A turbojet is an air-breathing gas turbine that produces thrust by accelerating its exhaust through a nozzle. Air passes through an inlet, compressor, combustion chamber and turbine before reaching the nozzle. The turbine powers the compressor, while energy remaining in the gas produces the high-speed propulsive jet.[1]
The engine cycle
The compressor raises the pressure of incoming air. Fuel is added and burned, increasing the gas temperature before it flows through the turbine. A shaft links turbine and compressor, allowing the engine to sustain its compression process after starting. The nozzle converts available gas energy into increased exhaust velocity.[1]
Most of the exhaust mass originated as atmospheric air; the fuel adds a smaller contribution. Net thrust depends on the difference between outgoing and incoming momentum flow, together with any pressure difference at the nozzle exit. When exit pressure matches ambient pressure, NASA expresses net thrust as gross exhaust momentum thrust minus the incoming airflow's ram drag.[1]
Afterburning
An afterburner injects additional fuel downstream of the turbine. Flameholders stabilise combustion in the exhaust stream, raising its temperature and increasing nozzle thrust. Because this additional burning occurs after the main turbine, it provides a way to augment thrust without passing the hotter combustion products through that turbine.[2]
The extra thrust carries a substantial fuel penalty. With the afterburner switched off, the engine operates as a basic turbojet. Both turbojets and low-bypass turbofans can employ afterburning; the feature does not by itself identify the underlying engine type.[2]
Relationship to the turbofan
A turbofan adds a fan and a bypass airflow around the core. Moving that additional air gives turbofans better fuel efficiency than basic turbojets in many aircraft applications. Even low-bypass turbofans can improve efficiency while retaining an installation suitable for fast aircraft, helping explain their extensive adoption in later military and civil designs.[3]