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The Martin XB-48

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The Martin XB-48 (Martin Model 223) was the last of four designs (XB-45, XB-46, XB-47, XB-48) evaluated by the Army Air Force for a 1944 all-jet bomber requirement. The following description is from a June 1947 press release issued by the Glenn L. Martin Public Relations Department:

" ... the six-jet Martin XB-48, newest Army Air Forces high speed jet bomber made its initial flight Sunday (22 June 1947) from the Glenn L. Martin Company airport in Baltimore to the Patuxent River (Maryland) Naval Air Station, remaining aloft 37 minutes. Powered by six General Electric J-35 gas turbine engines housed three in each wing, the Martin XB-48 has a speed of over 480 miles per hour and carries a bomb load of more than 10 tons. It employs a new type "bicycle" landing gear, because of the difficulty of retracting heavy gears into extremely thin wings required for high speeds. Two pairs of main wheels are located tandem-style under the fuselage, and two smaller "outrigger" wheels farther out under the wing, to give stability during ground operations. The large main gear folds into the fuselage and the smaller wheels retract into the wings."

The tandem landing gear arrangement was initially tested on the Martin XB-26H, a modified B-26 with the basic tandem main landing gear with outriggers arrangement.

Because of the urgency of the jet bomber requirement, the two four-engine bombers competed against each other (XB-45 and XB-46) for an immediate production contract. The XB-45 won this competition. The XB-48 lost to the Boeing XB-47 in a performance fly-off of the two six-engine bombers. The XB-47 with its swept wing had much better performance. Only two prototype XB-48s were completed for flight testing before the entire program was canceled.
 

Type Number built/
converted		 Remarks
XB-48 prototype 2 Six-engine jet bomber


Notes:
· Serial numbers: 45-59585 and 45-59586
· First flight was June 22, 1947


SPECIFICATIONS:
Span: 108 ft. 4 in.
Length: 85 ft. 8 in.
Height: 26 ft. 6 in.
Weight: 102,600 lbs. (maximum takeoff weight)
Armament: Designed for two .50-cal. machine guns in a radar-controlled tail turret and up to 22,000 lbs. of bombs (one 22,000-lb. "Grand Slam" or 14 1,000-lb. bombs maximum loading)
Engines: Six Allison J35-A-5 axial flow turbojet engines of 4,000 lbs. thrust each maximum
Crew: Three (pilot, copilot-radio operator-gunner, bombardier-navigator)

PERFORMANCE:
Maximum speed: 495 mph
Cruising speed: 437 mph
Range: 2,500 miles with 8,000 lbs. of bombs
Service ceiling: 43,000 ft.

 

 

 

Air Force Museum

 

The XB-48

 

The Martin XB-48 was a medium jet bomber developed in the mid-1940s which never saw production or active duty. Only two prototypes were built.

In 1944 the U.S. War Department was aware of aviation advances in Germany and issued a requirement for a range of designs for medium bombers weighing from 80,000 to more than 200,000 pounds. Other designs resulting from this competition, sometimes named the class of '45, included the North American XB-45 and the Convair XB-46. Production orders finally went to the B-45 Tornado and even this airplane only served for a couple of years before again being replaced by the much more modern B-47 Stratojet.

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In retrospect, the class of '45 were transitional aircraft combining the power of turbojets with the aeronautical knowledge of world War II. The XB-48 was no exception, as its round fuselage and unswept wings show a distinct patronage from the earlier Martin B-26 medium bomber. Still, where the B-26 had enough thrust with two massive 18-cylinder radial engines, the XB-48 needed no less than 6 of the new jet engines. Although the pictures make it look like the aircraft has three engine gondolas under each wing, the jet engines were actually clustered in two flat three-engined gondolas with an intricate system of air canals between the engines providing cooling. At the time of the XB-48's design, jet propulsion was clearly still in is infancy.

 

Specifications (XB-48)

Data from "Encyclopedia of US Air Force Aircraft and Missile Systems, Volume II" by Knaack, Marcelle Size (Washington: Office of Air Force History, ISBN 0-16-002260-6), 1988.

General characteristics

Performance

Armament

 

References

  1. "Encyclopedia of US Air Force Aircraft and Missile Systems, Volume II" by Knaack, Marcelle Size (Washington: Office of Air Force History, ISBN 0-16-002260-6), 1988.

 

 

The XB-48

Joe Baugher

The Martin XB-48 officially originated back in 1944, at a time when the USAAF was already aware of German advances in the field of jet propulsion, especially as applied to the development of jet bombers. Alarmed by German developments, the War Department called for bids on a new family of jet-powered bombers, with gross weights ranging from 80,000 pounds to more than 200,000 pounds. These new aircraft were to be powered either by TG-180 or TG-190 engines which were then under development at General Electric. The TG-180 was eventually built by the Allison Division of General Motors as the J35, and the TG-190 was built by the General Electric company as the J47.

On November 17, 1944, the USAAF issued a specification calling for a bomber with a range of 3000 miles, a service ceiling of 45,000 feet, a tactical operating altitude of 40,000 feet, and a maximum speed of 550 mph. On January 29, 1945 these requirements were amended to stipulate that the aircraft would have to carry specific types of bombs, including the conventional M-121, a 10,000-pound "dam-buster" earthquake bomb.

The Glenn L. Martin company of Baltimore, Maryland came up with the Model 223 in response to this requirement. The Martin proposal was submitted to the Air Technical Service Command on December 9, 1944, and led to Letter Contract W33-038 ac-7675. Approved on December 9, 1945, this initial contract called for one mockup of the Martin Model 223. The designation XB-48 was assigned.

At the same time, three other contractors were awarded development contracts, North American for the XB-45, Convair for the XB-46, and Boeing for the XB-47.

The end of the Second World War resulted in the cancellation of many projects and the delay of others. However, the War Department felt that the development of a jet-powered bomber should still be pressed forward with the utmost speed, and the XB-45, XB-46, XB-47, and XB-48 contracts were left untouched. In 1946, the USAAF decided to forego the competition that would ordinarily be held between the four entries and opted instead to review the available designs to see which of the contestants could be produced first. By that time, the XB-45 and XB-46 were nearing completion, but the XB-47 and XB-48 were still at least two more years away. Since the USAAF was guided by what it felt to be a sense of great urgency, it decided to appraise the XB-45 and XB-46 right away and choose one of them for immediate production. Any consideration of the XB-47 and XB-48 would be deferred until after they had flown. if either the XB-47 or XB-48 turned out to be markedly superior to the plane that was then being produced, then that aircraft would be purchased and the currently-produced version would be phased out. This is indeed what happened when the XB-47 appeared.

On December 13, 1946, the original contract was superseded by W33-038 AC-13492 which called for two XB-48 prototypes, spare parts, and a bomb bay mockup. The first XB-48 was to be flight tested and delivered by September 30, 1947, with the second being delivered by June 30, 1948.

The XB-48 was to be powered by six General Electric TG-180 turbojets, later to be redesignated J35. The six engines were encased three each in lifting aerofoil section pods housed underneath each wing. The lift pods had air ducts between the pods and had adjustable tailpipes on the engines. The pilot and copilot were seated in tandem underneath a canopy-type enclosure, and the bombardier/navigator sat in the extreme nose. The wings were too thin to house a conventional landing gear, so The aircraft had a bicycle-type tandem undercarriage, with tandem twin-wheel units retracting into the fuselage ahead and behind the bomb bay. The aircraft had a pair of smaller outrigger wheels underneath each wing outboard of the engine pods. This arrangement had been tested on a XB-26H and had been found to be feasible. The armament was to have been a pair of 0.50-inch machine guns housed in a remotely-controlled tail turret and guided by an AN/APG-27 radar.

The first XB-48 (serial number 45-59585) made its maiden flight on June 22, 1947. It took off from Martin's company airfield at Baltimore and landed at the Patuxent Naval Air Station some 80 miles away. It was powered by six TG-180-B1 (J35-GE-7) engines. Development and testing of the XB-48 was delayed by engine difficulties. The first XB-48 went through no less than 14 engines during its first 44 flights.

In the spring of 1948, after early flight test data had been obtained on both the Boeing XB-47 and the Martin XB-48, the Air Force concluded that the XB-47 had an appreciably better performance and showed greater development potential. In addition, the Martin design was over 50 mph slower than its guaranteed speed, and no production of the XB-48 was ordered. The end of the line for the XB-48 became official in September of 1948, when the Air Force ordered its first lot of B-47 Stratojets.

The second XB-48 flew for the first time on October 16, 1948, some three months behind schedule. However, this delay did not matter very much, since by that time the fate of the XB-48 program had already been decided. It was powered by six J35-GE-9 turbojets.

In early 1949, Martin attempted to revive the B-48 program by proposing that the second XB-48 be re-engined with four XT40 turboprops installed in reconfigured nacelles. This converted XB-48 was to have been a prototype for the Martin Model 247-1, an airplane which the contractor insisted was capable of competing with the B-47, B-50, and B-54. The Air Force felt that the contractor's cost and performance estimates were too optimistic, and, in addition, since the XT40 was a Navy-developed engine, it was unlikely that Martin would be able to get enough engines to meet the schedules. Moreover, the Air Force was now of the opinion that turbojets, not turboprops, were the wave of the future for bombers, and on March 31, 1949, Martin was formally notified that the Model 247-1 would not be proceeded with.

Flight tests with the XB-48s continued even after the formal end of the program. In the fall of 1949, the first XB-48 was cannibalized to keep the second flying. The latter aircraft was scheduled for a series of tests on the F-1 autopilot, jet engine cooling systems, and a hydraulic system for jet engines. However, these tests were cancelled before any could be carried out. The second XB-48 was used instead for the testing of thermal de-icing systems. In September 1951, the aircraft was flown to Phillips Field at the Aberdeen Proving Grounds in Maryland where it was static tested to destruction.

 

Specification of Martin XB-48:

Engines: Six General Electric J35-GE-7 axial-flow turbojets, each rated at 3820 lb.s.t. Performance (contractor's estimate): Maximum speed 479 mph at 35,000 feet, 516 mph at 20,000 feet, and 486 mph at sea level. Average cruising speed 415 mph. Combat radius 500 miles with maximum bombload. Takeoff run 7900 feet at 102,600 pounds takeoff weight. Initial rate of climb 3250 feet per minute at takeoff weight of 102,000 pounds. Combat rate of climb 4200 feet per minute at combat takeoff weight of 86,000 pounds. An altitude of 30,000 feet could be attained in 21.5 minutes. Service ceiling 39,400 feet. Dimensions: Wingspan 108 feet 4 inches, length 85 feet 9 inches, Height 26 feet 6 inches, wing area 1330 square feet. Weights: 58,500 pounds empty, 92,600 maximum takeoff. 102,600 pounds combat with 4968 gallons of fuel included. Armament: Two 0.50-inch machine guns in extreme tail in remotely-controlled turret (not actually fitted). Maximum bombload 22,000 pounds.

 

Sources:
  1. American Combat Planes, Third Enlarged Edition, Ray Wagner, Doubleday, 1982.
     
  2. Post World War II Bombers, Marcelle Size Knaack, Office of Air Force History, 1988.

Joe Baugher

 

 

The XB-48 Martin

 

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The XB-48, like the more fortunate XB-45, originated in 1944, when the War Department concluded that jet propulsion was promising enough to warrant extension of the program, thus far centered on fighters and light bombers, to heavier aircraft with gross weights ranging from 80,000 to more than 200,000 pounds. Realizing that such an ambitious project could be fraught with difficulties, Army Air Forces (AAF) headquarters informed the Materiel Command and Air Services Command on 10 Augusts that in the beginning contracts for jet bombers of the medium and heavy categories would have to be let on a phased basis so that they could be readily terminated upon completion of any one stage of development. This cautious procedure was formalized on 15 August. About 2 weeks later the 2 commands merged to form the AAF Technical Service Command, which was redesignated Air Technical Service Command on 1 July 1945. This organization became the Air Materiel Command on 9 March 1946.

On 17 November 1944, the AAF issued military characteristics calling for a bomber with a range of 3,000 miles (minimum acceptable, 2,500); a service ceiling of 45,000 feet and a tactical operating altitude of 40,000 feet (minimums acceptable, 40,000 and 35,000 feet, respectively); and an average speed of 450 miles per hour with a high speed of 550. These characteristics were amended on 29 January 1945 to reemphasize that such aircraft needed to carry specific types of bombs, including the conventional M-121, a 10,000-pound "dam-buster" developed during World War II. The M-121, sometimes called the "Earthquake" bomb, was more often referred to as the "Grand Slam" bomb, a totally misleading nickname. Actually "Grand Slam" was the code name of a highly classified modification project strictly concerned with atomic matters. The "Grand Slam" modifications would allow the Convair B-36 to carry atomic bombs, which the Air Force believed might weigh more than 40,000 pounds. Since the 10,000-pound M-121, when properly dropped, could inflict the damage of a 40,000-pound bomb, curiosity and rumors most likely explained the ensuing confusion. As a matter of fact, the "Grand Slam" designation was also loosely applied to other conventional bombs of the M-121 category.

In accordance with the AAF's endorsement of "phase" contracts and based on the military characteristics of November 1944, a Martin proposal, submitted to the Air Technical Service Command on 9 December 1944, led to Letter Contract W33-038 ac-7675. Approved on 29 December, this initial document covered certain engineering services and completion by 1 May 1945 of 1 mockup of Martin's Model 223, designated XB-48 by the Air Technical Service Command. Tentative costs were set at $574,826. The letter contract of December 1944 was replaced on 27 March 1945 by a definitive contract, which reduced estimated costs to $569,252, including Martin's fixed-fee of $16,500.

Procurement of the XB-48 overcame many vicissitudes. In June 1945, 2 months after inspection of the XB-48 mockup, Martin submitted a proposal for 1 stripped and 1, 2, or 3 complete XB-48s. Accompanying cost figures, however, were immediately questioned. The Air Technical Service Command's surprise, it was soon ascertained that the estimated cost of $80.09 per pound for the XB-48 compared favorably to the $105.68 for the XB-45, but the AAF remained dissatisfied because the XB-48's engineering lagged behind the XB-45 and XB-46. Despite these concerns, the XB-48 project survived, and the initial contract was supplemented many times while negotiations went on. In March 1946, the contractor introduced a new proposal and offered to furnish 1 stripped and 1 complete XB-48 for about $10 million. This proposal was made on a fixed-price rather than a cost-plus-fixed-fee basis in order to conform to the policy set forth by the Air Technical Service Command in December 1945 on the procurement of experimental airplanes. Just the same, the Martin proposal of March 1946 had to be revised, and negotiations were not consummated until the end of the year. The final contract (W33-038 ac-13492), approved on 13 December 1946, superseded Contract W33-038 ac-7675 which, as amended, had reached an estimated future cost of $10.9 million. Only some $500,000, covered by the initial letter contract, were unaffected. For the same amount, the new contract promised 2 XB-48s, spare parts, and a bomb-bay mockup. Also, the first XB-48 was to be flight tested and delivered by 30 September 1947; the second one, by 30 June 1948. Finally, all wind tunnel tests were to be completed by 1 January 1947.

Development and testing of the 2 XB-48s were delayed by engine difficulties. General Electric turbojet engines were installed, the first XB48 being powered by 6 J35-GE-7 (TG-180-B1) engines; the second, by 6 J35-GE-9s (TG-180-Cls). Since the engines were in an even more experimental stage than the airplanes, it took time to get them to operate properly. Also, like every new engine, the J35s were in short supply. Still, the first XB-48 would go through 14 engines during its first 44 flights.

The sleek, all-metal, high-wing XB-48 presented many special features, the most outstanding one being the tandem bicycle landing gear necessitated by the airplane's wings, too thin to house conventional landing gear with bulky retracting mechanisms. Martin had experimented with a 4-wheel bicycle landing gear on an XB-26H and concluded that such an arrangement was feasible. Bicycle-type landing gears were later used by other jet bombers, including the B-47. Other novel features were the number of engines, 6 as compared to 4 on the other proposed medium bombers; the turbojet engine's installation, encased in pods (3 under each wing) in a lift section with air ducts between the pods; and also adjustable tail pipes on the engines. The 3-crew arrangement was also unusual. The pilot and co-pilot were seated in tandem under a canopy-type enclosure, similar to that found in high-speed fighter planes, while the bombardier-navigator was seated in the aircraft's nose. The XB-48 had retractable bomb-bay doors, a feature that sprang from the fact that all new medium and heavy bombers had to be capable of carrying the so-called "Grand Slam" bombs, as well as the cumbersome atomic bombs of the period.

The XB-48, the first US. 6 jet bomber to fly, made its initial flight on 22 June 1947. The experimental plane took off from Martin's airfield at Baltimore and landed some 80 miles away at the Patuxent Naval Air Station, also in Maryland. The 38-minute flight was not a great success. At 10,000 feet, the Martin pilot discovered that the right spoiler aileron snapped up too rapidly. On landing, the XB-48 drifted across the runway. Rudder steering was attempted, but the rudder was ineffective with the full use of brakes. In addition, the brakes overheated and stopped working. The aircraft finally came to a halt off the runway with no damage, even though both tires were worn through.

The second XB-48 did not fly until 16 October 1948, some 3 months behind schedule. The 30-minute flight was satisfactory, but of relative unimportance since the future of the experimental program had already been decided. The Air Force gave the contractor the option to eliminate all flaws or to pay a lump-sum penalty of $25,000. In January 1950, Martin agreed to pay the penalty.

Martin pilots tested the first XB48 52 times, for a total of 41 hours; the Air Force, 50 times for a total of 64 hours. The second XB48 was also thoroughly tested. The contractor put in 14 hours, accumulated in 15 flights; the Air Force, 49 hours, reached in 25 flights. Results of the first XB-48's flight test program revealed that the aircraft did not meet the Martin guarantees. The XB48 was 14,000 pounds overweight; the nose wheel was too sensitive; turbulence occurred in the bomb bay when the doors were open; and metal chips, deposited by disintegrated test stand hydraulic pumps, shattered the hydraulic system.29

The experimental B-48 program agreed upon in December 1946 was not curtailed. Yet, in spite of the contractor's efforts, no production program followed. Although no firm commitment would be made before many months, planning for the procurement of B-47 production models began in December 1947, right after the XB-47's first flight-a poor omen for the B-48, initially flown in June of the same year.

In the spring of 1948, after early experimental flight information had been obtained for both the XB-47 and the XB-48, the Air Force conducted an evaluation to determine which of the 2 planes could best satisfy the urgent need for a high-speed, high-altitude medium bomber. The evaluation confirmed that the performance of the XB-47 was appreciably better than that of the XB-48. It was also apparent that the XB-47 design provided possibilities for growth which surpassed those of the XB-48. The XB-47's swept-back wing would enable it to attain higher speeds, and its simpler pod-nacelle arrangement minimized the problem of incorporating newer and more efficient jet engines as they became available. The end of the B-48 production program became official in September 1948, when the Air Force ordered the first lot of 10 B-47s.

Early in 1949 Martin attempted to rescue the B-48 production program and proposed to modify the second XB-48 by removing the J35 engines and nacelles and installing 4 XT-40A propeller turbines in new and repositioned nacelles, at an estimated cost of $1.5 million. Actually, the reconfigured XB48 would become a prototype of the Martin Model 247-1, an airplane, the contractor insisted, capable of competing with the B-47, B-50, and B-54. On paper, Model 247-1's performance looked good, but the Air Force did not believe the proposed reconfiguration could be accomplished for the amount of money estimated by the contractor. In addition, since the XT-40A turboprop was a Navy-developed engine, it was doubtful that Martin could obtain enough engines to complete the reconfiguration on schedule. Finally, and of overriding importance, senior Air Force officials believed that turbojet aircraft "currently offered greater promise than turboprop installations." Thus, on 31 March 1949, Martin was formally told that the Model 247-1, like the original XB-48, was a dead issue.

The Air Force accepted the first XB-48 on 26 October 1948, but only conditionally. The acceptance became final in 1950, when Martin paid the $25,000 penalty assessed by the Air Force because of the aircraft's several defects. The second XB-48, also conditionally accepted on 26 October 1948, was finally accepted on 23 February 1949, after the contractor completed various modifications.

The total cost of the XB-48 development program reached $11.5 million. Of this amount, less than $500,000 pertained to the letter contract of December 1944. The rest covered the final contract of December 1946 and represented an increase of about $100,000, justified by various changes ordered by the Air Force.

In the fall of 1949, the first XB-48 was cannibalized to provide parts for the second XB-48. The latter aircraft was scheduled for many tests, including tests on the F-1 autopilot, jet engine cooling system, and a hydraulic system for jet engines. The proposed tests, however, were canceled. The Air Force decided to use the second XB48 as a test-bed for "bad weather" flight items, including a badly needed deicing system. Completion of the thermal anti-icing survey test program in mid-1951 paved the way for the second XB-48's end. In September, the aircraft was flown to Phillips Field, Aberdeen Proving Ground, Maryland, where the strength of the XB-48 structure was tested until the aircraft was totally destroyed.

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