fan and core flow mixing function turning now to fig2 there is shown a schematic representation of the coolant pressure regulator apparatus which is located in the exhaust system of a turbojet turbofan engine the pressure regulating valve 20 is positioned in the closed cavity suppressor unit 22 the closed cavity suppressor unit 22 is located at the far end of the forward fan duct 24 and after the mixer control unit 26 the pressure regulating valve 20 prevents the coolant pressure in the annulus 28 between engine exhaust casing 30 and liner 32 from becoming greater than that required for effective cooling of the liner 32 and the exhaust nozzle 34 this coolant pressure regulation is accomplished without affecting the pressure in the forward fan duct 24 which delivers fan air to the exhaust cooling annulus 28 and the fan core flow mixer control unit 26 the benefit of limiting the coolant pressure is lower weight lower cost and more easily maintained exhaust system liners in fig3 a b c there is shown a schematic representation of the pressure regulating valve in different operational modes in fig3 a the forward fan duct is operating at low coolant pressure in fig3 b the operational mode is in the excess coolant pressure operating condition in fig3 c the operational mode is in a transient low core pressure mode it should be well understood that while the present discussion is with respect to a single valve 20 that in actual operation a series of valves are arranged around the circumference of the liner 32 the following description of the operation of the pressure regulating valve 20 will be with respect to the operational modes that are shown in fig3 a b c and fig2 in operation shown in fig3.