XB-51 | |
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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.
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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]
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]
General characteristics
Performance
Armament
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