|First prototype, 46-685 during testing|
|Manufacturer||Glenn L. Martin Company|
|First flight||28 October 1949|
|Retired||25 March 1956|
|Status||Canceled in 1952|
|Primary user||United States Air Force|
US$12.6 million for the program
The Martin XB-51 was an American trijet ground-attack aircraft. It was designed in 1945 and made its maiden flight in 1949. It was originally designed as a bomber for the United States Army Air Forces under specification V-8237-1 and was designated XA-45. The "A" ground-attack classification was eliminated the next year, and the XB-51 designation was assigned instead. The requirement was for low-level bombing and close support. The XB-51 lost out in evaluation to the English Electric Canberra which - built by Martin - entered service as the Martin B-57 Canberra.
Design and development
This unorthodox design, first flying on 28 October 1949, was fitted with three General Electric J47 engines - an unusual number for a combat aircraft - two underneath the forward fuselage in pods, and one at the extreme tail with the intake at the base of the tailfin. The innovative, variable incidence wings, swept at 35° and with 6° anhedral, were equipped with leading edge slats and full-width flaps. Spoilers gave most of the roll control and undersized ailerons provided feel for the pilot. The combination of variable incidence adjustment and slotted flaps allowed for a shorter takeoff run. Four 954 lb (4.24 kN) thrust Rocket-Assisted Take Off (RATO) bottles with a 14-second burn duration could be fitted to the rear fuselage to improve takeoff performance. Spectacular launches were a feature of later test flights.
The main landing gear consisted of dual wheel sets in tandem in the fuselage, similar to the Boeing B-47 Stratojet, with outrigger wheels at the wingtips (originally proved on a modified Martin B-26 Marauder named "Middle River Stump Jumper"). The B-51 was a large but aerodynamically "clean" design which incorporated nearly all major systems internally. The aircraft was fitted with a rotating bomb bay, a Martin trademark; bombs could also be carried externally up to a maximum load of 10,400 lb (4,700 kg), although the specified basic mission required only a 4,000 lb (1,814 kg) bombload. Eight 20 mm cannon mounted in the nose would have been installed in production aircraft.
Crew provision was for a pilot under a "fighter"-type bubble canopy and a Short-range navigation and bombing system (SHORAN) operator/navigator in a compartment located lower than and to the rear of the cockpit (only a small observation window was provided). Both crew members were provided with a pressurized, air conditioned environment, equipped with upward-firing ejection seats. The XB-51 was the first Martin aircraft equipped with ejection seats, these being of their own design.
In 1950, the United States Air Force issued a new requirement based on early Korean war experience for a night intruder/bomber to replace the Douglas A-26 Invader. The XB-51 was entered, as well as the Avro Canada CF-100 and English Electric Canberra; the XB-51 and Canberra emerged from these as the favorites.
Test flights showed the XB-51 to be highly maneuverable at low altitudes and substantially faster than the Canberra (i.e. its "turn-of-speed" was faster than most fighter aircraft of the era). However, the XB-51's endurance was substantially poorer than that of the Canberra and this factor was decisive in its cancellation. In addition, a load limiting factor of only 3.67 g (36 m/s2) meant that the general strength of the airframe – especially its load-bearing capabilities – were relatively poor and would prevent tight turns while fully loaded. Additionally, the tandem main gear plus outriggers of the XB-51 were thought unsuitable for the requirement to fly from emergency forward airfields.
While the XB-51 was not selected for procurement, it was decided that Martin would build 250 Canberras under license, under the designation B-57. Furthermore, Martin's rotating bomb bay would be incorporated into production variants of the B-57. A "Super Canberra", incorporating other XB-51 features, such as swept wings and tail-planes, was also proposed. This aircraft – although it promised much better speed and performance than the B-57 – never reached the prototype stage, mainly because the many changes would have taken too long to implement and test, before it could be put into production.
Flights by the XB-51 prototype, 46-685, continued, for general research purposes, following the project's official cancellation by the USAF. A second prototype, 46-686, which first flew in 1950, crashed during low-level aerobatics on 9 May 1952. 46-685 continued to fly, including an appearance in the film Toward the Unknown as the "Gilbert XF-120" fighter.[note 1] The surviving prototype was en- route to Eglin AFB to shoot additional footage when it crashed during takeoff, following a refueling stop in El Paso, Texas, on 25 March 1956.
Data from U.S. Standard Aircraft Characteristics
- Crew: 2
- Length: 85 ft 1 in (25.9 m)
- Wingspan: 53 ft 1 in (16.2 m)
- Height: 17 ft 4 in (5.3 m)
- Wing area: 548 ft² (50.9 m²)
- Empty weight: 30,906 lb (14,019 kg)
- Loaded weight: 57,874 lb (26,251 kg)
- Max. takeoff weight: 62,452 lb (28,328 kg)
- Powerplant: 3 × General Electric J47-GE-13 turbojets, 5,200 lbf (23 kN) each
- Maximum speed: 644 mph (560 kn, 1,037 km/h) at sea level
- Range: 1,075 mi (934 nmi, 1,730 km)
- Ferry range: 1,444 mi (1,255 nmi, 2,324 km)
- Service ceiling: 41,750 ft (12,725 m)
- Rate of climb: 6,600 ft/min (33.5 m/s)
- Wing loading: 105.6 lb/ft² (515.6 kg/m²)
- Thrust/weight: 0.27
- Guns: 8 × 20 mm M24 cannon (0.79 in) cannon with 1,280 rounds
- Rockets: 8 × High Velocity Aerial Rockets (HVAR) or
- Bombs: Up to 10,400 lb (4,720 kg) carried internally
Aircraft of comparable role, configuration and era
- A few seconds of test flight footage of an XB-51 also appeared in the 1951 Tales of Tomorrow episode "Plague From Space". Note: Although the XB-51 did not receive an official name, "Panther" had been suggested by the company.
- Knaack, Marcelle Size. Post-World War II bombers, 1945-1973. Washington, DC: Office of Air Force History, 1988. ISBN 0-16-002260-6.
- Winchester 2005, p. 144.
- The Martin XB-51, Air Force Legends Number 201, Scott Libis, published by Steve Ginter 1998, ISBN 0-942612-00-0, p.5
- Winchester 2005, p. 145.
- "Pivoting Bomb-bay Door Permits Accurate Drops at High-Speeds." Popular Mechanics, February 1954, p. 126.
- Tuttle, Jim. Eject! The Complete History of U.S. Aircraft Escape Systems. St. Paul, Minnesota: MBI Publishing Company, 2002. ISBN 0-7603-1185-4.
- "Standard Aircraft Characteristics: XB-51" (PDF). US Air Force. 11 July 1952. Retrieved 8 June 2017.
- Andrade, John M. U.S. Military Aircraft Designations and Serials since 1909. Earl Shilton, Leicester, UK: Midland Counties Publications, 1979. ISBN 0-904597-22-9.
- Boyne, Walter. "Attack, The Story of the XB-51, Martin's Phantom Strike Ship!" Airpower, Volume 8, No. 4, July 1978.
- Jones, Lloyd S. U.S. Bombers, B-1 1928 to B-1 1980s. Fallbrook, California: Aero Publishers, 1962, second edition 1974. ISBN 0-8168-9126-5.
- Mikesh, Robert C. 'B-57 Canberra At War 1964-1972. London: Ian Allan, 1980. ISBN 0-7110-1004-8.
- Winchester, Jim. "Martin XB-51." Concept Aircraft: Prototypes, X-Planes and Experimental Aircraft. Kent, UK: Grange Books plc., 2005. ISBN 978-1-84013-809-2.
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