In the tail or aft section, each pod contains 12 primary engines and 2 vernier engines. All primary thrusters and vernier engines were designed for a 100-mission lifespan. It is pressurized to Earth's air pressure at sea level. 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.