Rolls-Royce, Boeing and BAE Systems are investing in Reaction Engines Limited, the UK-based aerospace technology company. The focus of these investments is the company's Synergetic Air-Breathing Rocket Engine (SABRE). SABRE engines are unique in delivering the fuel efficiency of a jet engine with the power and high-speed ability of a rocket that is capable of Mach 5 in air-breathing mode and Mach 25 in rocket mode for space flight. The company is about to start building a new UK test site to test critical subsystems and aim to test a fully integrated engine core in 2020.