Look at an SR-71 head-on and the most striking thing is the pair of sharp cones in the front of its engine nacelles. They are not decoration. At Mach 3 those spikes, and the doors and ducts behind them, did a job no jet engine of the time could do on its own: they slowed air arriving at more than 2,000 mph to below the speed of sound before it reached the compressor.

A reel doing the rounds on Instagram explains the basic idea correctly. Here is how the system actually worked, using figures from NASA’s technical history of the Blackbird.

Why the Air Had to Be Slowed

A jet engine’s compressor cannot cope with supersonic air. NASA’s history puts it simply: air velocity had to be reduced because no existing engine could run on supersonic flow. The usual way to slow air down at supersonic speed is a shock wave. Air passing through a shock slows abruptly and its pressure and temperature jump. Do it in one big shock and you waste a lot of energy; do it in a series of weaker shocks and you recover much more pressure. That is what the SR-71’s inlet was designed to do.

The principle is the same as the shock cone on a MiG-21’s nose, but the Blackbird pushed it much further. Its inlet was a mixed-compression design: some compression happened outside the inlet lip on the spike’s cone, and the rest inside the duct, ending in a final normal shock that dropped the flow to subsonic speed.

The Moving Spike

The spike was the key control. According to NASA, the cone translated forward and aft to control the position of the shock wave and the Mach number inside the inlet. At low speed it sat fully forward. As the aircraft accelerated beyond about Mach 1.6, it moved progressively rearward, by roughly 26 inches in total by cruise speed, keeping the oblique shock from the spike’s tip positioned on the inlet lip and the inner shock in the right place in the duct.

Two sets of doors worked with it. Forward bypass doors opened and closed automatically, based on pressures measured in the ducts, to hold the shock in position. Aft bypass doors, set by the pilot according to Mach number and the position of the forward doors, controlled the airflow at the face of the engine. A bleed system also removed the slow, low-energy boundary layer that built up along the spike, so that the whole inlet passage carried clean, high-energy air.

An Inlet That Made Most of the Thrust

At cruise, the J58 behaved less like a pure turbojet and more like a ramjet. A large share of the air entering the inlet bypassed the engine core through ducts and went straight to the afterburner, where at Mach 3 fuel burned more efficiently than in the main combustor. Engineers called the powerplant a turbo-ramjet. We look at what that meant for the aircraft’s top speed in SR-71 at Mach 4.

NASA’s numbers show how dramatic the shift was. At Mach 2.2 the inlet provided 13 percent of the thrust, the engine 73 percent and the exhaust ejector 14 percent. At Mach 3 cruise the inlet provided 54 percent, the ejector 29 percent and the turbojet itself only 17 percent. The air was compressed about 40 to 1 by the time it reached the engine.

When It Went Wrong: The Unstart

The system depended on keeping the shock wave inside the inlet. Large airflow disturbances or a fault in the inlet control system could make the inlet expel the shock instead. That was an unstart. Pressure and airflow to the engine collapsed, thrust on that side vanished, and the aircraft lurched violently in yaw, pitch and roll. Crews were thrown hard against their straps, and their helmets could hit the canopy.

Unstarts were usually recovered by the inlet control system or the crew. But on 25 January 1966 an unstart during a test flight contributed to the loss of an SR-71 at Mach 3.17 over New Mexico, the accident that Lockheed test pilot Bill Weaver survived after the aircraft broke up around him. Later Blackbirds received a digital automatic flight and inlet control system that made unstarts far less frequent.

What the Viral Reel Gets Right

The reel’s short caption is accurate: the J58 could not take supersonic air directly, and the variable-geometry inlet with its movable spike controlled the shock waves, compressed the air and delivered stable flow to the engine. What it leaves out is the most surprising part: at Mach 3, that inlet was producing more of the Blackbird’s thrust than the engine behind it.

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Sources: Peter W. Merlin, “Design and Development of the Blackbird: Challenges and Lessons Learned”, NASA Dryden / AIAA 2009-1522; Wikipedia, Pratt & Whitney J58 and Lockheed SR-71 Blackbird.