The Lockheed YF-12 is a prototype high-performance interceptor aircraft designed by the United States. It is one of the developments of the Lockheed A-12 Oxcart program launched by the CIA in the late 1950s, which notably led to the famous SR-71 Blackbird spy plane.
Despite good results during flight tests, the YF-12 was never mass-produced: its only real use was participation in a research program conducted by NASA until 1979. In September 1959, for budgetary reasons, the US Air Force abandoned the XF-108 Rapier project, an interceptor capable of flying at Mach 3 and intended to replace the F-106 Delta Dart. Lockheed then proposed a derivative of the Lockheed A-12 Oxcart spy plane, arguing that the essential design elements of the A-12 had already been developed and therefore the aircraft would be less expensive. In October 1960, it was decided to take three aircraft from the A-12 assembly line and convert them into YF-12 prototypes.
The nose was modified to accommodate the Hughes AN/ASG-18 radar, originally developed for the F-108 and with a range exceeding 320 km. The size of the radome necessitated reducing the wing extensions, which were truncated compared to those of the A-12. A second cockpit for the gunnery officer was added. The bays containing the A-12's reconnaissance equipment were modified to carry three Hughes AIM-47A Falcon air-to-air missiles (also initially intended for the F-108), with a range of 180 km and capable of carrying a nuclear warhead. The modifications to the aircraft's aerodynamics required the addition of ventral fins under the fuselage and engine nacelles to maintain stability. The first of three YF-12s made its maiden flight on August 7, 1963. Missile drop tests, followed by live-fire tests against remotely operated targets, began in 1964: 6 of the 7 live-fire tests were successful, including one launched from a YF-12 flying at Mach 3.2 and 23,000 m at a target flying significantly lower. On May 1, 1965, a YF-12 set a simultaneous speed and altitude record, reaching 3,330 km/h at 24,462 m.
While the USAF was ready to sign a production contract for about 100 aircraft, the Secretary of Defense refused to commit the funds and instead financed the F-106X (an F-106 Delta Dart with a new radar and other improvements that promised to be even cheaper… but ultimately failed). The F-12 project was therefore abandoned in early 1968.
In 1969, two YF-12s were loaned to NASA for a research program: exploration of phenomena encountered in high-speed flight (including thermal studies), aerodynamic studies, etc. The two aircraft completed nearly 300 test flights until 1979, when the program was terminated. The last flight of a YF-12 took place on November 7, 1979.
Of the three YF-12s built, one was lost following an onboard fire on June 24, 1971, one is on display at the USAF Museum, and the last was used to build the sole SR-71C (a two-seat trainer).
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