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Space Shuttle

The Space Transportation System (STS), also known as the Space Shuttle is a reusable spacecraft designed to be launched into orbit by rockets and then to return to the Earth's surface by gliding down and landing on a runway. The Shuttle was selected in the early 1970s as the principal space launcher and carrier vehicle to be developed by NASA. It was planned as a replacement for the more expensive, expendable booster rockets used since the late 1950s for launching major commercial and governmental satellites. Together with launch facilities, mission control and supporting centers, and a tracking and data relay satellite system, it would complete NASA's new Space Transportation System.

After various delays, the program commenced in the early 1980s. Despite several problems, the craft demonstrated its versatility in a series of missions until the fatal disaster during the launch of the Challenger on January 28, 1986 forced a long delay. The program resumed in late 1988 and the modifications to the shuttle affected neither the basic design of the craft nor its overall dimensions.

Although the shuttle launched a few military payloads in its early days, such as the Defense Support Program satellite, the USAF abandoned it as a launch vehicle after the Challenger disaster. However, it could conceivably be used for military missions again if the decision were made to do so.

In 2002, the space agency restructured its Shuttle upgrades program to better align investments with agency goals and the ISTP. Now called the Service Life Extension Program, the program's primary objective is to reduce risk and preserve Shuttle safety and viability through investments in Shuttle upgrades and infrastructure revitalization. NASA will consider factors including safety, reliability, supportability, performance, and cost reduction in prioritizing improvement projects.

The Space Shuttle 's day-to-day operations have been managed by United Space Alliance, a Boeing-Lockheed Martin joint venture, since 1996. NASA, as well as others in the space community,has raised the possibility that even more responsibility for the Shuttle might be shifted from government to private sector control. A task force commissioned by NASA to examine this possibility suggested in 2002 a variety of options through which the space agency could further competitive outsourcing of Shuttle operations. NASA has not yet announced how it will proceed.

On its first mission in 2003, the Space Shuttle Orbiter Columbia suffered a catastrophic failure on reentry that resulted in the loss of its seven astronauts and the vehicle. Fortunately, there were no casualties on the ground as the breakup of the vehicle and the resultant debris field covered a long swath of land primarily through sparsely inhabited Northeast Texas. The Columbia Accident Investigation Board (CAIB)s completed its report on the cause of the accident and its recommendations on actions required to be completed by NASA to return the STS safely to flight status.

The three remaining Orbiters, Discovery, Atlantis, and Endeavour were grounded since the Columbia accident. The Space Shuttle is the only means available today for completing assembly of the ISS. Intending to fly the Shuttle until 2010, NASA is committed to investing in the Space Shuttle fleet to maintain safety and reliability and extend orbiter service life until its responsibilities constructing the ISS are complete.

Shuttle Operations

The three main components of the Space Shuttle are the orbiter, the external tank and the solid rocket boosters. The Shuttle weighs 4.5 million pounds at launch and stands 184.2 feet tall and can carry up to 55,000 pounds of cargo to LEO on one mission.

The orbiter, 78 feet across the wing tips and 122.2 feet long, is the portion resembling a delta-winged jet fighter. It is a rocket stage during launch, a spaceship in orbit and a hypersonic glider on reentry and landing. A three-deck crew compartment and an attitude thruster module are in the nose, the mid-body is the cargo hold or payload bay (15 ft wide and 60 ft long) and the tail holds the three main engines plus maneuvering engine pods.

Each engine, burning hydrogen and oxygen, produces up to 394,000 pounds of thrust. The external tank, actually an oxygen tank and a hydrogen tank joined by a load-bearing intertank, is the structural backbone of the Shuttle. Measuring 27.56 feet wide and 154.2 feet tall, it carries 1,520,000 pounds of liquefied propellants for the main engines. The shuttle's main engines produce over 27 million horsepower and empties the external tank in about eight and one-half minutes.

Two solid rocket boosters, each slightly over 12 feet wide and 149 feet tall, provide the Shuttle with a lift to the upper atmosphere so the main engines can work more efficiently. Each produces an average thrust of 3.3 million pounds. The propellant in the solid rocket motors consists of ammonium perchlorate, aluminum powder, iron oxide and a binding agent. Total thrust of the vehicle at liftoff (two solid motors and three liquid engines) is 7.78 million pounds.

The Shuttle's main engines are ignited, the booster rockets are ignited about six seconds before lift-off at T-6 seconds and the hold-down bolts are released at T-0. The Shuttle lifts off vertically about 2.5 seconds later with all five engines operating. As soon as it clears the gantry, it rolls and pitches to fly with the orbiter upside down, as the craft's design puts the thrust vector off-center. At T+2 minutes 12 seconds, the boosters burn out and are jettisoned from the external tank at an altitude of approximately 26-27 statute miles. The boosters then parachute into the sea for recovery, refurbishing and reuse. Meanwhile, the Shuttle continues on under the power of the main engines. Just short of orbital velocity, the engines are shut down (T+8 min 32 sec) and the tank is jettisoned (T+8 min 50 sec). The tank burns up as it reenters the atmosphere.

Once the vehicle is in space, it maneuvers using two different systems, the Orbital Maneuvering System (OMS) and the Reaction Control System (RSC). The orbiter's own OMS engines act as the third stage that puts the craft into orbit. The OMS uses two bipropellant, 6,000 pound thrust rocket engines mounted in pods on the aft end of the orbiter fuselage. The hypergolic propellants consist of monomethylhydrazine and nitrogen tetroxide, with about 21,600 pounds of propellant stored within the orbiter in titanium tanks. The OMS is used for orbit insertion or transfer, orbit circulation, rendezvous and deorbit.

The RCS uses 38 bi-propellant liquid rocket engines and six bipropellant liquid rocket vernier thrusters. Fourteen of the engines are on the orbiter's nose, together with two verniers. The remaining engines and verniers are split equally between the two OMS pods of the aft end of the orbiter fuselage. The RCS used the same type propellants as the OMS, but carries 2,413 pounds of fuel in separate tanks. There is a system to transfer fuel to and from the RCS to the OMS. The RCS is used to maneuver in space during rendezvous and deorbit maneuvers.

The vehicle is normally manned by a crew of four (minimum two, maximum eight except as noted): the commander, pilot, mission specialist and payload specialist. In an emergency ten people can fit in the orbiter. The interior of the orbiter is pressurized, allowing the astronauts to operate in a short-sleeve environment without spacesuits. Passengers can fly on the shuttle without extensive astronaut training because of the relatively light 3G acceleration during launch and the pressurized cabin. The self-contained crew module is supported within the fuselage by four attachment points, the entire module being welded to create the pressure tight vessel. The module has a fuselage side hatch for access, a hatch into the airlock from the mid-section and a hatch from the airlock into the payload bay. As previously mentioned, the crew module is divided into three levels. The upper flight deck has seats for the mission and payload specialists, the commander, and the pilot. There are dual flight controls and the controls for the Remote Manipulator System (RMS) which extracts payloads from the Shuttle's cargo bay. The mid-level deck has additional seating, a galley, electronics bays and crew sleeping and comfort facilities. The lowest level houses environmental equipment and storage.

At the end of the orbital mission, the orbiter is protected from the heat of reentry by heat-resistant ceramic tiles. As dynamic pressure from the air increases, control of the vehicle switches from the RCS to aerodynamic surfaces and the orbiter glides to a landing.

The Future of the Shuttle

Ending the Shuttle and Station programs is necessary to free up funds for other aspects of the proposal and to avoid Shuttle recertification in 2010, an expensive process called for by the Columbia Accident Investigation Board.

The Administration acknowledges that it has not yet figured out how to get crews to the Station between the retirement of the Shuttle in 2010 and the first flight of the CEV in 2014. The Shuttle may also be unavailable for crew transfer earlier, if its schedule needs to be devoted entirely to Station construction.

The US is already using the Russian Soyuz spacecraft for crew transfer while the Shuttle is grounded. However, it is doing so under an agreement that the Russians will have fulfilled by 2006. Renewing the agreement may require a change in the Iran Nonproliferation Act (INA), which Congress passed in 2000. That Act attempts to prevent the spread of weapons of mass destruction to Iran by prohibiting the purchase of Russian rockets by the U.S. unless the President certifies that no Russian entity is engaged in any sales of missiles or missile systems to Iran. The INA does not apply to the current agreement.

How will NASA carry cargo to and from the Station after the Shuttle is retired? Similar to the crew situation, NASA has no current plan for getting cargo to the Station after the Shuttle is retired. NASA is using Russian Progress vehicles while the Shuttle is grounded, but continuing to do so indefinitely could require amending the Iran Nonproliferation Act. (See above.) NASA might also rely on Europe or Japan, which are partners in the Space Station and which are developing cargo-carrying spacecraft of their own. But those craft have not yet been flight-tested.

Some have suggested that NASA could convert the space shuttle itself into a cargo-only craft that could deliver huge loads of cargo to the ISS. But critics have said that such an approach would be much more expensive than flying smaller loads on existing rockets. Finally, NASA might try to purchase the services of commercial rocket firms. But at present no firm has a rocket that can supply the Station, although several have indicated a willingness to try to carry small amounts of cargo there. Another complication is that some cargo for the Space Station is very large-major replacement parts, for example-and most craft other than the Shuttle are not big enough to carry such cargo.

The Administration described the Hubble cancellation as a ''close call'' made by the Administrator because of safety concerns. The Hubble, which has been enormously successful, is expected to go dark around 2007 without a servicing mission. Many astronomers are lobbying for that mission to occur, and, indeed, before the President's initiative was announced, a panel assembled by the National Academy of Sciences, called for another servicing mission to be added to extend the telescope's life even further. That request became moot with the decision to discontinue the Shuttle in 2010. However, some experts contend that ground-based telescopes have advanced so much in recent years that they can now make up for at least some of the capability that would be lost if the Hubble ceases to function.

A Shuttle mission to the Hubble is a special case because Hubble missions cannot reach the Space Station, which could be used as a ''safe haven'' in case of an emergency or the need to inspect or repair the Shuttle. The Columbia Accident Investigation Board said that the Shuttle should fly to destinations other than the Space Station only when NASA had developed an ''autonomous'' inspection and repair capability-that is, a way to inspect without using the Space Station. NASA believes such a capability is probably many years away. As a substitute, NASA examined having a second Shuttle ready to fly a rescue mission, but viewed that as dangerous and prohibitively expensive.

NASA examined alternative configurations for the Space Station that meet the goals of the Vision and the needs of international partners. In May 2005, NASA initiated the Shuttle/Station Configuration Options Team (SSCOT). This team conducted a 60-day study of the configuration options for the ISS and assessing the related number of flights needed by the Space Shuttle before it retires no later than the year 2010. The scope of the Shuttle/Station Configuration Options Team study spans ISS assembly, operations, and use and considers such factors as international partner commitments, research utilization, cost, and ISS sustainability. This team was expected to complete its work in June 2005, with those results integrated into the ongoing Exploration Systems Architecture Study (ESAS).

On 31 July 2005 NASA Administrator Mike Griffin said there would be 19 or 20 more Shuttle flights, including the Hubble repair mission. Previously, NASA had said the Space Station would require 28 more flights, indicating the baseline assembly sequence had been reduced by 9 shuttle flights.

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Page last modified: 21-07-2011 00:48:55 ZULU