The UK Sabre engine is an innovative hybrid engine that has the potential to revolutionize air travel. Developed by a British aerospace company called Reaction Engines, the engine combines the best features of a jet engine and a rocket engine. It is designed to take off and land like a conventional jet engine, but once it reaches the edge of the Earth's atmosphere, it can switch to rocket mode and reach speeds up to Mach 5 or even higher. The Sabre engine works by using a closed-loop cooling system to cool incoming air from 1,000°C to minus 150°C in just a fraction of a second. This process allows the engine to extract oxygen from the air, which it then uses to burn stored hydrogen fuel. The UK Sabre engine has the potential to reduce flight times by a significant margin, making it a game-changer for the aerospace industry.

The spacecraft must be powered solely by SABRE engines; it should be a single stage craft that takes off under the power of SABRE, and reaches orbit the same way.