Inlet Cone - Operation

Operation

At subsonic flight speeds, the conical inlet operates much like pitot intake, it sucks air. However, as the vehicle goes supersonic a conical shock wave appears, emanating from the cone apex. Conical (and oblique) shock waves are akin to the bow wave on a ship. As the flight Mach number increases, the conical shock wave becomes more oblique and eventually impinges on the intake lip.

With increasing mach number the cone is not moved out of the inlet as it would make sense for the external compression, but inwards, because the cone sits in the cowl like a plug. By moving it inwards the path between the cone and the inlet narrows as needed for higher speed. The compression occurring in this path is called "internal compression" (opposed to the "external compression" on the cone) and can be subject to stall.

Care must be taken to prevent the normal shock wave, which lies behind the throat of the diffuser, coming forward through the throat, replacing the oblique shocks and reducing stagnation pressure and leading to excessive inlet temperatures burning the compressor.

• The cone is moved in and out of the pitot intake controlled by a computer with temperature and pressure sensors in the intake to match the throat to the mach number.
• With a ramjet, this occurs if excessive fuel is injected into the combustor, raising internal pressure too far. However, with a turbojet or turbofan, the problem arises when the engine is throttled back, causing a mismatch between intake airflow and engine mass flow. A trapdoor is needed to dump excess flow overboard. Or a variable geometry for the stators of the fan is used.

To restore the engine after a surge the computer has to change inlet, turbine and nozzle geometry, fuel injection and counteract the yaw on a two engine airplane, by means of control surfaces and throttling back the second engine.

The boundary layer on the cone is stretched as it moves up the cone preventing flow separation, but for the internal compression and the subsonic compression the boundary layer still tends to separate and usually is sucked through tiny holes in the wall. As a side note on the aerospike engine the boundary layer gets thicker towards the end of the cone as needed for the greater speed difference between the air molecules just on the surface of the cone and the fully accelerated stream of air.