SABRE (rocket engine)

SABRE (Synergistic Air-Breathing Rocket Engine) [1] is a concept by Reaction Engines Limited for a hypersonic precooled hybrid air breathing rocket engine [4] for propelling the proposed Skylon launch vehicle into low Earth orbit (LEO). SABRE is the logical continuation of Alan Bond's series of liquid air cycle engine (LACE) and LACE-like designs that started in the early/mid-1980s for the HOTOL project.

The design comprises a single combined cycle rocket engine with two modes of operation.[2] The air breathing mode combines a turbo-compressor with a lightweight air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram compressed air leading to an unusually high pressure ratio within the engine. The compressed air is subsequently fed into the rocket combustion chamber where it is ignited with stored liquid hydrogen. The high pressure ratio allows the engine to continue to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed which gives a very lightweight engine — essential for reaching orbit. In addition, unlike the LACE concept that preceded it, SABRE’s precooler does not liquefy the air letting it run more efficiently.[3]

After shutting the inlet cone off at Mach 5.14, 28.5 km altitude,[2] the system continues as a closed cycle high performance rocket engine combusting liquid oxygen and liquid hydrogen from on-board fuel tanks allowing Skylon to reach orbital velocity after leaving the atmosphere on a steep climb.

An engine derived from the SABRE concept called Scimitar has been designed for the company’s A2 hypersonic passenger jet proposal for the EU-funded LAPCAT study.[5]

Alan Bond says that the technology readiness level of the SABRE engine is, as of May 2009, 2-3.[6] A project called the Technology Demonstration Programme, funded through private finance as well as an ESA grant, began in February 2009 with the aim of validating key aspects of the engine by the end of 2011. The company anticipates this will bring the system to a technology readiness level of 4-5.[7][8]