Pneumatic Jet Nozzle Control: Modulation of the Throat Area of a Turbojet Propelling Nozzle by Controlling Pressure Ratio across the Turbine Stages

THE principal reason for fitting a variable throat area nozzle to a turbojet engine, instead of a simple, fixed pipe, is to permit afterburning (fig. 1), whereby the temperature of the propelling gas may be more than doubled. The throat area of the nozzle must be increased approximately in proportion to the square root of the temperature, so that the jet pipe pressure is not much changed. Otherwise, the pressure rises sharply when the afterburner is lit, and throttles the compressor: the drop in airflow may then cause the engine to overheat and surge. The aim of a nozzle control system is thus to keep the rotating components unaware of whether the afterburner is alight or not, and safe from damage due to pressure and temperature transients.