Free-turbine turboshaft

The compressor and its turbine are connected by a common shaft which, together with the combustor, is known as a gas generator, which is modelled using the Brayton Cycle.

Such a failure resulted in the 1954 accident of the second prototype Bristol Britannia, G-ALRX, which was forced to land in the Severn Estuary.

A failure in the Bristol Proteus propeller reduction gearbox led to an overspeed and release of the power turbine of Nº3 engine.

[4][5] The Proteus gears were redesigned and an emergency fuel shut-off device was fitted to prevent a similar reoccurrence.

However, certification regulations allow other methods for preventing excessive overspeed such as disc rubbing and blade interference.

Piston helicopters of this period had barely adequate performance; the switch to a turbine engine could both reduce several hundred pounds of engine weight, 600 lb (270 kg) for the Napier Gazelle of the Westland Wessex,[6] and also allow considerably more payload weight.

It does not need a clutch, as the gas generator may be started while the output shaft remains stationary.

It also allowed contra-rotating engines, where the gas generator core and power turbine revolved in opposite directions, reducing the overall moment of inertia.

[8] This was the first Bristol gas turbine and its broad design had been produced by Frank Owner at Tockington Manor.

This places the turbine output close to the propeller gearbox, avoiding the need for a long driveshaft.

[9] The M1 Abrams main battle tank is powered by a Honeywell AGT1500 (formerly Textron Lycoming) two-spool gas turbine engine.

A commercial derivative has been designed as the TF15 for marine and railroad applications,[11][12] and a flight-rated version, the PLT27, was also developed but lost a major contract to the GE T700 turboshaft.

[13] Turboshaft engines were used to power several gas turbine locomotives, most notably using the Turbomeca Turmo in Turbotrain (France) and Turboliner (United States) service.