The plug nozzle is a type of rocket engine nozzle that belongs to a member of the class of altitude compensating nozzles much like the aerospike which, unlike traditional designs, maintains its efficiency at a wide range of altitudes.
The ideal contour of a plug nozzle is a long tapering 'spike' with a doughnut-shaped combustion chamber situated at the base, hence sometimes this nozzle is also called a "spike nozzle". To save weight, this design is shortened without a large drop in efficiency.
The exhaust is confined by atmospheric pressure so that at different altitudes the varying pressures will allow the exit area to change. This allows perfect atmospheric compensation. With the shortened (or truncated) nozzle, the recirculation of trapped gases at the base of the plug causes a small thrust which offsets the loss due to the non-ideal shape.
Published in July 2009.
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