The engine inlet.. A very big part of the a-12 design was the air inlet system. This system must accommodate the needs of the J-58 turbo-ramjet engine, from subsonic speeds to more than Mach 3, at altitudes from sea-level to 90,000'. Quite a task, but Lockheed took it on and made it work.
The main parts of the inlet include the nacelle which houses the J-58 engine. The moving parts of the inlet include the spike, forward bypass doors, and the aft bypass doors. The spikes may be positioned manually, as well as the forward bypass doors.
If the system is working normally, the spike and forward bypass are operated in the auto mode. The aft bypass doors are only controlled manually. Since the inlet normally operates in the "auto" mode, the sequence starts after the airplane is supersonic and starting to intercept the climb schedule.
The airplane is normally taken off with a half fuel load, then we went a top-off tanker within a hundred mile of the base. This was always done.
Since the inlet is a "working" system, it is said to be "started" when you intercept 1.6 Mach on the climbout maneuver. In a way, anything that "started" can "unstart" the subject of my story.
Once upon a time, I was scheduled to fly a training sortie out of area 51, the home base. This sortie was planned to exercise both me, the airplane and the tanker force that supported us.
The mission called for an early launch from home, a top-off tanker nearby, then a long run to the west for another tanker near Hawaii. Of course this included an "accel" maneuver on the way west toward Hawaii.
This is where it got interesting. The "accel" maneuver went well, and I intercepted the climb schedule normally. The spikes started moving aft normally at Mach 1.6, the forward bypass doors opened and modulated normally too. Looks like an easy run out to speed
At about Mach 2.5 in the "accel" the inlet got very smooth. Normally a bit of duct rumble is tolerated to stay safe from "unstart" a smooth duct at the point of max "q" is a warning!! Sure was a nice ride so far*********then***************right duct "unstart", banged my head against the canopy, left side unstarted too. My day went from blissful flying to how can I get this thing safely home.
Got the thing back subsonic after a while and headed home. The damage I could see in the cockpit included a broken over the shoulder camera, and a broken helmet sun-visor. Scared me as well, and I am fearless. I obviously, aborted that mission and flew home to lick my wounds. Most of the time the A-12 is a pure joy to fly, either in the pattern back home or on an overflight of an adversary country.
In retrospect, an "unstart" is most likely in the region where the duct is most efficient, the duct pressure is highest, the spike positioning is most critical etc. For these reasons the pilot is acutely aware of too "smooth" duct feeling. A bit of rumble is tolerated in order to avert the unstart.
The inlet control system was made better over time and "unstarts" became more rare than matter of fact. The experiences of the a-12 pilots led to much better inlet control systems in the SR-71 aircraft.