the F-15, F-16, B-1B. Additional features Thrust-vectoring Nozzles for vectored thrust include fixed geometry Bristol Siddeley Pegasus and variable geometry F119 (F-22). Thrust-reversing The thrust reversers on some engines are incorporated into the nozzle itself and are known as target thrust reversers. The nozzle opens up in two halves which come together to redirect the exhaust partially forward. Since the nozzle area has an influence on the operation of the engine (see below), the deployed thrust reverser has to be spaced the correct distance from the jetpipe to prevent changes in engine operating limits. Examples of target thrust reversers are found on the Fokker 100, Gulfstream IV and Dassault F7X. Noise-reducing Jet noise may be reduced by adding features to the exit of the nozzle which increase the surface area of the cylindrical jet. Commercial turbojets and early by-pass engines typically split the jet into multiple lobes. Modern high by-pass turbofans have triangular serrations, called chevrons, which protrude slightly into the propelling jet. Further topics The other purpose of the propelling nozzle The nozzle, by virtue of setting the back-pressure, acts as a downstream restrictor to the compressor, and thus determines what goes into the front of the engine. It shares this function with the other downstream restrictor, the turbine nozzle. The areas of both the propelling nozzle and turbine nozzle set the mass flow through the engine and the maximum pressure. While both these areas are fixed in many engines (i.e. those with a simple fixed propelling nozzle), others, most notably those with afterburning, have a variable area propelling nozzle. This area variation is necessary to contain the disturbing effect on the engine of the high combustion temperatures in the jet pipe, though the area may also be varied during non-afterburning operation to alter the pumping performance of the compressor at lower thrust settings. For example, if the propelling nozzle were to be removed to convert a turbojet into a turboshaft, the role played by the nozzle area is now taken by the area of the power turbine nozzle guide vanes or stators. Reasons for C-D nozzle over-expansion and examples Overexpansion occurs when the exit area is too big relative to the size of the afterburner, or primary, nozzle. This occurred under certain conditions on the J85 installation in the T-38. The secondary or final nozzle was a fixed geometry sized for the maximum afterburner case. At non-afterburner thrust