A new air-breathing rocket engine is ready for a major round of testing in the next 18 months after having passed a preliminary design review by the European Space Agency (ESA).

The Synergistic Air-Breathing Rocket Engine (SABRE), which is being developed by the U.K. company Reaction Engines, can switch between two modes. In aircraft-engine mode, it uses oxygen from the atmosphere, and in rocket-engine mode, it burns an oxidizer carried onboard together with the fuel liquid hydrogen.

The technology, deemed particularly promising for suborbital spaceflight and supersonic intercontinental travel, could one day revolutionize space transportation, advocates say.

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SABRE is a hybrid jet and rocket engine that was originally designed for a single-stage-to-orbit space plane. (Image credit: Reaction Engines Ltd.)

The engine uses atmospheric air up to an altitude of 15.5 miles (25 kilometers) before switching to rocket mode. The launch vehicle, or space plane , would therefore need to carry less oxygen and so could be lighter and cheaper. SABRE is also designed to be reusable, which could offer further cost reductions.

"The positive conclusion of our preliminary design review marks a major milestone in SABRE development," Mark Ford, the head of ESA's Propulsion Engineering section, said in a statement . "It confirms [that] the test version of this revolutionary new class of engine is ready for implementation."

The testing of the demonstrator engine core will take place in Westcott, in the English county of Buckinghamshire, at a new facility that is currently being built by Reaction Engines.

The company previously received 10 million euros ($11.3 million at current exchange rates) from ESA and 60 million euros ($67.8 million) from the U.K. government to fund the development of the technology.

ESA earlier helped Reaction Engines validate the design of the so-called precooler, an essential component of SABRE, which cools the hot airstream entering the engine at hypersonic speed during the aircraft-like portion of SABRE's operation.

"One of the great advantages of the SABRE propulsion concept is that it is totally modular from both design and operational perspectives," Reaction Engines Chief Technology Officer Richard Varvill said in the same statement.

"Therefore, it is possible to subject each of the key components of the engine to rigorous ground testing, which fully mimic the operational conditions the engine will face up to Mach 5 flight [five times the speed of sound ] at 25-km altitude," Varvill added.

ESA said launchers equipped with SABRE could be up to 50% lighter than current launchers and possibly offer a higher launch rate due to their reusability.