SABRE (Synergistic Air-Breathing Rocket Engine) is a GSTP activity with the UK that functions as a combined cycle engine, which can operate in both air breathing and pure rocket mode. When moving at speeds up to Mach three it will still use the on-board compressor. Beyond that it will switch to being a ram jet. In 2018, the final design concept for an air intake and a bypass burner system was developed with Germany.