Propelling nozzle

A propelling nozzle converts a gas turbine or gas generator into a jet engine. Energy available in the gas turbine exhaust is converted into a high speed propelling jet by the nozzle. Turbofan engines may have an additional and separate propelling nozzle which produces a high speed propelling jet from the energy in the air that has passed through the fan. In addition, the nozzle helps to determine how the gas generator and fan operate as it acts as a downstream restrictor.

Propelling nozzles accelerate the available gas to subsonic, transonic, or supersonic velocities depending on the power setting of the engine, their internal shape and the pressures at entry to, and exit from, the nozzle. The internal shape may be convergent or convergent-divergent (C-D). C-D nozzles can accelerate the jet to supersonic velocities within the divergent section, whereas a convergent nozzle cannot accelerate the jet beyond sonic speed.

Propelling nozzles may have a fixed geometry, or they may have variable geometry to give different exit areas to control the operation of the engine when equipped with an afterburner or a reheat system. When afterburning engines are equipped with a C-D nozzle the throat area is variable. Nozzles for supersonic flight speeds, at which high nozzle pressure ratios are generated, also have variable area divergent sections.