Taken from the October 2020 issue of Physics World, where it appeared under the headline "Eliminating the boundary between sky and space".
Reusable vehicles are vital to make access to space more affordable, but conventional rocket engines have their limits. Oliver Nailard describes how UK firm Reaction Engines hopes to revolutionize space access with a new class of propulsion system, the Synergetic Air Breathing Rocket Engine (SABRE)