The B-47C was the initial designation for a design project initiated in January 1950 to redesign the aircraft for four Allison J35-A-23 turbojets in place of the six General Electric J47s. The B-47C was redesignated the XB-56 and subsequently cancelled because of problems with development and a shift of interest towards the budding B-52 program.
However, during its existence, the B-47C program was based closely on the design of the B-47B. The difference was that the B-47C was to receive four Allison J35-A-23 turbojets in place of the six General Electric J47s. The C model configuration was nearly identical to the B-47B with the exception of the inboard engine nacelles -- one J35 was to be mounted in place of the two J47s on the production B model.
Features incorporated in the B-56A preliminary design included cabin pressurization, thermal anti-icing, bombing-navigation radar, gun laying radar, anti-skid brakes, bail-out spoiler door, internal fuel purging, camera provisions, braking parachute, single point refueling, air refueling provisions and a maximum fuel capacity of 17,350 gallons.
One B-47B-20-BW (S/N 50-082) was scheduled for modification and the first flight was estimated for April 1951. The first flight date of April 1951 was delayed because of problems with the new engine system. They did not produce as much thrust as first estimated. The original Allison J35-A-23s turbojets had estimated performance of 9,700 pounds thrust each at maximum power (with afterburners) and a normal power rating of 8,200 pounds thrust each. The actual J35 performance was no where near the estimates (about 7,400 pounds maximum power and 5,600 pounds normal) so alternative engines were proposed. The J35 was phased out of production in 1956, but was followed by several illustrious successors.
Because of delays and problems with the engines, a number of alternatives were proposed. The Allison J71-A-5 was the first engine considered for replacing the J35. The J71 had maximum thrust above 10,000 pounds at military power but other problems and delays forced it to be dropped from the B-47C project as well. The 9700-pound thrust, 16-stage axial-flow engine was 16 feet long, 4 feet in diameter, and weighed 4450 pounds. The J71 replaced the Westinghouseâ€™s J40 on several Navy fighters.
Another program also encountered problems with the J71. The 1954 test flight of the first B-66 variant, designated RB-66A, showed numerous aircraft handling deficiencies. The aircraft's major problem, which continued to plague it until its retirement in 1974, was engines. The manufacturer, Douglas Aircraft, favored the Pratt and Whitney J57, but, because it was earmarked for several other aircraft, the USAF instead settled for the Allison J71. The engine, used in the B/RB-66 fleet, produced an under-powered aircraft. The navy did switch to the J71, and the F-3H became one of the navy’s primary fighters until the early 1960s.
The Pratt & Whitney J57 was evaluated for the B-56 project, but it was still in development and delivery delays were unacceptable. The J57 was so far ahead of the competition that virtually every aircraft manufacturer in the United States designed an airplane around them. Some improvements were made to the turbojet engine in the postwar years, but its capabilities advanced exponentially from 1948 through 1955. It was not a great leap from Wright’s R-3350 3,500-pound-thrust piston engine to General Electric’s (GE’s 4,000-pound I-40 turbojet. New engines, such as Pratt & Whitney’s J57, however, produced 13,500 pounds of thrust.
The most powerful and successful of the postwar jet engines was the Pratt & Whitney J57. It was the first engine to exceed 10,000 pounds of thrust and was used to power the military’s premier aircraft in the 1950s and 1960s, including the B-52 Stratofortress, the F-100 Super Sabre, the Lockheed U-2, and the F-102. A variant of the J57, the JT3, was used on Boeing’s 707 and Douglas’s DC-8 commercial airliners.
The J57 was a twin-spool, axial flow configuration, a substantial departure from earlier centrifugal-flow designs. It was an immediate success, and its performance was described in superlavitves. Pratt & Whitney had leapfrogged the industry with its first turbojet design. In 1952, the J57 won the prestigious Collier Trophy for greatest achievement in American aviation. The military turned to the J57 for its fighter squadrons. In May 1953, a J57-powered North American F-100 Super Sabre became the first production aircraft to exceed the speed of sound in level flight, a feat accomplished on its maiden flight. The popular Convair F-102 Delta Dart was the next J57-powered aircraft. The Navy's Chance Vought F8U-1 used its power to set the first official speed record in excess of 1,000 miles per hour. Other aircraft included Lockheed's U-2 reconnaissance plane, the prototype of Republic's F-105 Thunderchief fighter-bomber and Northrop's Snark intercontinental guided missile.
The J57 was unique in that it had a dual axial-flow compressor that allowed it to break through the barrier that seemed to exist at 10,000 pounds of thrust. The engine employed two coaxial compressors, corresponding coaxial turbines, and a fixed-area nozzle. The inner spool had a seven-stage axial-flow compressor and a single-stage turbine. The outer spool had a nine-stage axial-flow compressor and two-stage shrouded turbine. The original version of the engine, the J57-P-1, was studied in late 1953 during the run-up to the first flight of the B-52 bomber. The B-52 had been significantly redesigned in 1949 to include the then-secret J57 engines.
The Allison J71-A-5 and the Pratt & Whitney J57 were the engines considered, but by the time testing came about, the interest in the program died due to shifted interest in the B-52 program (the B-52 eventually recieved the Pratt & Whitney J57 engines). The Boeing B-52 project was well underway by this time and had higher priority for the J57s when they became available. The B-52 was designed by Boeing in the late 1940s as a successor to the B-47 bomber and remains an important U.S. strategic bomber. Despite the design similarities, the B-52, powered by four pairs of Pratt & Whitney J57 engines, was a much heavier and faster aircraft.
The initial prototype conversion of the B-47B was first designated YB-47C then YB-56. A production version of the aircraft was designated B-56A and a photo reconnaissance version was designated RB-56A. The entire B-47C/B-56 project was canceled in December 1952 before conversion of the B-47B began. The unconverted B-47 was relegated to a ground instructional airframe and in the late 1960s was in use at the Naval Air Facility El Centro as an egress trainer.
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