Reaction Engine's Synergetic Air-Breathing Rocket Engine (SABRE) is closer to reality with ESA and the UK Space Agency (UKSA) green lighting the preliminary design of the demonstrator engine core. The successful review of the hypersonic engine, which is designed to act as both a jet and a rocket, means that the company can move on the major testing milestones in the next 18 months.

It's been more than 30 years since the ancestor of the SABRE engine began development, with significant investments by Boeing, Rolls-Royce, ESA, and UKSA to the tune of tens of millions of pounds. This may seem like a long time, but SABRE, which is the power plant of the Reaction Engine Skylon spaceplane , is a significant step forward in orbital launcher design.

SABRE is a hypersonic hybrid engine. It draws in air to act like a conventional jet while accelerating to speeds of up to Mach 5 (3,704 mph, 5,961 km/h), then converts to a pure rocket engine burning hydrogen and liquid oxygen, making speeds of up to Mach 25 (17,521 mph, 29,808 km/h) possible.

Cutaway view of the latest iteration of the SABRE engine ESA

Key to this is a revolutionary heat exchanger that protects the engine as it approaches hypersonic velocity. At high supersonic speeds, the air is coming into the engine with the force 25 times that of a category 5 hurricane, generating temperatures that would melt any material that the engine might be made of. To prevent this, SABRE has a precooler that uses recirculated cryogenic hydrogen fuel to cool incoming air down from 1,000° C (1,832° F) to -150° C (-302° F) in 1/100th of a second.

This precooler was tested in 2012 by Reaction Engine and ESA. Now it's the turn of the engine core where combustion takes place, and where the turbomachinery modules are located. When this is tested and combined with the precooler and rocket nozzle, SABRE will be a complete engine that has the potential to significantly lower the cost of boosting payloads into orbit by vastly reducing the amount of liquid oxygen that needs to be carried to provide thrust.

Air flow inside the SABRE engine ESA

ESA says the tests will take place at its ground-test facility at Westcott, Buckinghamshire, where new firing facilities are currently under construction. This is an historic site that in the past saw such iconic British rockets at the Blue Streak and Black Arrow developed.

"One of the great advantages of the SABRE propulsion concept is that it is totally modular from both design and operational perspectives" says Richard Varvill, Chief Technology Officer of Reaction Engines. "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 at 25 km (16 mi) altitude."

The video below shows the SABRE engine design.

Source: ESA