Propelling Nozzle - Afterburners

Afterburners

Non-afterburning engines have nozzles of a fixed size because variable geometry nozzles are expensive and the change in atmospheric pressure with changing altitude has little effect on engine aerodynamics. However, the afterburner present on many combat aircraft's exhaust systems increases exhaust flow volume to the point that upstream turbomachinery can aerodynamically rematch; although some engines achieve maximum net thrust by allowing a modest rematch, all others must have a nozzle that can accommodate this upsurge by increasing its throat area. To accomplish this task, the nozzle consists of a series of moving, overlapping petals with a nearly circular nozzle cross-section.