The Space Shuttle s Main Engine (SSME) was a highly innovative, high performance, liquid propelled rocket engine with a variable thrust and mixture ratio. It was controlled electronically by an automatic system that could perform checkout, start, mainstage and shutdown functions. Ignited on the ground, the three engines on the orbital vehicle burned for an average of ten minutes during the boost phase. The SSME used a staged combustion cycle in which propellants were burned at high pressure and relatively low temperature in the preburners, and then completely combusted at high temperature and high pressure in the main chamber before expanding through the nozzle. Hydrogen fuel was usedduring combustion to cool vulnerable parts of the engine. Originally created in 1973 during the development of the Space Shuttle Program by NASA prime contractor Rockwell International, this informative booklet provides an overview of the SSME s design and components and their physical characteristics.