subsonic compression, or diffusion, up to the engine face. 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. Certain circumstances can cause the shock wave to be expelled from the intake. This is known as an unstart. Alternative shapes Some air inlets feature a biconic centrebody (MIG-21) to form two conic shock waves, both focused on the lip of the intake. This improves pressure recovery. Some aircraft (F-104, Mirage III) use a semi-conic centrebody. The F-111 has a quarter cone, which moves axially, followed by an expanding cone section. 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. Inlet ramps allow for swept inlet cowls (F-22 Raptor, F-35 Lightning II) to avoid shocks. Some other supersonic aircraft (Eurofighter Typhoon) use a variable lower cowl lip for high angle of attack operation and a bleed system (porous wall) incorporated on the intake ramp to facilitate stabilization of the shock system at supersonic Mach numbers. For the improvement of the intake flow (reduced distortion), air is dumped via an intake bleed slot on the ramp side downstream of the intake. The ramp, which is separated from the fuselage by a diverter, produces an oblique shock in order to decelerate the flow. The leading edge of the splitter plate separating the two intakes is located downstream of this oblique shock. Many supersonic aircraft (F-16 Fighting Falcon) dispense with the conical centrebody and employ a simple pitot intake. A detached, strong normal shock appears directly in front of the inlet at supersonic flight speeds, which leads to poor pressure recovery. NASA has tested an alternative to the external/internal, or mixed compression inlet, needed for speeds above about Mach 2.2 (below that speed inlets with all-external compression are used). The mixed-compression inlet is susceptible to unstarts or expulsion of the internal shock to in front of the inlet. The NASA inlet, which they call a Parametric Inlet, does all the supersonic compression externally so there is no shock inside the ducting in a potentially unstable location. Different types of inlet cone See also Aerospike engine Nose cone design Splitter plate References Grundlagen der Gasdynamik; Klaus OSWATITSCH. Springer, 1976. Benson, T. (2004). High Speed Aerodynamics Index. Retrieved Nov. 19, 2004. Eden,