The Sabre engines have a dual mode capability. In rocket mode the engine operates as a closed cycle Lox/Lh2 high specific impulse rocket engine. In airbreathing mode (from takeoff to Mach 5) the liquid oxygen flow is replaced by atmospheric air, increasing the installed specific impulse 3-6 fold. The airflow is drawn into the engine via a 2 shock axisymmetric intake and is cooled to cryogenic temperatures prior to compression. The hydrogen fuel acts as a heatsink for the closed cycle helium loop before entering the combustion chamber.