A jet engine produces thrust by accelerating a stream of fluid and experiencing the reaction force. In everyday aviation usage, the term usually refers to air-breathing gas-turbine engines, particularly turbojets and turbofans. The basic physical principle is the same as that used by a rocket, but an air-breathing engine obtains oxygen from the surrounding atmosphere.[1]
Gas-turbine operation
A turbojet draws air through an inlet and raises its pressure in a compressor. Fuel is added and burned in the combustion chamber. The resulting hot gas then passes through a turbine, which extracts enough mechanical energy to drive the compressor through a shaft. The remaining energy accelerates the exhaust through a nozzle and produces thrust.[2]
The compressor and turbine perform opposite energy transfers. The compressor requires shaft power to increase the air's pressure, while the turbine supplies shaft power by extracting energy from the hot gas. This continuous flow differs from the repeated intake, compression, combustion and exhaust events occurring inside an individual piston-engine cylinder.[2]
Thrust and airflow
Thrust depends on the mass of fluid passing through the propulsion system and the change in its velocity. Most of the mass flowing through an air-breathing jet comes from the atmosphere, rather than from its fuel supply. The engine changes the momentum of that flow; it does not need to push against a solid surface behind the aircraft.[1]
Engines without turbines
Not every air-breathing jet uses a rotating compressor. A ramjet relies on the vehicle's forward motion and inlet geometry to compress the air before combustion. It therefore cannot produce useful static thrust in the manner of a turbojet and needs another means of acceleration before it can operate effectively.[3]
A conventional ramjet slows its internal flow to subsonic speed before combustion. A scramjet instead burns fuel in a supersonic flow. These designs address very high-speed atmospheric flight and differ fundamentally from the machinery used in ordinary passenger-aircraft engines.[3]
Aircraft applications
Aircraft missions lead to different propulsion choices. NASA uses transport aircraft, fighters and trainers to illustrate the distinction: the required combination of cruise efficiency, acceleration, payload and speed determines the appropriate engine arrangement. A gas turbine may also deliver mechanical power to a propeller or rotor, so the broader category of turbine engines extends beyond engines whose main output is direct exhaust thrust.[1]