There are four main types of gas turbine: turbojet, turbofan, turboprop, and turboshaft. The turbojet and turbofan are both reaction engines which derive power from the reaction to the exhaust stream. The turboprop and turboshaft operate differently by using the exhaust stream to power an additional turbine which drives a propeller or output shaft.

Turbojet
The original concept, the turbojet, is the simplest form of gas turbine and relies on the high velocity hot gas exhaust to provide the thrust. Its disadvantages today are its relatively high noise levels and fuel consumption. Examples: Olympus 593 in the Concorde supersonic airliner; Viper in a variety of military aircraft.

Turbofan
In the turbofan or ‘bypass’ engine the partly compressed airflow is divided, some into a central part - the gas generator or core - and some into a surrounding casing - the bypass duct. The gas generator acts like a turbojet whilst the larger mass of bypass air is accelerated relatively slowly down the duct to provide ‘cold stream’ thrust. The cold and hot streams mix to give better propulsive efficiency, lower noise levels, and improved fuel consumption. In the high bypass ratio turbofan, as much as seven or eight times as much air bypasses the core as passes through it. It achieves around 75% of its thrust from the bypass air and is ideal for subsonic transport aircraft. A low bypass ratio turbofan, where the air is divided approximately equally between the gas generator and the bypass duct, is well suited to high-speed military usage. Examples: in commercial usage - Trent in the Airbus A330; RB211-535 in the Boeing 757: in military usage - RB199 in the Tornado and EJ200 in the Typhoon. The vectored thrust Pegasus in the Harrier is a variation of the turbofan.

Turboprop
As its name implies, a turboprop uses a propeller to transmit the power it produces. The propeller is driven through a reduction gear by a shaft from a power turbine, using the gas energy which would provide the thrust in a turbojet. Turboprop power is measured in total equivalent horsepower (tehp), or kilowatts (kW). Examples: Dart in BAe748 and Fokker F27; AE2100 in the Saab 2000.

Turboshaft
The turboshaft is a powerplant for helicopters. Like the turboprop, it also uses a power turbine and gearbox, though in this case the power is transmitted to the helicopter’s rotor system. This type of engine is also used in industrial and marine applications. Turboshaft power is measured in shaft horse power (shp), or kilowatts (kW). Examples: Model 250 in the Jet Ranger; RTM322 in the Merlin.