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The RB545 was an air-breathing rocket engine that was proposed to propel a British space shuttle (see HOTOL) to orbit using a single stage. The Rolls-Royce company was involved, while British Aerospace (BAe) worked on the vehicle.
The exact details of this engine were covered by the UK's Official Secrets Act and there is consequently little public information about its operation, although it has apparently been declassified.
Within the atmosphere, hydrogen fuel would be used in a heat exchanger to precool air entering a high overall pressure ratio turbojet-like engine cycle, before being burnt with the air in a rocket motor, to produce a very high velocity propulsive jet.
Once out of the atmosphere, the RB545 would be capable of burning the hydrogen with on-board liquid oxygen as a high-efficiency hydrogen/oxygen rocket. The engine was given the Rolls Royce name "Swallow".