Stock Photo - The so_called YF_12C on the NASA Flight Research Center ramp. Following the loss of a YF_12A in a non_fatal accident in June 1971, NASA acquired the second production SR_71A 61_7951 from the Air Force. Because the SR_71 program was shrouded in the highest secrecy, the Air Force restricted NASA to using the aircraft solely for propulsion testing with YF_12A inlets and engines. It was designated the YF_12C, and given a bogus tail number 06937. The two YF_12As in the program had actual tail numbers 06935 and 06936. The first NASA flight of the YF_12C took place on 24 May 1972.The Flight Research Center´s involvement with the YF_12A, an interceptor version of the Lockheed A_12, began in 1967. Ames Research Center was interested in using wind tunnel data that had been generated at Ames under extreme secrecy. Also, the Office of Advanced Research and Technology OART saw the YF_12A as a means to advance high_speed technology, which would help in designing the Supersonic Transport SST. The Air Force needed technical assistance to get the latest reconnaissance version of the A_12 family, the SR_71A, fully operational. Eventually, the Air Force offered NASA the use of two YF_12A aircraft, 60_6935 and 606936. A joint NASA_USAF program was mapped out in June 1969.NASA and Air Force technicians spent three months readying 935 for flight. On 11 December 1969, the flight program got underway with a successful maiden flight piloted by Col. Joe Rogers and Maj. Gary Heidelbaugh of the SR_71/F_12 Test Force. During the program, the Air Force concentrated on military applications, and NASA pursued a loads research program. NASA studies included inflight heating, skin_friction cooling, coldwall research a heat transfer experiment, flowfield studies, shaker vane research, and tests in support of the Space Shuttle landing program.Ultimately, 935 became the workhorse of the program, with 146 flights between 11 December 1969 and 7 November 1979. The second YF_12A, 936, made 62 flights. It was lost in a non_fatal crash on 24 June 1971. It was replaced by the YF_12C.The Lockheed A_12 family, known as the Blackbirds, were designed by Clarence Kelly Johnson. They were constructed mostly of titanium to withstand aerodynamic heating. Fueled by JP_7, the Blackbirds were capable of cruising at Mach 3.2 and attaining altitudes in excess of 80,000 feet. The first version, a CIA reconnaissance aircraft that first flew in April 1962 was called the A_12. An interceptor version was developed in 1963 under the designation YF_12A. A USAF reconnaissance variant, called the SR_71, was first flown in 1964. The A_12 and SR_71 designs included leading and trailing edges made of high_temperature fiberglass_asbestos laminates.The NASA YF_12 research program was ambitious, the aircraft flew an average of once a week unless down for extended maintenance or modification. Program expenses averaged $3.1 million per year just to run the flight tests. NASA crews for the YF_12 included pilots Fitzhugh Fulton and Donald Mallick, anf flight test engineers Victor Horton and Ray Young. Other NASA test pilots checked out in the YF_12A included John Manke, William Dana, Gary Krier, Einar Enevoldson, Tom McMurtry, Steve Ishmael, and Michael Swann. Only Fulton, Mallick, Ray, and Horton flew the YF_12C.