THE 456th FIGHTER INTERCEPTOR SQUADRON

THE PROTECTORS OF  S. A. C.

 

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The X-15 Program In Retrospect

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by John V. Becker
Chief, Aero-Physics Div.,
NASA Langley Research Center,
Hampton, Va., U.S.A.

Fellow, American Institute of
Aeronautocs and Astronautics

 

Presented at the 1st Annual Meeting,
Deutsche Gesellschaft für Luft- und Raumfahrt
Bonn, Germany
December 4-5, 1968

 

 

Introduction

 

The X-15 program was the first major investment of the United States in manned aerospace flight technology. During the long 15-year lifetime of the program, hundreds of people have contributed importantly to its success. It is a great privilege for me to represent this outstanding team at this meeting of so many distinguished members of your society, including Frau Dr. Sänger-Bredt. We sincerely appreciate the award of the Sänger medal.

Professor Sänger's pioneering studies of long-range rocket-propelled aircraft had a strong influence on the thinking which led to initiation of the X-15 program. Until the Sänger and Bredt paper (ref. 1) became available to us after the war we had thought of hypersonic flight only as a domain for missiles. The concept of manned rocket aircraft flying efficiently at hypersonic speeds for very long ranges was new and highly stimulating. The remarkably detailed analyses of many aspects of their new concept which Sänger and Bredt undertook in their paper gave real substance to the idea. From this stimulus there appeared shortly in the United States a number of studies of rocket aircraft investigating various extensions and modifications of the Sänger and Bredt concept. These studies provided the background from which the X-15 proposal emerged.

By 1954 we had reached a definite conclusion: the exciting potentialities of these rocket-boosted aircraft could not be realized without major advances in technology in all areas of aircraft design. In particular, the unprecedented problems of aerodynamic heating and high-temperature structures appeared to be so formidable that they were viewed as "barriers" to hypersonic flight. Thus no definite requirements for hypersonic vehicles could be established or justified. In today's environment this inability to prove "cost-effectiveness" would be in some quarters a major obstacle to any flight vehicle proposal. But in 1954 nearly everyone believed intuitively in the continuing rapid increase in flight speeds of aeronautical vehicles. The powerful new propulsion systems needed for aircraft flight beyond Mach 3 were identifiable in the large rocket engines being developed in the long-range missile programs. There was virtually unanimous support for hypersonic technology development. Fortunately also, there was no competition in 1954 from other glamorous and expensive manned space projects. And thus the X-15 proposal was born at what appears in retrospect as the most propitious of all possible times for its promotion and approval.

The broad objective of the X-15 was to take a long step forward in developing the new technologies needed for rocket-boosted hypersonic aircraft (ref. 2). Fortunately, it was not proposed as a prototype of any of the particular concepts in vogue in 1954, which have since largely fallen by the wayside. It was conceived rather as a general tool for manned hypersonic flight research,, able to penetrate the new regime briefly, safely, and without the burdens, restrictions, and delays imposed by operational requirements other than research. The merits of this approach had been convincingly demonstrated by experiences with the previous research air planes, notably the X-1 series.

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Figure 1. Typical X-15 research flight paths.

The plan called for two different types of flight profile. The first consisted of a variety of constant angle-of-attack, constant altitude, and maneuvering flights within the altitude corridor of interest to hypersonic gliders. In the second type of trajectory it was proposed that the vehicle should explore for the first time some of the problems of manned space flight by making long leaps out of the sensible atmosphere (fig. 1). Our analyses of 1954 (ref. 2) showed that such excursions into space were feasible provided that an Inconel X heat-sink structural concept was used together with employment of high lift and low L/D during the reentry pull-up maneuver. This latter maneuver in itself was recognized as a prime problem for manned space flight from both the heating and the piloting viewpoints.

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Figure 2. Velocity and altitude of the X-15 flights.

While the hypersonic aeronautics aspect of the X-15 proposal enjoyed virtually unanimous approval, it is interesting to note that the space flight aspect was viewed in 1954 with what can best be described as cautious tolerance. There were'few then who believed that space flight was imminent, and even the most sanguine believed that manned space flight was many decades in the future, probably not before the 21st century (ref. 3). Several of the senior research consultants who reviewed the proposal counseled that the space flight maneuver was premature and should be eliminated. Fortunately it remained in the program as a prime objective, not because of superior vision on the part of the X-15 planners, but rather because of their recognition that the problems involved were basic and would have to be solved before true mannea space flight could be achieved, and that it was now possible in the X-15 Project to take the first steps toward their solution.

An idea of the scope of the program is obtainable from figure 2. The dots show the speed and altitude of all of the flights. Of the 199 flights accomplished in the program 109 exceeded Mach 5, and four exceeded Mach 6. The highest speed, Mach 6.7 (2020 meters/sec), was reached in 1967. In altitude, it can be seen from figure 2 that the space-trajectory type flights constituted a major part of the program with a peak altitude of 108,000 meters, well above the 82,000 meters vhich was the goal set in the original prospectus.

The results of the program have been widely disseminated and previous reviews have been given periodically (refs. 4, 5, and 6, for example). There is no need for a detailed review here. It will be of interest, however, now that the program has been completed, to examine in retrospect its principal accomplishments against a background of the original hopes and aims of 1954.
 

 

Piloting Aspects

One of the major initial goals of the program which has been most richly achieved was to explore the capabilities, and the limitations, of the human pilot in an aerospace vehicle. There were those in 1954 who speculated that man had no place in hypersonic or space flight. And there were others who believed that he would prove indispensable. In either event, the space trajectory and reentry maneuver which the X-15 pilot was asked to negotiate were guaranteed to provide a convincing test.

From the outset simulators of all kinds were used to an unprecedented extent in pilot training., flight planning, and also in vehicle design. There was no two-seated version of the X-15 in which pilots could be taught to fly. Twelve pilots trained on the simulators with outstanding success. These experiences paved the way for similar all-out use of simulators in the space program.

It is well known that for greatest effectiveness the use of simulators requires careful correlation with flight testing. In the early stages of X-15 design, of course, flight data were not available, and some of the design features decided upon on the basis of the simulated experiences alone proved to be wrong and had to be altered. One of these was the large ventral tail employed during the first phase of the program. In the original vehicle configuration developed by RACA in 1954 it had been found that this axrangement suffered at high angles of attack at hypersonic speeds from a nearly complete loss of effectiveness of the upper tail, and a large increase in effectiveness of the laver tail, leading to very high and undesirable negative dihedral effect. Thus, our original proposal suggested that only a small ventral tail should be used. The early simulator studies, however, revealed that the large ventral tail was necessary for law angle of-attack controllability to cope with feared thrust misalinement effects of the rocket engine. Furthermore, as shown in figure 3, left side, the simulator studies with the large ventral indicated that the machim could be controlled without dampers at high angles of attack in spite of the negative dihedral. Thus the decision was made to use this symetrical tail configuration.

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Figure 3. Handling characteristics of X-15 with dampers inoperative.

This condition of "dampers-off" controllability was an essential design requirement because of the doubtful reliability of the damper system. In the first flights of the program, contrary to the simulator results, the machire was found to be unflyable at angles of attack above about 8° with dampers inoperative (fig. 3,, center). This discrepancy was traced, in part, to the influence of secondary aerodynamic effects (such as trim, for example) on the stability derivatives, effects which were not included in the original simulation. In addition, the pilots naturally felt less secure in flight than in the simulator and were not willing to accept vehicle motions which they had rated "acceptable for emergency" on the early simulator. With flight "calibration" of this kind together with a continuing program of other improvements, the fixed-base simulator eventually achieved satisfactory simulations of instrument flight.

Early in the flight program when the state of affairs shown in figure 3, center graph, had been established there was serious doubt as to vhether the high altitude "space flight" missions of the X-15 could be flown safely. These missions typically required angles of attack in excess of 17° on reentry. One of the major constraints in the problem was eliminated when operational experience with the XLR99 rocket engine revealed that it had no significant thrust misalinement as originally feared. Thus the underlying reason for the large ventral disappeared, the ventral rudder was removed., and the problem was solved by a return to a tail configuration similar to that recommended by NACA in the original 1954 study (fig. 3. right graph). As an added safety measure, a back-up damper system was installed to provide high reliability. With this system the "uncontrollable" region above 20° could be safely penetrated, and reentry trajectories up to 26° were flown.

And so it vas that the absence of flight "calibrations" of the early fixed-base simulator, together with -unfounded worries over thrust misalinement led to a costly excursion in configuration design. A consoling thought in retrospect is that more was learned than if this mistake had somehow been avoided.

The capabilities demonstrated by the pilots in the principal areas of interest are summarized briefly as follows:

Exit phase

The program shows clearly that, given precise displays, the pilot can fly rocket-boosted vehicles into space with great accuracy (refs. 7, 8). He cannot do any better than completely automated systems, however. Perhaps his best role will be as a monitor of automatic systen able to contend with malfunctions or to make trajectory changes as needed.

Attitude control in space

This was considered a major research problem area in 1954. Development of a workable reaction control system was achieved with the aid of a ground-based simula or and flight tests at low dynamic pressure in the X-lB airplane. As a result of this program it became clear that attitude control without aerodynamics and with threshold aerodynamics were skills readily acquired by pilots, and the X-15 high-altitude flights fully confirmed this finding.

Maneuvering reentry

The steep reentries of the X-15 with flight path angles up to -38°, Mach numbers approaching 6, and angles of attack up to 26° presented a more difficult piloting problem than the shallow entries of lifting manned vehicles returning from orbital or lunar missions. The prime requisite, of course, is a flyable vehicle, which means in general for hypersonic flight a vehicle incorporating artificial damping systems. When the X-15's damping systems were operative the pilots could perform the reentry maneuver readily (refs- 7, 9). The "self-adaptive" damping system was preferred over the simple rate-responsive dampers. (Footnote: The basic feature of the "self adaptive" system is its automatic gain changer which maintains the desired dynamic response characteristics of the airplane for a wide range of dynamic pressures. Added capabilities of the installation in the X-15-3 airplane were dual redundancy, integra tion of aerodynamic and reaction controls, and automatic stabilization in pitch, roll and yaw. The system was developed under sponsorship of the USAF, Aeronautical Systems Division, and it represents one of the noteworthy advances associated with the X-15 program.) Transitions from reaction to aerodynamic controls were made without difficulty and a control mode in which the two systems were blended was also developed satisfactorily.

Gravity effects on pilot performance

With a few exceptions these proved small - essentially negligible. Weightlessness, which was one of the largest fears of the unknown in the early system studies, produced no difficulties in the few minutes it existed in the high altitude flights. This result, of course, shottly lost its impact after the first Mercury flight in 1962 involving a much longer period of weightlessness.

Pilot plus redundancy

An analysis of the first 44 flights showed that 13 would have failed in the absence of a human pilot together with the various redundant systems provided in the vehicle (refs. 5, 7). Against these'figures in favor of the pilot there were only a few examples where the pilot's error degraded the mission performance, and only one catastrophic accident out of 199 flights. The NASA-USAF board investigating this accident reported that in its judgement, the pilot confused roll and yaw indicators and inadvertently yawed the airplane to 90° or more at the start of the reentry, possibly as a result of display misinterpretation, distraction, or vertigo. This condition apparently lead to complete loss of control and subsequent breakup of the X-15-3 airplane (ref. 10).

The broad positive finding of the program, however, is clear: the capability of the human pilot for sensing, judging, coping with the unexpected, and employing a fantastic variety of acquired skills remains essentially undiminished in all of the key problem areas of aerospace flight. It is equally clear that there are many new areas in aerospace flight in which the pilot's capabilities must be supplemented. The need for artificial damping of hypersonic vehicles is one example.
 

 

Hypersonic Aerodynamics

 

Hypersonic aerodynamics was in its early infancy in 1954. The few small hypersonic wind tunnels then in existence had been used almost entirely for fluid mechanics studies. They were unable to simulate either the high temperatures or the high Reynolds numbers of flight. Because of strongly interacting flow fields, viscous interactions with strong shocks, and possible real gas effects, it was generally feared that testing in these limited wind tunnels would not produce valid results. And it was expected that the X-15 would reveal large discrepancies between flight and ground test data (ref. 2). Our inability to devise ground facilities capable of true-temperature simulation was in fact in 1954 regarded as a sort of "facility barrier". All-out efforts were launched during that period to try to develop high-temperature facilities.

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Figure 4. Typical comparison of wind-tunnel and flight aerodynamic data.

The X-15 program helped to expose the fallacies of this "facility barrier". Virtually all of the flight pressures and forces were found to be in excellent agreement with the low-temperature wind-tunnel predictions (refs. 11, 12, 13, and 14; fig. 4 shows two typical examples). Prior to the start of flight operations it was learned by analysis that the "real gas" high-temperature effects in themselves were for the most part negligible below Mach 10. Thus the agreement noted above implies primarily an absence of any important scale effects on the pressures and forces (other than skin friction) for the X-15 configuration. (Footnote: Other configurations, notably the highly swept delta wing with trailing-edge flaps, have been found to exhibit important scale effects, not only hyper sonically but throughout the speed range.)

Concuxrent with the first years of the X-15 flight program, a number of missile and space vehicle configurations were also successfully developed in small low-temperature hypersonic wind tunnels, and in a few cases limited flight data were obtained which provided some additional confirms tion of the wind-tunnel results. With this broad general validation, the bulk of which came from the X-15 results, the conventional low-temperature hypersonic wind tunnel became the accepted tool for configuration develop ment. The "true-temperature" hypersonic aerodynamics tunnel on the other hand, with its enormous operational and interpretational difficulties, has proved useful only for limited special problems where full temperature simulation is mandatory (ramjet combustor development, for example).

 

 

Laminar Flow

 

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Figure 5. Boundary-layer transition location on X-15 wingcompared with wind-tunnel prediction, and roughness Reynolds
number of X-15 leading-edge joint compared with critical roughness Reynolds number .

In the early studies of hypersonic aircraft, there was a widely held belief that the hypersonic laminar boundary layer would prove infinitely stable because of the heat flow out of the boundary layer. It was thought likely that hypersonic vehicles would be the first true laminar flow aircraft. Actually there was little substance to this belief other than an early theoretical indication of a stabilizing effect due to heat flaw from the boundary layer. Nevertheless it was the hopeful practice in the early fifties to compute performance and heating for the all-laminar case. One respected research group even suggested that hypersonic research to reduce laminar friction deserved top priority.

This technical superstition has now almost entirely disappeared, largely as a consequence of the X-15 findings (ref. 15; fig- 5). At the Mach 6 flight Reynolds number, which was about 2.7 million/meter, wind tunnel data for a smooth model, not including any benefits from heat transfer indicate extensive laminar flows. But only the leading-edge region was found to be laminar in flight. A "step" irregularity existed behind the leading edge which, although very small, was sufficient according to low-speed criteria to trigger transition. Thus the small surface irregularities which have proved the nemesis of laminar flaw at lower speeds, are apparently equally adverse on the blunt-edged wing at Mach 6.
 

 

Turbulent Heat Transfer

 

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Figure 6. Turbulent heat-transfer data for wing
of X-15 compared with prediction method.

Fortunately the X-15 structure was designed conservatively to cope with turbulent heating. Hypersonic turbulent heating rates were recognized from the outset as an area of special importance for flight measurements because of the weakness of the available semi-empirical prediction methods and the almost complete lack of reliable hypersonic wind-tunnel data for turbulent flow. It should not have been any cause for real surprise, therefore, when the X-15 flight results of 1961 (ref. 16) showed a marked departure from the predictions available at that time, averaging about 35 percent below the van Driest and the Eckert T' predictions (refs. 17, 18) for the low wall temperature conditions of these flights (fig. 6).

At first this result was received with disbelief by most fluid mechanics specialists, who are by nature skeptical of flight measurements. The result has been thoroughly substantiated., however, by repeated measurements at several locations on the airplane together with local flow f ield surveys to aid in analysis of the data. The important highlighting of this weakness of the prediction methods by the X-15 stimulated comprehensive studies in a variety of ground facilities. New cold-wall data have been obtained which fully confirm the X-15 results. Improved prediction methods are being sought and new investigations of the structure of the cooled turbulent boundary layer are in progress (ref. 19, for instance). This is an excellent example of one of the greatest values of an exploratory research airplane - the highlighting of an important problem and the stimulating of ground-based research for its solution.
 

 

 

Structures

 

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Figure 7. Wing skin buckle on flight to Mach 5.3 and corrective modification.
Figure 8. Temperature distribution aft of leading-edge expansion slot.
Figure 9. Cracked windshield glass in flight to Mmax= 6.04
Figure 10. X-15 windshield retainer modifications.
Figure 11. Wing covered with spray-on ablator prior to sanding.
Figure 12. Telemetry antenna showing effect of bow
shock heating on fuselage, Mmax= 6.7.
Figure 13. Cavity heating revealed by charring of ablator, Mmax= 6.7.
Figure 14. Tail-fuselage juncture heating, Mmax= 6.7.

The X-15 was the first manned aircraft in which aerodynamic heating was the dominant problem of structural design. It was, of course, out of the question in 1954 to hope to be able to identify an optimum structural concept representative of distant future developments and no attempt was made to do so. The approach taken was to utilize the concept best suited to the short duration X-15 mission itself, with its particular requirements for heat-transfer research for drastically different trajectories ranging from equilibrium glides to steep high-angle-of-attack space reentries. A thick-skinned heat-sink approach was adopted. This has proved satisfactory both as a structure and as a calorimeter for heat-transfer measurements. Aside from its relatively thick. heat-sink skin, the X-15 structure contains many features representative of current advanced concepts for Mach 6-8 cruise aircraft, such as corrugated shear webs, combined use of superalloys and materials of a lower expansion coefficient such as Titanium, and the use of segmented leading edges. In retrospect, the major advances in the area of primary structure occurred during design and development. Great reliance was placed on the results of structural tests in which heat was applied electrically and loads mechanically according to schedules representing actual flight environmental histories. Measurements of the behavior of the primary structure in actual flight have verified these ground simulations. Thus the X-15 confirmed that ccMplex high-temperature structures can be reliably developed with ground-based "partial simulation" techniques using only rather simple and inexpensive equipment. Here again, early notions that a true-temperature aerodynamic type of structures test facility would be required were dispelled by the X-15 findings.

Although no surprises were found in flight for the primary structure, many unanticipated problems came to light in the secondary structures. Early in the program the pilot reported a rumbling noise at high dynamic pressures. This turned out to be panel flutter of large areas of the skin on the side fairings and tails. It was found to be related to certain design features incorporated to reduce thermal stress. Solution of this problem produced important advances in flutter prediction criteria (ref. 20).

Unanticipated leading-edge distortions and buckling of the adjacent wing skin developed during the first flight to Mach 5.3 (fig. 7). The slots, perhaps through vortex action and partly through triggering transi tion, cause intense local heating. Covering the slot and providing an internal shear tie between segments solved this problem. Evidence of the local heating is seen in figure 8 (ref. 5).

Although the soda-lime glass windshields of the X-15 were designed conservatively from the standpoint of glass temperatures, a failure occurred, fortunately involving only the right outer pane (fig. 9). Close examination of the failure revealed that it was initiated by buckling of the retainer frame. This problem was solved by replacing the Inconel X frame with Titanium, for which buckling does not occur because of the lower expansion coefficients of Titanium (fig. 10). The aft portion of the frame was removed because it was apparently causing a shock-induced hot spot on the aft part of the window (ref. 7). These fixes have proved successful.

One could argue that these local problems could have been solved by more comprehensive ground testing and analysis of the local flow field and heating mechanisms. True, if someone had had the foresight to identify these new problems in advance of their disclosure by the flight tests. 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.

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Figure 15. Local heating on wing due to
impingement of fuselage bow shock and side-fairing shock.
Figure 16. Dummy ramjet engine installation.
Figure 17. Failure of pylon due to interference
heating from dummy ramjet.

During the past year one of the airplanes has been flown with a radical change in the structural concept (ref. 10). The airplane was covered with ablative insulation designed to permit flights to Mach 7.4 (fig. 11). A silicone elastomeric ablator was sprayed on in variable thickness appropriate to the local heat loads. Leading edges were protected by a related erosion-resistant material applied in preformed sections. This program was of interest to lifting reentry vehicle development, because the spray-on technique has often been advanced as a possible means of refurbishing the incremental protection required for metal reentry structures. In the initial phases of a lifting reentry where the heating rates are beyond the limits of the metallic radiation cooled structure, the material would ablate. Then, as the speed reduces the bare metal would be exposed and radiation cooling would take over.

The X-15 experience rather clearly shows that this approach to a refurbishable structure is impractical. Some 5 weeks time and over 2000 hours were required for the total job including special treatment of removable hatches, covers, etc., hardly a practical operation for a refurbishable logistics vehicle.

Performance of the material was generally satisfactory on flight which reached Mach 6.7. Charring occurred only in regions of highest heating, and a few local failures occurred where the thickness of the material was inadequate or where bond failures occurred due to back surface heating.

A most interesting by-product of the ablative insulation flights was the revelation of a number of heating phenomena not detectable on previous flights with the metal heat-sink structure. The ablator tended to show up areas of intense heating by charring. Many of these areas did not become very hot in the bare-metal flights because of heat-sink cooling, but the ablation material blocked the cooling effect and showed up the hot spots. An example is the high-pressure heating zone behind the detached bow shock of the telemetry antenna (fig. 12).

The supporting pylon was subject to strong shock fields from the spike and from the closed cowl inlet. Heat protection material was applied to the leading edge of the pylon in roughly the same thicknesses as for the wing leading edge but the complex local heating phenomena on the pylon were not accounted for in the design. A serious failure of the system occurred with actual burn-through of the pylon skin (fig. 17). Pylon leading-edge heating rates of the order of seven times those without interference were estimated. This is the closest the X-15 ever came to a major structural failure in flight due to heating. Again these results underscore the need for maximum attention to aerothermodynamics detail in design and preflight testing.

 

 

Operational Problems With Subsystems

 

In order to achieve maximum penetration into the realms of hypersonic and space flight, the X-15 necessarily had to use many newly conceived, partially developed subsystems fabricated of new materials by new processes. The XLR99 rocket engine, the auxiliary power units (APU), and the stability augmentation (damper) system are examples. A host of new problems were encountered with the subsystems after the start of flight operations, and flight schedules were subject to innumerable delays (refs. 5, 7, and 21).

In retrospect, a great deal of money and lost time could have been saved through more imaginative and more elaborate ground testing of all major subsystems. This sounds like a serious general criticism. But one must remember that the only precedents available in the mid-fifties were those based on experience with ordinary aircraft and with the much simpler previous research airplanes. As it was, the initial environmental testing of the X-15 subsystems far exceeded that of any previous vehicle. In this area perhaps more than any other, the X-15 brought to light innumerable "real" problems which could not have been foreseen by any means other than involvement with an actual flight vehicle. The program revealed the true nature of the difficulties encountered in an aerospace environment and identified many specific new requirements for developmental testing that were of value in subsystem development for the space program
 

 

Follow-On Scientific Experiments

 

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Figure 18. Solar spectrum instrumentation in wing-tip pod.
Figure 19. Saturn insulation on X-15 dive break showing
flow-field rake and photographic reference grid..

It became apparent early in the program that the X-15's could perform a valuable function, not foreseen in the original planning, as reusable carriers for a wide variety of scientific experiments. Some 15 such experiments which have returned useful data are tabulated below:

A quick scan of the list reveals that all but a few of the experiments deal with space problems.

Multiple experiments were carried on many flights, and the X-15 system, unlike space rocket testing, permitted full recovery of the equipment, recalibration, and repeat runs where needed. In nearly all cases use of the X-15 was the least costly and quickest means of achieving the desired data.

An illustration of the solar spectrum equipment is shown as an example in figure 18. The equipment is mounted in a wing tip pod, and is exposed after the aircraft reaches the high altitude environment. A second type of experiment involved testing the insulation of the Saturn booster on the dive brakes of the X-15 where the severe real-environmental heating situation of the Saturn could be duplicated (fig. 19).


 

Other Contributions

 

One of the important general claims of the original prospectus for the program was that it would stimulate aerospace research and development. A measure of how well this claim was realized is seen in the following tabulation of technical documents:
 

Principal Technical Documents Associated with the X-15 Program
Development of X-15 system and supporting R and D 276 documents
Flight test results 290 documents
General research inspired by X-15 program 200 documents
 
Total 766 documents


The total production of over 700 technical documents is equivalent to the output of a typical 4000-man federal research center for a period of some 2 years. Of special interest and significance are the 200 papers reporting general research inspired by the X-15. These latter papers were identified by personal knowledge of various members of the X-15 team and by contacts with-the authors. They are reports of research that would not have been undertaken had it not been for the inspiration provided in one way or another by the X-15. Thus we see statistical confirmation of the massive stimulus and the focus provided by the program.

It is most important to note that this displtty of new technology represents, in addition to specific research results, the acquisition of new manned aerospace flight "know how" by many teams in government and industry. They had to learn to work together, face up to unprecedented problems, develop solutions, and make this first manned aerospace project work. These team were an important national asset in the ensuing space program.

At the start of the Mercury program there was, of course, a significant foundation of missile technology that was heavily utilized for launch propulsion, blunt-body aerodynamics, and heat protection. However, the innumerable added constraints and requirements of a manned aerospace system were present and in this area the X-15 team and the X-15 developments were the principal technology sources. The later developments of the USAF X-20 Dyna-Soar Program, particularly the large advances it provided in radiation-cooled structures and materials, also became available for post-Mercury systems.

If one takes a broad look at all of the contributions of the program and considers relative values based on the actual applications that have been made of the results, it is quite clear that the space-oriented results have been of greater value than the hypersonic aeronautics contributions. This is the reverse of what was expected in the beginning and is primarily a consequence of the unanticipated early arrival of the space age. It is interesting and important to note that any attempt at a cost-effectiveness evaluation of research aircraft in 1954 would almost certainly have either ignored or grossly undervalued the space flight aspects. At the same time, the boost-glide missions which have since been displaced by unmanned space systems, would have been exaggerated in value in any 1954 assessment. Who could have foreseen in 1954 that within 5 years the top priority goal of Western technology would be to put a man in orbit in space without delay?

Without the advantage of X-15 technology lead time, would Project Mercury and the subsequent manned space projects have been delayed? What mis fortunes might have occurred? What losses in national prestige might have resulted? Haw much was it worth to have X-15 technology in these critical times? No specific answers are possible, of course. But the existence of intangible and initially unforeseeable values is undeniable.

 

 

Implications For Other Research Aircraft

A large new area of potential interest for exploration by future research aircraft has been opened up by the advent of hypersonic air breathing propulsion systems. These systems, which were not generally believed feasible in the early years of the X-15 program, employ some combination of the ramjet or scramjet for cruise, with the turbojet or rocket for acceleration and climb. The systems of interest include space launch vehicles, military strike or reconnaissance vehicles, and commercial transports, with speeds extending upward to about Mach 12. Many new problems not covered by X-15 research are found in the propulsion system, cryogenic fuel tankage, lightweight radiation-cooled structures, and in the piloting and operations areas. While it is not clear at present that a new air-breathing research airplane system will prove justifiable, it is desirable to explore and evaluate some of the possibilities.

If we look to the X-15 experience as a guide, what prime features does it suggest? What features were most vital to the role it has played in aerospace history? Clearly the decision in 1954 to proceed quickly with a general research tool as opposed to a configuration fully optimized with respect to the 1954 vision of the feature mission was vital. If the optimized pseudo-prototype route had been followed one can see with the advantage of hindsight that we would have picked the wrong mission, the wrong structure, the wrong aerodynamic shapes, and the wrong propulsion.

More important, in the 3 or 4 years which all of this optimization might have consumed we would have dissipated our technology lead time, and with the start of the space age in 1958, in all probability the X-15 would never have been attempted. A second basic feature of the X-15 that proved vital was the design of the system with great latitude in performance so that it would reach well beyond the hypersonic aerodynamic corridor into simulated space flight.

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Figure 20. Airbreathing research airplane study configuration.

A possible new research airplane system conforming to these and other X-15 guidelines is illustrated in figure 20. (Footnote: This is one of several systems receiving preliminary study in the United States.) It is a lifting-body cruise configuration designed for Mach numbers up to 12. Its acceleration engine is a hydrogen fueled J-2S rocket adapted from an upper-stage engine of the Saturn vehicle. Integrated into the lower surface is a research scramjet engine sized to paver the airplane in cruise. Following guidelines from our X-15 experience the vehicle is kept as small as possible, aboub 25 meters in length, and it remains in the Mach 12 environment only long enough for research purposes, about 5 minutes. As we have leaxned from the X-15, a new hypersonic research airplane system is likely to have a long lifetime of perhaps 15 years, during which many new unsuspected ideas for research and changes in configuration are likely to appear as the program develops. Accordingly, we are proposing here actually three different vehicle arrangements. We would start the program with the lifting-body rocket glider without the air-breathing research engine. Later, a delta-winged version using the same subsystems would be flown. And, finally, the integrated scranjet research engine shown here would be installed. Provision for structural cooling schemes including direct fuel cooling, air-film cooling, and other schemes likely to appear in these vehicles might also be made.

The case for approval of such a new research airplane is no longer simple as it was in 1954. A major difference is the high level of confidence now enjoyed by the partial-simulation techniques of ground-based research and development. The X-15 program itself, together with the successfully developed reentry vehicle systems, has thus tended to eliminate a major justification vhich existed in 1954. In the author's opinion 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. Thus there is not likely to be a future research airplane unless a high valuation is placed on the other vital but less tangible contributicns - the focusing of the countless detailed efforts in many areas, the revelation of new and unsuspected problems for research, the overall stimulation of technology development, and the early availability of new technology for important but initially unforeseeable new applications. The X-15 experience affirms that the ex ploratory research airplane is a most effective device for producing these values.

 

Acknowledgements

I have been assisted in this review by several other members of the X-15 team. The extensive help of Mr. Jack Fischel of the NASA Flight Research Center and Mr. Jim A. Penland of the NASA Langley Research Center is especially noted. I would also like to remind readers that although NASA exercised technical responsibility for most of the flight research discussed here, the United States Air Force played a major role in the program. They provided much of the business management, the bulk of the funding, and they were also deeply involved in all of the flight operations.

 

References

1. Sänger, Eugen; and Bredt, Irene:
Über einen Raketenantrieb für Fernbomber.
Deutsche Luftfahrtforschung U93538, 1944.
(Translation CGD-32, Tech. Info. Branch, Bur. Aero.., Navy Department.)

2. Becker., John V.:
The X-15 Project - Origins and Research Background.
Astronautics and Aerospace Engineering, Feb. 1964.

3. Dryden, Hugh L.:
Toward the New Horizons of Tomorrow.
First von Karman Lecture, Astronautics, Jan. 1963.

4. Beeler, D. E.:
The X-15 Research Program.
AGARD Rept. 289, NATO, Paris, October 1960.

5. Weil, Joseph:
Review of the X-15 Program.NASA TN D-1278, 1962.

6. Toll, T. A. I., and Fischel, J.:
The X-15 Project - Results and New Research.
Astronautics and Aerospace Engineering, March 1964.

7. Walker, J. A.; and Weil, J.:
The X-15 Program Proceedings, Second Manned Space Flight Meeting,
Co-sponsored by AIAA and NASA,
April 22-24, 1963, Dallas, Texas.

8. Holleman, E. C.:
Piloting Performance During Boost of the X-15 Airplane to High Altitude.
NASA TN D-2289, April 1964.

9. Holleman, E. C.:
Control Experiences of the X-15 Pertinent to Lifting Entry.
NASA TN D-3262, February 1966.

10. Knight, W. J.:
Increased Piloting Tasks and Performance of the X-15-2 in Hypersonic Flight.
The Aeronautical Journal, Sept. 1968.

11. Keener, E. R.; and Pembro, C.:
Aerodynamic Forces on Components of the X-15 Airplane.
NASA TM X-712, 1962.

12. Walker,H. J.; and Wolowicz, C. H.:
Stability and Control Derivative Characteristics of the X-15 Airplane.
NASA TM X-714, 1962.

13. Yancey, R. B.:
Flight Measurements of Stability and Control Derivatives
of the X-15 Research Airplane to a Mach Number of 6.02
and an Angle of Attack of 25.
NASA TN D-2532, November 1964.

14. Pyle, J. S.:
Flight Pressure Distributions on the Vertical Stabilizers
and Speed Brakes of the X-1.5 Airplane at Mach Numbers from 1 to 6.
NASA TN D-3o48, October 1965.
(See also, same author, TN D-2241 (fuselage pressures),
and TN D-2602 (wing pressures)..)

15. Braslow, A. L.:
Analyses of Boundary Layer Transition on X-15-2 Research Airplane.
NASA TN D-3487, July 1966.

16. Banner, R. D.; Kuhl; A. E.; and Quinn, R. D.:
Preliminary Results of Aerodynamic Heating Studies on the X-1.5 Airplane.
NASA TK x-638, March 1962.

17. van Driest., E. R.:
The Problem of Aerodynamic Heating.
Aero. Engr. Review, Vol. 15, No. 10, October 1956, pp. 26-41.

18. Eckert, E. R. G.:
Survey of Boundary Layer Heat Transfer at
High Velocities and High Temperatures.
WADC TR 59-624, Wright Air Development Center, USAF, April 1960
(available from ASTIA as AD-238292).

19. Bertram, M. H.; Cary, A. M., Jr.; and Whitehead, A. H. Jr.:
Experiments with Hypersonic Turbulent Boundary layers on Flat Plates and Delta Wings.
Presented at AGARD Specialists Mtg. on Hypersonic Boundary Layers and Flow Fields,
May 1-3, 1968, Lcndon, England.

20. Jordan, G. H.; McLeod, N. J.; and Guy, L. D.:
Structural Dynamic Experiences of the X-15 Airplane.
NASA TN D-1158, 1962.

21. Row, P. V.; and Fischel, J.:
X-15 Flight Test Experience.
Astronautics and Aerospace Engineering, June-1963.
 

 

 

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