higher supersonic speeds the pressure loss through a normal shock wave in front of the inlet becomes prohibitive and a protruding spike or cone has to be used to produce oblique shock waves in front of a final normal shock which now occurs at the inlet entrance lip. The diffuser in this case consists of two parts, the supersonic diffuser, with its shock waves external to the inlet, followed by the internal subsonic diffuser. At higher speeds still, part of the supersonic diffusion has to take place internally so there are external and internal oblique shock waves. The final normal shock has to occur in the vicinity of a minimum flow area known as the throat, which is followed by the subsonic diffuser. Combustor As with other jet engines, the combustor has to raise the temperature of the air by burning fuel. This takes place with a small pressure loss. The air velocity entering the combustor has to be low enough such that continuous combustion can take place in sheltered zones provided by flame holders. Since there is no downstream turbine, a ramjet combustor can safely operate at stoichiometric fuel: air ratios, which implies a combustor exit stagnation temperature of the order of for kerosene. Normally, the combustor must be capable of operating over a wide range of throttle settings, for a range of flight speeds and altitudes. Usually, a sheltered pilot region enables combustion to continue when the vehicle intake undergoes high yaw/pitch during turns. Other flame stabilization techniques make use of flame holders, which vary in design from combustor cans to simple flat plates, to shelter the flame and improve fuel mixing. Over-fuelling the combustor can cause the final (normal) shock in the diffuser to be pushed forward beyond the intake lip, resulting in a substantial drop in engine airflow and thrust. Nozzles The propelling nozzle is a critical part of a ramjet design, since it accelerates exhaust flow to produce thrust. Subsonic ramjets accelerate exhaust flow with a nozzle. Supersonic flight typically requires a convergent–divergent nozzle. Performance and control Although ramjets have been run as slow as , below about they give little thrust and are highly inefficient due to their low pressure ratios. Above this speed, given sufficient initial flight velocity, a ramjet will be self-sustaining. Indeed, unless the vehicle drag is extremely high, the engine/airframe combination will tend to accelerate to higher and higher flight