The Sabre – the Synergistic Air-Breathing Rocket Engine - design overcomes the problems encountered by conventional engines, which cannot "breathe" air when travelling at more than 3,200km/h. Sabre switches in flight to become a rocket engine that could boost the Skylon spaceplane to faster than Mach 5 - more than five times the speed of sound. Reaction Engine’s Sabre design slows down the air entering the engine at hypersonic speeds using a series of pipes filled with liquid helium. This cools the air, which would normally heat it up to around 1,000C. The Sabre design cools it down to 140C in just a fraction of a second, before it enters the engine.