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Space shuttles have many important subsystems, including: the Orbital Maneuvering System (OMS); the Reaction Control System (RCS); Electrical Power; Hydraulic Power; Environment Control and Life Support System (ECLSS); and Thermal Protection.
Orbital Maneuvering System (OMS): Two rocket units at the orbiter's aft end, at the base of the vertical tail, are used to place the vehicle onto its final orbital path, as well as for extended maneuvering while in space. The OMS is also used to slow the orbiter at the end of a mission. As the orbiter slows down, gravity pulls it back into the atmosphere, and it glides back to Earth for a runway landing. The OMS uses nitrogen tetroxide and monomethyl hydrazine for fuel. Each engine produces 6,000 pounds of thrust.
Reaction Control System (RCS): This system consists of 44 nozzles on both sides of the nose and each side of the aft fuselage pod near each OMS engine. The RCS is used throughout the mission to move or roll the orbiter as the crew carries out tasks which require the vehicle to be pointed certain ways for experiments and photography. The RCS uses the same types of fuel as the OMS. Thirty-eight of the thrusters produce 870 pounds of thrust each. The six others each produce 25 pounds of thrust.
Electrical Power: Three fuel cells supply electrical power on the orbiter during all phases of a mission. The units are located in the mid-body area of the payload bay. Electrical power is produced by the chemical reaction of hydrogen and oxygen, which are supplied continuously as needed to meet output requirements. A by-product of this reaction is drinking water for the crew. Each fuel cell is connected to one of three independent electrical distribution systems. During peak and average power loads, all three systems are used. During minimum loads, only two are used and the third is on standby, but can be brought back on line instantly if needed. The system provides up to 24 kilowatts of power, ranging from 27.5 to 32.5 volts of direct current.
Hydraulic Power: Three auxiliary power units (APUs) furnish power to operate hydraulic systems on the orbiters, such as the main engine gimbaling controls, the nose and main landing gear and brake systems, and the rudder, speed brake, and eleven flight control surfaces. The APUs are fueled by hydrazine which is changed into a hot gas by a granular catalyst. The momentum of the expanding gas spins turbine blades and this energy is transferred to gearboxes on the hydraulic pump units. All three APUs operate during launch, but only two are needed for reentry and landing.
Environment Control and Life Support System (ECLSS): The orbiter's environmental control and life-support system purifies the cabin air, adds fresh oxygen, keeps the pressure at sea level, heats and cools the air, and provides drinking and wash water. The system also includes lavatory facilities. The cabin is pressurized to sea level (14.7 psi) with 21 percent oxygen and 79 percent nitrogen, comparable to earth's atmosphere. The air is circulated through lithium hydroxide/charcoal canisters which remove carbon dioxide. The canisters are changed on a regular basis. Excess heat from the cabin and flight-deck electronics is collected by a circulating coolant water system and transferred to radiator panels on the payload bay doors where it is dissipated. The fuel cells produce about seven pounds of water each hour. It is stored in tanks, and excess water is dumped overboard. The lavatory unit collects and processes body waste, and also collects wash water from the personal hygiene station. The lavatory unit, located in the mid deck area, operates much like those on commercial airlines but is designed for a weightless space environment.
Thermal Protection: The thermal protection system is designed to limit the temperature of the orbiter's aluminum and graphite epoxy structures to about 350 degrees (F) during reentry. There are four types of materials used to protect the orbiter. Reinforced carbon-carbon is a composite of a layer of graphite cloth contained in a carbon matrix. It is used on the nose cap and wing leading edges where temperatures exceed 2,300 degrees (F). High-temperature reusable surface insulation consists of about 20,000 tiles located mainly on the lower surfaces of the vehicle. The tiles are about six inches square and made of a low-density silica fiber insulator bonded to the surface in areas where temperatures reach up to 1,300 degrees (F). Low-temperature reusable surface insulation also consists of tiles. There are about 7000 of these on the upper wing and fuselage sides where temperatures range from 700 to 1,200 degrees (F). Flexible reusable surface insulation (coated Nomex felt) is sheet-type material applied directly to the payload bay doors, sides of the fuselage and upper wing areas where heat does not exceed 700 degrees (F).
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