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Orbiter Performance

The Space Shuttle Orbiter has proven itself to be a versatile, reliable vehicle capable of carrying out a number of tasks.

Employing a payload bay measuring 60 feet long by 15 feet wide, the Orbiter is designed to carry payloads into space and perform missions at orbital altitudes ranging from 115 to 250 miles.

Payload capability averages about 37,800 pounds per mission, but under certain conditions heavier payloads may be carried. An Orbiter performing a mission at a lower altitude would be able to carry a heavier payload than one performing a mission at a higher altitude.

Given Space Shuttle performance enhancements like lighter weight External Tanks and improved Main Propulsion System, Space Shuttle payload capability is expected to peak at about 60,000 pounds.

The Orbiter is designed to carry a maximum crew of eight astronauts, although it can carry up to ten astronauts in an emergency. The Orbiter carries all of the supplies and equipment necessary for the crew to perform its mission.

The mission duration of the Orbiter is typically seven to nine days, although certain Orbiters have been modified to allow missions of up to 16 days. The Orbiter affords its occupants a shirt-sleeve environment, and never produces stresses in excess of three g's, which is less than many amusement park thrill rides.

The Orbiter is launched in an upright position, with thrust provided by three Space Shuttle Main Engines (SSME) and two Solid Rocket Boosters (SRB). SRB separation occurs about two minutes after launch, and the SSME's burn for about 8.5 minutes after launch.

The SSME's shut down just before the Orbiter reaches orbit. The External Tank (ET) that provides fuel for the SSME separates from the Orbiter shortly after SSME cutoff. Combinations of the 38 reaction control thrusters and six vernier thrusters are fired to stabilize the Orbiter during ET separation and help clear the Orbiter from the ET.

Combinations of the reaction control thrusters and vernier thrusters are also fired to support attitude pitch, roll and yaw maneuvers as the Orbiter continues its ascent after ET separation. The two orbital maneuvering system engines are then fired to place the Orbiter on its proper orbit.

Once the Orbiter reaches its proper orbit, the orbital maneuvering system engines can be fired again to support any major velocity maneuvers that become necessary. Combinations of the reaction control thrusters and vernier thrusters may be fired to support precision operations such as rendezvous and docking operations.

Once the mission is completed, the orbital maneuvering system engines are fired to slow the Orbiter in what is called the deorbit burn, or deorbit maneuver. Once in orbit, the Orbiter travels at a speed of about 25,400 feet per second. The deorbit burn decreases the Orbiter speed to about 300 feet per second as it prepares for re-entry.

The unpowered Orbiter then re-enters the atmosphere, and is guided to a precision landing like a traditional aircraft. There are three Space Shuttle landing sites available in the United States. These are located at the Kennedy Space Center in Florida, Edwards Air Force Base in California and White Sands in New Mexico.

Once on the ground, a number of highly specialized vehicles approach the Orbiter to perform a variety of servicing and safety tasks prior to crew egress. Great care is taken to provide for the safety of the crew and prevent toxic fuels and gases from harming the environment.

The Orbiter is then routinely serviced for its next mission in a turnaround that typically takes two to three months. In certain circumstances, the Orbiter may be ferry-flown to California for factory modifications or major servicing.

In the event an emergency is encountered during flight, the Orbiter has several flight options available. These include:

1. Return To Launch Site (RTLS), in which the Orbiter may return to the Kennedy Space Center if all thrust is lost from one Space Shuttle Main Engine (SSME) between liftoff and launch plus 4 minutes, 20 seconds, after which time there is not enough fuel available to support this type of abort.

An RTLS abort consists of a powered stage, at which time the SSME's are still firing. This is followed by an External Tank separation stage, which can not occur until after the Solid Rocket Boosters are jettisoned. Finally, the Orbiter is maneuvered into a glide stage for a return to the launch site.

2. Transatlantic Abort Landing (TAL), in which the Orbiter may land at an overseas abort landing site if a Space Shuttle Main Engine (SSME) fails after the last RTLS abort opportunity but before any other abort can be accomplished. The TAL abort would also be attempted if an Orbiter system failure prevented any other type of abort.

Using the TAL abort, the Orbiter would complete a powered flight on a ballistic path across the Atlantic Ocean, and would then perform a glide landing at a pre-selected runway located in either the city of Moron in Spain, the city of Dakar in Senegal or the city of Ben Guerur in Morocco.

3. Abort To Orbit (ATO), in which the Orbiter may reach a lower, but safe, orbit if a propulsion failure does not allow it to reach its intended orbit. An ATO was performed during Space Shuttle Mission STS-51F, in which Challenger was able to successfully complete its mission at a lower orbital altitude.

4. Abort Once Around (AOA), in which the Orbiter may travel once around the Earth before making a landing in the United States. The AOA would be used if a propulsion failure did not allow the Orbiter to maintain any orbit, even one lower than intended.

The AOA would also be used if for some reason a system failure required the Orbiter to land quickly after it reached orbit. For all intents and purposes, an AOA would be performed in a similar manner to a normal re-entry and landing.

5. Contingency Abort, which would be used if the Orbiter could not land on a runway. During a Contingency Abort, the Orbiter would ideally be guided to a safe glide path to allow the astronauts to use the Inflight Crew Escape System. If a safe glide path was not possible, the Orbiter would have to be ditched with the crew aboard.

The Space Shuttle Orbiter has proven itself to be a versatile vehicle, and has supported a number of diverse mission applications. These have included the deployment of a variety of scientific, military and commercial satellites and the deployment of scientific space probes.

A vast number of scientific investigations have been conducted aboard Space Shuttle Orbiters, including those performed inside pressurized laboratory modules housed in the Orbiter payload bay. Many scientific payloads have been carried in the Orbiter payload bay, including free-flying satellites that were deployed and retrieved.

Many scientific accomplishments have been made through spacewalks conducted from the Orbiter. In addition to rehearsing construction techniques for the International Space Station, astronauts have demonstrated that on-orbit repair and maintenance of satellites is possible, as are a number of on-orbit troubleshooting activities.

Space Shuttle Orbiters also have completed an ambitious docking program with the Russian Mir Space Station, helping to extend an unprecedented continuous U.S. presence in space while ushering in a new era of international cooperation in space.

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