Space Shuttle

The Space Shuttle, part of the Space Transportation System (STS), is an American spacecraft operated by the National Aeronautics and Space Administration (NASA) for orbital human spaceflight missions. The first of four test flights occurred in 1981, which were followed by operational flights beginning in 1982. The system is scheduled to be retired from service in 2011 after 135 launches. Major missions have included launching numerous satellites and interplanetary probes, conducting space science experiments, and servicing and construction of space stations. The Shuttle has been used for orbital space missions by NASA, the U.S. Department of Defense, the European Space Agency, and Germany. The United States funded STS development and shuttle operations. At launch, the Space Shuttle consists of the shuttle stack which includes a dark orange-colored external tank (ET); two white, slender Solid Rocket Boosters (SRBs); and the STS Orbiter Vehicle (OV) which contains the crew and payload. The ET was painted white for the first 2 missions, but was discontinued to save weight. Payloads can be launched into higher orbits with either of two different booster stages developed for the STS (1 stage Payload Assist Module or 2 stage Inertial Upper Stage). The Space Shuttle is "stacked" in the Vehicle Assembly Building and the stack mounted on a mobile launch platform held down by four explosive bolts on each SRB which are detonated at launch. The shuttle stack launches vertically, like a conventional rocket, from a mobile launch platform. It lifts off under the power of its two SRBs and the three main engines which are fueled by liquid hydrogen and liquid oxygen from the external tank. The Space Shuttle has a two stage ascent. The SRBs provide additional thrust during liftoff and first stage flight. About two minutes after liftoff explosive bolts are fired releasing the SRBs which parachute into the ocean to be retrieved by ships for refurbishment and reuse. The shuttle orbiter and external tank continue to ascend on an increasingly horizontal flight path under power from the three main engines. Upon reaching 17,500 mph (7.8 km/s), necessary for low Earth orbit, the main engines are shut down. The external tank is then jettisoned downward to burn up in the atmosphere. It is, however, possible for the external tank to be re-used in orbit.[6] After jettisoning the external tank, the orbital maneuvering system (OMS) engines may be used to adjust the orbit. The orbiter carries astronauts and payload such as satellites or space station parts into low earth orbit, into the Earth's upper atmosphere or thermosphere.[7] Usually, five to seven crew members ride in the orbiter. Two crew members, the Commander and Pilot, are sufficient for a minimal flight, as in the first four "test" flights, STS-1 through STS-4. A typical payload capacity is about 22,700 kilograms (50,000 lb), but can be raised depending on the choice of launch configuration. The orbiter carries the payload in a large cargo bay with doors that open along the length of its top, a feature which makes the Space Shuttle unique among present spacecraft. This feature made possible the deployment of large satellites such as the Hubble Space Telescope, and also to capture and return large payloads back to Earth. When the orbiter's space mission is complete it fires its OMS thrusters to drop out of orbit and re-enter the lower atmosphere. During the descent, the shuttle orbiter passes through different layers of the atmosphere and decelerates from hypersonic speed primarily by aerobraking. In the lower atmosphere and landing phase, it acts as a glider with reaction control system (RCS) thrusters and fly-by wire controlled hydraulically actuated flight surfaces controlling its descent. It then makes a landing on a long runway as a spaceplane. The aerodynamic shape is a compromise between the demands of radically different speeds and air pressures during re-entry, subsonic atmospheric flight, and hypersonic flight. As a result the orbiter has a high sink rate at low altitudes, and transitions from using RCS thrusters in low pressure to flight surfaces at low altitudes.