Re-Heat Nozzles :

If re-heat was fitted to an engine with a standard sized fixed area propelling nozzle, the expansion of gases caused by the use of re-heat would increase the pressure in the jet pipe and reduce the pressure drop across the turbine (turbine expansion ratio). A reduced turbine expansion ratio will slow down the turbine and consequently lower the engine power. It would also increase the back pressure on the rear stage of the compressor which would cause the compressor to surge. To avoid a rise in pressure at the turbine outlet, the area of the propelling nozzle must be enlarged when re-heat is in use. Thus the propelling nozzle of a re-heat engine must be able to provide a nozzle area suitable for normal running without re-heat and a larger nozzle area when re-heat is used. Re-heat can usually be selected only after the throttle lever has passed through a normal 100% position. Therefore the smallest nozzle area must be efficient at normal maximum power and the large nozzle area must cater for the re-heat gas flow. If the amount of re-heat can be varied, then the re-heat nozzle must change to match the amount of re-heat selected.

Variable Area Nozzles

The variable propelling nozzle is suitable for use with controllable re-heat systems because it can provide a variable nozzle area to match the amount of re-heat selected. The circular continuity of the nozzle is maintained by a system of hinged flaps. The nozzle area is reduced by positive mechanical means but it is enlarged by the exhaust gas pressure acting upon the inside surface of the flaps.

A ring of hinged master flaps is interleaved with a ring of hinged sealing flaps to provide a variable area propelling nozzle. Each flap is hinged at its forward edge so that the rear edge can move inwards to reduce the nozzle area, or outwards to increase the nozzle area. 

Actuation of the nozzle system can be hydraulic using oil or fuel as the fluid medium, or an air motor driving screw jacks.
On selection of reheat the nozzle will move first to prevent back pressure on the engine, when it has moved the fuel will be supplied. With any increase in reheat the nozzle moves then the fuel follows. When reheat is reduced the opposite occurs first the fuel reduces then the nozzle closes. This ensures the nozzle area is too large rather than too small for any change in fuel flow.