[5][6] The engine is designed to achieve single-stage-to-orbit capability, propelling the proposed Skylon spaceplane to low Earth orbit.

The low temperature of the air permits light alloy construction to be employed and allow a very lightweight engine—essential for reaching orbit.

[2] After shutting the inlet cone off at Mach 5.14, and at an altitude of 28.5 km (17.7 mi),[3] the system continues as a closed-cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks, potentially allowing a hybrid spaceplane concept like 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 European Union-funded LAPCAT study.

The RB545's precooler had issues with embrittlement and excess liquid hydrogen consumption, and was encumbered by both patents and the UK's Official Secrets Act, so Bond developed SABRE instead.

A bypass system directs some of the air through a precooler into a compressor, which injects it into a combustion chamber where it is burnt with fuel, the exhaust products are accelerated through nozzles to provide thrust.

However, for a single-stage-to-orbit (SSTO) spaceplane, such heavy materials are unusable, and maximum thrust is necessary for orbital insertion at the earliest time to minimise gravity losses.

SABRE inserts a helium cooling loop between the air and the cold fuel to avoid problems with hydrogen embrittlement in the precooler.

[17] The cooler consists of a fine pipework heat exchanger with 16,800 thin-walled tubes,[18] and cools the hot in-rushing atmospheric air down to the required −150 °C (−238 °F) in 0.01 s.[19] The ice prevention system had been a closely guarded secret, but REL disclosed a methanol-injecting 3D-printed de-icer in 2015 through patents, as they needed partner companies and could not keep the secret while working closely with outsiders.

[20][21][22] Below five times the speed of sound and 25 kilometres of altitude, which are 20% of the speed and 20% of the altitude needed to reach orbit, the cooled air from the precooler passes into a modified turbo-compressor, similar in design to those used on conventional jet engines but running at an unusually high pressure ratio made possible by the low temperature of the inlet air.

This bypass ramjet system is designed to reduce the negative effects of drag resulting from air that passes into the intakes but is not fed into the main rocket engine, rather than generating thrust.

This distinguishes the system from a turboramjet where a turbine-cycle's exhaust is used to increase air-flow for the ramjet to become efficient enough to take over the role of primary propulsion.

Successful tests of an oxidiser (both air and oxygen) cooled combustion chamber were conducted by EADS-Astrium at Institute of Space Propulsion in 2010 In 2011, hardware testing of the heat exchanger technology "crucial to [the] hybrid air- and liquid oxygen-breathing [SABRE] rocket motor" was completed, demonstrating that the technology is viable.

[28][29] The tests validated that the heat exchanger could perform as needed for the engine to obtain adequate oxygen from the atmosphere to support the low-altitude, high-performance operation.

[36] In April 2015, the SABRE engine concept passed a theoretical feasibility review conducted by the U.S. Air Force Research Laboratory.

[40] In August 2015 the European Commission competition authority approved UK government funding of £50 million for further development of the SABRE project.

[42][43] In 2016, Reaction CEO Mark Thomas announced plans to build a quarter-sized ground test engine, given limitations of funding.

[49] In September 2017 it was announced the United States Defence Advanced Research Projects Agency (DARPA) had contracted with Reaction Engines Inc. to build a high-temperature airflow test facility at Front Range Airport near Watkins, Colorado.

[51][52] The HTX test unit was completed in the UK and sent to Colorado in 2018, where on 25 March 2019 an F-4 GE J79 turbojet exhaust was mixed with ambient air to replicate Mach 3.3 inlet conditions, successfully quenching a 420 °C (788 °F) stream of gases to 100 °C (212 °F) in less than 1/20 of a second.

[56] In March 2019, the UKSA and ESA preliminary design review of the demonstrator engine core confirmed the test version to be ready for implementation.

The testing was successfully completed by the company’s US subsidiary (Reaction Engines Incorporated – REI) and the US Air Force Research Laboratory (AFRL).

In October 2024, reports emerged that the company entered administration after failing to secure additional funding, leading to significant layoffs.

[61] Due to the static thrust capability of the hybrid rocket engine, the vehicle can take off under air-breathing mode, much like a conventional turbojet.

The engine had no air-breathing static thrust capability, relying on a rocket trolley to achieve takeoff Designed for use with Skylon A4.

In contrast, the SABRE engine permits a much slower, shallower climb (thirteen minutes to reach the 28.5 km transition altitude), while breathing air and using its wings to support the vehicle.