Two B-52Es were modified for use as engine test beds. One aircraft (B-52E-55-BW, S/N 57-0119) was used to test the General Electric TF-39 for the Lockheed C-5A Galaxy program. The TF-39 was mounted on the right inboard engine pylon in place of the two J57s normally installed. The single TF-39 turbofan, rated at about 40,000 pounds, had as much thrust as four J57 turbojets on a standard production B-52E.
Another aircraft (B-52E-85-BO, S/N 56-0636) was similarly modified to test the JT9D turbofan engine for the Boeing 747 program.
Various design studies were done to investigate the feasibility of modifying the B-52 airframe for the large turbofan engine. The most common design called for a four-engine configuration, but some had as many as eight engines. None of these designs were ever proceeded with; however, the B-52H did have upgraded TF-33 turbofan engines installed in place of the J57s. The TF-33 was much more powerful than the J57 it replaced, but nowhere near as powerful as the TF-39 or JT9D.
Engine test bed aircraft
TECHNICAL NOTES: Engines: Six Pratt & Whitney J57-P-19W turbojets of 10,500 lbs. thrust each (dry), 12,100 lbs. thrust each wet (water injection) and one General Electric TF-39 turbofan of 40,000 lbs. thrust (or one Pratt & Whitney JT9D turbofan) Maximum speed: 638 mph/551 knots at 20,000 ft. Cruising speed: 526 mph Range: 3,305 nautical miles combat radius at combat weight of 293,100 lbs. (without in-flight refueling); 8,338 miles maximum ferry range Service ceiling: 46,200 ft. at combat weight of 293,100 lbs. Span: 185 ft. 0 in. Length: 156 ft. 7 in. Height: 48 ft. 4 in. Serial numbers: B-52E-85-BO (56-0636) and B-52E-55-BW (57-0119) (S/N 56-0636 was used to test the Pratt & Whitney JT9D for the Boeing 747 program, and S/N 57-0119 was used to test the General Electric TF-39 for the Lockheed C-5A program)