With the YF-12, NASA researchers hoped to establish a technology base for the design of an efficient propulsion system for supersonic cruise aircraft, such as a Supersonic Transport (SST). Primary areas under investigation included inlet design analysis, propulsion system steady-state and dynamic performance, inlet engine control systems, and airframe/propulsion interactions. During the YF-12 research program, unscheduled unstarts were common on any given mission. Researchers studied the phenomenon of unstarts by inducing them intentionally and then using manual techniques and automatic systems to restart the engine. As a result of these important investigations into spike schedule refinements (coordinating spike position to retain the shock wave in the inlet) and hardware improvements, unstarts became a rare occurrence.
After entreaties from NASA officials, Air Force representatives agreed to loan NASA an SR-71A. For political reasons the aircraft’s identity was hidden under the fictitious designation “YF-12C.” By May 1971, Lockheed technicians undertook an inspection of the YF-12C in preparation for its addition to the NASA research program. They completed their work on the airplane by the middle of June, and prepared it to join the YF-12A aircraft already in NASA service. In June 1971, the program suffered a setback when one of the YF-12A aircraft was lost in a non-fatal accident. After the airplane caught fire due to a leaking fuel line, the crew ejected safely but the airplane plunged into the desert and was completely destroyed. While the remaining YF-12A continued to serve as a loads testbed, the YF-12C arrived at the FRC in July 1971. Propulsion research flights did not begin, however, until June 1972.
Propulsion research using the YF-12C included airspeed calibrations, collection of baseline data, and data collection at numerous flight conditions. To gather data on propulsion system performance, pilots performed such maneuvers as level accelerations and decelerations, constant power turns, and airspeed lag calibration roller coaster maneuvers. They also gathered data on engine bypass door and inlet spike performance and established speed-power points. Finally, they performed constant-speed climbs and descents at specific KEAS or Mach numbers and constant-power turns. As the crews operated the engine inlet controls in manual and automatic modes, instruments measured oscillations known as phugoids.