The SABRE (Synergetic Air-Breathing Rocket Engine) is a revolutionary engine concept designed by Reaction Engines Limited. It is intended to be a combined-cycle engine capable of operating as both a jet engine and a rocket engine. Due to its unique design, the traditional thermal efficiency formula for heat engines may not be directly applicable.
In a typical jet engine, the thermal efficiency is defined as the ratio of the useful work output (thrust) to the heat input. However, in the case of SABRE, the engine transitions between different modes of operation, including air-breathing (jet mode) and rocket mode.
The thermal efficiency is expressed mathematically as:
where:
- is the thermal efficiency,
- is the thrust produced by the engine,
- is the jet velocity,
- ṁp is the mass flow rate of Propellant,
- is the specific enthalpy of the fuel.