The ramjet engine began to compete with the turbojet when aircraft speeds increased beyond mach 2, thus entering a range where the compression produced by the airspeed becomes sufficient to perform the function of the compressor of the turbojet. For this reason the development of the ramjet also known as the “athodyd,” a contraction of “aerothermodynamic duct “has come into prominence in recent years. The air rushes into the inlet at supersonic speed and enters the combustion chamber, where it is heated by the combustion of fuel injected into the chamber. The heated air and the gases of combustion are discharged from a nozzle, thus producing the thrust. The main technical problem presented by the ramjet is to ensure steady combustion. For this it is generally necessary to have airflow speeds of less than about 100 m/sec (330 ft/sec) in the combustion chamber. This is a requirement difficult to fulfill at high air-speeds. For increasingly high speeds the ramjet evolves into something more resembling a rocket-propulsion Unit. In such engines the pressure developed in the combustion chamber is of the order of 100 atm. (about 1500 lb/in2), and the nozzle from which the jet emerges has to be made larger and larger. For very high speeds, in excess of mach 6, the engine evolves into the kind of system, where the inlet cone has become a specially shaped central body surrounded by an annular combustion chamber. As a result of allowing the gases of combustion to expand around the circumference of the conically tapering “tail” of the central body, a saving in overall construction weight of the engine is affected. From this example it is apparent how future high-speed ramjet engines are likely to become increasingly incorporated into the structure of the aircraft and thus become an integral feature thereof. The logical further development of the athodyd would consist in external combustion of fuel behind a shock wave.
The shock wave is formed at the nose of the aircraft and is associated with an abrupt increase in pressure. It could therefore serve theoretically as the “front wall” of a combustion chamber, fuel being injected into the air behind the shock wave. The fuel would ignite spontaneously in consequence of the high temperature that always develops behind the shock wave. External expansion of the gases of combustion at the rear part of the aircraft provides the propelling thrust. The appropriately shaped surfaces may be conceived as part of the aircraft’s fuselage or combination of fuselage and wing. This form of propulsion is in turn a transition to the athodyd with ultrasonic combustion. The main problem encountered here is that of stability of the flame. This may be achieved by enclosing it within a recirculation zone close to the surface of the aircraft. Alternatively, the propulsion system may take the form of a rocket motor which emits a stream of fuel-enriched gas into which air is injected and which is then brought to combustion. The main difference in relation to the conventional ramjet with subsonic combustion is that, instead of having to reduce the supersonic speed of the intake air to a subsonic value low enough to permit flame stability in the combustion chamber, the greater part of the kinetic energy of the intake air is now not converted into potential energy by adiabatic compression. This compression prior to combustion in the conventional ramjet reduces the efficiency of the ramjet at high mach numbers.