SABRE design is neither a conventional rocket engine nor jet engine, but a hybrid that uses air from the environment at low speeds/altitudes, and stored liquid oxygen (LOX) at higher altitude. The SABRE engine "relies on a heat exchanger capable of cooling incoming air to −150 °C (−238 °F), to provide oxygen for mixing with hydrogen and provide jet thrust during atmospheric flight before switching to tanked liquid oxygen when in space."
In air-breathing mode, air enters the engine through an inlet. A bypass system directs some of the air through a precooler into a compressor, which injects it into a combustion chamber where it is burnt with fuel, the exhaust products are accelerated through nozzles to provide thrust. The remainder of the intake air continues through the bypass system to a ring on flame holders which act as a ramjet for part of the air breathing flight regime. A helium loop is used to transfer the heat from the precooler to the fuel and drive the engine pumps and compressors.