Reheat or afterburning

In gas-turbine-propelled aircraft, there is frequently a requirement for short bursts of increased thrust, particularly for high performance military aircraft, which need to accelerate rapidly. Unlike the reciprocating engine, the gas tur­bine only uses, for combustion, a small proportion of the available oxygen in the air that passes through it. It is, therefore, possible to obtain a significant boost in thrust by burning more fuel in an extended tailpipe section known as an afterburner or reheat chamber, as illustrated in Fig. 6.30.

The thrust can be approximately doubled in this way with only a relatively small increase in weight. At low flight speeds, reheat is extremely inefficient, and is normally only used for take-off, and to produce short bursts of rapid acceleration. In supersonic flight, it becomes more efficient. Most modern supersonic aircraft use reheated low by-pass turbo-fans.

Reheat necessitates the use of a variable-area exhaust nozzle, and the extra tailpipe length and burner produce additional friction losses when not in use.