A propelling nozzle is a nozzle that converts the internal energy of a working gas into propulsive force; it is the nozzle, which forms a jet, that separates a gas turbine, or gas generator, from a jet engine. 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. Turbofan engines may have an additional and separate propelling nozzle which further accelerates the bypass air. Propelling nozzles also act as downstream restrictors, the consequences of which constitute an important aspect of engine design. Principles of operation A nozzle operates according to the Venturi effect to bring the exhaust gasses to ambient pressure, while forming them into a propulsive jet; if the pressure upstream of the nozzle is high enough, the flow will reach sonic speed (choke). The role of the nozzle in back-pressuring the engine is explained below. The energy to accelerate the stream comes from the temperature and pressure of the gas. The gas expands adiabatically with low losses and hence high efficiency. The gas accelerates to a final exit velocity which depends on the pressure and temperature at entry to the nozzle, the ambient pressure it exhausts to (unless the flow is choked), and the efficiency of the expansion. The efficiency is a measure of the losses due to friction, non-axial divergence as well as leakage in C-D nozzles. Airbreathing engines create forward thrust on the airframe by imparting a net rearward momentum to the exhaust gas. If thrust exceeds the resistance incurred by the aircraft moving through the air, it will accelerate. The jet may or may not be fully expanded. On some engines that are equipped with an afterburner the nozzle area is