Space shuttle main engine burns a combination of liquid oxygen and liquid hydrogen fed from the space shuttle external tank. The combustion process takes place in two steps, firstly the fuel is burnt at high pressure and low temperature and then it is burned completely at high pressure and high temperature.
Due to this staged burning of fuel, amount of fuel required would be comparatively less and hence, SSME's efficiency to produce thrust is about 99%. The space shuttle main engines are controlled during flight by digital computer systems mounted on each engine.
These operate in conjunction with engine sensors, valve actuators and spark igniters. These devices help in monitoring engine control, checkout and status of the combustion process of fuel.
Each SSME has one main engine controller consisting of two digital computers and their related electronics.