 1957 that the AMC Directorate of Procurement and Production authorized the Air Force Plant Representative at North American to proceed with an official change in the contract. ¹⁶ North American's formal contract change request was not made until 8 February 1957. ¹⁷

Although an ”operational” suit had been promised for the latter half of 1957, progress was not as rapid as had been contemplated at the meeting in July 1956. The Aero Medical Laboratory did not have an opportunity to conduct major tests until the week of 14–18 October 1957, and those were of the first prototype suit. ¹⁸

The Aero Medical Laboratory specifications which described the X-15 suit in terms approximating its final configuration, was not issued until 1 January 1958. ¹⁹ On 10 April 1958, the laboratory advised the X-15 project office that the first suit, scheduled for Crossfield's use would be delivered on 1 June 1958. At the same time, the laboratory advised that the four suits scheduled for delivery during the summer were the only suits programmed in support of the X-15 project in fiscal year 1958. The laboratory was to receive other full-pressure suits, but the additional suits had been designed for service testing in operational aircraft and were not compatible with the X-15 cockpit. Aero Medical Laboratory specialists cautioned the project group that ”… funding for further X-15 suit procurement … during FY58 must of necessity be furnished by your office.” ²⁰

The X-15 project office, faced with a scarcity of suits and funds, began to investigate the possibility of using a seat kit rather than a back kit for the X-15 suit. Such a change would permit the suits designed for service testing to be utilized by the X-15 pilots, would enable the pilots to try the suits in operational aircraft, and would eliminate the need to furnish each X-15 pilot with two suits--one for familiarization in operational aircraft and one for flight in the X-15. ²¹

The benefits to be derived from a program to make the X-15 and service test suits compatible would undoubtedly have been substantial, if such a program had been in effect from the beginning of the project. By May of 1958, however, the difficulties of obtaining compatibility outweighed the benefits. The X-15 project office continued to devote some thought to eventual compatibility, and the Aero Medical Laboratory actually carried out some preliminary design studies that were directed toward attainment of that goal. But despite these efforts, the suit utilized in the flight testing of 1959 was one that had been designed specifically for the X-15 and it was not suitable for use in operational aircraft. ²²

On 3 May 1958, representatives of the Aero Medical Laboratory and North American met at the David Clark Company, Worcester, Massachusetts and decided to freeze the configuration of the X-15 suit. The decision proved to be somewhat premature, however.

Three months later, project personnel and contractor representatives, meeting ^c at the Aero Medical Laboratory, discovered that the final configuration of the suit was still indefinite and could not be ”frozen” until more data were on hand. The suit schedule had already been delayed about a month and it was apparent that further tests would be needed at once.

The lag in the suit schedule, and the possibility that there would be still further delays before adequate solutions were found for the remaining problems, created a new threat to the X-15 schedule. On 19 August 1958, the X-15 project office informed Colonel J. P. Stapp, the newly assigned chief of the Aero Medical Laboratory, that failure to produce an acceptable full pressure suit could result in a serious delay of the X-15 program. The project office suggested that a complete suit, including helmet and controller assembly, should be tested immediately. Colonel Stapp was also asked to press for a decision on a final configuration for the entire suit. Simultaneously, Colonel Stapp learned that the 8 August meeting had revealed a controversy in regard to the use of a face seal versus a neck seal in the suit assembly. He was asked to arrange tests of both types and to determine which was superior. The Aero Medical Laboratory was also asked to furnish a delivery schedule for the completed suits. ²⁴

A meeting between the chiefs of the X-15 project office and the Aero Medical Laboratory, at the end of August, resulted in an agreement that a full discussion of the pressure suit program would take place on 8 September 1958. ²⁵ The September meeting, in turn, produced agreement that a fully qualified pressure suit was essential to the X-15 program and that the Aero Medical Laboratory was responsible for meeting this requirement. The first suit would be needed by 1 January 1959, a second suit would be needed by 15 February 1959, and four additional suits had to be ready by 15 May 1959. A suit with a neck seal and with provision for electrical defogging (of the faceplate) was to be the basic configuration, but a suit incorporating a face seal was to be considered as a backup because that configuration was nearer to qualification testing. Both configurations were to be tested, and both were to be procured in a quantity which would insure delivery of the first two suits on schedule--no matter what configuration proved superior. Ordering the components for the four suits scheduled for May was to be postponed until one of the two configurations had been proved superior. All of the pilots were to be furnished suits without the defogging provisions as quickly as possible, such suits being necessary for evaluation and familiarization flights. ²⁶ Three such suits were delivered by the first week in November. ²⁷

Although at that point agreement and mutual understanding seemed to have encompassed all participants, such was not entirely the case. An Air Force inspection team that visited WADC as September became October to consider the X-15 project, found much to concern it in the pressure suit area. Inspectors reported the existence of ”a serious disagreement between the North American Aviation Corporation and the Aero Medical Laboratory regarding certain design philosophies of the MC-2 suit assembly.” ²⁸ The reported disagreement was on the subject of a neck-seal versus a face-seal (actually an independently functioning oral-nasal mask inside the pressurized helmet), the Aero Medical Laboratory favoring the former and North American the latter. North American felt that the face seal could serve as an oxygen mask when the helmet face plate was raised and held that the pilot should be able to open his helmet. As the X-15 cockpit was pressurized by nitrogen, a pilot employing a suit with a neck-seal would be unable to raise the face plate, no matter what the emergency. Both North American and the X-15 project office had given some thought to pressurizing the cockpit with oxygen but this had not been done. At the time of the team's visit to Wright Air Development Center, the electrical defogging provisions for the face plate were not fully satisfactory and the plate itself had not yet been subjected to air blast tests. Still another area of disagreement had arisen over the laboratory's use of a fluid-filled ear cup in the helmet. North American advised the inspection team that the seal had failed during centrifuge tests and that a more satisfactory cup was needed.

In summary, the team listed inadequate testing of the regulator components, unqualified defogging provisions, the lack of a blast tested face plate, and the continuing controversy over the type of seal to be employed, as the major deficiencies of the pressure suit program.²⁹

Fortunately, pressure suit difficulties finally began to yield to the combined pressures of the project office, the Aero Medical Laboratory and the various contractors. The prototype helmet with electrical defogging provisions was delivered on 17 November 1958, and although the helmet was not completely satisfactory from an optical standpoint, it did pass the defogging tests. On 22 December, the helmet visor successfully withstood the wind blast tests, and by 16 January 1959, the Aero Medical Laboratory could report that the visor was ”fully qualified.” ³⁰

Scott Crossfield, the North American pilot who was scheduled to make the first X-15 flights, received a new suit of the face-seal type on 17 December and, two days later, the suit successfully passed nitrogen contamination tests at the Aero Medical Laboratory. On 30 January 1959, the project office reported that the Aero Medical Laboratory had furnished general qualification and test information on a complete suit. The X-15 project officer attributed much of the credit for the successful and timely qualification of the full-pressure suit to the early and intensive efforts of Mr. Crossfield. ³¹

Apparently another minor crisis had been met and overcome. After the first captive flights there were complaints about the poor optical qualities of the helmet and the first months of 1959 witnessed attempts to find a snap-on visor that would provide a temporary ”fix.” ³²

While not directly related to the pressure suit difficulties that threatened the over-all X-15 schedule, providing a means for successful escape from the aircraft, if that should become necessary, caused some concern during development. The type of escape system to be used in the X-15 had been the subject of debate at an early stage of the program; the decision to utilize the stable-seat, full-pressure-suit combination had been a compromise based largely on the fact that the ejection seat was lighter and offered fewer complications than the other alternatives.

As early as 8 February 1955, the Aero Medical Laboratory had recommended a capsular escape system, but the laboratory had also admitted that such a system would probably require extensive development. The second choice was a stable seat that incorporated limb retention features and one that would produce a minimum of deceleration. ³³

During meetings held in October and November of 1955, it was agreed that North American would design an ejection seat for the X-15 and would also prepare a study justifying the use of such a system in preference to a capsule. North American was to incorporate head and limb restraints in the proposed seat. ³⁴

Despite North American's plans to proceed with an ejection seat design, the Air Force was not convinced that such a seat was the best solution. At a specification meeting held at Wright Air Development Center on 2–3 May 1956, representatives of the X-15 project office and the Aero Medical Laboratory again pointed out the limitations of ejection seats. In the opinion of an NACA engineer who attended the meeting, the Air Force was still strongly in favor of a capsule – partly because of the additional safety a capsule system would offer and partly because the use of such a system in the X-15 would provide an opportunity for further developmental research. Despite this apparent preference for a capsule, the several participants finally agreed that because of the ”time factor, weight, ignorance about proper capsule design, and the safety features being built into the airplane structure itself, the X-15 was probably its own best capsule.” About the only result of the reluctance of the Air Force to endorse an ejection seat was another request that North American document the arguments for the seat. ³⁵

By November 1956, North American's seat had completed a number of tests in the wind tunnel at Massachusetts Institute of Technology. The results were encouraging although the seat had a tendency to stabilize in one of several positions instead of in a single position. ³⁶

The death of Captain Milburn G. Apt in the crash of the Bell X-2 in September 1956 renewed apprehension as to the adequacy of the X-15's escape system. Brigadier General Marvin C. Demler, ARDC's deputy commander for research and development, directed WADC to determine the best escape system for the X-15 and to conduct the study on an expedited basis. Evidently General Demler did not anticipate that the study would have any immediate effect on the design in progress, however, as he stated that the results of the study were to be incorporated in any ”future versions of the X-15.” ³⁷

By early 1957, North American's seat development efforts had indicated that several benefits could be derived from a change in the seat catapult originally specified. The company pointed out that the substitution of a contractor-furnished ballistic type rocket (Talco Number 1057-2) for the government furnished type T-18 ejection seat originally specified would increase energy of the catapult from 35,000 to 45,000 pound-feet, reduce frictional losses during the period of guided travel, increase the low altitude escape ability, eliminate binding of the catapult tubes as the seat entered the airstream, eliminate the forward pitching moment of the original T-18 type and extend the deceleration period because of forward thrust component in the ballistic rocket type. ³⁸ These arguments carried the day; the Air Force approved the change proposed by North American and the seat was equipped with the ballistic type rocket. ³⁹

Sled tests of the ejection seat began early in 1958 at Edwards Air Force Base, California, with the preliminary tests concluded on 22 April. During the fourth and final run of the preliminary tests, a shock wave generator catapult exploded, the malfunction being attributed to the high air loads at the beginning of the extension sequence. The accident occurred at Mach 1.26 and at a pressure of 2,192 pounds per square foot. The seat, suit, and test dummy were all damaged beyond repair. ⁴⁰ During a static firing on 24 April, the seat ejected successfully, but the post-ejection operation of the seat was a failure because a striker on the seat did not contact the striker plate on the seat frame. ⁴¹ A second static firing on May 14, 1958 was more satisfactory, but was not a complete success as the parachute and parachute lines wrapped around the seat. ⁴² Because of the high cost of sled runs, the X-15 project office advised North American to eliminate the planned incremental testing and to conduct the tests at just two pressure levels – 125 pounds per square foot and 1,500 pounds per square foot. The X-15 office felt that successful tests at these two levels would furnish adequate proof of seat reliability at intermediate pressures. ⁴³

The first sled run of the second test series took place on 4 June 1958. It was made at the 125-pounds-per-square-foot level conducted and appeared satisfactory. ⁴⁴ Three more sled runs were in June and July. The fourth test, which took place on 3 July revealed serious instability and North American decided to discontinue further testing until the cause of the instability could be determined. ⁴⁵ A detailed analysis of the fourth test revealed that the seat would have to be considerably modified, and by the latter part of September, consideration was being given to the utilization of a Convair ”B” or ”industry” seat. As test data was incomplete for both the X-15 seat and the Convair seat, the Aero Medical Laboratory and the Aircraft Laboratory undertook only a preliminary evaluation. A final decision was to be reached after further sled tests of both seats. ⁴⁶

North American's revised seat was ready for further tests and the postponed sled runs were resumed on 21 November. The revised seat included a trailing-boom modification, but the shock-wave generator that had been a part of the original design had been eliminated because the previous sled and wind tunnel tests had shown it to be unnecessary. ⁴⁷ The redesigned seat functioned properly during the test of 21 November but the failure of a number of test-sled rockets reduced the scheduled 1,500 pounds per square foot pressure to about 800 pounds per square foot. ⁴⁸ Two sled runs conducted in December were also marred by the failure of some of the test-sled rockets. ⁴⁹

Sled tests scheduled for January 1959 were delayed because of the unavailability of seat rockets. As the X-15 was nearly ready for captive flight, the X-15 project office arranged for a meeting with Aircraft Laboratory personnel on January 12 and requested that the laboratory approve the ejection seat for captive and glide flights, even though the sled tests had not been completed. The Aircraft Laboratory verbally approved the use of the seat for such flights but only within a range between Mach 0.377 and Mach 0.72, and with dynamic pressures limited to those between 195 and 715 pounds conducted during per square foot. ⁵⁰ (The only test that was January was a failure because the right-hand boom and right-hand fin both failed to deploy, with the result that the seat was highly unstable throughout most of the trajectory. The leg restraints of the seat failed during this same test, but this failure was attributed to the instability induced by the boom and fin malfunctions- The parachute functioned properly but did not open until just before the dummy reached the ground, too late to prevent a considerable amount of damage to the dummy.) ⁵¹

As a result of the January test, the booms were carefully rechecked and strengthened and the seat's gas system was pressure tested. ⁵² The final sled-test was conducted on 3 March 1959 with a dynamic pressure of about 1,600 pounds per square foot and at Mach 1.15; at conditions considerably in excess of requirements, it was by far the most successful test. The leg manacles broke during this test, but North American began an immediate program to correct this failure. ⁵³

Additional sled tests and a parachute jump program were proposed in April of 1959, but project personnel decided that further extensive testing was unnecessary. The possibility of parachute tests was not eliminated but neither was there any definite decision to conduct such tests. ⁵⁴

The third item in the X-15 program for which the Air Force retained direct responsibility (apart from the XLR99 rocket engine and the full-pressure suit) was the all-attitude inertial flight data system. Designers realized from the first that the X-15 s performance would necessitate a new means of determining altitude, speeds and aircraft attitude; the NACA had proposed a stable platform inertial integrating and attitude system as a means of meeting these needs. Unfortunately, not much thought seemed to have been given to the exact requirements of such a system or to the source from which it might be obtained.

An NACA report of meetings held at Wright Air Development Center and at North American in the fall of 1956 indicated that Wright Air Development Center agreed to furnish a stable platform. The NACA representative apparently assumed that the center had already developed a suitable platform, as his report stated that the instrument appeared to be a newly developed Bendix platform weighing only 28 pounds and occupying only one-half a cubic foot. ⁵⁵

North American and the NACA were not as certain about the platform, for during a visit to North American's plant, Mr. Walter Williams, the chief of NACA's High Speed Flight Station, specifically asked that the question of who was to supply the stable platform be clarified. ⁵⁶ It was not until 24 May 1956 that a meeting was held (at Langley) for the purpose of discussing the actual requirements for the proposed stable platform system. The May meeting was attended by representatives of North American Aviation, the NACA, the Eclipse-Pioneer Division of Bendix, and Wright Air Development Center. One of the center's representatives was Mr. M. L. Lipscomb of the Instrument Branch in the Flight Control Laboratory. Mr. Lipscomb was subsequently to play an important role in the selection and development of the system that was eventually procured. The attendance of Eclipse-Pioneer representatives indicated that Bendix was still being considered as the potential contractor. The consensus of those attending the meeting was that a suitable platform could be developed in twenty four months. North American presented the weight and size requirements for the system, and the NACA agreed that since the platform would provide research information, 40 pounds of the estimated 65 pound weight should be considered as a part of the allotted weight of the research instrumentation. ⁵⁷

During the summer of 1956, Eclipse-Pioneer failed to display any further interest in providing the desired equipment and the Flight Control Laboratory invited the Sperry Gyroscope Company to submit proposals for a stable platform system. By August, Sperry had prepared the requested proposal, and on 4 October Sperry personnel participated in a briefing at Wright Air Development Center. ⁵⁸

On 26 December 1956, Mr. Lipscomb asked the Air Materiel Command to start the procurement of eight all-attitude flight data systems. Two of the requested items were designated ”B” type and were to be utilized for research; six were to be assigned to the X-15 program and were designated type ”A.” The systems were described in detail in an accompanying exhibit, dated 12 December 1956. The laboratory recommended that the contract be given to the Sperry Gyroscope Company, estimating the cost at $1,030,000. ⁵⁹ The request for a proposal from Sperry was not made until 6 February 1957. ⁶⁰

Sperry replied on the 20th of the same month, and by 28 March the Flight Control Laboratory had evaluated and approved Sperry's proposals. In the meantime, however, the Air Materiel Command, the Flight Control Laboratory, the X-15 Weapon System Project Office, and Sperry had become involved in a controversy over a number of details. Some of the points at issue were the total amount of the contract, the amount of the fixed fee, the contractor's cost criteria, and the provisions for travel in connection with the proposed contract. By 11 April 1957, the contract negotiations seemed to have reached a deadlock, and the Air Materiel Command buyer notified the Flight Control Laboratory and the project office that he intended to solicit sources other than Sperry in an effort to secure the desired system at a reduced cost. The laboratory and the project office responded to this development by reiterating their reasons for considering Sperry to be the only contractor capable of producing the required system within the time period available. The laboratory's position was that Sperry was the only concern with experience in components, systems, and applications, and the project office emphasized that Sperry was the only supplier who could produce the equipment in time to meet the schedule of the X-15 program. ⁶¹

The Air Materiel Command still refused to concede the validity of the justifications for considering Sperry as a sole source and it was evident that the patience of all the parties concerned was rapidly being exhausted when the entire controversy was brought to a head on 22 April 1957. On that date, General Haugen, then director of development at Wright Air Development Center, advised the Air Materiel Command that ”sole source procurement from Sperry provides the only possibility of obtaining the specific equipment to meet the time schedule of the X-15 program.” General Haugen added that the importance of the X-15 program justified an award of the contract to Sperry ”at the earliest possible date.” ⁶² General Haugen's intervention proved the needed catalyst, for while negotiations continued, they were conducted only with the Sperry Gyroscope Company and a contract was ready for final negotiation by 26 April 1957. The cost-plus-fixed-fee contract, completed on 5 June 1957, provided an estimated cost of $1,213,518.06, with a fixed fee of $85,000. ⁶³

By May 1958, the cost had risen to $2,498,518, and in June a further increase brought the cost to $2,741,375 and raised the fee to $102,000. No further increases took place during 1958, but several were permitted in early 1959. By mid-April 1959, costs had reached $3,234,188.87 and the fixed fee had risen to $119,888.56. ⁶⁴

By April 1958, the Flight Control Laboratory and the X-15 project office had concluded that the scheduled delivery of the first Sperry unit in December of that year would not permit adequate testing to be performed prior to the first flights of the X-15. Consequently the several participants decided to install an interim gyroscopic system in the first two aircraft and to install the completed system in the third. ⁶⁵

As the development of the stable-platform progressed, it became apparent that its weight had been seriously underestimated. The increase in weight was obvious by May 1958, when Sperry undertook a program of weight reduction which, unhappily, was not as successful as the Flight Control Laboratory and the project office had hoped. In August, the project office reported that the weight was then approximately 100 percent greater than had been originally anticipated. ⁶⁶

As a result of the concern over the weight increase, the laboratory requested that Sperry be asked to justify the weight increase. ⁶⁷ On 7 August 1958, the Air Materiel Command advised Sperry of the laboratory's desire for additional information on the company's weight reduction program and for a justification of the weight increases that had taken place. ⁶⁸ Sperry's reply revealed that with a shock mount which would meet the vibration specification, the weight of the system had increased to 185.25 pounds and that with a less satisfactory but possibly adequate shock mount, the weight would be 165.25 pounds. Sperry stated that the company had been fully aware of the weight problem throughout the program and that it had ”… designed and developed an optimum system considering the present state of the art.” A number of detail changes that had been made in the effort to eliminate excess weight were also itemized. These included the substitution of aluminum for stainless steel whenever possible, the reduction of the thickness of cases and covers, the development of the less satisfactory but lighter shock mount, and a careful reduction of component weights whenever such reduction proved feasible. Sperry also pointed out that it had been necessary to include power supplies in the final design. Finally, Sperry had compared the X-15 system to similar systems made by other concerns and felt that the Sperry equipment was ”… lighter, more accurate, and required less total aircraft volume …” than any of the equipment to which it was compared. ⁶⁹

Apparently Sperry's letter of justification was satisfactory, because project people thereafter accepted the fact that the system was overweight and was going to remain overweight.

By the end of November 1958, the two major system components, the stabilizer and the computer, had completed the individual tests and were ready to be tested as a complete system during the following month. The ground test equipment was also nearing completion and was scheduled for year-end delivery. The first system completed its acceptance test in December; the system and ground test cart were shipped to Edwards Air Force Base in mid-January 1959. ⁷⁰ During the spring of 1959, the original plans to utilize the carrier B-52 as a test vehicle for the stable platform system were changed and arrangements made to test the equipment in a KC-97 that was already in use as a test aircraft in connection with the B-58 program. ⁷¹ The first test flights in the KC-97 were carried out in late April. ⁷² By June, North American had made a successful test installation of the Sperry system in the third X-15 and the stable-platform program seemed to be moving toward a successful conclusion with no major obstacles or difficulties foreseen. ⁷³
 

 

Section V  Tying Up Loose Ends

 

Apart from the major problems encountered during the development of the X-15, there arose less critical items of concern, some technical, some administrative, and some financial. portions of the program were routine, but even those portions demanded time and attention if they were to remain in the routine category. For instance, the ground range presented a problem that had no connection with the selection of radars, with geography, or with building and equipping the stations. The procurement of the high-altitude tracking equipment was expedited by transferring the procurement responsibility to Patrick Air Force Base and the equipment was obtained by the modification of an existing contract. ¹ The detail design and fabrication of the range was undertaken by the Electronic Engineering Company of Los Angeles, California. ² The range was completed and ready for operation in late 1958. ³ The difficulty in connection with the ground range stemmed from the joint nature of the program and consisted of a dispute between the Air Force and NACA over the operation of the range after its completion.

As early as 7 April 1955, Brigadier General B. S. Kelsey, the Air Force representative on the Research Airplane Committee, wrote to Dr. H. L. Dryden, director of NACA, and requested that an understanding be reached on the construction and operation of the range. ⁴ At a meeting of the Research Airplane Committee on 17 May 1955, the NACA agreed to cooperate with WADC and the Air Force Flight Test Center (AFFTC) in planning the range; the Air Force was to be given the task of building and equipping the range, and the NACA would operate the range after its completion. ⁵

These decisions were not favorably received by Air Force Flight Test Center personnel, who felt that their center was ”… being relegated to the position of procurement agent for NACA. The Air Force also had some reservations about the ”… adequacy of equipment NACA had selected for the range.” ⁶ Despite the flight test center's lack of enthusiasm for the arrangements, an amendment of the original development directive, issued on 28 July 1955, spelled out the flight test center's responsibility for establishing the range. As neither the amendment nor the original directive assigned the responsibility for the operation of the completed range, the Air Force Flight Test Center renewed its attempts to acquire this responsibility. On 2 December 1955, Lieutenant Colonel B. H. Harris Jr., deputy chief of staff for operations at the flight test center, wrote to the commander of ARDC and formally requested that his center ”… be assigned the responsibility for operating, as well as developing, the test range.” ⁷

The NACA, determined to retain the responsibility for the operation of the range, simply reminded the Air Force that the matter had been settled by an agreement between Dr. Dryden and General Kelsey. As the Air Force's strongest argument was that Air Force operation would permit the use of the range during tests of such advanced fighters as the F-107, the NACA quickly agreed to make the range available for such tests – providing such use did not interfere with the X-15 program. ⁸

Fortunately this dispute over range operation and the similar disagreement with the Navy in relation to engine procurement were exceptions rather than the rule. The division of responsibility was usually arranged without difficulty and such disputes never offered a serious threat to the ultimate success of the project.

Another aspect of the X-15 program which occasionally caused concern was in the field of public relations. With numerous government agencies and contractors taking part in a program which was certain to arouse a great deal of public interest, there was bound to be some conflict. Each agency and each contractor had an information service or a publicity department, and it was to be expected that each such organization would seek to insure proper recognition of its own parent. Ordinarily such competition would have been considered unimportant, but after the success of the Soviet satellites in late 1957, the X-15 program became intimately associated with national prestige. The successful launchings of space vehicles made everything connected with space exploration a matter of vital interest to a world that was deeply concerned about the technological race between the United States and the Soviet Union. The X-15 ceased to be just an advanced research airplane-it suddenly became an entry in an international race.

As the roll-out of the first X-15 did not occur until 15 October 1958, most of the early publicity burden was carried by the pilots who had been selected to fly the aircraft. At one stage, the demands upon these pilots became so serious as to interfere with their training and indoctrination program. Some of those involved actually reported physical incapacitation as a result of extensive travel, irregular meals, and a lack of proper rest.

In addition to actual interference with the X-15 schedule, the publicity efforts sometimes created ill will and misunderstandings. For instance, on 13 January 1958, the Office of Information Services in the Office of the Secretary of the Air Force issued a release which stated that after company demonstrations of the X-15, the airplane would be flown by Air Force pilots and then turned over to NASA. ^a As an arrangement had already been made for the Air Force and NASA pilots to share in the research flights and for NASA to plan and direct the flights, the release confused the NASA and Air Force personnel at Edwards who had been planning the joint research effort and the relegation of NASA to last place displeased that agency. ⁹

One of the least comprehensible facets of the public relations program was the insistence that the X-15 would reach an altitude of 100 miles. The 100-mile figure was repeated in almost every article and broadcast that dealt with the X-15, and it was also used in speeches by Air Force and North American personnel. While it was more than probable that the X-15 would exceed its design altitude of 250,000 feet, the constant reiteration of a maximum altitude figure seemed very questionable public relations. The Air Force might find itself in the unenviable position of having to confess that the X-15 could not meet its advertised goals. On the other hand, if the design altitude had been used in the various releases, it is very possible the Air Force could have proudly announced that the X-15 had exceeded the goals that were set for it. The X-15 project office consistently reduced the altitude claims in its contacts with news media, but apparently the 100-mile figure was firmly established in the public mind. The inflated figure seemed particularly unnecessary in view of the fact that attainment of the design altitude would approximately double the existing record. If the X-15 failed to reach the goals announced in the preflight publicity releases, it could only result in a general impression that the project was a partial failure, or create doubts about the veracity of the information services that had persisted in publicizing the maximum performance. ¹⁰

Throughout the development of the X-15 there was a considerable body of opinion favoring an extension of the program beyond the original three aircraft. Although this opinion did not prevail, the proposals for such an extension were of more than passing interest. At one point, North American suggested the X-15 be utilized as a training vehicle and that an extensive training program be established. The company pointed out that such a program would prove useful in familiarizing Air Force pilots with rocket powered aircraft, the use of reaction controls, and some of the physical problems and sensations of space flight. Naturally such a training program would have necessitated production of additional X-15s.

Early in 1958, the NACA expressed the hope that at least one additional X-15 could be produced and that it would be devoted specifically to flight-control research. ¹¹ However, the most serious consideration for an extension of the X-15 program came in mid-1958. On 8 April of that year, Air Force headquarters asked ARDC to consider the wisdom of investing additional funds in an expansion of the X-15 program. The letter in which the request appeared asked that ARDC weigh the cost of a possible extension against the probable value of such an extension and suggested that the requested recommendations should ”… include configuration changes, estimated costs, aircraft availability, the increased performance expected, the test results to be obtained and a brief substantiation of their value.” It also urged that the results of the extension studies be made available to Air Force headquarters at an early date--because any decision for an extension would have to be made before North American broke up the engineering team that had been assembled for the original program. ¹²

In response to this request for a study of extension possibilities, the X-15 project office conferred with the Directorate of Laboratories at WADC, with North American, and with the Air Force Flight Test Center at Edwards Air Force Base. Conferences with these groups took place between 17 and 21 April 1958 in an attempt to determine the future research requirements that might be met by an extension of the X-15 program. There were discussions on a possible change of structural materials of the X-15 airframe and attempts to estimate preliminary costs, design changes, production schedules, and performance figures for some of the more promising modifications that were envisioned. By 29 April, ARDC, the X-15 project office, and the Directorate of Laboratories had concluded that the best approach to an extension program would be to prove out the existing aerodynamic design and to consider the possibilities of improving performance by the use of new structural materials and the substitution of an improved rocket engine for the XLR99. The suggestion for a new engine was evidently influenced by the then-current difficulties with the XLR99; Air Force planners emphasized that any new engine should ”… be obtained as a result of across the board BMD (Ballistic Missile Division) and other effort, and not as a sole X-15 effort.” ¹³

On 19–20 May, the Air Force obtained verbal concurrence on the proposed extension from the Navy and NASA. The recommendation submitted to ARDC by the X-15 project office on June 13 was that the X-15 program be extended by the construction of three additional airplanes employing structural materials capable of withstanding higher temperatures than the materials utilized in the original design. ARDC approved the recommendation and forwarded it to Air Force headquarters on 16 June. ¹⁴

The urgency expressed in the original headquarters letter of April 8 had apparently evaporated, for it was not until 18 November 1958 that General Demler, director of research and development at Air Force headquarters, advised the commander of ARDC that no further consideration was to be given to an extension of the X-15 program. The Research Airplane Committee had not even met until October 31 and at that time Dr. Dryden, the NASA representative, stated that NASA had reached the conclusion that the original aircraft were adequate for the research contemplated by his agency and that any increase in program effort should be directed toward the maximum exploitation of the three X-15s already procured. He held that further development of additional aircraft was not warranted; the Navy and Air Force representatives on the Research Airplane Committee concurred with Dr. Dryden. ¹⁵

Not everyone was convinced that the decision was final, as there was still some interest in at least one additional airplane for flight-control research. However, as all three of the original aircraft were substantially completed by early 1959, it seemed most unlikely that there would ever be any additional extension proposals.

Responsibilities of the X-15 project office were many and varied. The office had to maintain close liaison with NASA on such subjects as the spherical nose being developed under the supervision of that agency. It had to make the arrangements for procuring and modifying the two B-52s that were to replace the B-36 carrier that had been contemplated originally. Other aircraft had to be scheduled for pilot indoctrination and for chase planes. Ground equipment had to be scheduled so that components could be tested and the aircraft maintained. Pilots had to be selected for the program. It was necessary to arrange for wind tunnel and centrifuge time at facilities already operating on tight schedules. Difficulties in the fabrication of some of the pressure tanks had to be considered and decisions made as to whether it was better to accept the weight penalties involved in a change of materials or the time penalties involved in further development. The decision to switch from a B-36 to a B-52 carrier necessitated that the X-15 be carried under a wing rather than the fuselage of the carrier aircraft and this change introduced new problems of sonic fatigue and flutter that had to be met and overcome. Testing revealed that some components of the stability augmentation system were 'not satisfactory and time was lost in redesign and retesting. A second industry conference, held 28–30 July 1958 at Los Angeles, had to be arranged. In addition to all of these items, routine paper work had to be accomplished, reports reviewed, and everyone concerned with the program advised of the progress. The paper work was more burdensome than usual because of the participation of two additional government agencies--the Navy and the NACA(NASA).

Two incidents, both connected with the XLR99 engine, revealed the variety of details with which the project office had to concern itself. They also illustrated the problems and frustrations that occurred when that office was not adequately or promptly informed. The first incident involved an aft-fuselage section which was furnished to Reaction Motors for 'the purpose of determining engine-airframe compatibility and the effects of engine vibration on the fuselage structure. When Reaction Motors had completed the tests, the company in accordance with existing procurement directives advised the nearest Air Force procurement office of this fact and asked for instructions as to its disposition. The Air Force office in quest£on, without consulting the X-15 project office, instructed Reaction Motors to destroy the item and the company proceeded to do so, completely unaware that the $300,000 airframe section had been scheduled for further use at Edwards.

The second incident involved the shipment of the first XLR99 engine, associated ground test equipment, and spare parts to Edwards Air Force Base. In an attempt to expedite the engine program, the project office had arranged for a military aircraft to transport the engine, the test equipment, and the spares from Reaction Motors to Edwards. Everything was ready for shipment.when the Air Force inspector noted that the boxes containing the spare parts were not labeled in accordance with regulations. The part 'numbers had been inked rather than typed on the box labels. The inspector refused to release the spares for shipment, with the result that the military aircraft proceeded to California with the engine and test equipment but without any spares. Subsequently, the project office had to arrange for shipment of the spares by commercial air freight. ¹⁶

In addition to the various technical tasks, the X-15 project office was under almost constant pressure to secure additional funds. This was because the original cost estimates for the X-15 and the XLR99 were grossly inaccurate. Initial ”planning” figures – for everything – totaled $12,000,000. Between October 1955 and the beginning of 1959, the airframe estimates rose from $50,063,500 to $64,021,146, and in the first 6 months of 1959 the estimates continued to rise, first to $67,540,178, and then to $68,657,644. ¹⁷ By 1 June 1959, North American's informal estimate of the airframe's cost had risen to $74,500,000. ¹⁸

The engine program involved even greater relative increases. In 1955, it had been estimated that the engine costs would ultimately be about $6,000,000. By the time an engine contract had been signed, the estimate had risen to $10,000,000. At the end of fiscal 1958, engine costs had risen to over $38,000,000 and expenditures in fiscal year 1959 brought the cost to $59,323,000. Estimated engine costs for fiscal 1960 were $9,050,000 – almost as much as the total estimate of 1955. As of June 1959, it appeared that engine costs would be at least $68,373,000 – over five times the original estimate for the entire X-15 program and almost a seven-fold increase over the costs contemplated when the engine contract was signed. ¹⁹ With the cost of the stable platform totaling more than $3,000,000, and with an estimated $6,000,000 needed for support costs in fiscal 1961, the total cost of the X-15 was going to exceed $150,000,000, even if no further increases occurred. (The Navy's contribution to the X-15 program totaled $6,400,000 at the end of fiscal 1959 and the project office hoped that an additional $1,000,000 could be obtained from the Navy in fiscal 1960.) ²⁰ All the remaining funds had to be furnished by the Air Force, as NASA's contribution was in the form of wind tunnel testing and evaluation. The program was never halted by a lack of funds, but there were occasions when the funds only became available at the last possible moment; the files of the project office revealed that appeals for funds, justifications for additional funds, and the explanation of increased costs absorbed much of that office's time and energy.

Despite the technical problems, the paper work, the necessity of seeking more and more funds, and the recognition that the first flight would be several months. late, the X-15 program did move ahead.

It was still too early to predict the ultimate success of the X-15 in its research role, but the development program was rapidly drawing to a close in 1959. The airplane took slightly longer to reach the flight stage than had originally been contemplated, and the costs were far in excess of the estimates, but it would appear that the vehicle would be able to equal or surpass the performance for which it had been designed, and that it would prove to be a valuable research instrument.

 

 

Section VI  Research At The Edge Of Space

 

The first of the three X-15s (serial 56-6670) arrived at the Air Force Flight Test Center at Edwards Air Force Base, California, in mid-October 1958, trucked over the hills from the plant in Los Angeles for testing at the NASA High-Speed Flight Station (subsequently redesignated the NASA Flight Research Center). It was joined by the second airplane (serial 56-6671) in April 1959. In contrast to the relative secrecy that had attended flight trials with the XS-1 (X-1) a decade before, the X-15 program offered the spectacle of pure theater.

The X-15's contractor program lasted two years, from mid-1959 through mid-1960. North American had to demonstrate the craft's general airworthiness during flights above Mach 2, and successful operation of its new XLR99 engine before delivering the craft to NASA. Anything beyond Mach 3 was considered a part of the government's research obligation. The task of flying the X-15 during the contractor program rested in the capable hands of Scott Crossfield, who had left NACA to join North American and help shepherd the craft through its long development. Crossfield completed the first captive flight on 10 March 1959 and first glide flight on 8 June. Just prior to landing, the plane began a series of increasingly wild pitching motions; thanks to Crossfield's instinctive corrective action, the plane landed safely; Crossfield feared for the plane's design, but fortunately, for naught. North American's engineers subsequently modified its boosted control system to increase the control rate response, and the X-15 never again experienced the porpoising motions that had threatened it on its first flight. On 17 September, the X-15 completed its first powered flight, when Crossfield flew the second airplane to Mach 2.11. ¹ 

A series of ground and in-flight accidents marred the X-15's contractor program, fortunately without injuries or even greatly delaying the program. On 5 November 1959 an engine fire – always extremely hazardous in a volatile rocket airplane – forced an emergency landing on Rosamond Dry Lake; the X-15 landed with a heavy load of propellants and broke its back, grounding this particular X-15 for three months. During a ground engine test with the third X-15 (the first one equipped with the large Thiokol engine), a stuck pressure regulator caused the craft to explode, necessitating virtual rebuilding. The second X-15 was actually the first of the series to test-fly the large XLR99 engine, and after adding the engine to the other two craft, North American delivered the last of the X-15s to NASA in June 1961. By that time, NASA, Air Force and Navy test pilots had been operating the X-15 on government research flights for just over a year. ² The research phase of the X-15's flight program involved four broad objectives: verification of predicted hypersonic aerodynamic behavior and hypersonic heating rates, study of the X-15's structural characteristics in an environment of high heating and high flight loads, investigation of hypersonic stability and control problems during atmospheric exit and reentry, and investigation of piloting tasks and pilot performance. By late 1961, these four areas had been generally examined, though detailed research continued to about 1964 using the first and third aircraft, and to 1967 with the second (the X-15A-2). Before the end of 1961, the X-15 had attained its Mach 6 design goal and had flown well above 200,000 feet; by the end of the next year the X-15 was routinely flying above 300,000 feet. Within a single year, the X-15 had extended the range of winged aircraft flight speeds from Mach 3.2 to Mach 6.04, the latter achieved by Air Force test pilot Bob White on 9 November 1961.

The intensive flight program on the X-15 revealed a number of interesting things. physiologists discovered the heart rates of X-15 pilots varied between 145 and 180 beats per minute in flight, as compared to a normal of 70 to 80 beats per minute for test missions in other aircraft. Researchers eventually concluded that prelaunch anticipatory stress, rather than actual post launch physical stress, influenced the heart rate. They believed, correctly, that these rates could be considered as probable baselines for predicting the physiological behavior of future pilot-astronauts. Aerodynamic researchers found remarkable agreement between the tunnel tests of exceedingly small X-15 models and actual results, with the exception of drag measurements. Drag produced by the blunt aft end of the aircraft proved 15% higher on the actual aircraft than wind-tunnel tests had predicted. At Mach 6, the X-15 absorbed eight times the heating load it experienced at Mach 3, with the highest heating rates occurring in the frontal and lower surfaces of the aircraft, which received the brunt of airflow impact. During the first Mach 5+ excursion, four expansion slots in the leading edge of the wing generated turbulent vortices that increased heating rates to the point that the external skin behind the joints buckled. As a solution, technicians added small Inconel alloy strips over the slots, and the X-15 flew without further evidence of buckling. It offered ”… a classical example of the interaction among aerodynamic flow, thermodynamic properties of air, and elastic characteristics of structure.” ³

Heating and turbulent flow generated by the protruding cockpit enclosure posed other problems; on two occasions, the outer panels of the X-15's heavy glass cockpit windshields fractured because heating loads in the expanding frame overstressed the soda-lime glass. NASA solved the difficulty by changing the cockpit frame from Inconel to titanium, modifying its configuration, and replacing the outer glass panels with high-temperature alumina silica glass. Another problem concerned an old aerodynamics and structures bugaboo, panel flutter. Panels along the flanks of the X-15 fluttered at airspeeds above Mach 2.4, forcing engineers to add longitudinal metal stiffeners to the panels. ^a All this warned aerospace designers to proceed cautiously. John Becker, writing in 1968, noted of the X-15 experience that: ⁴

The really important lesson here is that what are minor and unimportant features of a subsonic or supersonic aircraft must be dealt with as prime design problems in a hypersonic airplane. This lesson was applied effectively in the precise design of a host of important details on the manned space vehicles.

A serious roll instability predicted for the airplane under certain reentry conditions posed a serious challenge to flight researchers. To simulate accurately the reentry profile of a returning winged spacecraft, the X-15 had to fly at angles of attack of at least 17°. Yet the cruciform ”wedge” tail, so necessary for stability and control in other portions of the plane's flight regime, actually prevented it from being flown safely at angles of attack greater than 20° because of potential rolling problems. By this time, FRC researchers had gained enough experience with the XLR99 engine to realize that fears of thrust misalignment – a major reason for the large vertical fin – were unwarranted. The obvious solution was simply to remove the lower half of the ventral fin, a portion of the fin that X-15 pilots had to jettison prior to landing anyway so that the craft could touch down on its landing skids. Removing the ventral produced an acceptable tradeoff. While it reduced stability by about 50%. at high angles of attack, it greatly improved the pilot's ability to control the airplane. With the ventral off, the X-15 could now fly into the previously ”uncontrollable” region above 20° angle of attack with complete safety. Eventually the X-15 went on to reentry trajectories of up to 26°, often with flight path angles of -38° at speeds up to Mach 6, a much more demanding piloting task than the shallow entries flown by manned vehicles returning from orbital or lunar missions. Its reentry characteristics were remarkably similar to those of the later NASA Space Shuttle orbiter. ^{5 b}

When Project Mercury took to the air, it rapidly eclipsed the X-15 in glamour, but the two programs really were complementary in nature, though Mercury dominated some of the research areas that had first interested X-15 planners, such as ”zero g” weightlessness studies. The use of reaction controls to maintain a vehicle's attitude in space proved academic after Mercury flew, but the X-15 had already proved them and would also furnish valuable design information on the use of blending reaction controls with conventional aerodynamic controls during an exit and reentry, a matter of concern to subsequent Shuttle development. The X-15 experience clearly demonstrated the ability of pilots to fly rocket-propelled aircraft out of the atmosphere and back in to precision landings. Flight Research Center director Paul Bikle saw the X-15 and Mercury as a: ⁶

parallel, two-pronged approach to solving some of the problems of manned space flight. While Mercury was demonstrating man's capability to function effectively in space, the X-15 was demonstrating man's ability to control a high-performance vehicle in a near-space environment … considerable new knowledge was obtained on the techniques and problems associated with lifting reentry.

Operationally, the X-15 gave its team a number of headaches. Because of the complexity of its systems, the plane experienced a number of operational glitches that delayed flights, aborted them before launch, or forced abandonment of a mission after launch. Early in the program, the X-15's stability augmentation and inertial guidance systems were two major problem areas. NASA eventually replaced the Sperry inertial unit with a Honeywell unit first designed for the Dyna-Soar. The plane's propellant system had its own weaknesses. Pneumatic vent and relief valves and pressure regulators gave the greatest difficulties, followed by spring pressure switches in the auxiliary power units, the turbopump, and the gas generation system. NASA's mechanics routinely had to reject 24 to 30% of spare parts as unusable, a clear indication of the difficulties of devising industrial manufacturing and acceptance test procedures when building for use in an environment at the frontier of science. ⁷ Weather posed a critical factor. Many times Edwards enjoyed fine weather, the lakebed bone-dry, while upcountry the High Range was covered with clouds, alternate landing sites were flooded, or some other meteorological condition postponed a mission. In one case, weather and minor maintenance kept one X-15 grounded from mid-October 1961 to early January 1962. When it finally flew, the pilot had to make an emergency landing up range. Weather and maintenance then grounded the plane until mid-April. ⁸ On an average, the X-15 completed 1.77 flights per month – a figure comparing well within the shuttle's own subsequent experience (until the loss of Challenger.)

The X-15 had its share of accidents, one of which killed an Air Force test pilot; another seriously injured a NASA research pilot. As previously mentioned, Scott Crossfield once made an emergency landing on Rosamond Lake with an X-15 damaged by an engine fire; the plane broke its back on landing, necessitating lengthy repairs. The third X-15 blew up during ground testing of its XLR99 engine, but it, too, was rebuilt. In November 1962, an engine failure forced Jack McKay to make an emergency landing at Mud Lake, Nevada, in the second X-15; its landing gear collapsed and the X-15 flipped over on its back. McKay was promptly rescued by an Air Force medical team standing by near the launch site, and eventually recovered to fly the X-15 again. But his injuries, more serious than at first thought, eventually forced his retirement from NASA. In November 1967, Mike Adams was killed in a strange accident in the third X-15 that will be discussed later in great detail. One of the most remarkable close calls in the X-15 program involved Air Force. test pilot Major William J. ”Pete” Knight. in June 1967 he experienced a complete electrical failure while climbing through 100,000 feet at Mach 4+. With no computed information and guidance, Knight continued to climb, suddenly reduced to ”seat of the pants” flying technique. During reentry he managed to restart one of the auxiliary power units, restoring some instruments, and made an emergency landing at Mud Lake, for which he received the Distinguished Flying Cross. ^c Within NACA and later NASA, developing the X-15 had been left largely in the hands of Langley, the center most closely involved in determining its mission and configuration, with important inputs from the other centers, especially the High-Speed Flight Station. The flight research program was the province of the Flight Research Center with liaison and support from the Air Force Flight Test Center at Edwards. In the summer of 1961, as the X-15 approached its maximum performance during test flights, a new initiative began, one that sprang jointly from the Air Force's Aeronautical Systems Division at Wright-Patterson AFB and from NASA Headquarters: using the X-15 as a ”testbed” or carrier aircraft for a wide range of scientific experiments unforeseen in its original conception.

Pressures had existed even before the X-15 first flew to extend the scope of the program beyond aerodynamics and structural research. Researchers at the Flight Research Center had proposed using the airplane to carry to high altitude some experiments related to. the proposed Orbiting Astronomical Observatory; others suggested modifying one of the planes to carry a Mach 5+ ramjet for advanced air-breathing propulsion studies. Over 40 experiments were suggested by the scientific community as suitable candidates for the X-15 to carry. In August 1961, after consulting with Bikle at FRC, NASA headquarters, and the Air Force Aeronautical Systems Division, NASA and the Air Force formed an X-15 Joint Program Coordinating Committee to prepare a plan for a follow-on experiments program. Most of the suggested experiments were in space science, such as ultraviolet stellar photography. Others supported the Apollo program and hypersonic ramjet studies. A series of meetings held at NASA headquarters over the fall of 1961 between the joint committee. Hartley Soule, and John Stack, then NASA's director of aeronautical research, culminated in approval of the proposed follow-on research program and the classification of two groups of experiments. Category A experiments consisted of well-advanced and funded experiments having great importance; category B included worthwhile projects of less urgency or importance. ⁹

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Figure 1 X-15/BLUE SCOUT LAUNCH SYSTEM

In March 1962 the X-15 committee approved the ”X-15 Follow-on Program,” which NASA announced 13 April in a Headquarters news conference presided over by Stack and FRC planner Hubert Drake. Drake announced that the first task would be to fly an ultraviolet stellar photography experiment from the University of Wisconsin's Washburn Observatory. NASA had investigated the possibility of the X-15 carrying a Scout booster that could fire small satellites into orbit, the entire B-52/X-15/Scout becoming in effect a multistage satellite booster, but that the agency finally rejected the idea for reasons of safety, utility, and economy. The X-15's space science program eventually included twenty-eight experiments running from astronomy to micrometeorite collection, using wingtop pods that opened at 150,000 feet, and high-altitude mapping. Two of the follow-on programs, a horizon definition experiment from the Massachusetts Institute of Technology and tests of proposed insulation for the Saturn launch vehicle, directly benefitted navigation equipment and the thermal protection used on Apollo-Saturn launch vehicle.

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Figure 2 PROFILE OF PACIFIC MISSILE RANGE LAUNCH

FRC quickly implemented the follow-on program. In 1964, fully 65% of all data returned from the three X-15 aircraft involved follow-on projects; this percentage increased yearly through conclusion of the program.¹⁰

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Figure 3 PROFILE OF ATLANTIC MISSILE RANGE LAUNCH

NASA's major X-15 follow-on project involved a Langley developed Hypersonic Ramjet Experiment (HRE). ^d FRC advanced planners had long wanted to extend the X-15's speed capabilities, perhaps even to Mach 8, by adding extra fuel in jettisonable drop tanks and some sort of thermal protection system. Langley researchers had developed a design configuration for a proposed hypersonic ramjet engine. The two groups now came together to advocate modifying one of the X-15s as a Mach 8 research craft that could be tested with a ramjet fueled by liquid hydrogen. The proposal became more attractive when the landing accident to the second X-15 in November 1962 forced the rebuilding of the aircraft. The opportunity to make the modifications was too good to pass up. In March 1963 the Air Force and NASA authorized North American to rebuild the airplane with a longer fuselage. Changes were to be made in the propellant system; two huge drop tanks and a small tank for liquid hydrogen within the plane were to be added; the drop tanks could be recovered via parachute and refurbished, as with the Space Shuttle's solid-fuel boosters nearly two decades later.

Forty weeks and $9 million later, North American delivered the modified plane, designated the X-15A-2, in February 1964. ¹¹ The X-15A-2 (Figure 4) first flew in June 1964, piloted by Air Force test pilot Major Bob Rushworth. Early proving flights demonstrated that the plane retained satisfactory flying qualities at Mach 5+ speeds, though on three flights, thermal stresses caused portions of the landing gear to extend at Mach 4.3, generating "an awful bang and a yaw," but Rushworth landed safely despite (in one case) blow-out of the heat-weakened tires upon touchdown. In November 1966, Air Force pilot Pete Knight set an unofficial world's airspeed record of Mach 6.33 in the plane. NASA then grounded it for application of an ablative coating to enable it to exceed Mach 7. ¹²

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Figure 4 NORTH AMERICAN X-15A-2 RESEARCH AIRCRAFT

Flight Research Center's technical staff had evaluated several possible coatings that could be applied over the X-15'.s Inconel structure to enable it to withstand the added thermal loads experienced above Mach 6. NASA hoped that such coatings might point the way toward materials that could be readily and cheaply applied to reusable spacecraft, minimizing refurbishment costs and turn-around time between flights. Such a coating would have to be relatively light; have good insulating properties; be easy to apply, cure, and then remove; and be easy to reapply before another flight. On FRC's advice, a joint NASA-Air Force committee selected an ablator developed by the Martin Company, MA-25S, in connection with some corporate studies on reusable spacecraft concepts. Consisting of a resin base, a catalyst, and a glass bead powder, it would protect the X-15's structure from the expected 2000°F heating as the craft sped through the upper atmosphere. Martin estimated that the coating, ranging from .59 inches thick on the canopy, wings, vertical, and horizontal tail down to 0.015 inches on the trailing edges of the wings and tail, would keep the skin temperature down to a comfortable 600°F. The first unpleasant surprise came, however, with the application of the coating to the X-15A-2: it took six weeks. Because the ablator would char and emit a residue in flight, North American had installed an ”eyelid” over the left cockpit window. It would remain closed until just before approach and landing. During launch and climbout, the pilot would use the right window, but residue from the ablator would render it opaque above Mach 6. ¹³

Late in the summer of 1967, the X-15A-2 was ready for flight with the ablative coating. It had already flown with a dummy ramjet affixed to its stub ventral fin; the ramjet, while providing a pronounced nose-down trim change, actually added to the plane's directional stability. The weight of the ablative coating – 125 pounds higher than planned – -together with expected increased drag reduced the theoretical maximum performance of the airplane to Mach 7.4, still a significant advance over the Mach 6.3 previously attained with the plane. The appearance of the X-15A-2 was striking, an overall flat off-white finish, the huge external tanks a mix of silver and orange-red with broad striping. NASA hoped that early Mach 7+ trials would lead to tests with an actual ”hot” ramjet rather than the dummy now attached to the plane. On 21 August 1967 Knight completed the first flight in the ablative coated plane, reaching Mach 4.94 and familiarizing himself with its handling qualities. His next flight, on 3 October 1967, was destined to be the X-15's fastest flight and the most surprising as well. ¹⁴

That day, high over Nevada, Knight dropped away from the B-52, the heavy X-15A-2 brimming with fuel. The following is an extract from the official AFFTC summary of the X-15A-2's envelope expansion program: ¹⁵

The launch transients were very mild with a bank angle excursion of 14 degrees. During the rotation the pilot had good control of the aircraft and increased the angle of attack to 15 degrees and felt the onset of buffet. The remainder of the rotation to the planned pitch angle was made at 12 to 13 degrees angle of attack. During this period the roll control was excellent and the bank angle did not deviate more than 8 degrees. The maximum dynamic pressure experienced during the rotation was 560 psf, close to the 540 psf observed on the simulator. The planned pitch angle of 35 degrees was reached in 38 seconds and was maintained within plus/minus one degree.

The external tanks were ejected 67.4 seconds after launch. Tank separation was satisfactory, however, the pilot felt the ejection was "harder" than the last one he had experienced (Flight No. 2-50-89). The longitudinal trim change to the aircraft was from 4.2 to -2 degrees angle of attack. The external tank recovery system performed satisfactorily and the tanks were recovered in repairable condition.

After tank ejection the planned 2 degree angle of attack was maintained within +1 degree. As the aircraft came level at an indicated altitude of 99,000 feet, the pilot increased the angle of attack to 6 degrees to maintain zero rate of climb. During this task the pilot reported that the pitch control was very sensitive and it was difficult to hold a constant angle of attack.

The pilot reported shutting down the engine at 6500 fps; however, the final radar data analysis revealed the maximum velocity to be 6630 fps. The total engine burn time was 141.4 seconds, which compared favorably with the 141 seconds planned. However, the aircraft had achieved a velocity which was 130 fps faster than that of the simulator during this time.

During the deceleration the pilot was concentrating on performing stability and control maneuvers and as a result the profile was not exactly as planned. After shutdown the aircraft did not descend at the rate planned, resulting in a lower dynamic pressure between 5500 and 4000 fps. This anomaly, along with the higher maximum velocity, presented the pilot with the task of managing higher energy in approaching the high key position. The region of largest dispersion from the planned ranging occurred at the time when the dynamic pressure was lower than planned. To regain the desired high key energy conditions, the pilot delayed the retraction of the speed brakes and flew the remainder of the deceleration at a higher dynamic pressure (a maneuver commonly used on X-15 flights).

The ability of the ablative material to protect the aircraft structure from the high aerodynamic heating was considered good except in the area of the dummy ramjet where the heating rates were significantly higher than predicted. Considerable heat damage occurred on the dummy ramjet and the ramjet pylon. The ramjet instrumentation ceased approximately 25 seconds after engine shutdown indicating that a burn through of the ramjet/pylon structure had occurred. Shortly thereafter the heat propagated upward into the lower aft fuselage area causing the engine hydrogen peroxide hot light to illuminate in the cockpit. Ground control, assuming a genuine overheat condition, requested the pilot to jettison the remaining engine peroxide. The high heat in the aft fuselage area also caused a failure of a helium control gas line allowing not only the normal helium source gas to escape, but also the emergency jettison control gas supply as well (because of the failure of a check valve). Thus, the remaining residual propellants could not be jettisoned. The aircraft was an estimated 1500 pounds heavier than normal at landing, but the landing was accomplished without incident.

The pilot performed a rudder pulse with the yaw damper off 71 seconds after engine shutdown and noted that the sideslip indicator did not oscillate as expected. Post-flight analysis of the maneuver revealed that the aircraft did in fact experience a reasonable yaw rate and lateral acceleration. The maneuver was performed at approximately the time of maximum temperature for the unprotected Ball Nose. It was concluded that the sphere of the Ball Nose experienced binding, possibly due to differential expansion.

The heat in the ramjet pylon area became high enough to ignite 3 of the 4 explosive bolts retaining the ramjet to the pylon at some time during the flight. As the pilot was performing a turn to downwind in the landing pattern, the one remaining bolt failed structurally and the ramjet separated from the aircraft. The pilot did not feel the ramjet separate. Since the landing chase aircraft had not yet joined up, the pilot was not aware that the unit had separated.

The position of the aircraft at the time of separation was established by radar data and the most likely trajectory estimated. A ground search party discovered the ramjet impact point on the Edwards AFB bombing range. Although it had been damaged by impact, it was returned for study of the heat damage that had occurred.

 

RAMJET SEPARATION CONDITIONS FLIGHT NO. 2-53-97 Velocity 980 fps Angle of attack 8° Altitude 35,500 feet Roll angle 57° left Mach Number 0.98 Normal accel. 1.6 g Dynamic Pressure 340 psf

The unprotected right-hand windshield was, as anticipated, partially covered with ablation products. With the pilot's visibility being restricted (the left window was still covered by the eyelid) his guidance to the high key position was based on radar vectors from ground control. The eyelid was opened at approximately 1.6 Mach number as the aircraft was over Rogers Lake and the visibility out this window was good.

Knight landed at Edwards, the plane resembling burnt firewood. It had been an eventful flight; now the engineers sat down and took a long look at what it all meant.

What it really meant was the end of the refurbishable spray-on ablator concept. It was the closest any X-15 came to structural failure induced by heating. The plane was charred on its leading edges and nosecap. The ablator had actually prevented cooling of some hot spots by keeping the heat away from the craft's metal heat-sink structure. On earlier flights without the ablator, some of those areas remained relatively cool because of heat transfer through the heavy Inconel structure. Some heating effects, such as at the tail and body juncture and where shockwaves intersected the structure, had been the subject of theoretical studies, but had never before been seen on an actual aircraft in flight. To John Becker at Langley, the flight underscored ”… the need for maximum attention to aerothermodynamic detail in design and preflight testing.” ¹⁶ To Jack Kolf, an X-15 project engineer at the FRC, the X-15A-2's condition ”… was a surprise to all of us. If there had been any question that the airplane was going to come back in that shape, we never would have flown it.” ¹⁷ The ablator had done its job, but refurbishing for another flight near Mach 7 would have taken five weeks. Technicians would have had great difficulty in ensuring adequate depth of the ablator over the structure. Obviously, a much larger orbital vehicle would have had even greater problems. The sprayed-on refurbish ablator concept thus died a natural death. The unexpected airflow problems with the ramjet ended any idea of using that configuration on the X-15, as did the ramjet's own shortcomings as a design (as is discussed subsequently). After the flight, NASA sent the X-15A-2 to its manufacturer for general maintenance and repair. Though the plane returned to Edwards in June 1968, it never flew again. It is now on exhibit--in natural black finish – at the Air Force Museum, Wright-Patterson AFB, Ohio. The third X-15 (serial 56-6672) featured specialized flight instrumentation and displays that rendered it particularly suitable for high-altitude flight research. A key element of its control system was a so-called ”adaptive” flight control system developed by Honeywell; it automatically compensated for the airplane's behavior in various flight regimes, combining the aerodynamic control surfaces and the reaction controls into a single control ”package.” This offered much potential for future high-performance aircraft such as the anticipated Dyna-Soar and supersonic transports, should the latter be built.

By the end of 1963, this X-15 had flown above 50 miles, the altitude that the Air Force recognized as the minimum boundary of spaceflight. FRC pilot Joe Walker set an X-15 record for winged spaceflight by reaching 354,200 feet, a record that stood until the orbital flight of Columbia nearly two decades later. These flights, and others later, acquired reentry data considered applicable to the design of future ”lifting reentry” spacecraft. By mid-1967, the X-15-3 had completed sixty-four research flights, twenty-one at altitudes above 200,000 feet. It became the prime testbed for carrying experiments to high altitude, especially micrometeorite collection and solar-spectrum analysis experiments.

As had happened in some other research aircraft programs, a fatal accident signaled the end of the X-15 program. On 15 November 1967 at 10:30 a.m., the X-15-3 dropped away from its B-52 mothership at 45,000 feet near Delamar Dry Lake. At the controls was veteran Air Force test pilot, Maj. Michael J. Adams. Starting his climb under full power, he was soon passing through 85,000 feet. Then an electrical disturbance distracted him and slightly degraded the control of the aircraft. Having adequate backup controls, Adams continued on. At 10:33 he reached a peak altitude of 266,000 feet. In the FRC flight control room, fellow pilot and mission controller Pete Knight monitored the mission with a team of engineers. Something was amiss. As the X-15 climbed, Adams started a planned wing-rocking maneuver so an on-board camera could scan the horizon. The wing rocking quickly became excessive, by a factor of two or three. When he concluded the wing-rocking portion of the climb, the X-15 began a slow, gradual drift in heading; 40 seconds later, when the craft reached its maximum altitude, it was off heading by 15°. As the plane came over the top, the drift briefly halted, with the plane yawed 15° to the right. Then the drift began again; within 30 seconds, the plane was descending at right angles to the flight path. At 230,000 feet, encountering rapidly increasing dynamic pressures, the X-15 entered a Mach 5 spin. ¹⁸

In the flight control room there was no way to monitor heading, so nobody suspected the true situation that Adams now faced. The controllers did not know that the plane was yawing, eventually turning completely around. In fact, control advised the pilot that he was ”a little bit high,” but in ”real good shape.” Just 15 seconds later, Adams radioed that the plane ”seems squirrelly.” At 10:34 came a shattering call: ”I'm in a spin, Pete.” A mission monitor called out that Adams had, indeed, lost control of the plane. A NASA test pilot said quietly, ”That boy's in trouble.” Plagued by lack of heading information, the control room staff saw only large and very slow pitching and rolling motions. One reaction was ”disbelief; the feeling that possibly he was overstating the case.” But Adams again called out, ”I'm in a spin.” As best they could, the ground controllers sought to get the X-15 straightened out. They knew they had only seconds left. There was no recommended spin recovery technique for the plane, and engineers knew nothing about the X-15's supersonic spin tendencies. The chase pilots, realizing that the X-15 would never make Rogers Lake, went into afterburner and raced for the emergency lakes, for Ballarat, for Cuddeback. Adams held the X-15's controls against the spin, using both the aerodynamic control surfaces and the reaction controls. Through some combination of pilot technique and basic aerodynamic stability, the plane recovered from the spin at 118,000 feet and went into a Mach 4.7 dive, inverted, at a dive angle between 40 and 45 degrees. ¹⁹

Adams was in a relatively high altitude dive and had a good chance of rolling upright, pulling out, and setting up a landing. But now came a technical problem that spelled the end. The Honeywell adaptive flight control system began a limit-cycle oscillation just as the plane came out of the spin, preventing the system's gain changer from reducing pitch as dynamic pressure increased. The X-15 began a rapid pitching motion of increasing severity. All the while, the plane shot downward at 160,000 feet per minute, dynamic pressure increasing intolerably. High over the desert, it passed abeam of Cuddeback Lake, over the Searles Valley, over the Pinnacles, barrowing on toward Johannesburg. As the X-15 neared 65,000 feet, it was speeding downward at Mach 3.93 and experiencing over 15 g vertically, both positive and negative, and 8 g laterally. It broke up into many pieces amid loud sonic rumblings, striking northeast of Johannesburg. Two hunters heard the noise and saw the forward fuselage, the largest section, tumbling over a hill. On the ground, NASA control lost all telemetry at the moment of breakup, but still called to Adams. A chase pilot spotted dust on Cuddeback, but it was not the X-15. Then an Air Force pilot, who had been up on a delayed chase mission and had tagged along on the X-15 flight to see if he could fill in for an errant chase plane, spotted the main wreckage northwest of Cuddeback. Mike Adams was dead, the X-15 destroyed. NASA and the Air Force convened an accident board. ²⁰

Chaired by NASA's Donald R. Bellman, the board took two months to prepare and write its report. Ground parties scoured the countryside looking for wreckage, any bits that might furnish clues. Critical to the investigation was the cockpit camera and its film. The weekend after the accident, a voluntary and unofficial FRC search party found the camera; disappointingly, the film cartridge was nowhere in sight. Engineers theorized that the film cassette, being lighter than the camera, might be further away, to the north, blown there by winds at altitude. FRC engineer Victor Horton organized a search and on 29 November, during the first pass over the area, W. E. Dives found the cassette, in good condition. Investigators meanwhile concentrated on analyzing all telemetered data, interviewing participants and witnesses, and studying the aircraft systems. Most puzzling was Adams' complete lack of awareness Of major heading deviations in spite of accurately functioning cockpit instrumentation. The accident board concluded that he had allowed the aircraft to deviate as the result of a combination of distraction, misinterpreting his instrumentation display – and possible vertigo. The electrical disturbance early in the flight degraded the overall effectiveness of the aircraft's control system and further added to pilot workload. The X-15's adaptive control system then broke up the airplane on reentry. The board made two major recommendations: install a telemetered heading indicator in the control room, visible to the flight controller, and medically screen X-15 pilot candidates for labyrinth (vertigo) sensitivity. As a result of the X-15's crash, FRC added a ground-based ”8 ball” attitude indicator, displayed on a TV monitor in the control room, which furnished mission controllers with ”"real time” pitch, roll, heading, angle of attack, and sideslip information available to the pilot, using this for the remainder of the X-15 program. ²¹

Click on Picture to enlarge

Figure 5 THE PROPOSED DELTA-WING X-15, NOVEMBER 1964

The X-15 program itself did not long survive the loss of the X-15 #3. The X-15A-2, grounded for repairs, soon remained grounded forever. The first X-15 continued flying, with sharp differences of opinion about whether the research results returned were worth the effort and expense. The ramjet program had offered hope to zealots that the program might continue, but the X-15A-2's experience really ended all that. A proposed delta wing X-15 modification had offered supporters the hope that the program might continue to 1972 or 1973, but the loss of the third X-15 ended this hope as well, inasmuch as it would have been the third aircraft that would have been modified as a delta hypersonic testbed. The proposed delta wing X-15 (Figure 5) had grown out of studies in the early 1960s on using the X-15 as a hypersonic cruise research vehicle. Essentially, the delta X-15 would have made use of the third airframe with the adaptive flight control system, but also incorporated the modifications made to the X-15A-2 – lengthening the fuselage, revising the landing gear, adding external tankage, and provisions for a small-scale experimental ramjet. NASA proponents, particularly John Becker (chief of Langley's Aero-Physics Division) found the idea very attractive since, as Becker wrote in one internal memo: ²²

The highly swept delta wing has emerged from studies of the past decade as the form most likely to be utilized on future hypersonic flight vehicles in which high lift/drag ratio is a prime requirement i.e., hypersonic transports and military hypersonic cruise vehicles, and certain recoverable boost vehicles as well.

Despite such endorsement, support remained lukewarm at best both within NASA and the Air Force (indeed, only within the flight testing and hypersonic communities of both organizations was there ever much support for the X-15 program at all); the loss of Mike Adams and the third X-15 sealed the fate of the delta proposal, though the idea did influence in a roundabout way the subsequent attempts to build hypersonic sustained cruise technology demonstrators in the 1970s such as the National Hypersonic Flight Research Facility (NHFRF).

Perhaps because of the generalized feeling that the X-15 had long passed the point of productive and timely research – a feeling that program participants would have contested – support for the X-15 dropped dramatically after 1963. As early as March 1964, in consultation with NASA Headquarters, Brig. Gen. James T. Stewart, director of science and technology for the Air Force, had determined to end the program in December 1968. ²³ The first X-15, the only one of the three still flying after the Knight and Adams' flights, had just about exhausted its research ability, and it cost roughly $600,000 per flight. Other NASA programs could benefit from this funding, and thus NASA did not request a continuation of X-15 funding after December 1968. ²⁴ During 1968 Bill Dana of NASA and Pete Knight of the AFFTC took turns flying the X-15, though a variety of weather, maintenance, and operational problems caused rescheduling and cancellation of a number of flights. On 24 October 1968, Dana completed the first X-15's 81st flight, the 199th flight of the series. The plane attained Mach 5.38 at 255,000 feet, carrying a variety of follow-on experiments. Though researchers tried to get a 200th flight before the end of the year, weather, maintenance and operational problems dictated otherwise. The X-15 program, after nearly a decade of flight operations, came to an end.

 

 

Section VII  The Legacy Of The X-15

 

The conclusion of the X-15's flight test program brought an era in flight testing history to a close. In 199 flights, the X-15 spent eighteen hours above Mach 1, twelve hours above Mach 2, nearly nine hours above Mach 3, nearly six hours above Mach 4, one hour above Mach 5, and scant minutes above Mach 6. It flew to Mach 6.72 (4,520 mph) and an altitude of 67 miles. Twelve pilots flew it, and one of them died. Beginning as a hypersonic aerodynamics research tool, the X-15 eventually became much more than that. What, then, did it accomplish?

In October 1968 John Becker enumerated 22 accomplishments from the research and development work that produced the X-15, 28 accomplishments from its actual flight research, and 16 from testbed investigations. As of May 1968, the X-15 had generated 766 technical reports on research stimulated by its development, flight testing, and test results, equivalent to the output of a typical 4,000-man federal research center working for two years. As the X-I had provided a focus and stimulus for supersonic research, the X-15 furnished a focus and stimulus for hypersonic studies. A sampling of its accomplishments indicates their scope: ¹

• Development of the first large restartable ”man-rated” throttleable rocket engine, the XLR99.
• First application of hypersonic theory and wind-tunnel work to an actual flight vehicle.
• Development of the wedge tail as a solution to hypersonic directional stability problems.
• First use of reaction controls for attitude control in space.
• First reusable superalloy structure capable of with-standing the temperatures and thermal gradients of hypersonic reentry.
• Development of new techniques for the machining,forming, welding, and heat-treating of Inconel X and titanium.
• Development of improved high-temperature seals and lubricants.
• Development of the NACA ”Q” ball ”hot nose” flow-direction sensor for operation over an extreme range of dynamic pressures and a stagnation air temperature of 1.900°C.
• Development of the first practical full-pressure suit for pilot protection in space.
• Development of nitrogen cabin conditioning.
• Development of inertial flight data systems capable of functioning in a high-dynamic pressure and space environment.
• Discovery that hypersonic boundary layer flow is turbulent and not laminar.
• Discovery that turbulent heating rates are significantly lower than had been predicted by theory.
• First direct measurement of hypersonic skin friction, and discovery that skin friction is lower than had been predicted.
• Discovery of ”hot spots” generated by surface irregularities.
• Discovery of methods to correlate base drag measurements with tunnel test results so as to correct wind tunnel data.
• Development of practical boost-guidance pilot displays.
• Demonstration of a pilot's ability to control a rocket-boosted aerospace vehicle through atmospheric exit.
• Development of large supersonic drop tanks.
• Successful transition from aerodynamic controls to reaction controls, and back again.
• Demonstration of a pilot's ability to function in a weightless environment.
• First demonstration of piloted, lifting atmospheric reentry.
• First application of energy-management techniques.
• Studies of hypersonic acoustic measurements used to define insulation and structural design requirements for the Mercury spacecraft.
• Use of the three X-15 aircraft as testbeds carrying a wide variety of experimental packages.

The X-15 also made its mark in many other ways. When NACA began its development, the science of hypersonic aerodynamics was in its infancy; the few existing hypersonic tunnels were used largely for studies in fluid mechanics. Aerodynamicists feared that there might be a hypersonic ”facility barrier,” much like the earlier transonic tunnel trouble that led to the Bell X-1 and Douglas D558, so that hypersonic tunnel tests might prove of little value in predicting actual flight conditions. The X-15 disproved this; predicted wind tunnel data and data flight testing of the airplane generally showed remarkable agreement. Proving that hypersonic laminar flow conditions did not develop led to the disappearance of this ”technical superstition,” and recognition that the small surface irregularities that prevent laminar flow at low speed also prevent its formation at hypersonic speeds. Like the earlier X-l, the X-15 encouraged a great deal of ground research and simulation techniques. So successful were these methods and so great was the engineers' confidence in these methods and the X-15's flight results that the X-15 wound up actually decreasing the likelihood of NASA's developing any future hypersonic research aircraft with the prime justification being the generation of unique and otherwise unobtainable data. Any future research aircraft would be built more for ”proof of concept” purposes than for acquiring information unobtainable by other means. At the conclusion of the X-15 program, the German Society of Aeronautics and Astronautics presented the NASA X-15 team with the Eugen Sanger Medal – a fitting and appropriate honor. In his acceptance address on behalf of the team, John Becker stated that "no new exploratory research airplane can ever again be successfully promoted primarily on the grounds that it will produce unique flight data without which a successful technology cannot be achieved.” ²

Nearly ten years after Becker's assessment, Capt. Ronald G. Boston of the U.S. Air Force Academy's history department reviewed the X-15 program for ”lessons learned” that might be applied or benefit the development of the National Hypersonic Flight Research Facility Program, an effort that itself died shortly thereafter. Boston's study, presented in clipped outline style, offers an interesting perspective on the X-15 both from the vantage point of history, as well as giving an inkling of the state of the art in hypersonic studies in the mid-1970s on the eve of the Shuttle in light of the X-15's experience. Reprinted here in full, it provides an interesting complementary viewpoint to that of X-15's originator John Becker: ³


 

The X-15's Role In Aerospace Progress

 

This outline presents a synopsis of X-15's contributions to aerospace technology and is intended as a preliminary report on the X-15 historical study conducted as part of the National Hypersonic Flight Research Facility (NHFRF) feasibility study. Specifically, this study looks to see of what value the developments and lessons of the X-l5 program have been. It is a case study of the X-15 program intended to show the value of research aircraft.

Covered in this study are two general types of contributions made by the X-15: revolutionary and evolutionary. Revolutionary contributions are those technological breakthroughs that open new fields, that are dependent upon the advanced capabilities of the research aircraft, and that are sometimes totally unexpected. Evolutionary contributions include those for which the research vehicle represents the latest and most advanced stage in the developmental process. While the latter may not be dependent upon the particular aircraft's capabilities, the demands of the research program nonetheless drive the technology toward a greater degree of perfection. The two types are often confused; yet, only the former provides legitimate justification for undertaking a research program. But in an evaluation in retrospect, both forms of contribution make up the ultimate worth of a program.

The study begins with the~X-15 program's goals and examines the degree of success achieved. It covers the lessons learned, both intentional and unintentional in origin. It then looks to the present time to see what, if any, uses have been made of the knowledge gained. Lastly, this study poses the questions raised but left unanswered in the conduct of this program.
 

1. Program Overview:
   
    
   1. Goals and Design Philosophy. Using near-state-of-the-art (1954) technology to propel a conservative Mach 2 design out to Mach 6 and 250,000 feet to explore the hypersonic and near-space environments:
      (1) To verify existing theory and wind-tunnel techniques.
      (2) To study aircraft structures under high (1,200°F) heating.
      (3) To investigate stability and control problems associated with high-altitude boost and reentry.
      (4) To investigate the biomedical effects of both weightless and high-g flight.
      
       
   2. Achievements and Ultimate Utilization. All design goals were met; most were surpassed: Mach 6.7, 354,200 feet, 1,300 degrees F, and 2,000 pounds per square foot (psf). In addition, once the original research goals were accomplished, the X-15 became a handy high-altitude, hypersonic testbed for which 46 follow-on experiments were designed--majority flown before the program was abruptly terminated in 1968. Many proposals for modifying or optimizing the basic airframe surfaced during the course of the program, and the X-15 was envisioned as a hypersonic facility for the 1970s. Due to the absence of a subsequent hypersonic mission, aircraft applications of X-15 technology have been few. In space, however, the X-15 paved the way for manned, orbital and lunar flight.

   
   
    

2. Hypersonic Aerodynamics:
   
    
   1. Hypersonic Flow. The X-15 program remains the most thoroughly tested aircraft program to date and offered an excellent opportunity to compare actual flight data with theory and wind tunnel predictions. The X-15 verified existing wind-tunnel techniques for approximating interference effects for high-Mach, high-angle-of-attack hypersonic flight, thus giving increased confidence in small scale techniques for hypersonic design studies. Wind-tunnel drag measurements were also validated, except for the 15 percent discrepancy found in base drag--masked by the "sting" support used in the tunnel. The laminar boundary layer theory for hypersonic flight was disproven, the flow actually being almost entirely turbulent. X-15 flight-test data indicated that hypersonic flow phenomena are linear above Mach 5, allowing us to design with confidence craft like the Mach 25-30 Shuttle Orbiter that must fly as expected without the cautious "buildup" program of the X-15.
      
       
   2. Stability and Control. X-15's experience disproved the existence of ”barriers” to hypersonic flight as were suspected after the X-1 and X-2 aircraft encountered extreme, high-supersonic instability.
      (1) ”Wedge Tail.” A redesigned vertical stabilizer reduced the instability that plagued the X-1 series and X-2 aircraft.
      (2) ”Rolling Tail.” Differentially deflected horizontal stabilizers gave precise roll control and allowed for elimination of ailerons out on a hot wing section. This design concept was later incorporated into the ”swing wing” of the B-1 bomber to simplify wing construction.
      (3) Tunnel Parameter Verification. X-15 data verified wind-tunnel parameters used for aerodynamic stability prediction above Mach 2.Flight test results also pointed out the need for an ”error band” or degree of uncertainty to be put on such predictions. AFFTC and NASA Dryden Flight Research Center have both made inputs to the Shuttle program in this regard based on past flight test experience, the X-15 providing the only parameter's experience above Mach 2.
      (4) Side-Stick Controller. The first modern application of the side-stick concept for more precise, ”wrist-action” control – as now comes standard in the F-16.
      (5) Augmentation. Some phases of flight, such as reentry, were marginally stable, and pilots required artificial augmentation (damping) to achieve satisfactory stability. The X-15 necessitated the development of one of the earliest stability augmentation systems (SAS). Originally equipped with a simple fail-safe, fixed gain system, one of the three ships was later equipped with a triple redundant adaptive flight control system (AFCS). Here the pilot flew via inputs to the electrical augmentation system. Though a point of continuing debate, the X-15 did not incorporate ”fly-by-wire” if meant to denote a non-mechanically linked control system. A purely electric side stick had been developed under contract for the X-15 and test flown in a F-101B. Thus the X-15 did advance ”fly-by-wire” technology.
      
       
   3. Simulation Techniques. The art of simulation grew with the X-15 program, not only for pilot training and mission rehearsal, but for research into controllability problems. Subject to continuous updating based on flight-test results, the simulator was programmed to ”fly” like the aircraft. Thus the simulator could be used to explore those areas of the flight envelope too risky for actual flight. The demands of the X-15's wide velocity and altitude envelope necessitated development of the first full six-degree-of-freedom flight simulator. The X-15 program showed the value of good wind-tunnel testing and simulation in maximizing the knowledge gained from each of the 199 short, expensive test flights.
      
       
   4. Aerodynamic Heating Effects. In a major discovery, the existing Sommer-Short and Eckert T-prime heating prediction theories (laminar flow) were found to be 30 to 40 percent in excess of flight-test results. (Hence the X-15's structure was over designed for heating effects.) This discovery led to renewed wind-tunnel testing leading to NASA-Langley's choice of the empirical Spaulding-Chi model for hypersonic heating. Lighter, more optimum vehicles are now possible, the Apollo command and service modules being a case in point. Based on their X-15 experience, Rockwell International devised a computerized mathematical model for aerodynamic heating called HASTE--Hypersonic and Supersonic Thermal Evaluation – which gives a workable ”first cut” approximation for design studies. HASTE was, for example, used directly in the initial Apollo design study.

   
    

3. Structures:
   
    
   1. Development. X-15 was designed as a ”heat sink” structure to absorb heat pulses, not to withstand hypersonic cruise heating. Development showed the validity of ground ”partial simulation” testing of primary members stressed under high temperature. A facility was since built at DFRC for heat-stress testing of the entire structure. X-15's development pioneered the use of corrugations and beading to relieve thermal expansion stresses (as now used on YF-12/SR-71, though Lockheed disclaims any X-15 inputs). Metals with dissimilar expansion coefficients were also used to alleviate stresses. The leading edges were segmented, much like a concrete sidewalk, to allow for expansion. The X-15 required the perfection of fabrication (milling and welding) techniques for high temperature alloys: Inconel X (skin) and titanium (structural members) had heretofore not been extensively used to such fine specifications. Such is now routine in aircraft and spacecraft construction.
      
       
   2. Flight Stresses. Though the primary structure proved sound, several surface design problems were uncovered during early flight tests.
      (1) Local Hot Spots. A surprise lesson came with the discovery of heretofore unconsidered local heating phenomena. Tiny slots in the leading edge material, the abrupt contour change along the canopy, and the wing root caused flow disruptions that produced excessive heating and adjacent material failure. The X-15, tested in ”typical” panels or sections, demonstrated the problems encountered when those sections are joined and thus precipitated an analytical program designed to predict such local heating stresses. Today, from this experience, Rockwell engineers are closely scrutinizing the segmented, carbon-carbon composite leading edge of the Shuttle Orbiter's wing. The bimetallic ”floating retainer” concept designed to dissipate stresses across the X-15's windshield carried over to Rockwell's Apollo and Shuttle windshield designs as well.
      (2) Hot Air Leaks. Hot boundary-layer air on several occasions seeped into the nose-gear compartment, damaging gear and compartment and causing high-speed extension of the gear. The need for very careful examination of all seals thus became apparent, and closer scrutiny of surface irregularities, small cracks, and areas of flow interaction became routine. Consequently, Rockwell engineers are now examining the seal around the Orbiter's thermal surface tiles.
      (3) Panel Flutter. Incidences of X-15's panel flutter led to an industry-wide reevaluation of panel flutter design criteria in 1961–62. Stiffeners and reduced panel sizes alleviated the problems on the X-15's upper vertical stabilizer and side fairings. Similar techniques later found general application in the high speed aircraft of the 1960s.
      (4) Boundary Layer Noise. The X-15 provided the first opportunity to study the effects of acoustical fatigue over a wide range of Mach and dynamic pressures. In these first inflight measurements, ”noise” related stresses were found to be a function of g-force, not Mach number. Such fatigue was determined to be no great problem for a structure stressed to normal inflight loading. This knowledge has allowed for more optimum structural design of missiles and space capsules that experience high velocities.
      
       
   3. Fabrication Techniques. Working with the hard nickel alloy Inconel X required new fabrication techniques. New welding, drilling, forming, and milling methods were perfected and are commonplace with the tough aerospace alloys now in use. The ”Chem-Mill,” or chemical milling, was a North American Aviation development that got its first test in reducing the center portions of skin panels to reduce weight. North American also pioneered a new spar construction to combat thermal expansion: the X-15's ”hat” – spar construction, which gave compressive strength while reducing secondary stresses, has evolved into the ”sine wave” spar used on the B-1 and other supersonic aircraft. To remedy the thermal buckling along the side fairings, North American also pioneered the use of expansion joints that nonetheless retained fuselage structural integrity. Indeed, the fuselage itself was used as the fuel tanks, advancing the concept of integral tankage to reduce weight.

   
    

4. Manned Flight:
   
    
   1. Bioastronautics. Coming at a time when serious doubts were being raised concerning man's ability to handle complex tasks in the high-speed, weightless environment of space, the X-15 program became the first program for repetitive, dynamic monitoring of pilot heart rate, respirations, and EKG under extreme stress over a wide range of speeds and forces. When preexisting, theoretical limits for heart rate were exceeded, all estimates of man's ability to endure stress had to be revised upward. Accelerated heart rates therefore caused no undue alarm or mission aborts for the subsequent manned space program. In fact, X-15's success gave the confidence to go ahead with early manned Mercury flights – the downrange ballistic shots being similar to the X-15's mission profile-at a time of great political concern over the success of America's first space program. Biomedical monitoring as begun with the X-15 has continued at DFRC. Pilot functions are being studied with an eye to devising the means to monitor pilot response and alertness from the ground as a function of vital measurements.
      (1) Instrumentation. The bio-instrumentation developed for the X-15 program has allowed similar monitoring of all subsequent flight test programs. Incorporated in the pressure suit, pickups are unencumbering and compatible with aircraft electronics. The flexible, spray-on wire leads have since found use in monitoring cardiac patients in ambulances.
      (2) pressure Suit Development.. The A/P-22S-2, the first single-piece, full pressure suit, was developed for the X-15 program. Later it was refined as the A/P-22S-6 suit, which remains the standard USAF operational suit for high-altitude flight.
      
       
   2. Manned Flight Operations. America's space and advanced manned vehicle programs are all indebted to the X-15 for some aspect of their training, command and control, or recovery procedures. The X-15 not only demonstrated the value of man at the controls, but provided the accepted methodology for experimental manned programs.
      (1) pilot-in-the-Loop. The X-15 provided for no ground-based control input or override; the pilot remained constantly ”in the loop,” controlling and correcting aircraft attitude. He provided a highly sophisticated onboard ”computer” and also served as the primary backup system for redundancy. Statistics show that without a pilot in control, the 3 aircraft would have sustained 15 losses on the first 47 flights alone. Overall mission success rate stood at 96 percent, versus 80 percent for component reliability. The pilots were able to recognize and override malfunctions to complete the primary or alternate missions to greatly enhance the worth of the program.
      (2) Crew Training. The opportunity to observe the pilot's performance under high-stress and high g-forces also dictated that an extensive ground training program be instituted to prepare pilots to handle the complex tasks and mission profiles. The result was a simulation program that became the foundation for crew training for all manned space work. The program depended on four types of training simulation.
        (a) Six Degree-of-Freedom Fixed-Base. A static cockpit mockup provided the means for extensive mission rehearsal-averaging 20 hours per 10 minute flight. Such preparation was directly responsible for the high degree of mission success achieved as pilots rehearsed their primary, alternate, and emergency diversion mission profiles.
        (b) Dynamic Simulation. Prior to the first X-15 mission, the ability of the pilot to function under the high g-forces expected on boost and reentry was tested in a closed-loop, six degree-of-freedom simulation using the centrifuge at the Naval Air Development Center, Pa. This simulation ”first” had the pilot controlling the g-forces and demonstrated pilot ability to function under 12 to 15 g's – more than ever experienced on actual flights. This project became the prototype for programs set up at the Ames Research Center and the Manned Spacecraft Center at Houston.
        (c) Variable Stability Aircraft. X-15 pilots maintained proficiency and adaptability by practicing on T-33 and F-100 aircraft whose handling characteristics could be varied in flight, simulating the varied response of the X-15 traversing a wide range of velocities and atmospheric densities.
        (d) Approach and Landing. Pilots practiced the exacting, low L/D landing maneuver in F-104 aircraft. With gear and speed brakes extended, the F-104's power-off glide ratio approximated that of the unpowered X-15. Shuttle Orbiter crews continue this same practice.
      (3) Command and Control. The ”NASA 1” control room located atop DFRC was the model for establishing the Mission Control Center (MCC) at Houston. Back up systems monitors and flight trackers were duplicated. Astronaut Capsule Communicators, ”Cap-Comms,” were a direct outgrowth of the X-15's practice of using an X-15 pilot as the ground communicator for all X-15 missions. Of course, all subsequent work at Edwards relied on X-15's spawned methodology. The X-15 program required an elaborate tracking network known as ”High Range.” Operational techniques were established for real-time monitoring and trajectory correction. These were carried over to the space program – the very same NASA personnel went on to set up the world-wide MCC tracking system.
      (4) Reentry and Landing. By demonstrating the operational feasibility of high angle-of-attack, ”lifting” reentries to unpowered, low L/D recoveries and landings, the X-15 paved the way for the lifting-body programs and the current Shuttle Orbiter concept. Accordingly, landing-assist rockets intended to ease the touchdown of the Shuttle Orbiter were ultimately eliminated from the Orbiter design. X-15 pilots routinely landed within 1,000 .feet of target with 70 percent reliability. The techniques for ground-monitored energy management to arrive overhead the landing spot at a ”high key” originated with the X-15 program. Here the extreme altitudes and distances from touchdown exceeded the pilot's ability to make a visual, ”deadstick” recovery as in preceding rocket aircraft programs. The terminal approach for the Shuttle Orbiter is a variation of the 360-degree, overhead pattern flown by the X-15: the Orbiter will enter figure-eight ”energy-dissipation circles” overhead the approach end of the field until energy is reduced to within landing limits. Thus X-15 operations experience, more than any other source, provides the basic framework for the research programs of the 1970s and 80s. In fact, in 1958 North American Aviation proposed launching an X-15 into orbit for subsequent recovery.

   
    

5. Component Systems: The extreme speed and altitude demands of the X-15 program forced development of a number of advanced subsystems that continue to yield dividends long after the program's termination.
   
    
   1. Flight Data Systems. The X-15 required a choice be made between four possible approaches to flight data: 1) pressure instruments; 2) ground-based radar monitoring; 3) simple gyroscopic instruments; or 4) true inertial systems. The inertial approach, then very primitive, augmented with pressure instruments and radar, was selected.
      (1) Air-Data Sensors. For subsonic flight the X-15 relied on simple pilot-static pressure instruments. (Later in the program, an extendable pitot tube was added when the velocity envelope was expanded beyond Mach 6.) Mach, dynamic pressure, static pressure, and altitude for hypersonic flight were telemetered from the ground where "High Range" computers evaluated radar inputs and ambient atmospheric conditions gathered by sounding rockets. Angle of attack and yaw were derived from a null-seeking "ball nose" which measured the pressure differentials felt across ports in the ball. The ball nose was later modified to measure static pressure to monitor dynamic pressure [q = f(Pt)] which gave the pilot the ability to limit or hold a constant dynamic pressure. Thus far the ball nose has not found subsequent application. The Shuttle Orbiter will rely on redundant, onboard inertial systems backed up by ground radar.
      (2) Inertial Flight Data Systems (IFDS). Onboard measurement of velocity was handled by inertial systems. All three aircraft were initially equipped with analog-type systems which proved to be highly unreliable. Later, two aircraft, including the one aircraft with the adaptive control system, were modified with digital systems. In the subsequent parallel evaluation of analog versus digital IFDS, the latter was found to be superior. It was far more flexible and could make direct inputs to the adaptive flight control system; it was also subject to less error. This type is now the accepted approach, as will be used on the Shuttle Orbiter.
      
       
   2. Landing Gear. The main landing gear represented a marked departure from the standard pneumatic tire and retractable strut-as were retained in the nose-gear assembly. To reduce storage and heating problems, Inconel X skids were spring-loaded along the aft underside of the fuselage. This highly successful arrangement was programed for the X-20 Dyna-Soar and will be seen on the Rockwell HiMAT (High Maneuvering Aircraft Technology) RPV. One surprise lesson on the slap-down loading problems that low L/D aircraft with extremely aft-mounted main gear can experience was learned: the nearly immediate loss of lift as the nose lowered on touchdown caused unexpectedly high gear loads which resulted in gear failure and a major accident in 1962.
      
       
   3. Aerospace Hardware. The combination of high aerodynamic heating and cryogenic liquids posed severe problems for the X-15's designers. From their efforts have come thermal insulators for hydraulic lines and actuators which are used today in the Shuttle Orbiter, high-temperature hydraulic fluids, cryogenic tubing as used directly in Apollo components, and experience with Inconel and titanium pressure vessels to withstand extreme temperature and pressure gradients. By way of costly aborts, engineers learned almost embarrassing lessons such as the need to pressurize the gear boxes of auxiliary power units taken to the low ambient pressure of space – where foaming of the lubricant caused material failures.
      
       
   4. Cabin Environmental Systems. The X-15 presented the first requirement for full space-environment human engineering. While life support was provided by the full pressure suit, cockpit and electronics bay air-conditioning used the first cryogenic (liquid nitrogen) cooling system – designed by Garrett-Air Research, who went on to do the environmental controls for the Mercury capsules.
      
       
   5. Reaction Controls. The X-15 provided the first operational test of hydrogen-peroxide reaction controls outside the earth's atmosphere. Designed by Bell Aerospace and flown on their X-1B to 75,000 feet in 1958 (not outside aerodynamic control effects), this system represented a true technological leap when included in the X-15 design in 1956. It later went into the Mercury spacecraft as the primary control system.
      
       
   6. Propulsion. The X-15 was powered by the XLR99 liquid fueled rocket motor. Produced specifically for the X-15 mission, this complicated motor pioneered the concept of a throttleable, restartable motor with an idle-power feature. At idle, the XLR99 could complete 55 percent of its start and light-off sequence before drop. This complexity also resulted in many aborted missions (approximately one-tenth of all mission aborts). The requirement for a ”man-rated” fail-safe system further compromised reliability. Through hindsight a number of X-15 engineers now feel the throttleable future to have been a needless luxury that complicated and delayed the development of the XLR99 – this feature has not been used on subsequent motors. However, the production effort did give confidence in the concept, and six XLR99 throttleable motors yet remain in storage for some future reuse.

   
   The full value of X-15's experience to the designing of sub-systems for advanced aircraft and especially spacecraft can only be guessed at. At Rockwell International Corporation (Los Angeles Aircraft Division) many of the same people from the X-15 project worked on the Shuttle Orbiter. Yet X-15's experience is overshadowed by more recent projects and becomes exceedingly difficult to trace as systems evolve through successive programs. Nonetheless, those engineers are confident that they owe much to the X-15, even though many are at a loss to give any concrete examples.
   
    

6. Follow-on Experiments: By roughly 1963 the X-15 had completed its original research objectives. There was talk of terminating the program entirely; there was even talk of closing DFRC for want of further flight research programs. New life was given to both as proposals for research needing either the speed or altitude of the X-15 surfaced. In the early 1960s the X-15 alone had the capability to carry a payload of much weight (or size) above the atmosphere. And unlike in missile research, the X-15 returned equipment and results for reevaluation, recalibration, and reuse. Perhaps the earliest true ”follow-on” experiment came in 1961: a coating material designed to reduce the infrared emissions of the B-70 was tested to Mach 4.43 (525°F) on the exterior surface of an X-15 stabilizer panel. Thus began a series of 46 additional experiments concerning the physical sciences, space navigation aids, reconnaissance studies, and advanced aerodynamics--many of the 46 were left unfinished when the X-15 program ended in 1968.
   
    
   1. Physical Sciences. Of special concern to scientists was the X-l5's ability to carry experiments above the attenuating effects of the earth's atmosphere.
      (1) Ultraviolet Stellar Photography. This astronomical study required photometering of the ultraviolet brightness of several of the brighter stars to study the material make-up of stars. The X-15 carried four cameras (on a gimbaled platform in the instrument bay behind the pilot) above the filtering effects of the ozone layer – approximately 40 miles up. Conducted in 1963 and again in 1966, this work was subsequently continued on improved sounding rockets.
      (2) Atmospheric Density Measurement. The X-15 was ideally suited to measure densities of the 30 to 74 kilometer altitudes, crosschecking measurements on ascent with those on descent. Using the ball nose to take measurements, flow-angularity errors were eliminated. The X-15 provided atmospheric density profiles of seasonal variation.
      (3) Micrometeorite Collection. Designed to collect samples at various altitudes, this experiment was part of a larger NASA study to build a particle-impact data base for spacecraft design criteria. Only on the last of six flights did this experiment ”catch” any particles, those being so contaminated by reaction control jet particles that the project was canceled.
      (4) Rarefied Gas Flow. This experiment failed to provide any useful information despite repeated attempts.
      (5) Solar Spectrum Measurements. The X-15 provided the first direct measurement from above the atmosphere of the sun's irradiance. A scientific revelation, this data allowed refinement of the Solar Constant of Radiation which was revalued 2.5 percent lower than existing ground-based determinations. This vital constant provides a measure of thermal energy incident on the earth and upon which all photochemical processes depend. It is also useful for the design of thermal protection for spacecraft.
      
      
       
   2. Space Navigation:
      (1) Horizon Definition. The X-15 supported two – MIT and NASA-Langley – projects to determine the earth's infrared horizon radiance profile. This information has been used in attitude referencing systems for orbiting spacecraft. The MIT work was part of an Apollo support program seeking alternative means for earth's orbit reinsertion guidance in case of radar or communications failure. The space sextant designed for this task was checked enroute on Apollo missions 8, 10, and 11 with relatively good accuracy when compared to radar position.
      (2) High-Altitude Daytime Sky Brightness. This successful program to collect data on radiation characteristics of the daytime sky background was part of an effort to develop a ”star tracking” navigational system. Such an automatic electro-optical tracking system is now used on SAC reconnaissance planes and has applications in satellite positioning and space travel.
      
       
   3. Reconnaissance Systems. The X-15's speed and altitude combined to make it an ideal testbed for high-speed aircraft and satellite systems development.
      (1) Ultraviolet Studies. Ultraviolet (UV) sensors were studied as ICBM early warning detectors. This three-part project yielded promising results, but to date UV systems remain overshadowed by the more advanced infrared systems.
        (a) UV Earth's Background. Good data was obtained on the UV background against which the UV signature of an ICBM's exhaust could be detected.
        (b) Exhaust Plume Characteristics. To determine the signature of a typical rocket exhaust above the ozone layer, the exhaust plume of the X-15 itself was scanned.
        (c) Pacific Missile Range Monitor. To test the feasibility of detecting a missile launch by its UV signature, an actual launch from Vandenberg AFB was to be monitored. However, due to equipment malfunctions, scheduling problems, and ultimately a snow storm which prevented the last scheduled X-15 flight, this test was never possible.
      (2) Infrared Studies. Infrared (IR) work was devoted to two separate projects:
        (a) Space Detection Systems. The current satellite detection systems began as X-15 IR experiments. Early (1963) experiments studied the IR exhaust plume characteristics of the X-15. The follow-up project to measure the earth's IR background using an IR scanner never flew before the X-15 program ended. Nonetheless, the equipment developed therein contributed directly to successful tests later carried by U-2 aircraft and thus to the eventual satellite program.
        (b) IR Scanner. This experiment produced the first IR picture taken through a "hot" window. Though only a crude, two dimensional image was obtained, the notion that hypersonic IR reconnaissance was impossible was disproven. This work also advanced the development of operational line scanners for mapping carried on RF-4, EF-111, and Navy aircraft. The Earth's Resources Development Agency (ERDA) even uses this technology to monitor pollution levels.
      (3) Optical Background. This effort to determine daytime background interference effects to laser optics produced good data showing the feasibility of high-altitude laser surveillance. No actual pictures or images resulted, and this work has moved on to satellite testbeds.
      (4) Aerial Photographs. Optical degradation experiments determined that the shock wave, boundary-layer flow, and temperature gradients across windows caused negligible degradation to visual, near-IR, and radar aerial photography to Mach 5.5 and 125,000 feet. However, improved photographic equipment and much faster-speed films may very well invalidate these findings, hence the need for renewed flight testing. Toward the end of this experiment several tests of near-IR color photography produced the first successful inflight use of color films. Such were later used in reconnaissance work over Southeast Asia where colored emissions could denote enemy activity under dense foliage. ERDA now uses this technique via satellites to study the earth's resources.
      
       
   4. Advanced Aerodynamic Research. The X-15 served to carry aloft aerodynamic projects that were impractical for wind-tunnel study.
      (1) Several tests of flow distortion over surface irregularities were run to verify wind-tunnel studies; little disparity between the two was noted.
      (2) Attempts to measure cold-wall effects on coefficients of heating produced only marginal results, and this effort is still underway using the YF-12/SR-71 at DFRC. ^a
      (3) The feasibility of using fluidic (cavity) temperature probes to measure total temperature at high Mach, where standard probes burn away, was demonstrated.
      (4) A complete reentry guidance system for onboard, computerized energy management incorporating digital inputs to the adaptive flight control system was under study until the one aircraft so equipped was destroyed in 1967.
      
       
   5. X-15A-2 Modification Program. The 1962 crash of aircraft number two opened the door for extensive modification since considerable rebuilding was required. The resultant modification, as the X-15A-2, was primarily aimed at providing a testbed for development of a Mach 8 hypersonic, air breathing engine – the Hypersonic Ramjet Engine (HRE). Then, as now, no tunnel facility existed wherein such an engine could be realistically tested, and rocket boosters could not give steady-state tests or return the equipment.
      (1) HRE Program. The actual prototype engine was to be carried attached to the lower ventral of the X-15. Twenty-nine inches were added to the fuselage' between the existing tanks for the liquid hydrogen to power the HRE. This compartment could also be used to carry other experiments and included a three-panel, high-heat resistant window in the belly. Two external fuel tanks were added alongside the fuselage and tucked under the wings to increase rocket-boost time to attain Mach 8. These tanks were jettisoned at about Mach 2. To withstand the added heating due to increased velocity, the entire aircraft surface was coated with an ablative-type insulator.
        (a) Flight Program. Garrett-Air Research contracted to provide six prototype engines by mid-1969. In the meantime flight-test evaluations were made of the modified aircraft itself and of a dummy or mock-up HRE attached to the X-15A-2. On the first and only maximum-speed test of the X-15A-2 in 1967, shock impingement off the dummy HRE caused severe heating damage to the lower empennage, and very nearly resulted in loss of the aircraft. Though quickly repaired, the X-15A-2 never flew again as the X-15A-2's already cautious supporters abandoned the project. Hindsight would place the blame for this design oversight on haste and insufficient flow interaction studies. A key lesson learned from this episode was not to hang external stores or pylons on hypersonic aircraft, at least not without far more extensive study of underside flow patterns. The HRE was eventually tunnel tested in 1969, and the primary objective of achieving supersonic combustion was met, though the thrust produced was less than the drag created. HRE engineers nonetheless claim a success in that the objective was supersonic combustion, not a workable engine. The X-15 program can claim credit for spawning the HRE project, which has been continued on to the present at NASA-Langley. Though no realistic testbed yet exists, futuristic designs for a hypersonic research aircraft now envision internally mounted engine test facilities.
        (b) Ablator Tests. Since Mach 8 exceeded the heating limits of the Inconel X, a spray-on ablator of silicone-based elastomeric material was chosen to protect the aircraft. The ablator was to limit skin temperatures to 500°F in the 1,900°F environment of Mach 7.4 in this first-ever test of such insulation for an aircraft. Except where HRE pylon shock impingement caused a ten-fold rise in temperature, the ablator worked successfully to Mach 6.7. However, this approach was found to be operationally infeasible. Extensive man hours (approximately 20 days) were required to refurbish the charred ablator surface, and then the integrity of the ablator-to-skin bonding was of concern for subsequent flights. Other operational problems argued against spray-on ablatives: the crew could not walk on the vehicle; access panels were hard to remove and recover without leaving surface cracks; liquid oxygen if spilled on the ablator damaged the surface, requiring a coat of white paint to seal the ablative material's surface.
        (c) Replaceable Wingtip. Though not a part of the HRE project, the right wing tip, damaged in 1962, was rebuilt to allow interchangeable wing-tip shapes. This facility portended valuable studies in the future, but was never utilized.

   
   Though never labeled as such, the X-15 began to function as a hypersonic, high-altitude ”facility” after the original research work was completed. A high percentage (perhaps half) of the follow-on experiments were failures. Critics have contended that in the rush to extend the life of the X-15 program, and DFRC, experiments of questionable value and hasty preparation were flown on the X-15. As early as 1964, NASA officials did begin questioning the cost effectiveness of the follow-on program. Yet the X-15 was the only facility available at the time, and some of the work produced results that contributed to vital programs of today, such as ballistic early warning. Unfortunately, the X-15 was not designed as a hypersonic facility, and thus was limited in its capability to do experimental work.
   
    

7. Conclusions:
   
    
   1. Comment On Contributions. The X-15 was certainly successful in fulfilling its original research goals. Upon X-15's experience rests all subsequent hypersonic study, and the manned space program owes much of its hardware and operations techniques to the X-15. Yet any evaluation of X-15's contributions to technology is tenuous at best. Most systems, knowledge, and especially experience derived from the program have evolved through successive programs over the past decade, and contribution by the X-15 is often obscured. In other cases, the old "X-15 hands" can simply no longer recall what became of the work started on the X-15; this is especially true with the follow-on experiments. Nor is it possible to determine any time factor for the delay between X-15's research and the appearance of useful technology or applications. The nature of the X-15's work was too varied; then too, there has been no subsequent hypersonic requirement outside the laboratory. From almost immediate payoffs in the manned space program on the one hand, X-15's technology sits dormant on the other. The X-15 did, at least, open the door for future hypersonic work, and in so doing sustained interest in manned aircraft at a time when all eyes were turning toward the capsule programs.
      
       
   2. Unfinished Work. Thanks to the X-15, hypersonic aerodynamics is well advanced. Thanks also to the X-15, the need for additional work in several key "stopper technologies"--areas which pose serious questions for future hypersonic vehicles--is evident. Since the program's abrupt termination in October 1968, the following areas have stood conspicuously in want of a testbed vehicle.
      (1) Scramjet Testing. Cruise capability is required of any operational hypersonic vehicle, and despite the advances being made in laboratories, development of a hypersonic, air-breathing power plant will require a flight-test facility.
      (2) Structural Cooling. Hypersonic cruise also requires advanced cooling methods, either active or passive, to dissipate the heat buildup. No existing hypersonic wind tunnel can handle sufficiently large prototype hardware and give reasonably accurate stagnation temperatures.
      (3) Aerodynamic Optimization. Despite enhanced ability to do accurate tunnel testing, interference-free testing of aerodynamic shapes can best be done on a hypersonic facility, as envisioned with the replaceable wing tip of the X-15A-2. Validation of proposed designs, such as the delta wing envisioned for the X-15 prior to its termination, ultimately requires flight testing.
      (4) Follow-on Projects. Since 1968, more experiments requiring a hypersonic testbed have been added to the list of projects left unfinished. Though referred to by at least one high ranking ASD officer as ”the first NHFRF,” the X-15 was an ill-suited testbed facility. It had not been designed as such, nor did it provide steady-state flight. It surpassed Mach 6 on only four occasions, the majority of its 199 flights being in the Mach 5 to 5.5 range. Yet what successes it did achieve point to the benefits a well-designed, Mach 6-plus facility could render such fields as hypersonic aerial photography and IR ”hot window” studies.
      
       
   3. Value Of The X-15. The commitment to go ahead with a flight-test program drove mid-1950s near-state-of-the-art technology toward perfection. Designed as a pure research vehicle with no operational prototype encumbrances or requirements for optimum design, the X-15 emerged from a short, three-year developmental program to return almost immediate data on the hypersonic environment. It gave the knowledge needed for today's designs for future hypersonic aircraft. Thanks to the X-15, we are able to do far more valuable laboratory research and testing. In a way, the X-15 reduced the urgency for a follow-on vehicle since so much more work can now be done with confidence in the wind tunnel, save the ultimate requirement for flight validation.

Thus wrote Ronald Boston with the perception of ten years after the program concluded.

When the X-15 quit flying, NASA was on the verge of initiating the first Phase A Shuttle studies. Yet, even before the X-15 had flown, a team of developers within the Air Force, industry, and NASA were busily at work on what would have been its immediate successor: an ambitious effort to develop an actual orbital hypersonic lifting reentry vehicle called Dyna-Soar.

 

Notes

 

NOTES: EDITOR'S INTRODUCTION

1. For general history of such studies in this time period, see NASA Langley Research Center staff report, "Conception and Research Background of the X-15 Project" (Hampton, VA: NASA LRC, June 1962), and NASA Langley Research Center staff report (draft document) "History of NACA-Proposed High-Mach Number, High-Altitude Research Airplane," (Hampton, VA: NASA LRC, n.d.), the latter hereafter referred to as NACA X-15 Origins, pp. 2-3. Both of these are in the files of the NASA History Office, Washington, D.C. Woods' proposals are in the meeting minutes of the NACA Committee on Aerodynamics, Oct. 4, 1951, Jan. 30, 1952, and June 24, 1952, in Record Group 255, National Archives, Washington, D.C. Hubert M. Drake and L. Robert Carman's "A Suggestion of Means for Flight Research at Hypersonic Velocities and High Altitudes" (Edwards, CA: NACA High-Speed Flight Research Station, n.d.), and "Suggested Program for High-Speed, High-Altitude Flight Research," (Edwards, CA: NACA HSFRS, August 1953), are in the NASA History Office archives, Washington, D.C. For SAB interest, see Thomas A. Sturm, The USAF Scientific Advisory Board: Its First Twenty Years, 1944-1964 (Washington, D.C.: USAF Historical Division Liaison office, Feb. 1, 1967), p. 59; The ONR-Douglas project is detailed in Douglas Aircraft Company Summary Report for Contract Nonr 1266(00), "High Altitude and High-Speed Study," (El Segundo, CA: DAC, May 28, 1954), in the corporate files of the McDonnell-Douglas Corporation, Douglas Aircraft division, Long Beach, California. I also wish to acknowledge the assistance of Douglas engineer Edward H. Heinemann's letter to me of Feb. 10, 1972. See also John V. Becker, "The X-15 Project," Astronautics & Aeronautics (February 1964), pp. 52-61. A good general summary of the X-15's development, including some excellent drawings of the various proposals of Bell, Douglas, Republic, and North American, can be found in Ben Guenther, Jay Miller, and Terry Panopalis, North American X-15/X-15A-2, Aerofax Datagraph 2 (Arlington, TX: Aerofax, Inc., 1985), pp. 1-8.

2. NACA Research Airplane Projects Panel meeting minutes, Feb. 4-5, 1954, from the files of the NASA Langley Research Center; NACA X-15 Origins, pp. 4-45; Interview with John V. Becker, NASA Langley Research Center, Nov. 12, 1971; Hallion, "American Rocket Aircraft;" Robert S. Houston, Development of the X-15 Research Aircraft, 1954-1959, v. III of History of Wright Air Development Center, 1958 (Dayton, OH: Wright-Patterson AFB, Office of Information Services, June 1959), pp. 3-13; 17-21; 82-127; 184-185.

3. John V. Becker, "Principal Technology Contributions of X-15 Program," (NASA Langley Research Center, Oct. 8, 1968); Becker, "The X-15 Program in Retrospect."

4. An excellent summation of early X-15 research can be found in Wendell H. Stillwell's X-15 Research Results, NASA SP-60 (Washington, D.C.: NASA, 1965), and Joseph Weil's NASA Technical Note D-1278, Review of the X-15 Program (Washington, D.C.: NASA, June 1962). See also James E. Love, "X-15: Past and Future," paper presented at the Fort Wayne Section, Society of Automotive Engineers, Dec. 9, 1964; and Walter C. Williams, "The Role of the Pilot in the Mercury and X-15 Flights," Proceedings of the XIVth AGARD General Assembly, NATO, Sept. 16-17, 1964, Lisbon, Portugal.

5. Richard P. Hallion, "X-15: Highest and Fastest of Them All," Flight International (Dec. 23, 1978), pp. 2256-2257, 2258, 2262. The X-15A-2 program is discussed in detail in Johnny G. Armstrong's Flight Planning and Conduct of the X-15A-2 Envelope Expansion Program, AFFTC-TD-69-4 (Edwards AFB: Air Force Flight Test Center, 1969), passim.

6. Becker, "X-15 Program in Retrospect;" Sänger-Bredt, "Silver Bird," pp. 223-224.


 

---

NOTES: SECTION I

a. A published summary of the July 9 NACA presentations did not appear until August 14.

b. By early February, the designation X-15 had been assigned to the proposed research aircraft, although in unclassified references it still carried the original title, "Project 1226."

c. The final evaluation ranked North American first, with Douglas, Bell, and Republic following in order of merit.

d. In August, the WADC Directorate of Weapon Systems Operations was transferred to the jurisdiction of headquarters ARDC and was given a new title.

e. The fee to Reaction Motors was greater than the original funds estimate for the total engine program; the definitive contract pro posed expenditure of 20 times as much as the original estimate and twice as much as the original program approval. As events later demonstrated, even this erred badly on the side of underestimation.

1. Res. Airplane Comm., Report on Conference on the Progress of the X-15 Project, compilation of the papers presented at the Langley Aero. Lab., 25-26 Oct. 1956, pub. by NACA-Langley, in files of X-15 WSPO, pp. xvii-xix and pp 3-9 (hereafter cited as Report on Progress of the X-15-1956).

2. Ibid; TT, RDTDP-6-15-E, Cmdr., ARDC, to Cmdr., WADC, 17 June 1954; TT, RDTDP-6-22-E, Cmdr., ARDC, to Cmdr., WADC, 21 June 1954.

3. Memo., J. W. Rogers, Liquid Propellant and Rocket Br., Rocket Propulsion Div., to Lt. Col. L. B. Zambon, Power Plant Lab., WADC, 13 July 1954.

4. Ltr., Col. P. F. Nay, Actg. Ch., Aero. and Propulsion Div., Dep. Cmdr/Tech. Ops., ARDC, to Cmdr., WADC, 29 July 1954, subj.: New Research Aircraft.

5. DF, E. C. Phillips, Ch., Ops. office, Power Plant Lab., to Dir/Labs., WADC, 5 Aug. 1954, subj.: NACA Conference on 9 July 1954 on Research Aircraft-Propulsion System; ltr., Col. V. R. Haugen, Dir/Labs., WADC, to Cmdr., ARDC, 13 Aug. 1954, subj.: New Research Aircraft.

6. Memo., J. W. Rogers, Liquid Propellant and Rocket Br., Rocket Propulsion Div., Power Plant Lab., to Ch., Non-Rotating Engine Br., Power Plant Lab., WADC, 11 Aug. 1954, subj.: Conferences on 9 and 10 August 1954 on NACA Research Aircraft-Propulsion System.

7. Ltr., Haugen to Cmdr., ARDC, 13 Aug. 1954.

8. Memo. , R. L. Schulz, Tech. Dir/Ac. , to Ch. , Ftr. Ac. D . iv. , Dir/WSO, WADC, about 13 Aug. 1954.

9. Ltr., Maj. Gen. F. B. Wood, Dep. Cmdr/Tech. Ops., ARDC, to Dir/R&D, USAF, 20 Sept. 1954, subj.: New Research Aircraft.

10. Ltr. (1st Ind.), Brig. Gen. B. S. Kelsey, Dep. Dir/R&D, DCS/D, USAF, to Cmdr., ARDC, 4 Oct. 1954, subj.: New Research Aircraft.

11. DF, Col. D. D. McKee, Ch., Ac. Lab., to Dir/Labs., WADC, 18 Oct. 1954, subj.: Trip Report.

12. DF, T. J. Keating, Ch., Non-Rotating Engine Br., Power Plant Lab. , to J. B. Trenholm, Ch., New Dev. Office, Ftr. Ac. Div., Dir/WSO, WADC, 15 Nov. 1954, subj.: New NACA Research Aircraft; memo., J. W. Rogers, Liquid Propellant and Rocket Br., Rocket Propulsion Div., to Ch., Power Plant Lab., WADC, 4 Jan. 1955, subj.: Selection of Engines for New Research Airplane.

13. Memo., T. Gardner, Special Asst/R&D, USAF, to Asst. Secy. Navy/Air, 9 Nov. 1954, subj.: Principles for the Conduct of a Joint Project for a New High Speed Research Airplane; ltr., J. H. Smith Jr., Asst. Secy. Navy/Air, to Dir/NACA, 21 Dec. 1954, no subj.; Report on Progress of the X-15-1956.

14. Memo. of Understanding, signed by H. L. Dryden, Dir/NACA, J. H. Smith Jr., Asst. Secy. Navy/Air, and T. Gardner, Special Asst/R&D, USAF, 23 Dec. 1954, subj.: Principles for the Conduct by the NACA, Navy, and Air Force of a Joint Project for a New High-Speed Research Airplane.

15. Memo., Gardner to Asst. Secy. Navy/Air, 9 Nov. 1954.

16. Ltr., Col. P. F. Nay, Ch., Aero. and Propulsion Div., Dep. Cmdr/Tech. Dev., ARDC, to Cmdr., WADC, approx. 30 Nov. 1954, subj.: Competition for New Research Aircraft.

17. TT, AFDRD-AN-43671, C/S, USAF, to Cmdr., ARDC, 8 Dec. 1954.

18. Memo., A. L. Sea, Asst. Ch., Ftr. Ac. Div., to Dir/WSO, WADC, 29 Dec. 1954, subj.: New Research Aircraft.

19. Ibid.

20. Ltr., Col. C. F. Damberg, Ch., Ac. Div., AMC, to Bell Ac. Corp. (hereafter cited as Bell), et. al., 30 Dec. 1954, subj.: Competition for New Research Aircraft; memo. , Sea, to Dir/WSO, WADC, 29 Dec. 1954; ltr. , J. B. Trenholm, Ch. , New Dev. Of f ice, Ftr. Ac. Div., Dir/WSO, WADC, to Cmdr., ARDC, 13 Jan. 1955, subj.: New Research Aircraft.

21. Ltr. , Brig. Gen. D. R. Ostrander, Dir/Dev. , Dep. Cmdr/Tech. Ops. , ARDC, to Cmdr., WADC, 2 Feb. 1955, subj.: X-15 Special Research Airplane.

22. Ltr. , Col. C. F. Damberg, Ch. , Ac. Div. , AMC, to Bell, et. al. , 2 Feb. 1955, subj.: Project 1226 Competition.

23. Memo., Rogers to Ch., Power Plant Lab., 4 Jan. 1955.

24. DF, T. J. Keating, Ch. , Non-Rotating Engine Br. , Power Plant Lab., to Lt. C. E. McCollough Jr., New Dev. Office, Dir/WSO, WADc, 14 Jan. 1955, subj.: Project 1226 -Information for Engine Contractors.

25. Ltr., R. W. Walker, Ch., Power Plant Dev. Sect., Power Plant Br., Aero. Equip. Div., AMC, to Reaction Motors, Inc. (hereafter cited as RMI), 4 Feb. 1955, subj. Power Plant for New Research Airplane.

26. Ltr., W. P. Turner, Mgr., Customer Relations and Contr. Div., RMI, to Cmdr., AMC, 3 Feb. 1955, subj.: XLR30 Rocket Engine-Information Concerning.

27. Ltr., J. B. Trenholm, Ch., New Dev. Office, Ftr. Ac. Div., Dir/WSO, WADC, to Bell, et. al., 22 Mar. 1955, subj.: Transmittal of Data, Project 1226 CompJi7tion.

28. Ltr., Ostrander to Cmdr., WADC, 2 Feb. 1955; ltr., Brig. Gen. H. M. Estes Jr., Dir/WSO, WADC, to Cmdr., ARDC, 11 Apr. 1955, subj.: X-15 Research Aircraft.

29. Ltr., Estes to Cmdr., ARDC, 11 Apr. 1955.

30. Ltr., Col. P. F. Nay, Ch., Aero. and Propulsion Div., Dep. Cmdr/Tech. Ops., ARDC, to Cmdr., WADC, 26 Apr. 1955, subj.:X-15 Research Aircraft.

31. Ltr., H. L. Dryden, Dir/NACA, to Dep. Dir/R&D, DCS/D, USAF, 20 May 1955, no subj.; ltr., Rear Adm. R. S. Hatcher, Asst. Ch/R&D, BuAer, USN, to Cmdr., WADC, 31 May 1955, subj.: Agreements Reached by "Research Airplane Committee," on Evaluation Procedure for X-15 Research Airplane Proposals.

32. Suborder, J. B. Trenholm, Ch., New Dev. Office, Ftr. Ac. Div., Dir/WSO, to Ch., Rocket Sect., Non-Rotating Engine Br., Power Plant Lab., WADC, 20 June 1955, subj.: X-15 Research Aircraft.

33. Memo., Brig. Gen. H. M. Estes Jr., Dir/WSO, to Dir/Labs., WADC, 28 June 1955, subj.: X-15 Evaluation.

34. Ltr., J. B. Trenholm Jr., Ch., New Dev. Office, Ftr. Ac. Div., Dir/WSO, WADC, to Cmdr., ARDC, I July 1955, subj.: X-15 Research Aircraft Evaluation Schedule; comments, Capt. C. E. McCollough Jr., Ftr. Ac. Div., Dir/WSO, WADC, July 1955.

35. Memo., Col. C. G. Allen, Ch., Ftr. Ac. Div., Dir/Sys. Mgmt., ARDC, to Cmdr., WADC 23 Aug. 1955, subj.: X-15 Evaluation. DF, Col. C. G. Allen, Ch., Ftr. Ac. Div., to Ch., New Dev. office, Ftr. Ac. Div., Dir/Sys. Mgmt., ARDC, 7 Sept. 1955, subj.: X-15 Evaluation.

36. DF, Allen to Ch., New Dev. Office, 7 Sept. 1955.

37. X-15 WSPO Weekly Activity Rpt. (hereafter cited as WAR), 22 Sept. 1955; interview, W. B. Underwood, NACA Liaison officer, 1 Oct. 1955, by Robert L. Perry, Ch., Hist. Br., WADC.

38. TT, AFDRD-30259, C/S, USAF, to Cmdr. ARDC, 20 Sept. 1955; TT RDZ-ISFF-9-514-E, Dir/Sys. Mgmt. to Cmdr., ARDC, 23 Sept. 1955; ltr., R. H. Rice, Vice Pres. and Ch. Engr., North American Avn., Inc. (hereafter cited as NAA), to Cmdr., ARDC, 23 Sept. 1955, subj Project 1226, Research Airplane.

39. Ltr., Col. C. F. Damberg, Ch., Ac. Div., AMC, to NAA, 30 Sept. 1955, subj.: X-15 Competition.

40. X-15 WSPO WAR, 13 Oct. 1955; 20 Oct. 1955; 27 Oct. 1955; and 15 Dec. 1955; DF, Col. B. C. Downs, Ch., Ftr. Br., to Ch., Ac. Div., Dir/Proc. and Prod., AMC, 7 Nov. 1955, subj.: Request for Permission to Negotiate a CPFF Type Contract -P.R. No. 636317 and 198558; Ltr., N. Shropshire, Dir/Contr. Admin., NAA, to Cmdr., AMC, 8 Dec. 1955, subj.: Letter Contract AF33(600)-31693.

41. DF, Downs to Ch., Ac. Div., 7 Nov. 1955.

42. DF, Capt. C. E. McCollough, Proj. Officer, New Dev. Office, Ftr Ac. Div. , Dir/Sys. Mgmt. , ARDC, to Ch. , Non-Rotating Engine Br. , Power Plant Lab., Dir/Labs., WADC, I Dec. 1955, subj.: Engine for X-15.

43. Ltr. Contr. AF33(600)-32248, 14 Feb. 1956, in files of Contr. Dist. and Files Sect., AMC (hereafter cited as AMC contr. files X-15 WSPO WAR, 13 Oct. 1955.

44. Contr. AF 33(600)-31693, 11 June 1956.

45. Contr. AF33(600)-32248, 7 Sept. 1956.


 

 

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NOTES: SECTION II

a. Eventually, an Air Force-NACA study team journeyed to France to study the Sud-Ouest Trident interceptor, which had such a tail con figuration.

b. A gyroscqpically stabilized mechanism that aligns itself to the local vertical to provide a reference plane that can be uti lized for the derivation of altitude, attitude, velocity, and rate-of-climb information.

c. The facilities were those of the NACA's Langley and Ames laboratories, of North American, and of the Massachusetts Institute of Technology.

d. A skip-flow generator was a deflector that directed the air flow so as to create a low velocity area around the pilot.

e. The engine was eventually to undergo numerous changes of detail but its basic design, as described to the conference, was not greatly altered.

1. Report on Progress of the X-15-1956, pp. 23-31; ltr. , Damberg to Bell, 30 Dec. 1954.

2. Memo., A. W. Vogeley, Aero. Res. Scientist, NACA, to Res. Airplane Proj. Leader, Langley Aero. Lab., NACA, 30 Nov. 1955, subi. : Project 1226 meetings to discuss changes in the North American Proposal - Wright -Patterson Air Force Base meeting of October 24 and 25, and North American Aviation meetings in Los Angeles on October 27 and 28 and November 14 and 15, 1955.

3. Memo. , W. C. Williams, Ch., NACA High Speed Flt. Station, Edwards AFB, Calif., to Res. Airplane Proj. Leader, Langley Aero. Lab., NACA, 27 Jan. 1956, subj.: Visit to North American Aviation, Inc. to discuss Project 1226.

4. Memo. , H. A. Soule, Res. Airplane Proj. Leader, Langley Aero. Lab., to Members, NACA Res. Airplane Proj. Panel, 7 June 1956, subj.: Project 1226-Progress report for month of May 1956.

5. Ibid.

6. DF, M. A. Todd, Actg. Ch. , Contr. Reporting and Bailment Br. , Support Div., to Ch., Ftr. Br., Ac. Div., Dir/Proc. and Prod., AMC, 15 June 1956, subj.: Confirmation of Serial Numbers Assigned.

7. Ltr. , Dr. H. L. Dryden, Dir/NACA, to Ch. , Ftr. WSPO, Dir/Sys. Mgmt., ARDC, 6 July 1956, no subj.

8. Report on Progress of the X-15-1956, pp. Iff.

9. Rpt., "Development Engineering Inspection of the X-15 Research Aircraft-13 December 1956," Dir/Sys. Mgmt., ARDC, in files of X-15 WSPO.


 

 

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NOTES: SECTION III

a. In fairness to the laboratory, it must be admitted that such estimates were accompanied by a statement that "less confidence in these estimates exists because the XLR30 engine is at present in a much earlier stage of development." It was this same XLR30 that was eventually to be turned into the XLR99 and which was to prove the laboratory's qualification of its estimates to have been justified.

b. Of the engines under consideration, only the Aerojet XLR73 was a funded development engine. Consequently, the XLR73 was the only engine which -- theoretically at least -- would have cost nothing additional.

c. In fact, of course, the X-15 eventually ~jLd require use of the XLR8 as an interim engine, when the XLR30's derivative, the XLR99, fell further and further behind schedule.

d. The designation became "official" at Wright-Field on March 6 and received Navy approval on March 29.

e. The term "spaghetti tube" graphically described the appearance of the injector devices that sent fuel to the combustion chamber.

f. The Power Plant Laboratory and Propeller Laboratory had been combined on June 17, 1957, the new organization being designated Propulsion Laboratory.

g. There was a clear distinction between proposals for an interim engine to permit flight trials before an XLR99 became available, and an alternate engine, to substitute for the XLR99 in the final X-15.

1 . DF, Phillips to Dir/Labs., 5 Aug. 1954; memo. , J. W. Rogers to Ch., Non-Rotating Engine Br., 11 Aug. 1954; DF, Keating to Ch., New Dev. office, 15 Nov. 1954; ltr., Col. C. F. Damberg, Ch., Ac. Div., AMC, to Bell, et al. , 2 Feb. 1955, subj. : Project 1226 Competition.

2. DF, Keating to Ch., New Dev. Office, 15 Nov. 1954.

3. Ibid.

4. Ibid.

5. Ltr., Damberg to Bell, 30 Dec. 1954.

6. Ltr., Turner to Cmdr, AMC, 3 Feb. 1955.

7. Ibid.

8. Ltr., Walker to RMI, 4 Feb. 1955.

9. Ltr., Lt. Col. W. K. Ashby, Ch., Power Plant Br., Aero. Equip. Div. , AMC, to RMI, 24 Feb. 1955, subj. : Power Plant for New Research Airplane.

10. Ltr., Trenholm to Bell, et al., 22 Mar. 1955.

11. Ibid.

12. Ltr., Nay to Cmdr., WADC, 26 Apr. 1955.

13. Suborder, Trenholm to Ch., Rocket Sect., 20 June 1955.

14. Rpt., "Evaluation of Engines - Project 1226," 15 July 1955, Power Plant Lab., WADC.

15. Ltr., Lt. H. J. Savage, Non-Rotating Engine Br., Power Plant Lab., WADC, to Rmi, 26 Oct. 1955, subj. : Engine for the X-15 Airplane.

16. DF, McCollough to Ch., Non-Rotating Engine Br., I Dec. 1955.

17. Ltr., R. W. Walker, Ch., Power Plant Dev. Sect., Power Plant Br., Aero. Equip. Div., AMC, to RMI, 8 Dec. 1955, subj.: Request for Proposal X-15 Aircraft Engine Development.

18. Ibid.

19. DF, T. J. Keating, Ch., Non-Rotating Engine Br., Power Plant Lab., WADC, to Proj. Officer, New Dev. Office, Ftr. Ac. Div., Dir/Sys. Mgmt, ARDC, 15 Dec. 1955, subj.: The X-15 Airplane Engine.

20. DF, J. W. Rogers, Liquid Propellant and Rocket Br. , Rocket Propulsion Div., Power Plant Lab., WADC, to Ch., New Dev. Office, Ftr. Ac. Div., Dir/Sys. Mgmt., ARDC, 23 Dec. 1955, subj.: Methods of Contracting for X-15 Engine.

21. Ltr., Rear Adm. W. A. Schoech, Asst. Ch/R&D, BuAer, USN., to C/S, USAF, 28 Nov. 1955, subj.: Cognizance over development of rocket power plant for NACA X-15 research airplane.

22. Ltr., Col. D. H. Heaton, Ch., Aero. Div., Dir/R&D, DCS/D, USAF, to Cmdr., ARDC, 9 Dec. 1955, subj.: Cognizance Over Development of Rocket Power Plant for NACA X-15 Research Airplane.

23. IT Conf. between personnel of Dir/Sys. Mgmt. and Hq., ARDC, 29 Dec. 1955, subj.: BuAer Letter on the XLR30.

24. Ltr. (lst Ind.), Col. E. N. Ljunggren, Asst/Ac. Sys., ARDC, to Dir/R&D, USAF, 3 Jan. 1956, subj.: Cognizance Over Development of Rocket Power Plant for NACA X-15 Research Airplane.

25. Ltr., Brig. Gen. V. R. Haugen, Dep. Cmdr/Dev., WADC, to NACA-Wash., 15 Feb. 1956, subj.: Engine Contract for the X-15.

26. Ibid.

27. Ltr., W. P. Turner, Mgr. Customer Relations and Contr. Div., RMI, to Cmdr., AMC, 7 Feb. 1956, subj.: Rocket Engine System for X-15 Research Aircraft.

28. Ibid.

29. Ltr., Haugen to NACA-Wash., 15 Feb. 1956.

30. Ltr. Contr. AF33(600)-32248, 14 Feb. 1956, in AMC contr. files.

31. Memo., J. L. Sloop, Ch., Rocket Br., Lewis Lab., to Hq., NACA, 16 Apr. 1956, subj.: Visit to Reaction Motors, Inc. re: Powerplant for the X-15.

32. Ltr., H. P. Barfield, Asst. Ch., Non-Rotating Engine Br., Power Plant Lab., WADC, to RMI, I Aug. 1956, subj.: Contract AF33(600)-32248.

33. Ltr., A. G. Thatcher, Mgr., Div Eng RMI, to Cmdr., WADC, 17 Aug. 1956, subj.: Contract AF33(600)-32248.

34. Ltr., R. H. Rice, Vice Pres. and Gen. Mgr., NAA, to Asst. Dep. Cmdr/Weap. Sys., ARDC, I Feb. 1957, no subj.

35. Ltr., Maj. Gen. H. M. Estes Jr., Asst. Dep. Cmdr/Weap. Sys., ARDC, to NAA, 7 Mar. 1957, no subj.

36. Rpt., H. A. Soule" , Res. Airplane Proj. Leader, to Members,

NACA Res. Airplane Proj. Panel, 19 Mar. 1957, subj.: Project 1226-Progress report for months of January and February 1957.

37. Interview, Capt. K. E. Weiss, XLR99 Proj. Officer, Propulsion Lab., 23 Sept. 1958, by D. R. McVeigh, Hist. Br., WADC.

38. ARDC Form III (Mgmt. Rpt.), Proj. 3116, 29 Mar. 1957, subj.: X-15 Propulsion Subsystem.

39. Memo., A. W. Vogeley, Aero. Res. Engr., to Res. Airplane Proj. Leader, NACA, 3 Aug. 1957, subj.: X-15 Airplane - Discussions at Air Research and Development Command, Detachment #I, Wright-Patterson Air Force Base, Dayton, Ohio, on July 29-30, 1957.

40. Rpt., Status of XLR99-RM-1, 9 Jan. 1958 through 27 June 1958, prep. by Propulsion Lab., WADC, (hereafter cited as Status Rpt., 9 Jan-27 June 1958).

41. X-15 WSPO WAR, 29 Jan. 1958.

42. Interview, C. E. McCollough, Asst. Ch., X-15 WSPO, Dir/Sys. Mgmt., ARDC, 14 May 1959, by R. S. Houston, Hist. Br., WADC.

43. Status rpt., 9 Jan-27 June 1958.

44. Memo., "NACA-ARDC Position 20th February 1958," unsigned and undated.

45. Status rpt., 9 Jan-27 June 1958.

46. Ibid.

47. Ltr. , Maj. Gen. S. T. Wray, Cmdr. , WADC, to Cmdr. , ARDC, 17 June 1958, no subj.

48. Ibid.

49. Status rpt., 9 Jan-27 June 1958.

50. Ltr., Lt. Col. L. Schaffer, ARDC Regional office, New York, N.Y., to Cmdr., ARDC, 7 Mar. 1958, subj.: Management Changes at Reaction Motors, Inc., Denville, N. J.

51. Porter, John Sherman, Ed., Moody's Industrial Manual for 1958, New York: D. F. Shea, Publisher.

52. Status rpt., 9 Jan-27 June 1958.

53. Ltr., F. W. Tangeman, Dep. Ch., Power Plant Dev. Sect., Power Plant Br., AMC, to Reactions Motors Div., Thiokol Chem. Corp. (hereafter cited as RMD), 29 May 1958, subj.: Contract AF33(600)-32248, XLR99-RM-1 Back-up Chamber Development.

54. Status rpt., 9 Jan-27 June 1958.

55. Ltr., Wray to Cmdr., ARDC, 17 June 1958.

56. Ltr. , Lt. Gen. S. E.. 4,nderson, Cmdr. , ARDC, to Pres. , Thiokol Chem. Corp., 27 June 1958, no subj.; ltr. (draft), Cmdr., ARDC, to Thiokol Chem. Corp., approx. 17 June 1958, no. subj., (utilized in composition of ltr. of 27 June 1958 but not actually sent to addressee).

57. Ltr., J. W. Crosby, Pres., Thiokol Chem. Corp., to Cmdr., ARDC, 3 July 1958, no subj.

58. Ltr. , Lt. Gen. S. E. Anderson, Cmdr. , ARDC, to Pres. , Thiokol Chem. Corp., 1 Aug. 1958, no subj.

59. "Red Flag Rpt., X-15 Powerplant XLR99-RM-1," D. McKee, on-Rotating Engine Br., Propulsion Lab., WADC, 18 July 1958.

60. "Red Flag Rpt., Engine Testing of the XLR-99," prep. in Non-Rotating Engine Br., Propulsion Lab., WADC, 7 Aug. 1958; interview, Maj. A. Murray, Ch. , X-15 WSPO, Dir/Sys. Mgmt., ARDC, 18 July 1959, by R. S. Houston, Hist. Br., WADC (hereafter cited as Murray interview).

61. See note above.

62. DF, Col. J. M. Silk, Ch., Propulsion Lab., to Cmdr., WADC, 20 Aug. 1958, subj.: XLR99 Engine for X-15 Aircraft.

63. X-15 WSPO WAR, 5 Sept. 1958, 13 Sept. 1958, 19 Sept. 1958, and 26 Sept. 1958.

64. X-15 WSPO WAR, 3 Oct. 1958, 10 Oct. 1958, 17 Oct. 1958, and 24 Oct. 1958.

65. X-15 WSPO WAR, 31 Oct. 1958, and 14 Nov. 1958.

66. X-15 WSPO WAR, 28 Nov. 1958.

67. X-15 WSPO WAR, 30 Jan. 1959.

68. X-15 WSPO WAR, 20 Feb. 1959, and 6 Mar. 1959.

69. X-15 WSPO WAR, 24 Apr. 1959.

70. X-15 WSPO WAR, 8 May 1959.

71. DF, Col. F. A. Holm, Dep/Advanced Sys., Dir/Sys. Mgmt., to Ch. Programs and Evaluations Office, IG, ARDC, 13 Feb. 1958, subj. Survey of the X-15 Research Aircraft, 30 September-7 October 1958.


 

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NOTES: SECTION IV

a. A wetting agent is any substance added to a liquid in order to increase the dispersion and penetrability of the liquid. In this case, it was hoped that starting times could be reduced by increasing the contact between the hydrogen peroxide and the catalyst.

b. Crossfield's interest was not at all impersonal. He had joined North American for the purpose of becoming the first pilot of the X-15.

c. Attendees included representatives of the Aero Medical Laboratory, the X-15 project office, the WADC Crew Station Office, North American, the David Clark Company, the Firewel Company, and the Bill Jack Company.

1. X-15 WSPO WAR, 2 May 1958.

2. X-15 WSPO WAR, 21 May 1958.

3. X-15 WSPO WAR, 29 May 1958.

4. X-15 WSPO WAR, 11 June 1958.

5. X-15 WSPO WAR, 25 June 1958.

6. Interview, Lt. J. S. Worley, X-15 WSPO, Dir/Sys. Mgmt. , ARDC, 22 May 1959, by R. S. Houston, Hist. Br., WADC.

7. X-15 WSPO WAR, 5 Sept. 1958.

8. Remarks, Maj. A. Murray, Ch., X-15 WSPO, Dir/Sys. Mgmt., ARDC, to Bureau of Budget personnel at WADC, 6 May 1959.

9. Res. Airplane Comm., Report on Conference on the Progress of the X-15 Project, compilation of the papers presented at the IAS Bldg., Los Angeles, Calif., 28-30 July 1958, in files of X-15 WSPO, p. 117 (hereafter cited as Report on Progress of the X-15 - 1958).

10. DF, F. Harris, Ch., Plans Office, Dir/Res., to Ch., Aero Med. Lab., WADC, 23 Aug. 1955, subj.: Planning for Support of X-15 Development.

11. Rpt., "Detail Specifications NA 55-447," revised 2 Mar. 1956 in files of X-15 WSPO.

12. NAA Spec. No. NA5-4077, 8 Apr. 1956, in files of X-15 WSPO.

13. DF, Lt. Col. K. F. Troup, Ch., Aircrew Effectiveness Br., Aero Med. Lab., WADC, to Ch., New Dev. WSPO, Ftr. Ac. Div., ARDC, 4 May 1956, in files of X-15 WSPO.

14. X-15 WSPO WAR, 28 June 1956.

15. AMC Form 52 (Record of Verbal Coordination), 12 July 1956, subj.: Personal Equipment for X-15 Weapons System, in files of X-15 WSPO, Murray interview, 16 July 1959.

16. Ltr., R. L. Stanley, Dep. Ch., Ftr. Ac. Br., Ac. and Missiles Div., Dir/Proc. and Prod., AMC, to AFPR, NAA, 16 Jan. 1957, subj.: Contract AF33(600)-31693, X-15 Airplane - ECP's NA-X15-7, NA-X15-1, NA-X15-8, NA-X15-12.

17. Ltr., S. C. Hellman, Mgr., Contr. and Proposals, NAA, to Cmdr., AMC, 8 Feb. 1957, subj.: Contract AF33(600)-31693 (3 X-15) NA-240 Contractual Document Request for Full Pressure Pilot's Suit-Change from CFE to GFAE, ECP NA-X-15-8.

18. X-15 WSPO WAR, 30 Oct. 1957.

19. Exhibit, "Full Pressure Suit Assembly," Physiology Br., Aero Med. Lab., WADC, I Jan. 1958.

20. DF, G. Kitzes, Asst. Ch., Physiology Br., Aero Med. Lab., WADC, to Ch., Ftr. Ac. Div., Dir/Sys. Mgmt., ARDC, 10 Apr. 1958, subj.: Status of MC-2 Full Pressure Altitude Suits for the X-15 research aircraft.

21. X-15 WSPO WAR, 2 May 1958.

22. Interview, Capt. J. E. Schaub, X-15 WSPO, Dir/Sys. Mgmt., ARDC, 28 May 1959, by R. S. Houston, Hist. Br., WADC.

23. X-15 WSPO WAR, 9 May 1958.

24. DF, Lt. J. E. Schaub, X-15 WSPO, Dir/Sys. Mgmt. , ARDC, to Ch. , Aero Med. Lab., WADC, 19 Aug. 1958, subj.: X-15 Full Pressure Suit Program.

25. X-15 WSPO WAR, 5 Sept. 1958.

26. X-15 WSPO WAR, 13 Sept. 1958.

27. X-15 WSPO WAR, 7 Nov. 1958.

28. Rpt., "Survey of the X-15 Research Aircraft, 30 September - 7 October 1958," IG, ARDC, undated.

29. ibid.

30. X-15 WSPO WAR, 21 Nov. 1958, 5 Dec. 1958, and 9 Jan. 1959; DF, Holm. to Ch., Programs and Evaluations Office, IG, ARDC, 13 Feb. 1958.

31. X-15 WSPO WAR, 30 Jan. 1959; Murray interview, 16 July 1959.

32. X-15 WSPO WAR, 3 Apr. 1959.

33. DF, H. E. Savely, Ch., Biophysics Br., Aero Med. Lab., WADC, to Ch., New Dev. Office, Ftr. Ac. Div., Dir/WSO, 8 Feb. 1955, subj.: Acceleration Tolerance and Emergency Escape.

34. Memo., Vogeley to Res. Airplane Proj. Leader, 30 Nov. 1955.

35. Memo., Soule to Members, NACA Res. Airplane Proj. Panel, 7 June 1956.

36. Memo. , H. A. Soule, Res. Airplane Proj. Leader, Langley Aero. Lab. , to Members, NACA Res. Airplane Proj. Panel, NACA, 15 Nov. 1956, subj.: Project 1226 - Progress Report for months of September and October 1956.

37. Memo., Brig. Gen. M. C. Demler, Dep. Cmdr/R&D, to Dep. Cmdr/Weap. Sys., ARDC, 2 Jan. 1957, subj.: Escape Systems for Research Vehicles such as. the X-15.

38. Ltr., R. H. Rice, Vice Pres. and Gen. Mgr., NAA, to Cmdr., AMC, 31 Jan. 1957, subj. : Contract AF 33(600)-31693, X-15 Airplane, GFAE Ejection Seat Catapult - Change to CFE Ballistic Rocket Type-ECP NA-X-15-19.

39. Interview, Lt. R. L. Panton, X-15 WSPO, Dir/Sys. Mgmt. , ARDC, 1 June 1959, by R. S. Houston, Hist. Br., WADC.

40. X-15 WSPO WAR, 2 May 1958.

41. Ibid.

42. X-15 WSPO WAR, 21 May 1958.

43. Ibid.

44. X-15 WSPO WAR, 11 June 1958.

45. X-15 WSPO WAR, 11 July 1958.

46. X-15 WSPO WAR, 3 Oct. 1958.

47. X-15 WSPO WAR, 7 Nov. 1958, and 28 Nov. 1958.

48. X-15 WSPO WAR, 28 Nov. 1958.

49. X-15 WSPO WAR, 9 Jan. 1959.

50. X-15 WSPO WAR, 16 Jan. 1959.

51. X-15 WSPO WAR, 6 Feb. 1959.

52. X-15 WSPO WAR, 13 Feb. 1959.

53. X-15 WSPO WAR, 13 Mar. 1959.

54. X-15 WSPO WAR, 17 Apr. 1959.

55. Memo., Vogeley to Res. Airplane Proj. Leader, 30 Nov. 1955.

56. Memo., Williams to Res. Airplane Proj. Leader, 27 Jan. 1956.

57. Memo., Soule to Members, NACA Res. Airplane Proj. Panel, 7 June 1956.

58. Proposal No. A. E. 1752, "Development of Flight Research Stabilized Platform," Sperry Gyroscope Co., Aug. 1956, in AMC Contr. files (AF33-600-35397); memo., Soule' to Members, NACA Res. Airplane Proj. Panel, 15 Nov. 1956.

59. DF) M. L. Lipscomb, Instr. Br., Flt. Control Lab., WADC, to Ch.. Accessories Dev. Sect., Accessories Br., Aero. Equip. Div., AMC, 26 Dec. 1956, in AMC contr. files (AF33-600-35397).

60. Ltr., H. L. Kimball, Ch., Accessories Dev. Sect., Accessories Br., Aero. Equip. Div., AMC, to Sperry Gyroscope Co., 6 Feb. 1957, in AMC contr. files (AF33-600-35397).

61. Negotiation Summary, C. E. Deardorff, Accessories Dev. Sect., Accessories Br., Aero. Equip. Div., AMC, 25 Apr. 1956, in AMC contr. files (AF33-600-35397).

62. DF, Brig. Gen. V. R. Haugen, Dir/Dev., WADC, to Ch., Aero. Equip. Div., Dir/Proc. and Prod., AMC, 22 Apr. 1957, subj.: Flight Data System for the X-15; Purchase Request DE-7-S-4184, in AMC contr. files (AF33-600-35397).

63. Contr. AF33(600)-35397, 5 June 1957, in AMC contr. files.

64. Supp. Agmt. I through 10, Contr. AF33(600)-35397, 5 June 1957 and subsequent, in AMC contr. files.

65. X-15 WSPO WAR, 2 May 1958.

66. DF, W. W. Bailey, Programming Br., Flt. Control Lab., WADC and Chester E. McCollough, Asst. Ch., X-15 WSPO, to Ch., Flt. Data Sect., Accessories Br., Aero. Equip. Div., AMC, 5 Aug. 1958, in files of X-15 WSPO.

67. Ibid.

68. Ltr., J. J. Slamer, Dep. Ch., Flt. Data Sect., Accessories Br., Aero. Equip. Div., AMC, to AFPR, Sperry Rand Corp., 7 Aug. 1958, subj.: Ltr. Contract AF33(600)-35397, in AMC contr. files.

69. Ltr., G. W. Schleich, Aero. Equip. Div., Sperry Gyroscope Co., to Cmdr., AMC, 4 Sept. 1958, subj.: Contract AF33(600)-35397, in AMC contr. files.

70. X-15 WSPO WAR, 23 Jan. 1959.

71. X-15 WSPO WAR, 13 Mar. 1959.

72. X-15 WSPO. WAR 1 May 1959.

73. Interview, Lt. R.L. Panton, X-15 WSPO Dir/Sys. Mgmt., ARDC, 1 June 1959, by R.S. Houston, Hist. Br., WADC.


 

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NOTES: SECTION V

a. National Aeronautics and Space Administration, successor to NACA.

1. X-15 WSPO WAR, 21 June 1956.

2. Report on Progress of the X-15 - 1956.

3. Report on Progress of the X-15 - 1958.

4. Ltr. , Brig. Gen. B. S. Kelsey, Dep. Dir/R&D, DCS/D, USAF, to Dir/NACA, 7 Apr. 1955, no subj.

5. Ltr., Dryden to Dep. Dir/R&D, 20 May 1955.

6. Memo. for record, Maj. J. E. Downhill, Asst. Ch., Test Proj. Sect., Aer. & Propulsion Div., ARDC, 28 July 1955, subj.: Flight Test Range for the X-15.

7. Ltr., Lt. Col. B. H. Harris Jr., DCS/O, AFFTC, to Cmdr., ARDC 2 Dec. 1955, subj.: X-15 Research Aircraft.

8. Ltr., H. A. Soule, Res. Airplane Proj. Leader, NACA-Langley, to NACA Liaison Officer, WPAFB, 20 Dec. 1955, subj.: Air Force-North American-NACA conference on Project 1226 Range.

9. Rpt., "Survey of the X-15 Research Aircraft, 30 September 7 October 1958," IG, ARDC, undated.

10. Remarks, Maj. A. Murray, Ch., X-15 WSPO, Dir/Sys. Mgmt., ARDC, to Bureau of Budget personnel at WADC, 6 May 1959; interview, C. E. McCollough, Asst. Ch., X-15 WSPO, Dir/Sys. Mgmt., ARDC, 11 June 1959, by R. S. Houston, Hist. Br., WADC.

11. Ltr., J. W. Crowley, Associate Dir/Res., NACA-Wash., to NACA High-Speed Fit. Station, Edwards AFB, Calif., 28 Feb. 1958, subj.: Flight control research for hypersonic airplanes.

12. X-15 WSPO WAR, 23 Apr. 1958.

13. X-15 WSPO WAR, 2 May 1958.

14. X-15 WSPO WAR, 18 June 1958.

15. Ltr., Maj. Gen. M. C. Demler, Dir/R&D, USAF, to Cmdr., ARDC, 18 Nov. 1958, subj.: Further Development of X-15 Aircraft.

16. Interview, C. E. McCollough, Asst. Ch., X-15 WSPO, Dir/Sys. Mgmt., ARDC, 12 June 1959, by R. S. Houston, Hist. Br., WADC.

17. Cost Projection, Contr. AF33(600)-31693, prep. quarterly by NAA Dept. of Pricing, 1955-1959, in files of X-15 WSPO.

18. Interview, C. E. McCollough, Asst. Ch., X-15 WSPO, Dir/Sys. Mgmt., ARDC, 12 June 1959, by R. S. Houston, Hist. Br., WADC.

19. Ibid.

20. Ibid.


 

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NOTES: SECTION VI

a. Concerns over panel flutter resulted in extensive redesign of the proposed X-20 Dyna-Soar, and played a major part in the research rationale behind the ASSET program, as will be seen.

b. In fact, one way of envisioning the Space Shuttle is to ima gine a transpor,t the size of a McDonnell Douglas C-9 Nightingale (civilian DC-9), carrying the payload of a Lockheed C-130 Hercules, and flying like the X-15.

c. Knight subsequently related his thoughts as he began his descent; looking down at Mud Lake he muttered, "Take a good look, Pete, that's probably where you'll plant it!" Such, fortunately, was not the case.

d. See the Hypersonic Ramjet Experiment case study for the "ground" story of this interesting effort.

1. Robert Mulac, X-15 flight chronology (NASA-Langley Research center, n.d.) Copy in the files of the NASA History Office, Washington, D.C. See also A. Scott Crossfield and Clay Blair, Always Another Dawn: The Story of a Rocket Test Pilot (Cleveland, OH: World Publishing Co., 1960), pp. 307-366.

2. Mulac, X-15 flight chronology: Crossfield and Blair, pp. 366-405; NASA, X-15 series flight log assembled by Betty J. Love, NASA DFRC.

3. Wendell Stillwell, X-15 Research Results (Wash., D.C.: NASA, 1965), P. 65. Stillwell's book is useful mostly for the information on the program prior to 1965. Interview with R. Dale Reed, 30 Nov. 1976.

4. Becker, "The X-15 Program in Retrospect." Raumfahrtforschung. See also Stillwell, p. 66; and James E. Love, "X-15: Past and Future," paper presented to the Fort Wayne Section, Society of Automotive Engineers, 9 Dec. 1964.

5. Stillwell, pp. 51-52, 75-78, for a general technical review of the X-15 effort, see Joseph Weil, NASA Technical Note D-1278, Review of the X-15 Program (Washington, D.C.: NASA, June 1962); Becker, "The X-15 Program in Retrospect."

6. Stillwell, p. iv; see also Walter C. Williams, "The Role of the Pilot in the Mercury and X-15 Flights." Proceedings of the Fourteenth AGARD General Assembly, 16-17 Sept. 1965, Portugal.

7. Weil, Review of the X-15 Program; James E. Love and W. R. Young, "Operational Experience from the X-15 Program," n.d., files of the DFRC Safety Office.

8. Betty Love, X-15 flight chronology, DFRC Pilots' Office files.

9. Love, "X-15: Past and Future," Becker, "The X-15 Program in Retrospect," memo on X-15 follow-on program by Homer Newell to Hugh Dryden, 18 Dec. 1961; undated memo, Bikle to Soule' (believed Nov. 1961); memo, Stack to Dryden, 3 Jan. 1962; ltr., Dryden to Maj. Gen. Marvin C. Demler, USAF, 12 Jul. 1962; NASA news release 61-261.

10. Air Force Systems Command, X-15 System Package Program, 6-37-48; Love, "X-15: Past and Future;" ltr., Dryden to Demler, 23 Mar. 1962; NASA news release 62-98; X-15 news release 62-91; ltr., Dryden to Lt. Gen. James Ferguson, 15 Jul. 1963; NASA news release 64-42; Carlton R. Gray, MIT/IL X-15 Horizon Definition Experimental Final Report (Cambridge, Mass.: MIT Instrumentation Laboratory, Oct. 1969), passim. ; NASA George C. Marshall Space Flight Center news release 68-69.

11. AFSC, X-15 System Package Program, 1-2; Robert A. Hoover and Robert A. Rushworth, "X-15A-2 Advanced Capability," paper presented at the annual symposium of The Society of Experimental Test Pilots, Beverly Hills, CA, 25-26 Sept. 1964.

12. Love, X-15 flight chronology; cockpit voice transcription for Rushworth flight, 14 Aug. 1964; X-15 Operations Flight Report, 19 Aug. 1964; Rushworth flight comments, n.d.

13. Hoover and Rushworth, "X-15A-2 Advanced Capability," NASA news release 66-11; C. M. Plattner, "Insulated X-15A-2 Ready for Speed Tests," Aviation Week & Space Technology (24 July 1967), pp. 75-81.

14. William J. Knight, "Increased Piloting Tasks and Performance of X-15A-2 in Hypersonic Flight," paper presented at the annual symposium of the SETP, Beverly Hills, CA, 28-30 Sept. 1967; Love, X-15 chronology.

15. Johnny G. Armstrong, Flight Planning and Conduct of the X-15A Envelope Expansion Program, AFFTC-TD-69-4; (Edwards AFB: AFFTC, 1969), pp. 52-56.

16. Becker, "The X-15 Program in Retrospect."

17. Interview with Jack Koll, 28 Feb. 1977.

18. Donald R. Bellman, et al., Investigation of the Crash of the X-15-3 Aircraft on November 15, 1967 (Edwards: NASA FRC, Jan. 1968), pp. 8-15.

19. Ibid.; passim; confidential interviews.

20. X-15 accident report, passim.

21. Ibid.; R. Dale Reed, "RPRV's: The First and Future Flights," Astronautics & Aeronautics (Apr. 1974), pp. 26-42.

22. For example, see the file "X-15: Mtg re research related to Hypersonic Cruise Vehicle," NASA History office, Washington, D.C., particularly memo, John Becker to Floyd Thompson, 29 Oct 1964; ltr. , Paul Bikle to C. W. Harper, 13 Nov. 1964; and Becker, et. al. , "Proposed Aerodynamic and Heat Trans f er Experiments," n. d. (transmitted by Floyd Thompson to A. J. Evans, 25 Jan 1965).

23. USAF Hq. Development Directive No. 32, 5 Mar. 1964, reprinted in X-15 System Package Program, 13-7.

24. James E. Love and William R. Young, NASA TN D-3732, "Survey of Operation and Cost Experience of the X-15 Airplane as a Reusable Space Vehicle" (Washington, D.C.: NASA, Nov. 1966), p. 7; Bikle interview; Finch interview.


 

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NOTES: SECTION VII

a. Ed. note: The NASA YF-12 program concluded in 1979.

1. John V. Becker, "Principal Technology Contributions of X-15 Program," (Hampton, VA: NASA Langley Research Center, 8 Oct 1968). Copy in the files of the NASA History Office.

2. Becker, "The X-15 in Retrospect."

3. Ronald G. Boston, "Outline of the X-15's Contributions to Aerospace Technology," (Colorado Springs, CO: USAF Academy, 21 Nov. 1977), pp. 1-15.