The Concorde’s variable geometry intake was designed to deliver the correct mass of air at all aircraft speeds and to reduce air velocity to an acceptable subsonic value prior to compressor entry.
The only supersonic intake in commercial use was to be found on the Concorde.
These intakes relied on the correct positioning of the oblique and normal shocks which, as we know, can be moved by changes in engine demand.
The variable geometry intake has a moveable ramp and auxiliary spill and inlet doors.
During acceleration through Mach 1.3 into the supersonic flight. The ramp progressively lowered to accurately position and maintain several oblique shock waves and a normal shock wave. The intake duct forms convergent and a divergent section during this process. The air decelerates gradually through the oblique shocks whilst decelerating supersonically in the converging duct section.
On passing the normal shock normal subsonic deceleration and pressure rise occur in the divergent section up to compressor entry. The ramp raised and the auxiliary inlet door opens to provide extra air flow on rapid forward engine throttling and take-off. The intake duct is a subsonic duct. The auxiliary door will close at about Mach 0.7.
At Mach 1.3 the ramp is lowered under hydraulic power and computer control. And a series of oblique shocks appear starting at the lower intake lip. These gently and smoothly decelerate the airflow as it passes supersonically through the now convergent duct section. On passing the final, normal shockwave. The air enters the divergent section as subsonic flow and diffuses up to compressor inlet. By Mach 2 the ramp has moved about half its travel.
The air temperature in the intake reaches around 200°C by this stage. The computer senses any changes to the intake air temperature and pressure and makes fine adjustments to the ramp position. Movement of the ramp doors also provides access to an air spill void. This permits air spill when sub-critical conditions try to push the shock waves out.
If the intake does go sub-critical. Air can intermittently spill over the intake lower lip causing a condition called intake ‘Buzz’.If the throttle suddenly pulled back to a lower level an auxiliary spill door opens and dumps excess air overboard. The same action will occur if the engine fails. The ramp extends fully down and the air immediately dumped through the auxiliary spill door.
This restricts airflow to the failed engine and prevents excessive wind-milling speeds. The Concorde intake copes with all situations it placed in and the engine will not even ‘blip’ under even the most rapid changes in engine power.