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SR-71 Facts: 29 Details That Might Surprise You

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Sympathetic Unstart

SR-71 Facts
NASA

It was critical that the shockwaves stay inside the inlet. If a disturbance allowed shockwaves to exit the inlet, it could cause an one engine/inlet to produce 80% thrust while the other only produces 20% thrust. This resulted in an unstart in one of the engines that sent the plane into a violent slice. To combat this, a program was developed that forced the other engine to unstart sympathetically in order to keep thrust symmetrical. Theoretically, this forced both the spikes to return to their subsonic position and level the aircraft.