How Things Work: Supersonic Inlets

“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.

During some Blackbird flights, however, the harmonious working of the spike and the forward and aft bypass doors broke down, and all too quickly the inlet was filled with more air than it could handle. When the air pressure inside the inlet became too great, the normal shock wave was suddenly belched out of the inlet in an unstart, accompanied by an instantaneous loss of air flow to the engine, an enormous increase in drag, and a significant yaw to the side with the affected inlet. Unstarts occurred “when you least expected them—all relaxed and taking in the magnificent view from 75,000 feet,” wrote Graham in SR-71 Revealed. If the crew’s attempts to restart the inlet’s supersonic flow failed, they would have to slow their aircraft to subsonic speeds.

With a top speed of Mach 1.6, the Lockheed Martin F-35 Joint Strike Fighter has an inlet design that is far simpler than that of the Mach 3-plus SR-71; the single-engine JSF inlet cannot vary its geometry. The F-35’s engineers could get away with a less complicated design because at vehicle speeds up to about Mach 2, the shape of the inlet itself can slow down much of the supersonic air before it enters the inlet. The JSF inlet is, however, a breakthrough design: It has no diverters. Traditional fighter inlets, such as those found on the F/A-18 and F-22, have slots, slats, and moving parts to divert or channel airflow. The F-15 inlet has ramps and doors that alter its external and internal shape to adjust airflow as needed.

Many other currently operational fighters also have boundary layer diverters. Air that clings to the surface of an aircraft in flight is known as boundary layer air, and it tends to cause turbulence in the air flowing into the engine, especially when it interacts with shock waves. Inlet designers try to keep out as much boundary layer air as possible, frequently positioning the inlet several inches away from the surface of the fuselage and its boundary layer air and employing a duct system to whisk the undesirable air away. (The SR-71 inlet rids itself of boundary layer air by sucking it in through slots on the spike and passing it through ducts that exit the nacelle.)

The F-35 inlet, however, is positioned flush against the fuselage, and just in front of the inlet opening is a raised surface, or bump, that pushes much of the boundary layer air off to the sides and away from the inlet. The bump serves another purpose: During supersonic flight, it compresses and slows the air passing over it into an oblique shock wave. The air is still moving supersonically, however, and it is slowed down to subsonic speeds after passing through a normal shock wave that forms at the mouth of the inlet. The simplicity of the JSF design makes for an inlet that requires less maintenance, reduces aircraft weight by 300 pounds, and costs $500,000 less than a traditional fighter inlet.