The military J75 is similar to the commercial JT4A-11 that powers some variants of the Boeing 707 and Douglas DC-8. However, most of these early aircraft employed the less powerful Pratt & Whitney JT3C engine, which is basically a civil version of the military J57 turbo jet used by such aircraft as the Boeing B-52 bomber and the North American F-100 fighter.
The Republic F-105 Thunderchief was the first supersonic tactical fighter-bomber developed from scratch. The Air Staff endorsed the F-105 in May of 1952 instead of ordering an improved version of the F-84F. In an August 1953 warning, the National Advisory Committee for Aeronautics informed Republic that the overall efficiency of the J-71 engine was only 80 percent of that predicted by the contractor. Interim use of the Pratt & Whitney J57 engine was then considered. On 10 August 1954, the Air Force authorized the modification of four F-105s "as required to incorporate the YJ75-P-1 in lieu of the J57-P-25 engine." Finally, the USAF dictated that the higher-thrust J75 engine be installed to qualify the fighter-bomber for first-line service from 1958 through the sixties. On 22 October 1954, the YF-105A started a 45-minute flight, during which time it managed to exceed mach 1 despite the limited power of the J57 engine.
The first flight of the YF-105B powered by the J75 took place on 26 May 1956. The wing was highly swept and incorporated low-speed ailerons and high-speed spoilers for lateral control, and a droop-snoot leading edge. With the designation F-105B came the engine change to a Pratt & Whitney J75-P-3. Other changes were made in this model too, including the use of a unique type of swept-forward air intake to control the shock-wave and introduction of "area rule" on the fuselage.
If any single factor won over even the hardest-to-convince, it was the F-105's ruggedness that enabled it to sustain extensive battle damage and return the pilot to friendly territory, an ability owed largely to its tough J75 engine and dry wing. The F-105 wing, unlike that of many contemporary aircaft, contained no fuel tanks, thus sharply reducing the vulnerability area of the aircraft. Ever since Vietnam, everyone involved with the F-105 has called it by the affectionate nickname Thud.
In early 1956, at Edwards AFB, the F-105 was named overall winner in a competitive flyoff against the North American XF-107, a J75-powered derivative of the F-100 Super Sabre.
The F-106 was carefully area ruled to reduce the drag rise accompanying an increase in Mach number from subsonic to supersonic values. This careful attention to drag reduction, together with the large 24 000-pound-thrust (with afterburning) Pratt & Whitney J75 turbojet engine, gave the Delta Dart a maximum speed of 1525 miles per hour (M=2.31) at 40000 feet and the capability of climbing to its combat ceiling of 51,800 feet in 6.9 minutes.
The history of the U-2 demonstrates CIA flair and imagination. The CIA adopted the J75 engine for better altitude, while the USAF did not. CIA had to give two J75-equipped U-2's to SAC for Cuban reconnaissance. The U-2R, first flown in 1967, was 40 percent larger than the original U-2 designed by Kelly Johnson in the mid fifties. The J75-P-13 of 17,000 lbs. thrust replaced the Pratt & Whitney J57-P-37A of 11,000 lbs. thrust for later models.
The ER-2 is a civilian version of the Air Force's U2-R reconnaissance aircraft. Powered by one Pratt & Whitney J75-P-13B , the Lockheed ER-2 was developed for NASA to serve as a high-altitude scientific research platform.
More recently the U-2 and ER-2 aircraft were refitted with a General Electric F-118-101 turbofan engine that burns less fuel, cuts weight and increases power over the previous Pratt & Whitney J75 engine. Other upgrades include a cockpit modernization program, complete rewire to improve power distribution and reduce electromagnetic interference, various sensors upgrades, and adding the Global Positioning System that will superimpose geo-coordinates directly on collected images.
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