Three Space Shuttle Main Engines (SSMEs) were mounted on the orbiter's aft fuselage in the pattern of an equilateral triangle. These three liquid-fueled engines could be swiveled 10.5 degrees vertically and 8.5 degrees horizontally during the rocket-powered ascent of the orbiter in order to change the direction of their thrust. Hence, they steered the entire Space Shuttle, as well as providing rocket thrust towards orbit. The aft fuselage also housed three auxiliary power units (APU). The APUs chemically converted hydrazine fuel from a liquid state to a gas state, powering a hydraulic pump which supplied pressure for all of the hydraulic system, including the hydraulic sub-system that pointed the three main liquid-fueled rocket engines, under computerized flight control. The hydraulic pressure generated was also used to control all of the orbiter's flight control surfaces (the elevons, rudder, speed brake, etc.), to deploy the landing gear of the orbiter, and to retract the umbilical hose connection doors located near the rear landing gear, which supplied the orbiter's SSMEs with liquid hydrogen and oxygen from the external tank.
