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 Mach numbers below about 2.2 all the shock compression is done externally. For higher Mach numbers part of the supersonic diffusion has to take place inside the duct, known as external/internal or mixed compression. In this case the rear part of the forward-facing conical surface, together with the internal surface profile of the duct, continues the supersonic diffusion with reflected oblique shocks until the final normal shock. In the case of the Lockheed SR-71 Blackbird with part of the supersonic compression taking place inside the ducting the spike and internal cowl surfaces were curved for gradual isentropic compression. The inlet cone also has different axial positions to control how the capture area varies with the duct internal throat area. For best intake operation this required area ratio gets bigger with increasing flight Mach number, hence the large inlet cone movement on the SR-71 which had to perform well from low speeds to Mach 3.2. On the SR-71 the cone moves back at higher speeds. Operation At subsonic flight speeds, the conical inlet operates much like a pitot intake or subsonic diffuser. However, as the vehicle goes supersonic a conical shock wave appears, emanating from the cone apex. The flow area through the shock wave decreases and the air is compressed. As the flight Mach number increases, the conical shock wave becomes more oblique and eventually impinges on the intake lip. For higher flight speeds a moving cone becomes necessary to allow the supersonic compression to occur more efficiently over a wider range of speeds. With increasing flight speed, in typical Oswatitsch-type supersonic moving-cone inlet - the cone is moved forward (MiG-21), and if it is non-Oswatitsch-type cone inlet (SR-71) it is moved to the rear, or into the intake. In both cases, due to the shape of the cone surface and the internal duct surface the internal flow area gets less as required to continue compressing the air supersonically. The compression occurring in this path is called "internal compression" (as opposed to the "external compression" on the cone). At the minimum flow area, or throat, a normal or plane shock occurs. The flow area then increases for