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SERN
From Wikipedia, the free encyclopedia
A SERN, which stands for Single Expansion Ramp Nozzle, is a type of linear expansion nozzle where the gas pressure transfers work only on one side. Traditional nozzles are axially symmetric, and therefore surround the expanding gas. Linear nozzles are not axially symmetric, but consist of a 2D configuration of two expansion ramps. A SERN could also be seen as a single sided aerospike engine.
SERNs can be constructed lighter than a standard linear nozzle, and, provided that Mach lines from the end of the combustion chamber do not reach the end of the expansion ramp, yield comparable efficiency to linear nozzles. A drawback is that the normal force component is usually not zero, and depends heavily on engine operation and the state of the surroundings, making pitch control of the vehicle difficult.
Many designs for space planes with scramjet engines make use of SERNs because of the weight reduction at large expansion ratios, or the additional lift at under-expansion. The X-43, a test vehicle in NASA's Hyper-X programmme, is a flying example.