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A test firing of twin linear XRS-2200 Aerospike engines. The aerospike engine is a type of rocket engine that maintains its efficiency across a wide range of altitudes through the use of an aerospike nozzle.
A standard rocket engine uses a bell shaped nozzle to contain and direct the exhaust gases. However the optimum shape of the bell depends on the air pressure which reduces as the rocket climbs. An aerospike uses the air flowing past the rocket to form half of a 'virtual bell' which automatically compensates for the reducing pressure.