How Things Work: Supersonic Inlets
- By Diane Tedeschi
- Air & Space magazine, November 2002
“We were in a turn and climbing when one of the inlets showed signs of instability. Shortly thereafter—KER BLAM!—the aircraft slammed my head against the side of the cockpit and then momentarily became unstable as it yawed, pitched, and vibrated.”
This is an account of a supersonic engine inlet failure, or “unstart,” recalled by retired reconnaissance systems officer Roger Jacks in SR-71 Revealed, a book by retired Lockheed SR-71 pilot Richard H. Graham. It shows what can happen when a supersonic inlet stops delivering the uniform stream of air upon which efficient jet engine operation depends.
When a jet airplane is flying faster than Mach 1—beyond the speed of sound—the air entering the engines is moving supersonically as well. But no turbojet engine compressor—the rotating disks and blades at the face of the engine that compress the air before it is mixed with fuel—is capable of handling supersonic air flow. The job of an engine inlet is to slow incoming air to subsonic speeds before it passes through the engine.
The inlet’s job is complicated by the fact that air moving supersonically behaves differently from subsonic air. An aircraft flying subsonically pushes through the air ahead of it, with each molecule of air having plenty of time to pass over its wings and fuselage. But as an airplane approaches Mach 1, it compresses the air ahead of it into shock waves—bands of air radiating from the airplane that are much hotter and denser than the ambient air.
Turbojet engines cannot digest the shock waves generated by their inlets, so a crucial role of the inlet is to keep the inevitable shock waves positioned so that they do no harm. The SR-71 Blackbird, a now-retired twin-engine reconnaissance aircraft, has an inlet design based on a cone-shaped body, or spike, that generates an oblique-angled, cone-shaped shock wave at the inlet’s entrance and a normal shock wave—one rising at a right angle from the direction of air flow—just aft of the internal inlet throat.
As the SR-71 increases its speed, the inlet varies its exterior and interior geometry to keep the cone-shaped shock wave and the normal shock wave optimally positioned. Inlet geometry is altered when the spike retracts toward the engine, approximately 1.6 inches per 0.1 Mach. At Mach 3.2, with the spike fully aft, the air-stream-capture area has increased by 112 percent and the throat area has shrunk by 54 percent.
The cone shape of the spike also incrementally reduces the speed of the incoming supersonic air without producing a drastic loss of pressure. The farther back over the cone the air moves, the more speed it bleeds off. As the slowed, but still supersonic, air continues to move farther into the inlet, the normal shock wave springs up between the inlet throat and the engine compressor—exactly where it is supposed to be. Once there, the normal shock wave slows the air passing through it to subsonic speeds, preparing it to enter the compressor.
It is a constant balancing act to keep the normal shock wave in the right position. The inlet has an internal pressure sensor, and when it detects that the pressure has grown too great, it triggers the forward bypass doors to open, expelling excess air. The inlet also has a set of aft bypass doors, controlled by the pilot. The forward and aft bypass doors work in opposition to each other: Opening the aft doors causes the forward doors to close, and when the pilot closes the aft doors, the forward doors open in turn.