Martin XB-51

XB-51
First prototype, 46-685 during testing
Role	Bomber
Manufacturer	Glenn L. Martin Company
First flight	28 October 1949
Retired	25 March 1956
Status	Cancelled in 1952
Primary user	USAF (NACA)
Number built	2
Unit cost	US$12.6 million for the program[1]

The Martin XB-51 was an American "tri-jet" ground attack aircraft designed to a 1945 United States Army Air Forces requirement. It was originally designed as an attack aircraft by the Air Force 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 entered service as the B-57.

Design and development

Both prototypes taking off in formation

Testing RATO

The resulting unorthodox design, first flying on 28 October 1949, was (unusually for a combat aircraft) fitted with three engines, General Electric J47s in this case: one at the extreme tail with an intake at the base of the tailfin, and two underneath the forward fuselage in pods.^[2] The innovative wings, swept at 35° and with 6° anhedral, were equipped with variable incidence, leading-edge slots, full-width flaps and spoilers instead of ailerons. The combination of variable incidence adjustment and slotted flaps allowed for a shorter takeoff run.^[3] 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 further improve takeoff performance. Spectacular launches were a feature of later test flights.^[2]

The main landing gear consisted of dual sets of wheels in tandem in the fuselage, similar to the B-47 Stratojet, with outrigger wheels at the wingtips (originally proved on a modified B-26 Marauder named "Middle River Stump Jumper"^[2]). The B-51 was a large but aerodynamically "clean" design which incorporated nearly all major systems internally.^[3] 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 only required a 4,000 lb (1,814 kg) bombload. Eight 20 mm (.79 in) cannons mounted in the nose would have been installed in production aircraft.^[3]

Crew provision was for a pilot under a "fighter"-type bubble canopy and a SHORAN (short-range navigation and bombing system) operator/navigator in a compartment located lower than and to the rear of the cockpit (only a small observation window was provided).^[3] Both crew members were provided with a pressurized, air-conditioned environment, equipped with upward-firing ejection seats.^[3] The XB-51 was the first Martin aircraft equipped with ejection seats; the ejection seats being of their own design.^[4]

Operational history

A screen image from Toward the Unknown

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 A-26 Invader. The XB-51 was entered, as well as the Avro Canada CF-100 and the English Electric Canberra. The Canberra and XB-51 emerged as the favorites. The XB-51 was a highly maneuverable aircraft at low level, and substantially faster than the Canberra (its "turn-of-speed" was faster than most fighter aircraft of the era^[3]). However, its load limiting factor of only 3.67 g (36 m/s²) restricted tight turns, and the XB-51's endurance was substantially poorer than the Canberra's; this latter proved to be the deciding factor. Additionally, the tandem main gear plus outriggers of the XB-51 was thought unsuitable for the requirement to fly from emergency forward airfields.

The Canberra was selected for procurement and the XB-51 program ended. Martin did not end up the loser, however, for they were selected to build the 250 Canberras ordered under the designation B-57A. Furthermore, the rotating bomb bay was incorporated in the B-57. A proposed B-57 Super-Canberra also included XB-51 features, such as swept wing and tailplane. In the end it was never built, mainly because it was a new design and would have taken too long to put in production, although it promised much better speed and performance.^[2]

Flight testing for research purposes continued after program cancellation. The second prototype, 46-686, which first flew in 1950, crashed on 9 May 1952 during low-level aerobatics. The first prototype, 46-685 continued to fly, including appearing in the film Toward the Unknown as the "Gilbert XF-120" fighter,^[5] then continuing its aerodynamic and weapons-handling tests until it crashed during takeoff on 25 March 1956.^[3]

Specifications (XB-51)

General 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: 29,584 lb (13,419 kg)
• Loaded weight: 55,923 lb (25,366 kg)
• Max takeoff weight: 62,457 lb (28,330 kg)
• Powerplant: 3 × General Electric J47-GE-13 turbojets

Performance

• Maximum speed: 645 mph (1,040 km/h)
• Range: 1,075 mi (1,730 km)
• Ferry range: 1,613 mi (2,596 km)
• Service ceiling: 40,500 ft (12,300 m)
• Rate of climb: 6,980 ft/min (35.5 m/s)
• Wing loading: 102 lb/ft² (498 kg/m²)
• Thrust/weight: 0.28

Armament

• Guns: 8 × 20 mm (0.79 in) cannon with 1,280 rounds
• Rockets: 8 × High Velocity Aerial Rockets (HVAR)
• Bombs: 2,000 (907 kg)

See also

Aircraft of comparable role, configuration and era

• English Electric Canberra
  

Related lists

• List of attack aircraft
• List of bomber aircraft
• List of military aircraft of the United States
  

References

Notes

1. ^ Knaack, Marcelle Size. Post-World War II bombers, 1945-1973. Washington, DC: Office of Air Force History, 1988. ISBN 0-16-002260-6.
2. ^ ^{a b c d} Winchester 2005, p. 144.
3. ^ ^{a b c d e f g} Winchester 2005, p. 145.
4. ^ Tuttle, Jim. Eject! The Complete History of U.S. Aircraft Escape Systems. St. Paul, Minnesota: MBI Publishing Company, 2002. ISBN 0-7603-1185-4.
5. ^ 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.

Bibliography

• 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, CA: Aero Publishers, 1962, second edition 1974. ISBN 0-8168-9126-5.
• Winchester, Jim. "Martin XB-51." Concept Aircraft: Prototypes, X-Planes and Experimental Aircraft. Kent, UK: Grange Books plc., 2005. ISBN 1-84013-309-2.
• Robert C. Mikesh. "B-57 Canberra At War 1964-1972". London, UK: Ian Allan, 1980 ISBN 0-7110-1004-8

External links

• USAF Museum: XB-51
• Several photographs of the Martin 234 XB-51 46-685
• Toward The Unknown at the Internet Movie Database