Concorde’s Rolls-Royce/Snecma Olympus 593 engines were turbojets, and like every jet engine their compressors needed subsonic air. At Mach 2 cruise, the air reaching the front of each nacelle was travelling at about 1,350 mph. The job of slowing it down, without wasting the energy it carried, fell to one of the cleverest parts of the aircraft: its variable-geometry air intakes.

Shock Waves as Brakes

A supersonic flow can only be slowed efficiently through shock waves. Pass it through one strong, straight-on (normal) shock and it slows abruptly but loses a lot of total pressure. Pass it instead through a series of weaker, angled (oblique) shocks, then a final weak normal shock, and far more of the pressure is recovered. Recovered pressure is free compression for the engine, and therefore thrust.

Concorde’s intakes did this with geometry. Two hinged ramps in the roof of each intake, one forward and one aft, moved up and down to set the angle of the surfaces the air met. The ramp angles positioned a system of shock waves running from the intake lip, so that by the time the air reached the engine it had been slowed to roughly Mach 0.5 and compressed.

Doors, Spill and Control

The intake also had to cope with very different conditions at take-off, in the transonic region and at cruise. An auxiliary inlet vane, part of the spill door assembly, was held open on take-off to let in extra air, and closed by about Mach 0.93. A spill door in the lower surface could dump excess air when the engine needed less than the intake was capturing. Late in development the intake control system was switched from analogue to digital, making it one of the first digital systems of its kind; Airways describes it adjusting the ramps automatically from sensor data.

An Intake That Made the Thrust

When the shocks sat in the right places, the intake contributed most of the propulsive effort at cruise. Heritage Concorde puts it at 63% of the powerplant’s net positive thrust at Mach 2, with the engine itself and the variable exhaust nozzle making up the rest. It is the same principle that made the SR-71’s spiked inlets so effective, which we explain in SR-71 Inlet Spike Explained. Concorde used its afterburners only for take-off and to push through the transonic region; at Mach 2 cruise the intakes and engines ran without reheat.

Getting it wrong was dangerous. If the shock system was expelled from the intake, an engine could surge, and at supersonic speed a sudden asymmetric loss of thrust was a serious event. Heritage Concorde notes that the design was self-starting, so the engines did not lock into surge.

What the Popular Explainer Gets Right

The point usually made, that shock waves are what slow the flow and that the subsonic compressor requirement drives the whole design, is correct. The Olympus 593 was a turbojet rather than a turbofan, a slip the creator of one widely shared video corrected herself.

Sources: Heritage Concorde, “Air Intake System”; Airways Magazine, “Rolls-Royce Olympus: The Engine Behind Concorde”; Grace’s Guide to British Industrial History, Rolls-Royce Olympus.