(Reuters) - Here is a look at the Synergetic Air-Breathing Rocket Engine (SABRE) developed by Reaction Engines Ltd over the last 20 years which could power a re-usable space plane.

Sabre will be a new engine class that can operate in both air-breathing and rocket modes.

* Alan Bond, a former British aircraft engineer, began his career with Rolls-Royce's Rocket Division, and went on to design the HOTOL space plane. The HOTOL or "Horizontal Take Off and Landing" project was to be fitted with an air-breathing engine, the RB454, which would extract the air's oxygen to burn. Once the air became too thin, a small supply of Liquid Oxygen would be used to place the craft in orbit. Around 7 tons of cargo could have been orbited for an estimated cost of 5 million pounds, less than for a Space Shuttle launch. The HOTOL study was launched in 1986, but two years later the government refused to fund it further. In 1989 Bond helped form Reaction and designed its new concept craft, Skylon.

* Designing a single stage to orbit propulsion system has been unsuccessful till now, largely due to the weight of the on-board oxidiser including liquid oxygen, needed by conventional rocket engines. A possible solution was to reduce the weight by using oxygen in the atmosphere in the combustion process just like an ordinary jet engine. The saving in weight would enable single stage launch vehicles to be re-usable. Ultra-lightweight heat exchangers are the key enabling components in the engines for Mach 5 cruising speed and aircraft-like access to space.

* This new approach would enable Sabre-powered vehicles to save carrying over 250 tons of on-board oxidant on their way to orbit, and removes the necessity for massive throw-away first stages that are jettisoned.

* Sabre could be the first engine to achieve this goal by operating in two modes: initially in air-breathing mode and then in conventional rocket mode. The rocket engine sucks in atmospheric air as a source of oxygen (as in a typical jet engine) to burn with its liquid hydrogen fuel in the rocket combustion chamber. Secondly in conventional rocket mode - the engine is above the atmosphere and transitions to using on-board liquid oxygen.

* The Skylon space plane only exists on paper. It would use the Sabre engine, could operate without a pilot, and be capable of transporting 15 tonnes of cargo into space. Lapcat is a concept passenger aircraft that could use the Sabre engine to cut flight time from Brussels to Sydney to less than 4 hours.

Sources: Reuters/http://www.reactionengines.co.uk/http://www.britain-in-space.co.uk

(Reporting by David Cutler, London Editorial Reference Unit)

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Factbox - Sabre Engine could revolutionise space flight