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Testing and proving the complex propulsion system of the Blackbird would continue for several years after the SR-71 was accepted into operational service, especially to overcome the violent eddies blocking airflow at the air-intake, accompanied by loud bangs (rumbling) which was traced to popping of the inlet shock. Trials were conducted until the start of the 1970s by NASA on the YF-12A and YF-12C. Specifically the introduction of SAS (stability augmentation system) and digital aircraft flight and inlets control systems (DAFICS) for the propulsion units allowed for improvement in engine functioning and thus aircraft handling…
According to NASA the analyses confirmed that an integrated flight and engine control system would, from then on, have a major influence on the flight characteristics of advanced-technology aircraft flying at high supersonic speeds. Tests carried out in 1978 showed that in-flight regulation of air-inlet and nozzle exhaust flow brought with it a reduction in the intake air withdrawal effect, and also permitted extension of operating range and shortening the time needed for crew training.
Shortly after development commenced, a wager was opened between Kelly Johnson and Bill Brown, over which of the two companies would succeed in producing the best fuel-injection system for the aircraft. It was settled that the loser would have to carry the costs for use of the wind-tunnel, which Brown estimated would be about 12,000 $. Pratt & Whitney won and true to his promise, Kelly Johnson sent a check of sufficient value. The event was used to play a prank. Bill Brown acquired a complete naval sailor's uniform, with cap and blue jacket, and thus attired had himself photographed in front of the 12.5 meters long private yacht of Bill Gordon. He sent the photograph to Johnson with the comment "thanks for the check". The photograph hung for many years afterwards on the wall of Lockheed's staff reunion room.
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