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Space Shuttle
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Each Space Shuttle consists of an Orbiter (of which three remain in service), an External Tank (ET), two Solid Rocket Boosters (SRBs), and the Space Shuttle Main Engines (SSMEs). All these components (see below for details) are reusable except for the External Tank. Designed to operate on land, in the atmosphere, and in space, the Shuttle combines features of a rocket, an aircraft, and a glider. No other flying machine is launched, serves as a crew habitat and cargo carrier, maneuvers in orbit, then returns from space for an unpowered landing on a runway, and is ready to fly again within a few weeks or months. Its main engines and solid rocket motors are the first ever designed for use on multiple missions.
The Space Shuttle can take up to eight astronauts into low Earth orbit to carry out a wide variety of tasks, from satellite-launching to construction of the International Space Station, on missions last up to two and a half weeks. It is used to support research in astronomy, biology, space medicine, and materials processing, and has delivered into orbit scientific, commercial, and military satellites and interplanetary probes. The Shuttle has also been used to carry Spacelab and to repair, refurbish, or recover satellites. After the first launch on Apr. 12, 1981 – 20 years to the day after Yuri Gagarin’s historic flight – Shuttles flew two to nine missions a year, except in 1986 and 1987 when flights were suspended following the Challenger disaster. Although its cost proved to be much greater and its practical launch frequency much lower than originally anticipated, it did represent a giant leap forward in manned spaceflight capability.
The Shuttle travels from the Vehicle Assembly Building at Kennedy Space Center (KSC), where its main components are put together, to its launch pad on a giant crawler-transporter vehicle – a trip which, at a maximum speed of 1.6 km/h, takes about five hours. Launch complexes 39A and 39B at Cape Canaveral were originally used for the Apollo missions and renovated for the Shuttle. A third intended Shuttle launch site built at Vandenberg Air Force Base, California by renovating Titan III launch complex SLC-6 (nicknamed “Slick-6”) has never been employed.
History
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At the outset, NASA envisaged a two-stage Shuttle with a smaller manned winged vehicle (the Orbiter) riding piggyback on a larger manned winged vehicle (the Booster). These would be pad-launched from a vertical position. The Booster would carry the Orbiter to a height of about 80 km, at which point the Orbiter would separate and fire its own engines to reach orbit. The Booster – essentially a winged fuel tank – would immediately descend and land near the launch site, while the Orbiter would return at the end of its mission. As described by NASA in 1970, this two-stage Shuttle would be able to carry a 11,300-kg payload to a maximum 480-km circular orbit.
In a time of recession, it soon became clear that NASA could not afford a fleet of complicated two-stage Shuttles together with a space station. Furthermore, although military funding for the Shuttle was vital, the Air Force specified a payload capability nearly three times what NASA had in mind. These factors led to a dramatic redesign of both the Shuttle and the proposed space station. From a Skylab-like, single-structure station to be launched by a Saturn V and serviced by the Shuttle, NASA switched to a modular concept: the space station would be built over several years from separate, Shuttle-launched elements. This not only spread the financial outlay over a longer period, it meant that the more powerful Shuttle required would be able to carry heavy military payloads. The redesign would also allow NASA to secure private funding to carry commercial satellites aboard Shuttles while cutting costs by phasing out the Atlas, Delta, and Titan fleets. With an event like the Challenger disaster never anticipated, NASA put all of its eggs in one basket: the Shuttle would become the Agency’s sole medium-to-heavy launcher into the next century. To cut costs further, NASA opted for a more traditional expendable system to carry the Orbiter into space, involving drop-away boosters and an expendable main engine fuel tank.
Rockwell began work on Orbiter Enterprise, officially known as Orbiter Vehicle-101 (OV-101), in June 1974. This was first rolled out of its hangar at Palmdale, California, on Sep. 17, 1976, and subsequently used for flight testing at the Dryden Flight Research Center. Meanwhile Rockwell continued to develop Orbiters Columbia (OV-102), Discovery (OV-103), and Atlantis (OV-104). Although Enterprise was originally intended to be refitted as an operational Shuttle, NASA opted instead to upgrade Challenger (OV-099) from its status as a high-fidelity structural test article. Upon the loss of Challenger in January 1986, it was decided to build a fifth operational Shuttle, Endeavour (OV-105), as a replacement.
The initial concept of flying 25-60 missions per year never proved realistic, and by the mid-1980s NASA was gearing toward a more modest launch rate of about 24 missions per year. Then the entire program was ground to a halt when, on Jan. 28, 1986, the 25th Shuttle mission ended tragically little more than a minute after takeoff. The explosion of Challenger made it stunningly clear that major changes in the entire Shuttle program were unavoidable. With the launch of Discovery on Sep. 29, 1988, NASA entered a new era of Shuttle operations, adopting a more relaxed pace averaging about eight launches per year. Learning from one of its greatest tragedies, NASA was able to rebuild and maintain a Shuttle program that had an outstanding setback - until the further setback of Columbia's loss. In April 1996 NASA began a four-phase plan, known as the Space Shuttle upgrade program, to keep the existing Shuttle fleet flying through at least 2012 and also proposed modifications and upgrades that might keep the fleet in action through 2030. Phase one of the plan called for improvements to the Shuttle to allow it to support construction and maintenance of the International Space Station – the program’s chief goal well into the 21st century. Phase two called for operational and cost improvements in ground operations that would decrease Shuttle servicing and maintenance time in order to support an average of 15 launches per year. Phase Three called for a number of modifications to Orbiter onboard systems that would also result in decreased processing and maintenance time. More ambitious elements of this plan envisaged the complete replacement of toxic fuels by nontoxic fuels in key Orbiter systems. Phase four called for significant redesign of the Shuttle fleet and its basic configuration.
External Tank (ET)
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The first six ETs delivered to NASA were called standard weight tanks (SWTs), each weighing 34,250 kg. SWTs were flown on missions STS-1 through STS-5 and STS-7. For STS-1 and STS-2, the ETs were painted white. Thereafter, hundreds of kg and thousands of dollars in preparation work were saved by leaving ETs unpainted, so that all ETs flown from STS-3 on have sported an orange-brown color. In 1979, even before a Shuttle had completed a spaceflight, NASA directed that ETs be lightened so that heavier payloads could be flown. The resulting ET, called the Lightweight Tank (LWT), shaved 4,500 kg off the SWT by using new materials and design changes. The 29,700-kg LWT was first flown on STS-6, and then from STS-8 through STS-90. Starting with STS-91, a new ET called the super lightweight tank (SLWT) has been flown. Weighing 26,300 kg, it enables still heavier payloads to be carried in support of the International Space Station.
| External Tank (SLWT version) statistics | |
|---|---|
| Length | 46.9 m |
| Diameter | 8.4 m |
| Mass | 757,000 kg (total); 730,000 (propellant) |
| LOX tank | |
| Size | 16.6 × 8.4 m |
| Capacity | 549,000 liters |
| LOX mass | 626,000 kg |
| LH2 tank | |
| Size | 29.5 × 8.4 m |
| Capacity | 1,470,000 liters |
| LH2 mass | 104,000 kg |
Orbiter
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The crew module is an independent, pressurized vessel of welded aluminum suspended within the forward fuselage of the Orbiter; it has two hatches and 10 windows, and consists of a two-level flight deck and crew quarters. During launch up to four astronauts may sit on the upper flight deck and up to four more on the middle crew quarters deck. The forward portion of the flight deck resembles the cockpit of a jet airliner but features separate controls for flying in space and flying in the atmosphere. The aft portion of the flight deck contains four stand-up duty stations including the controls for the RMS. The crew quarters deck is entered through an open hatch in the flight deck floor and provides eating, sleeping, and sanitary facilities. At the aft end of the crew quarters deck is an air lock, through which astronauts may enter the cargo bay for extravehicular activities. Electrical power for Orbiter systems is provided by fuel cells which produce, as a byproduct, water for drinking.
| Orbiter statistics | |
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| Wingspan | 23.79 m |
| Length | 37.24 m |
| Height | 17.27 m |
| Habitable volume | 71.5 cubic m |
| Dry mass | 78,448 kg (OV-102: 82,288 kg) |
| Landing mass | 104,328 kg max |
| Payload (to 204-km LEO) | 24,990 kg (OV-102: 21,140 kg) |
Solid Rocket Boosters (SRBs)
Twin solid propellant motors that provide just over 70% of the thrust needed to launch the Space Shuttle. They are the largest such motors ever built, and the first designed to be reused. The SRBs also support the entire weight of the Orbiter and ET prior to launch.
Each SRB is attached to the ET at the SRB’s aft frame by two lateral braces and a diagonal attachment. The forward end of each SRB is attached to the ET at the forward end of the SRBs forward skirt. On the launch pad, each SRB is fastened to the mobile launcher platform (MLP) at the SRB aft skirt by four large bolts that are severed by explosive charges at liftoff.
The propellant is a mixture of 69.6% ammonium perchlorate oxidizer, 16% aluminum (fuel), 0.4% iron oxide catalyst, 12.04% polymer binder, and 1.96% epoxy curing agent. This is contained within the SRB beginning with an 11-point star-shaped perforation in the forward segment to a double-truncated cone perforation in both of the aft segments and the aft closure segment. These varying shapes allow the SRB thrust to be reduced 50 seconds after launch by about 33% to relieve stress on the Shuttle as it goes through maximum dynamic pressure. The SRBs are ignited by electronic command from the orbiter at launch minus zero, provided that the Shuttle main engines (SSME) have built up enough thrust to support a launch. The SSMEs are ignited first, since the SRBs cannot be shut down once they are ignited and it would be catastrophic if the SRBs were ignited after an SSME failure on the launch pad. Each SRB exhaust nozzle may be gimbaled up to 8° to help steer the Shuttle during ascent.
About 125 sec after launch, at an altitude of about 45,000 m, the SRBs burn out and are jettisoned. The jettison command comes from the Orbiter, and jettison occurs when the forward and aft attach points between the SRBs and ET are blown by explosive charges. Milliseconds after SRB separation, 16 solid-fueled separation motors, four in the forward section and four in the aft skirt of each SRB, are fired briefly to help carry the SRBs away from the rest of the Shuttle. About 225 sec after separation, at an altitude of 4,800 m, the nose cap of each SRB is ejected and a pilot parachute deployed. This serves to pull out the 16.5-m-diameter drogue parachute which orients and stabilizes the descent of each SRB to a tail-first attitude ready for deployment of the main parachutes. At an altitude of 1,800 m, the three 41-m-diameter main parachutes open to slow each SRB to water impact at about 25 m/s. The SRBs splash down in the Atlantic about 225 km from the launch site. Retrieval ships locate each SRB by homing in on radio beacon signals transmitted from each booster and by flashing lights activated on each SRB. Once on location, recovery crews plug the SRB nozzles, empty the motors of water, and tow the rockets back to a receiving and processing site on Cape Canaveral. After inspection, the SRBs are disassembled, washed with fresh, deionized water to limit saltwater corrosion, and refurbished ready to fly.
| SRB statistics | |
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| Length | 45.5 m |
| Diameter | 3.7 m |
| Mass (per SRB) | 585,000 kg (with fuel); 85,000 kg (empty) |
| Thrust (per SRB) | 14.7 million N |
Space Shuttle Main Engines (SSMEs)
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The Main Engines burn a combination of liquid oxygen and liquid hydrogen fed from the ET and employ a staged combustion cycle, in which the fuels are first partially burned at high pressure and low temperature, then burned completely at high pressure and high temperature. This allows the SSMEs to produce thrust more efficiently than other rocket engines. SSME thrust can be varied from 65% to 109% of rated power at increments of 1%. A thrust value of 104%, known as full power, is typically used as the Shuttle ascends, although, in an emergency, each SSME may be throttled up to 109% power. All three SSMEs receive identical and simultaneous throttle commands which usually come from general purpose computers aboard the Orbiter.
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The SSMEs achieve full power at launch, but are throttled back at about launch plus 26 sec. to protect the Shuttle from aerodynamic stress and excessive heating. They are throttled back up to full power at about Launch Plus 60 sec. and typically continue to produce full power for about 8.5 min. until shortly before the Shuttle enters orbit. During ascent, each SSME may be gimbaled plus or minus 10.5° in pitch and yaw to help steer the Shuttle. At about launch plus 7 min. 40 sec. the SSMEs are throttled down to avoid subjecting the Shuttle and crew to gravitational forces over 3g. At about 10 sec. before main engine cutoff, a MECO sequence begins. About 3 sec. later, the SSMEs are commanded to begin throttling back at intervals of 10% thrust per second until they reach a thrust of 65% of rated power, called minimum power. This power is maintained for just under 7 sec., then the SSMEs shut down.
| SSME statistics | |
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| Length | 4.3 m |
| Diameter | 2.3 m |
| Rated thrust (per SSME) | 1,670,000 N |
Future
Designed in the 1970s and first flown in 1981, the Shuttle was envisioned as a workhorse vehicle that would make space travel commonplace. Nearly a quarter-century later, after two deadly accidents that have killed 14 crew, NASA has described the Shuttle as a test vessel whose useful days are numbered. Slated for retirement by 2010, the remaining three-ship (Atlantis, Discovery, and Endeavour) fleet is meant to spend the next years completing construction of the International Space Station, satisfying U.S. commitments to the space agencies of Europe, Russia, Japan and Canada. Given the Shuttles' unsettled flight schedule, the number of construction flights is limited. Before the Columbia accident, NASA figured 28 shuttle missions would be needed to complete the station. Just before Discovery's launch, Griffin estimated as few as 15 flights.
The Shuttle's replacement will be the Ares launch vehicles and an Apollo-like Orion spacecraft (Crew Exploration Vehicle), scheduled to make its first test flight in 2012. Orion will be developed to support NASA's ambitious goal to return Americans to the Moon by 2020 and send them eventually to Mars.
Related categories
• MANNED SPACEFLIGHT
• ROCKETS, MISSILES, AND LAUNCH VEHICLES
Archived news
'Retire shuttle early' says NASA (May 13, 2005)
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