I've been reading about the SR-71 and the J58 engine and I'm confused about what I've read about the inlet spike. The spike would start out extended 3 feet but as speed (Mach) went up it would retract, up to 26 inches when at Mach3.2. This confuses me as I thought you'd always want the shockwave to be at the lip of the engine inlet. As Mach goes up the shockwave angle increases so that in order to have the shockwave at the lip of the inlet, I'd think you would have to Extend the spike, not retract it. Maybe the lip of the inlet on the aircraft is still some distance away from the compressor face, Maybe its okay to have the initial shockwave in that inlet, because it will have reflection between the inlet wall and the back side of the spike. So many reflections that the shockwave will have 2 or more decelerations before the air becomes subsonic--at the compressor face. Anybody know about this or know where I can see Schlieren photos of this interaction?