Inlet cone

[4] A higher design speed may require two oblique shocks focussed on the lip to maintain an acceptable pressure recovery and pass maximum airflow.

In this case a biconic cone is required with two angles ( the Bristol Thor ramjet has 24 and 31 degrees for a design speed of Mach 2.5).

The rear of the cone beyond its maximum diameter, rear-facing and unseen inside the duct, is shaped for a similar reason to the protruding front part.

The visible cone is a supersonic diffuser with a requirement for low loss in total pressure, and the rear, streamlined part, together with the internal surface profile of the duct, forms the subsonic diffuser, also with a requirement for low loss in total pressure as the air slows to the compressor entry Mach number.

For higher Mach numbers part of the supersonic diffusion has to take place inside the duct, known as external/internal or mixed compression.

As the flight Mach number increases, the conical shock wave becomes more oblique and eventually impinges on the intake lip.

The position of the cone within the intake is usually controlled automatically to keep the plane shock wave correctly located just downstream of the throat.

Some air inlets feature a biconic centrebody (MIG-21) to form two conic shock waves, both focused on the lip of the intake.

Concorde, Tu-144, F-15 Eagle, MiG-25 Foxbat, and the A-5 Vigilante use so-called 2D inlets, where the nacelle is rectangular and a flat intake ramp replaces the dual cones.

A detached, strong normal shock appears directly in front of the inlet at supersonic flight speeds, which leads to poor pressure recovery.