Propelling nozzle
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A propelling nozzle is a component of a jet engine that is part of the tailpipe and operates to ensure that the exhaust gas's energy is maximised in the reaction throughout the engine so that the maximum thrust is achieved.
On non-afterburning engines the nozzle is a fixed size because the differing atmospheric pressure over the operating altitudes makes little difference to the engine aerodynamics and variable nozzle use is expensive. However the great range of mass flow rates an afterburning engine generates requires the use of variable nozzle diameters and shapes. Depending on the amount of afterburning, the variable nozzle may be simple or complex.
Engines capable of supersonic flight have convergent-divergent duct features that generate supersonic flow.
Engines that are required to generate thrust quickly, from idle, use a variable area propelling nozzle in its open configuration to keep thrust to a minimum while maintaining high engine rpm. When thrust is needed, initiating a go-around for example, it is simple and quick to close the nozzle to the high-thrust position.
