A nozzle cooling system having apparatus to overexpand the exhaust flow in a divergent section of an aircraft gas turbine engine nozzle, relative to the air in the engine nozzle's bay and a valve to allow ambient air from the bay to flow over divergent flaps and seals of the divergent section, and rapidly cool the divergent section of the nozzle for IR suppression. An area ratio control apparatus may be used to change a ratio of the nozzle exit area to the nozzle throat area to an overexpanded level that reduces static pressures along at least a longitudinally extending portion of the surfaces to below the ambient pressure of air outside the nozzle to draw the ambient air into the exhaust gas flowpath, and a valve apparatus to controllably flow the ambient air into the exhaust gas flowpath when the static pressures are below the ambient pressure. Further embodiments of the present invention provide alternative area ratio control apparatus, such as a variable exit area control apparatus to vary the exit area or a variable throat area control apparatus to vary the throat area, or both. Valve apparatus with a gap apparatus to controllably open gaps between the circumferentially adjacent divergent flaps and seals are provided such as spring loaded divergent seal retainers and flapper valves sealingly disposed over corresponding cooling apertures through the divergent seals.