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A turboshaft engine is a form of gas turbine that is optimized to produce shaft horsepower rather than jet thrust. In concept, turboshaft engines are very similar to , with additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power. They are even more similar to , with only minor differences, and a single engine is often sold in both forms.
Turboshaft engines are commonly used in applications that require a sustained high power output, high reliability, small size, and light weight. These include , auxiliary power units, and , , hovercraft, and stationary equipment.
In most designs, the gas generator and power section are mechanically separate so they can each rotate at different speeds appropriate for the conditions, referred to as a 'free power turbine'. A free power turbine can be an extremely useful design feature for vehicles, as it allows the design to forgo the weight and cost of complex multiple-ratio transmissions and .
An unusual example of the turboshaft principle is the Pratt & Whitney F135-PW-600 turbofan engine for the STOVL Lockheed F-35B Lightning II – in conventional mode it operates as a turbofan, but when powering the Rolls-Royce LiftSystem, it switches partially to turboshaft mode to send 29,000 horsepower forward through a shaftWarwick, Graham. " F-35B - The STOVL Challenges " Aviation Week & Space Technology, December 09, 2011. Accessed: April 10, 2014. and partially to turbofan mode to continue to send thrust to the main engine's fan and rear nozzle.
Large helicopters use two or three turboshaft engines. The Mil Mi-26 uses two Lotarev D-136 at 11,400 hp each," Mi-26 HALO" fas.org, September 21, 1999. Accessed: April 10, 2014. while the Sikorsky CH-53E Super Stallion uses three General Electric T64 at 4,380 hp each." About the GE T64" BGA-aeroweb, May 17, 2012. Accessed: April 10, 2014.
The T-80 tank, which entered service with the Soviet Army in 1976, was the first tank to use a gas turbine as its main engine. Since 1980 the US Army has operated the M1 Abrams tank, which also has a gas turbine engine. (Most tanks use reciprocating piston diesel engines.) The Swedish Stridsvagn 103 was the first tank to utilize a gas turbine as a secondary, high-horsepower "sprint" engine to augment its primary piston engine's performance. The turboshaft engines used in all these tanks have considerably fewer parts than the piston engines they replace or supplement, mechanically are very reliable, produce reduced exterior noise, and run on virtually any fuel: petrol (gasoline), diesel fuel, and aviation fuels. However, turboshaft engines have significantly higher fuel consumption than the diesel engines that are used in the majority of modern main battle tanks.
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