The thermal efficiency (ηt) of an aerospike engine measures how effectively the engine converts the chemical energy of the propellant into useful mechanical work or thrust. The thermal efficiency of any heat engine, including aerospike engines, can be calculated using the following formula:
ηt = (Useful Work or Thrust Output) / (Heat Input from Propellant)
In the case of an aerospike engine, “Useful Work or Thrust Output” refers to the actual thrust produced by the engine, typically in newtons (N) or pounds-force (lbf). “Heat Input from Propellant” represents the heat energy released by the combustion of the propellant, often measured in joules (J) or British thermal units (BTU).