Supersonic Inlet Analysis

Engine inlet:

Air enters the engines at supersonic speeds when a plane travels above Mach 1. A 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 unable to handle supersonic air flow. Thus, the purpose of an engine inlet is to slow incoming air to subsonic speeds before it flows through the engine.

The spike, shaped like a cone, also gradually decreases the velocity of the incoming supersonic air without leading to a huge loss of pressure. As the air travels farther along the cone, the more speed it bleeds off. The air is now slowed although still supersonic. As the slowed, but still supersonic, air continues to move farther into the engine, the normal shock wave occurs between the inlet throat and the engine compressor—the precise

Due to the fact that supersonic flow is unaware of what is ahead of it, it does not react to a change in slope gradient until the flow is downstream of the surface change. The line along which such a change occurs is a sloped line and is known as a compression ray or, for more substantial changes, an oblique shock. An abrupt change in flow direction, which produces an oblique shock, causes an attendant loss of ram pressure. Careful design, small, gradual changes in flow direction, holds these losses to small values.
A supersonic inlet is made up of two distinct parts. First, the flow is compressed supersonically from the velocity of the flight vehicle or, in other words, the free stream Mach number. This can be achieved by reducing the flow area as the flow proceeds downstream. In this area, the flow velocity is reduced via a series of compression waves and/or oblique shocks. Flow velocity is reduced to a minimum speed at the duct minimum area, also known as the throat of the inlet, where the