For 20 years Reaction Engines has been developing its Synergetic Air-Breathing Rocket Engine (SABRE), which works like a normal jet engine while in the Earth’s atmosphere, sucking in air to burn with its hydrogen fuel. However, once it hits five times the speed of sound – about 2,500mph at altitude - and is close to leaving the atmosphere where there is no air, it switches to being a conventional rocket engine, burning the liquefied oxygen it carries along with its fuel.