turboramjet

An aircraft engine that is a hybrid of a turbofan and a ramjet. When operated as a ramjet, the engine is capable of relatively efficient propulsion for flight at very high supersonic cruise speeds in the range of Mach numbers 5 to 6. The engine can also be operated as a turbofan engine to give it the capability of relatively efficient propulsion for the low-flight-speed segments of the aircraft's mission such as takeoff, acceleration, approach, and landing. One variation of the engine also includes a rocket engine, which gives the system the additional capability of transatmospheric propulsion. See also Ramjet; Rocket propulsion; Turbofan.

For operation at subsonic and transonic flight speeds, fuel together with an appropriate amount of an oxidizer such as liquid oxygen is introduced into the preburner in the middle of the engine, where the mixture is burned (see illustration). The resultant hot high-pressure gas stream is expanded through a turbine that drives the fuel and oxidizer pumps and also powers a large fan in the front of the engine. The front-fan discharge air bypasses the preburner and tubine and enters the main burner through a mixer, where it joins the gas stream exiting from the turbine. The stream of mixed gases is then accelerated through a variable-area exhaust nozzle to provide the required propulsive thrust. Thrust augmentation may be obtained by injecting an excess of fuel in the preburner so that, when the fan air is mixed with the fuel-rich turbine exhaust, additional combustion, or afterburning, takes place in the main burner. See also Afterburner.

Section drawing of an air turboramjet with rocket combustion chamber for exoatmospheric flight. Such a power plant would combine turbojet, ramjet, and rocket propulsion modes. (Aerojet)

At very high flight speed, with air at very high ram pressure entering the engine, the pumping action of the fan is no longer necessary and the fan may be feathered, or otherwise made inoperative, while permitting the ram air to pass through. Propulsion is now provided exclusively by the combustion of the ram air in the main burner with the fuel-rich gas stream from the preburner.

For aircraft that are designed to proceed from high-speed atmospheric flight to transatmospheric flight, a rocket chamber may be provided in the engine where fuel and oxidant are burned in greater quantity than is possible in the preburner, and the exhaust stream may be discharged through the thrust nozzle without having to pass through the turbine. See also Turbine propulsion.