Turboshaft

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Schematic diagram showing the operation of a simplified turboshaft engine. The compressor spool is shown in green and the free / power spool is in blue.

A Turboshaft engine is a form of gas turbine which is optimized to produce shaft power, rather than jet thrust. In principle a turboshaft engine is similar to a turbojet, except the former features additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power. Ideally there should be little residual thrust energy in the exhaust and the power turbine should be free to run at whatever speed the load demands.

The general layout of a turboshaft is similar to that of a turboprop, the main difference being the latter produces some residual propulsion thrust to supplement that produced by the shaft driven propeller. Another difference is that with a turboshaft the main gearbox is part of the vehicle (e.g. helicopter rotor reduction gearbox), not the engine. Virtually all turboshafts have a "free" power turbine, although this is also generally true for modern turboprop engines. At a given power output, compared to the equivalent piston engine, a turboshaft is extremely compact and, consequently, lightweight.

The name turboshaft is most commonly applied to engines driving ships, helicopters, tanks, locomotives and hovercraft or those used as stationary power sources.

The first true turboshaft engine was built by the French engine firm Turbomeca, led by the founder, Joseph Szydlowski. In 1948 they built the first French-designed turbine engine, the 100shp 782. In 1950 this work was used to develop the larger 280shp Artouste, which was widely used on the Aérospatiale Alouette II and other helicopters. The distinct whine of the Artouste is familiar to all those who have watched a 1967 UK television series The Prisoner, since an Alouette was featured in many of the episodes. Note that Artouste is also the name of an unrelated English design, the Blackburn Artouste.

Major efforts were underway in the United States and the United Kingdom to build similar engines. In the US Anselm Franz followed the same principles of simplicity that he used to develop the Jumo 004 in Germany, producing the T53 engine at Lycoming in 1953, and following this with the larger T55. General Electric beat his design into operation with their T58 series.

Today almost all engines are built so that power-take-off is independent of engine speed, using the free turbine stage. This has two advantages:

It allows a helicopter rotor or propeller to spin at any speed instead of being geared directly to the compressor turbine.
It allows the engine to be split into two sections, the "hot section" containing the majority of the engine, and the separate power-take-off, allowing the hot-section to be removed for easier maintenance.

This leads to slightly larger engines—compare the Pratt & Whitney PT-6 and similar models from Garrett Systems, for instance—but for the speed ranges served by these engines it is considered to be unimportant. Today practically all smaller turbine engines come in both turboprop and turboshaft versions, differing primarily in their accessory systems.