The design of Sabre evolved from liquid-air cycle engines (LACE) which have a single rocket combustion chamber with associated pumps, preburner and nozzle which are utilised in both modes. LACE engines employ the cooling capacity of the cryogenic liquid hydrogen fuel to liquefy incoming air prior to pumping. Unfortunately, this type of cycle necessitates very high fuel flow.

These faults are avoided in the Sabre engine, which only cools down the air to the vapour boundary and avoids liquefaction. This allows the use of a relatively conventional turbocompressor and avoids the requirement for an air condenser.

The Sabre engine is essentially a closed cycle rocket engine with an additional precooled turbo-compressor to provide a high pressure air supply to the combustion chamber. This allows operation from zero forward speed on the runway and up to Mach 5.5 in air breathing mode during ascent. As the air density falls with altitude the engine eventually switches to a pure rocket propelling Skylon to orbital velocity (around Mach 25).

Air collection is via a simple conical two shock inlet with a translating centrebody to maintain shock-on-lip conditions. The centrebody moves forward to close the inlet for re-entry. A bypass system is used to match the variable captured air flow to the engine demand. This bypass flow is reheated in order to recover the momentum lost through the capture shock system.

The thrust during airbreathing ascent is variable but around 200 tonnes. During rocket ascent this rises to 300 tonnes but is then throttled down towards the end of the ascent to limit the longitudinal acceleration to 3.0g.