Heavy-duty hydraulic systems provide power for the Orbiter's ascent, reentry, and landing operations. The hydraulic system is powered by three independent auxiliary power units, fueled by monopropellant hydrazine. Hydraulic actuators move wing aerodynamic control surfaces, a body flap, and the rudder. They also operate valves, lower landing gear, control main engine thrust direction, and provide power for Orbiter brakes. Electrical power operates everything else.
Electricity is provided by three fuel cells.
If the computers disagree, they, in effect, vote between themselves, and the data from the outvoted computer are ignored. Spacelab will be carried inside Orbiter's cargo bay on as many as three or more missions per year. As many as five pallets can be flown at a time. The igloo houses subsystems for operating experiments, including command, data management, and electrical power subsystems, which on other missions are located in the habitable module. One by one, the Shuttle's components were tested and pronounced ready.