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 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.
Under its research agreement with NASA, the Air Force provided the agency with two YF-12As in 1969. On June 24, 1971, one of the planes experienced an in-flight fuel line failure that led to a fire in the right engine. Unable to save the smoking aircraft, Air Force pilot Lt. Col. Ronald Layton and fire control officer Major Billy A. Curtis ejected and were not injured, but the YF-12A was lost in a fiery explosion in the desert. The plane was replaced by a "YF-12C." The YF-12C (so-called) differed from the YF-12A in that the A-model had a round nose while the C-model had its chine carried forward to the nose of the airplane (see three-views below). There were other differences in internal and external configuration, but the two aircraft shared common inlet designs, structural concepts, and subsystems.
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 History of NASA's YF-12 Project
On July 18, 1969, NASA and the Air Force announced joint involvement in a YF-12 research program. The agendas differed, with the Air Force focusing on combat research and NASA engineers initially focusing on a study of flight loads and structural heating. Much of the NASA research was concerned with the viability and development of supersonic cruise aircraft. Two YF-12As (tail numbers 935 and 936) were removed from Air Force storage for the program. On December 11, 1969, 935 successfully made its first flight as a NASA-USAF research plane and inaugurated the program. On June 24, 1971, 936 experienced the fuel line failure described above.
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.
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.