NASA's YF-12A
During mid-1967, Defense Affairs assisted Mr. Charles W. Harper, Deputy Associate Administrator for Advanced Research and Technology (Aeronautics), in lengthy negotiations to obtain release from the Air Force of certain technical data on the F-12/SR-71 aircraft series needed by NASA to fulfill effectively its research role in aeronautics and its supporting role in the development, under government programs, of both military and civilian supersonic aircraft. DOD and the Air Force concurred in the desirability of making the cats available to NASA, but difficulty was encountered in persuading the manufacturer to release the data, which were being held closely as proprietary information. NASA had formulated plans for studies of the F-12/SR-71 aircraft with the objectives of (1) helping to establish correlation between flight and propulsion system performance and predictions based upon ground facility tests and analog simulation, (2) assessing the technology advances as represented by these aircraft. and (3) identifying problem areas that had not been anticipated in early studies of other supersonic aircraft development programs.
Two YF-12 aircraft were flown in a joint Air Force-NASA research program at the NASA Flight Research Center (now the NASA Dryden Flight Research Center) between 1969 and 1979, although the second plane, piloted primarily by the Air Force, was lost to an in flight fire in 1971. The two YF-12 aircraft bore the serial numbers 60-6935 and 60-6936.
The NASA Dryden Flight Research Center (NASA-DFRC) 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.
Two YF-12s were flown in a joint Air Force-NASA research program at the NASA Flight Research Center (after 1976, the NASA Dryden Flight Research Center) between 1969 and 1979. A third shared plane, piloted primarily by the Air Force, was lost to an in-flight fire in 1971. The Air Force offered NASA the use of two YF-12A aircraft, 60-6935 and 60-6936. 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 in flight 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 YF-12C was delivered to NASA on 16 July 1971. From then until 22 December 1978, it made 90 flights. 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).
In fact, the "YF-12C" was a then-secret SR-71A (serial no. 61-7951) given the NASA tail no. 60-6937. The reason for this bit of subterfuge lay in the fact that NASA, while flying the YF-12A interceptor version of the aircraft, was not allowed to possess the strategic reconnaissance version for some time. The bogus tail number actually belonged to a Lockheed A-12 (serial no. 60-6937), but the existence of the A-12 remained classified until 1982. The tail number 06937 was selected because it followed in the sequence of tail numbers assigned to the three existing YF-12A aircraft: 06934, 06935, and 06936.
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, and 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. The YF-12C was flown only by Fulton, Mallick, Horton and Young during its NASA research missions.
The YF-12 allowed NASA researchers at all four of the agency's aeronautical centers (Langley, Lewis [now Glenn], and Ames as well as the Flight Research Center) to study the thermal, structural, and aerodynamic effects of sustained, high-altitude, Mach 3 flight. Painted flat black, the YF-12 was fabricated primarily from titanium alloy, which enabled it to withstand skin temperatures of over 500º F.
Unless grounded for maintenance or modification, the YF-12s flew nearly every week for most of the program's lifespan. The fiery end of 936 on the desert floor was the program's only crash, but flight crews were forced to make emergency landings at least twice because of in-flight problems. The planes were also prone to an airflow problem involving the engine inlets called an "unstart," which caused a thrust imbalance and resulted in violent yawing.
The YF-12's ability to sustain a cruise speed of greater than Mach 3 allowed NASA to expand its research capabilities. A large amount of flight research was performed in aerodynamics, propulsion, controls, structures, subsystems and other areas such as the physics of the upper atmosphere, noise tests and measurements, and handling qualities. The YF-12 flight research data was augmented by a series of wind tunnel tests, laboratory experiments, and analyses. As a result, the combined ground/flight research generated vast amounts of information that was later incorporated into the design of other supersonic aircraft. The program yielded over 125 technical reports.
YF-12 flight tests included propulsion studies, investigations of a flight path oscillation known as phugoid, studies of the plane's loads and handling capabilities, and performance tests that involved flights with the ventral fin removed. Other research included the use of attached vanes to investigate airflow and wind gusts, studies of jet wake dispersion, engine stalls, elevation-hold at high Mach speeds, boundary layer noise, and the effect of a boattail design on drag.
Although it yielded large amounts of research data, the YF-12 program was terminated in the late 1970s when NASA's research agenda shifted from speed to efficiency. During its nine-year life, the YF-12 research program logged 297 flights in the joint NASA-Air Force program and approximately 450 flight hours. The program was ordered terminated in 1977, but NASA used some residual funding to keep the project alive into 1979. Plane 935 made its last NASA flight on October 31, 1979. On November 7, 1979, it was ferried by an Air Force crew to the Air Force Museum at Wright-Patterson Air Force Base in Dayton, OH. Only one YF-12 remains in existence. It is displayed at the U.S. Air Force Museum at Wright-Patterson Air Force Base in Dayton, OH.
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