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Aturboshaft engine is a form ofgas turbine that is optimized to produceshaft horsepower rather thanjet thrust. In concept, turboshaft engines are very similar toturbojets, with additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power. They are even more similar toturboprops, 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 includehelicopters,auxiliary power units,boats andships,tanks,hovercraft, and stationary equipment.
A turboshaft engine may be made up of two major parts assemblies: the 'gas generator' and the 'power section'. The gas generator consists of thecompressor,combustion chambers with ignitors and fuelnozzles, and one or more stages ofturbine. The power section consists of additional stages of turbines, agear reduction system, and the shaft output. The gas generator creates the hot expanding gases to drive the power section. Depending on the design, the engine accessories may be driven either by the gas generator or by the power section.
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-ratiotransmissions andclutches.
An unusual example of the turboshaft principle is thePratt & Whitney F135-PW-600turbofan engine for theSTOVLLockheed F-35B Lightning II – in conventional mode it operates as a turbofan, but when powering theRolls-Royce LiftSystem, it switches partially to turboshaft mode to send 29,000horsepower forward through a shaft[1] 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. TheMil Mi-26 uses twoLotarev D-136 at 11,400 hp each,[2] while theSikorsky CH-53E Super Stallion uses threeGeneral Electric T64 at 4,380 hp each.[3]
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The first gas turbine engine considered for an armoured fighting vehicle, theGT 101 which was based on theBMW 003 turbojet, was tested in aPanther tank in mid-1944.[4][page needed]The first turboshaft engine forrotorcraft was built by theFrench engine firmTurbomeca, led by its founderJoseph Szydlowski. In 1948, they built the first French-designed turbine engine, the 100-shp 782. Originally conceived as anauxiliary power unit, it was soon adapted to aircraft propulsion, and found a niche as a powerplant for turboshaft-driven helicopters in the 1950s. In 1950, Turbomeca used its work from the 782 to develop the larger 280-shpArtouste, which was widely used on theAérospatiale Alouette II and other helicopters.[5] This was following the experimental installation of aBoeing T50 turboshaft in an example of theKaman K-225 synchropter on December 11, 1951, as the world's first-ever turboshaft-powered helicopter of any type to fly.[6]
TheT-80 tank, which entered service with theSoviet Army in 1976, was the first tank to use a gas turbine as its main engine. Since 1980 theUS Army has operated theM1 Abrams tank, which also has a gas turbine engine. (Most tanks use reciprocating piston diesel engines.) The SwedishStridsvagn 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.
