Inlet cones (sometimes called shock cones or inlet centerbodies) are a component of some supersonic aircraft and missiles. They are primarily used on ramjets, such as the D-21 Tagboard and Lockheed X-7. Some turbojet aircraft including the Su-7, MiG-21, English Electric Lightning, and SR-71 also use an inlet cone. Purpose An inlet cone, as part of an Oswatitsch-type inlet used on a supersonic aircraft or missile, is the surface on which supersonic ram compression for a gas turbine engine or ramjet combustor takes place through oblique shock waves. Slowing the air to low supersonic speeds using a cone minimizes loss in total pressure (increases pressure recovery). Also, the cone, together with the inlet cowl lip, determine the area which regulates the flow entering the inlet. If the flow is more than that required by the engine then shock position instability(buzz) can occur. If less than that required then the pressure recovery is lower which reduces engine thrust. An inlet with cone may be used to supply high pressure air for ramjet equipment which would normally be shaft-driven on a turbine engine, eg to drive turbopumps for the fuel pump on the Bristol Thor ramjet and hydraulic power on the Bristol Bloodhound missile. Shape The cone angle is chosen such that, at the design condition for the inlet (Mach 1.7 for the English Electric Lightning inlet), the shock wave that forms on its apex coincides with the cowl lip. The inlet passes its maximum airflow and achieves its maximum pressure recovery. 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). For higher speeds a more smoothly contoured transition between cone angles may be used in what is known as an isentropic spike (Marquardt RJ43 ramjet). The conical body may be a complete cone centerbody in a round inlet (MiG-21), a half cone in a side-fuselage inlet (Lockheed F-104 Starfighter) or a quarter cone in a side-fuselage/underwing inlet (General Dynamics F-111 Aardvark). 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