Part of a US$37.3 million Series B funding round that also included BAE Systems, it follows a £60 million (US$85.6 million) investment in the company by the British government in 2013.

Reaction Engines has been working on its plans for a hypersonic spaceplane long before the company was founded in 1989. Based on the work of engineer Alan Bond, it began life as the HOTOL project, which was a joint project by Rolls-Royce and British Aerospace. When that fell through due to technical problems, Bond, along with engineers John Scott-Scott and Richard Varvill, formed REL, which has concentrated for the past 29 years on developing variants of the Synergetic Air-Breathing Rocket Engine (SABRE) engine.

The main component of REL's Skylon spaceplane, SABRE is a hypersonic hybrid engine that acts like a conventional jet at speeds below Mach 5 (3,704 mph, 5,961 km/h), but at hypersonic speeds it converts into a pure rocket engine burning hydrogen and liquid oxygen to achieve speeds of up to Mach 25 (17,521 mph, 29,808 km/h). Key to this is a revolutionary heat exchanger that protects the engine against melting as it approaches hypersonic velocity.