J-58 Engine Inlet Spike

This sharp, pointed moveable cone called a "spike" was located in the engine inlet. It was locked in the full forward position on the ground or when in subsonic flight. During acceleration to high-speed cruise, the spike would unlock at Mach 1.6 and then begin a mechanical (internal jackscrew powered) travel to the rear. It moved back up to a maximum of 26 inches. By moving aft, the spike tip would "capture" the shock wave, forcing it closer to the inlet cowling until it just touched slightly inside the cowling lip. In this position the shock-wave was reduced and slowed to subsonic speeds. In doing so, shock pressures were maintained while slowing the air until a Mach 1 shock wave formed in front of the engine compressor. The backside of this "normal" shock wave was subsonic air for ingestion into the engine compressor. This capture of the Mach 1 shock wave within the inlet was called "Starting the Inlet." When the spike was in the wrong position and lost the shock wave, it was called an "unstart" and that was an excitng moment in the cockpit, as the engine lost thrust and the pilot's head was thrown left and right, often off the inside of the canopy.