SABRE (Synergic Air Breathing Engine) is a design for a hypersonic hydrogen-fueled air breathing combined cycle rocket engine/turbojet engine/ramjet engine for propelling the 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 SABRE design combines a lightweight turbine-cycle jet engine with an air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram compressed air, which allows the jet engine to continue to provide high thrust even at very high speeds. In addition, the low temperature of the air allows light alloy construction to be employed which gives a very lightweight engine  essential for reaching orbit.

The engine also includes rocket engine features which allow the vehicle to reach low earth orbit after leaving the atmosphere after shutting the inlet cone off at Mach 5.5, 26 km altitude.

History

The precooler concept is due to an idea originated by Robert P. Carmichael in 1955. This was followed by the Liquid Air Cycle Engine (LACE) idea which was originally explored by Marquardt and General Dynamics in the 1960s as part of the US Air Force's aerospaceplane efforts. This work eventually culminated in medium-thrust engines that ran for several minutes at a time. Although the program was generally successful, changing priorities and USAF funding led to the idea being abandoned.

In an operational setting with LACE, the system was to be placed behind a supersonic air intake which would compress the air through ram compression, then a heat exchanger would rapidly cool it using some of the liquid hydrogen fuel stored on board. The resulting liquid air was then processed to separate out the liquid oxygen for burning in the engine. The amount of warmed hydrogen was too great to burn with the oxygen, so most was to be simply dumped overboard (nevertheless giving useful thrust.)

HOTOL's engine, the RB545, was similar to the original LACE system, but much simpler in detail. Like the LACE system it used an air intake and a hydrogen heat exchanger, but was able to do away with much of the complexity of the USAF design, which included pumps and storage tanks along each step of the separation process.

The RB545 was a "straight-through" design that used careful arrangement of the components to dump the unwanted liquefied gasses directly overboard, instead of pumping it around from tank to tank. In addition, the RB545 did not liquefy the oxygen or nitrogen, but simply compressed the gas in a similar way to turbojet engines. From that point on, the RB545, like LACE before it, consisted of a fairly conventional rocket engine, but running on a variable mixture of compressed air and liquid oxygen.

In 1989, after funding for HOTOL ceased, Bond and several others formed Reaction Engines Limited to continue research. The RB545's liquid hydrogen precooler had issues with embrittlement, patents and The Official Secrets Act, so Bond went on to develop SABRE in its place.