For aircraft designed to fly mixed missions (i.e., at subsonic, transonic, and supersonic flight speeds) with low levels of fuel consumption, it is desirable to have an engine with the characteristics of both a high-bypass engine (for subsonic flight speed) and a low-bypass engine (for supersonic flight speed). This requirement is typical for many high-speed commercial airliners, including the Concorde, a type of supersonic transport built by the British and French that was in service from 1976 to 2003. The Concorde was capable of traveling over oceans and unpopulated land areas at supersonic cruise speeds, but it could not fly efficiently and quietly at subsonic flight speed for takeoff, ascent, cruising over populated areas, and approach and landing. This dual function is expected to be accomplished in the future by the variable-cycle engine (VCE). If the components of an engine are designed to accommodate the extreme limits of flow, pressure ratio, and other conditions involved in both high-bypass and low-bypass operation, the engine may be operated at either extreme of bypass ratio or at any bypass ratio between those extremes by means of a valve (or valves) in the bypass stream (in conjunction with a variable exhaust nozzle). When the valves are closed, they restrict the flow in the bypass stream to achieve low bypass for supersonic flight. When the valves are open, the bypass is increased to its maximum value for efficient subsonic flight.

Turboramjets

As noted above, the ramjet provides a simple and efficient means of propulsion for aircraft at relatively high supersonic flight speeds. It is, however, quite inefficient at transonic flight speeds and is completely ineffective at subsonic velocities. The turboramjet has been developed to overcome this inadequacy. In this system (shown in Figure 8), a turbofan engine is built into the inlet of a ramjet engine to charge the latter with a pressurized stream of air at subsonic flight speed where ram pressure is insufficient for effective ramjet operation. During supersonic flight the fan blades, if they are of variable pitch, may be feathered so that they do not interfere with the flow of ram air to the ramjet. A separate inlet to the core engine that drives the fan may be closed off so as not to expose the turbomachinery to the hostile environment of the high-temperature ram air.

Another variation of the turboramjet does without the core inlet and the core compressor altogether. Instead, the aircraft carries a tank of an oxidizer, such as liquid oxygen. The oxidizer is fed into the core combustion chamber along with the fuel to support the combustion process, which generates the hot gas stream to power the turbine that drives the fan. During supersonic flight, the fan may be feathered, and a surplus of fuel may be introduced into the core combustor. The unburned fuel passes through the fan turbine and undergoes combustion in the ramjet burner when it mixes with the fresh air entering via the bypass stream from the fan.