Space Shuttle

The Space Shuttle, part of the Space Transportation System (STS), is a spacecraft operated by NASA for orbital human spaceflight missions. Operational flights began in 1982 following four test flights. The system is scheduled to be retired from service in 2010 after 134 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 a dark orange-colored external tank (ET);^[1][2] two white, slender Solid Rocket Boosters (SRBs); and the STS Orbiter Vehicle (OV) which contains the crew and payload. Payloads can be launched into higher orbits with either of two different booster stages developed for STS (1 stage PAM or 2 stage IUS)

Space Shuttle

Space Shuttle Discovery launches at the start of STS-120.

The shuttle stack launches vertically like a conventional rocket from a mobile launch platform. It lifts off under the power of its two solid rocket boosters (SRBs) and its three main engines (SSMEs), the latter fueled by liquid hydrogen and liquid oxygen from the external tank. The Space Shuttle has a two stage ascent. The boosters are used only for the first stage, while the main engines burn for both stages. About two minutes after liftoff, staging occurs: the SRBs are released, and shortly begin falling into the ocean to be retrieved for reuse. The shuttle orbiter and external tank continue to ascend under power from the three main engines and their inertia. Upon reaching orbit, the main engines are shut down, and the external tank is jettisoned downward and falls to burn up in the atmosphere. However, it is possible for it to be re-used in orbit for various applications.^[3] At this point, the orbital maneuvering system (OMS) engines may be used to adjust or circularize the achieved 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.^[4] 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 Orbital Maneuvering System (OMS) thrusters to drop out of orbit and re-enter the lower atmosphere.^[4] 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.

Retirement

NASA's current plans call for the Space Shuttle to be retired from service in 2010, after nearly 30 years of service. Atlantis will be the first of NASA's three remaining operational Space Shuttles to be retired as the program winds down.^[43] To fill the void left by the Shuttle's retirement, a new spacecraft is being developed to ferry not only passengers and cargo to the ISS but also to travel beyond Earth orbit to the Moon and Mars.^[44] Originally called the Crew Exploration Vehicle, the concept has evolved into the Orion spacecraft and the project has been dubbed Project Constellation. This next-generation vehicle is targeted for first manned launch in 2014 at the earliest. Therefore, all crews traveling to and from the International Space Station during the intervening period will have to do so on board Russian spacecraft or possibly an American commercial spacecraft (see below).

Congress is discussing the possibility of delaying the retirement of the Space Shuttles. One proposal, which would cost about $10 billion, would have the shuttles make six or seven additional flights between 2010 and 2013 and would accelerate development of the Orion ships to be ready by then. A second proposal would keep the shuttles flying until 2015 and leave Orion's schedule intact.^[45]

Commercial replacement vehicles and services

NASA announced the awarding of contracts for the cargo resupply of the International Space Station (ISS) to SpaceX and Orbital Sciences Corporation on December 23, 2008.^[46] SpaceX will use its Falcon 9 launch vehicle and Dragon spacecraft.^[47] Orbital Sciences will use its Taurus II launch vehicle and Cygnus spacecraft.

Fleet history

Below is a list of major events in the Space Shuttle orbiter fleet.

OV-101 Enterprise takes flight for the first time over Dryden Flight Research Facility, Edwards, California in 1977 as part of the Shuttle program's Approach and Landing Tests (ALT).

Space Shuttle major events

Date	Orbiter	Major event / remarks
February 18, 1977	Enterprise	First flight; Attached to Shuttle Carrier Aircraft throughout flight.
August 12, 1977	Enterprise	First free flight; Tailcone on; lakebed landing.
October 12, 1977	Enterprise	third free flight; First with no tailcone; lakebed landing.
October 26, 1977	Enterprise	Final Enterprise free flight; First landing on Edwards AFB concrete runway.
April 12, 1981	Columbia	First Columbia flight, first orbital test flight; STS-1
November 11, 1982	Columbia	First operational flight of the Space Shuttle, first mission to carry four astronauts; STS-5
April 4, 1983	Challenger	First Challenger flight; STS-6
August 30, 1984	Discovery	First Discovery flight; STS-41-D
October 3, 1985	Atlantis	First Atlantis flight; STS-51-J
January 28, 1986	Challenger	Disaster starting 73 seconds after launch; STS-51-L; all seven crew members perished.
September 29, 1988	Discovery	First post-Challenger mission; STS-26
May 4, 1989	Atlantis	The first Space Shuttle mission to launch a space probe, Magellan; STS-30
April 24, 1990	Discovery	Launch of the Hubble Space Telescope; STS-31
May 7, 1992	Endeavour	First Endeavour flight; STS-49
November 19, 1996	Columbia	Longest Shuttle mission to date at 17 days, 15 hours; STS-80
October 11, 2000	Discovery	100th Space Shuttle mission; STS-92
February 1, 2003	Columbia	Disintegrated during re-entry; STS-107; all seven crew members perished.
July 25, 2005	Discovery	First post-Columbia mission; STS-114
August 28, 2009	Discovery	Carried Multi-Purpose Logistics Module (MPLM) Leonardo to ISS; STS-128
Planned fleet events
2010	Atlantis	Last planned Atlantis flight; STS-132
2010	Discovery	Last planned Discovery flight; last planned flight of the Space Shuttle program; STS-133
2010	Endeavour	Last planned Endeavour flight; STS-134[37]

Sources: NASA launch manifest,^[38] NASA Space Shuttle archive^[39]

Shuttle disasters

Main articles: Space Shuttle Challenger disaster and Space Shuttle Columbia disaster

On January 28, 1986, the Space Shuttle Challenger disintegrated 73 seconds after launch due to the failure of the right SRB, killing all seven astronauts on board. The disaster was caused by cold-temperature impairment of the SRB O-rings, a mission critical component. Repeated warnings from design engineers voicing concerns about the lack of evidence of the O-ring's safety when the temperature was below 53 °F (11.7 °C) were ignored by NASA managers.^[40]

In 2003, the Space Shuttle Columbia disintegrated during re-entry because of damage to heat tiles caused during launch. Ground control engineers made three separate requests for high-resolution images taken by the Department of Defense that would have provided a clearer understanding of the extent of the damage, while NASA's chief thermal protection system (TPS) engineer requested that astronauts on board the Columbia be allowed to leave the vehicle to inspect the damaged tiles. NASA managers intervened to stop the Department of Defense's assistance and refused the request for the spacewalk,^[41] and thus the feasibility of scenarios for astronaut repair or rescue by the Space Shuttle Atlantis were not considered by NASA management at the time.

Mission profile

Launch

All Space Shuttle missions are launched from Kennedy Space Center (KSC). The same weather criteria used for launch are also for end of mission landing at KSC, and include precipitation (none allowed at the launch pad or flight path), temperatures above 99 °F (37.2 °C) or below 35 °F (1.7 °C), a 20% or greater chance of lightning within 5 nautical miles and cloud cover allows direct visual observation of the shuttle through 8,000 feet.^[27] The shuttle will not be launched under conditions where it could be struck by lightning. Aircraft are often struck by lightning with no adverse effects because the electricity of the strike is dissipated through its conductive structure and the aircraft is not electrically grounded. Like most jet airliners, the shuttle is mainly constructed of conductive aluminum, which would normally shield and protect the internal systems. However, upon takeoff the shuttle sends out a long exhaust plume as it ascends, and this plume can trigger lightning by providing a current path to ground. The NASA Anvil Rule for a shuttle launch states that an anvil cloud cannot appear within a distance of 10 nautical miles.^[28] The Shuttle Launch Weather Officer will monitor conditions until the final decision to scrub a launch is announced. In addition, the weather conditions must be acceptable at one of the Transatlantic Abort Landing sites (one of several Space Shuttle abort modes) to launch as well as the solid rocket booster recovery area.^[27][29] While the shuttle might safely endure a lightning strike, a similar strike caused problems on Apollo 12, so for safety NASA chooses not to launch the shuttle if lightning is possible (NPR8715.5).

STS mission profile

Historically, the Shuttle was not launched if its flight would run from December to January (a year-end rollover or YERO). Its flight software, designed in the 1970s, was not designed for this, and would require the orbiter's computers be reset through a change of year, which could cause a glitch while in orbit. In 2007, NASA engineers devised a solution so Shuttle flights could cross the year-end boundary.^[30]

On the day of a launch, after the final hold in the countdown at T minus 9 minutes, the Shuttle goes through its final preparations for launch, and the countdown is automatically controlled by the Ground Launch Sequencer (GLS), software at the Launch Control Center, which stops the count if it senses a critical problem with any of the Shuttle's on-board systems. The GLS hands off the count to the Shuttle's on-board computers at T minus 31 seconds, in a process called auto sequence start.

At T minus 16 seconds, the massive sound suppression system (SPS) begins to drench the Mobile Launcher Platform (MLP) and SRB trenches with 300,000 US gallons (1,100 m³) of water to protect the Orbiter from damage by acoustical energy and rocket exhaust reflected from the flame trench and MLP during liftoff.^[31]

At T-minus 10 seconds, hydrogen igniters are activated under each engine bell to quell the stagnant gas inside the cones before ignition. Failure to burn these gases can trip the onboard sensors and create the possibility of an overpressure and explosion of the vehicle during the firing phase. The main engine turbopumps also begin charging the combustion chambers with liquid hydrogen and liquid oxygen at this time. The computers reciprocate this action by allowing the redundant computer systems to begin the firing phase.

Shuttle launch of Atlantis at sunset in 2001. The sun is behind the camera, and the plume's shadow intersects the moon across the sky.

The three Space Shuttle Main Engines (SSMEs) start at T minus 6.6 seconds. The main engines ignite sequentially via the shuttle's general purpose computers (GPCs) at 120 millisecond intervals. The GPCs require that the engines reach 90% of their rated performance to complete the final gimbal of the main engine nozzles to liftoff configuration.^[32] When the SSMEs start, the water from the sound suppression system flashes into a large volume of steam that shoots southward. All three SSMEs must reach the required 100% thrust within three seconds, otherwise the onboard computers will initiate an RSLS abort. If the onboard computers verify normal thrust buildup, at T minus 0 seconds, the 8 pyrotechnic nuts holding the vehicle to the pad are detonated and the SRBs are ignited. At this point the vehicle is committed to takeoff, as the SRBs cannot be turned off once ignited.^[33] The plume from the solid rockets exits the flame trench in a northward direction at near the speed of sound, often causing a rippling of shockwaves along the actual flame and smoke contrails. At ignition, the GPCs mandate the firing sequences via the Master Events Controller, a computer program integrated with the shuttle's four redundant computer systems. There are extensive emergency procedures (abort modes) to handle various failure scenarios during ascent. Many of these concern SSME failures, since that is the most complex and highly stressed component. After the Challenger disaster, there were extensive upgrades to the abort modes.

After the main engines start, but while the solid rocket boosters are still clamped to the pad, the offset thrust from the Shuttle's three main engines causes the entire launch stack (boosters, tank and shuttle) to pitch down about 2 m at cockpit level. This motion is called the "nod", or "twang" in NASA jargon. As the boosters flex back into their original shape, the launch stack pitches slowly back upright. This takes approximately six seconds. At the point when it is perfectly vertical, the boosters ignite and the launch commences.

Shortly after clearing the tower the Shuttle begins a roll and pitch program to set its orbital inclination and so that the vehicle is below the external tank and SRBs, with wings level. The vehicle climbs in a progressively flattening arc, accelerating as the weight of the SRBs and main tank decrease. To achieve low orbit requires much more horizontal than vertical acceleration. This is not visually obvious, since the vehicle rises vertically and is out of sight for most of the horizontal acceleration. The near circular orbital velocity at the 380 kilometers (236 mi) altitude of the International Space Station is 7.68 kilometers per second 27,650 km/h (17,180 mph), roughly equivalent to Mach 23 at sea level. As the International Space Station orbits at an inclination of 51.6 degrees, the Shuttle has to set its inclination to the same value to rendezvous with the station.

SSLV at Mach 2.46 and 66,000 ft (20,000 m). The surface of the vehicle is colored by the pressure coefficient, and the gray contours represent the density of the surrounding air, as calculated using the overflow codes.

Around a point called Max Q, where the aerodynamic forces are at their maximum, the main engines are temporarily throttled back to avoid overspeeding and hence overstressing the Shuttle, particularly in vulnerable areas such as the wings. At this point, a phenomenon known as the Prandtl-Glauert singularity occurs, where condensation clouds form during the vehicle's transition to supersonic speed.

126 seconds after launch, explosive bolts release the SRBs and small separation rockets push them laterally away from the vehicle. The SRBs parachute back to the ocean to be reused. The Shuttle then begins accelerating to orbit on the Space Shuttle main engines. The vehicle at that point in the flight has a thrust-to-weight ratio of less than one – the main engines actually have insufficient thrust to exceed the force of gravity, and the vertical speed given to it by the SRBs temporarily decreases. However, as the burn continues, the weight of the propellant decreases and the thrust-to-weight ratio exceeds 1 again and the ever-lighter vehicle then continues to accelerate towards orbit.

A picture taken of the afterglow from STS 119 with colors varying from white to orange.

The vehicle continues to climb and takes on a somewhat nose-up angle to the horizon – it uses the main engines to gain and then maintain altitude while it accelerates horizontally towards orbit. At about five and three-quarter minutes into ascent, the orbiter rolls heads up to switch communication links from ground stations to Tracking and Data Relay Satellites.

Finally, in the last tens of seconds of the main engine burn, the mass of the vehicle is low enough that the engines must be throttled back to limit vehicle acceleration to 3 g (30 m/s²), largely for astronaut comfort.

The main engines are shut down before complete depletion of propellant, as running dry would destroy the engines. The oxygen supply is terminated before the hydrogen supply, as the SSMEs react unfavorably to other shutdown modes. (Liquid oxygen has a tendency to react violently, and supports combustion when it encounters hot engine metal.) The external tank is released by firing explosive bolts and falls, largely burning up in the atmosphere, though some fragments fall into the ocean, in either the Indian Ocean or the Pacific Ocean depending on launch profile.^[23] The sealing action of the tank plumbing and lack of pressure relief systems on the external tank helps it break up in the lower atmosphere. After the foam burns away during reentry, the heat causes a pressure buildup in the remaining liquid oxygen and hydrogen until the tank explodes. This ensures that any pieces that fall back to Earth are small.

To prevent the shuttle from following the external tank back into the lower atmosphere, the Orbital maneuvering system (OMS) engines are fired to raise the perigee higher into the upper atmosphere. On some missions (e.g., missions to the ISS), the OMS engines are also used while the main engines are still firing. The reason for putting the orbiter on a path that brings it back to Earth is not just for external tank disposal but also one of safety: if the OMS malfunctions, or the cargo bay doors cannot open for some reason, the shuttle is already on a path to return to earth for an emergency abort landing.

In orbit

Once in orbit, the shuttle does any number of tasks, and usually some combination. In the 1980s and 1990s, many flights involved space science missions on the NASA/ESA Spacelab, or launching various types of satellites and science probes. By the 1990s and 2000s the focus shifted more to servicing space stations, with fewer satellite launches. Most missions involve staying in orbit several days to two weeks, although longer missions are possible with the Extended Duration Orbiter add-on or when attached to a space station.

Re-entry and landing

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Almost the entire Space Shuttle re-entry, except for lowering the landing gear and deploying the air data probes, is normally performed under computer control. However, the re-entry can be flown entirely manually if an emergency arises. The approach and landing phase can be controlled by the autopilot, but is usually hand flown.

The vehicle begins re-entry by firing the Orbital maneuvering system engines, while flying upside down, backside first, in the opposite direction to orbital motion for approximately three minutes, which reduces the shuttle's velocity by about 200 mph (322 km/h). The resultant slowing of the Shuttle lowers its orbital perigee down into the upper atmosphere. The shuttle then flips over, by pushing its nose down (which is actually "up" relative to the Earth, because it's flying upside down). This OMS firing is done roughly halfway around the globe from the landing site.

The vehicle starts encountering more significant air density in the lower thermosphere at about 400,000 ft (120 km), at around Mach 25, 8,200 m/s (30,000 km/h; 18,000 mph). The vehicle is controlled by a combination of RCS thrusters and control surfaces, to fly at a 40 degree nose-up attitude, producing high drag, not only to slow it down to landing speed, but also to reduce reentry heating. As the vehicle encounters progressively denser air, it begins a gradual transition from spacecraft to aircraft. In a straight line, its 40 degree nose-up attitude would cause the descent angle to flatten-out, or even rise. The vehicle therefore performs a series of four steep S-shaped banking turns, each lasting several minutes, at up to 70 degrees of bank, while still maintaining the 40 degree angle of attack. In this way it dissipates speed sideways rather than upwards. This occurs during the 'hottest' phase of re-entry, when the heat-shield glows red and the G-forces are at their highest. By the end of the last turn, the transition to aircraft is almost complete. The vehicle levels its wings, lowers its nose into a shallow dive and begins its approach to the landing site.

Simulation of the outside of the Shuttle as it heats up to over 1,500 °C during re-entry.

A Space Shuttle model undergoes a wind tunnel test in 1975. This test is simulating the ionized gasses that surround a shuttle as it reenters the atmosphere.

A computer simulation of high velocity air flow around the Space Shuttle during re-entry.

The orbiter's maximum glide ratio/lift-to-drag ratio varies considerably with speed, ranging from 1:1 at hypersonic speeds, 2:1 at supersonic speeds and reaching 4.5:1 at subsonic speeds during approach and landing.^[34]

In the lower atmosphere, the orbiter flies much like a conventional glider, except for a much higher descent rate, over 50 m/s (180 km/h; 110 mph). At approximately Mach 3, two air data probes, located on the left and right sides of the orbiter's forward lower fuselage, are deployed to sense air pressure related to the vehicle's movement in the atmosphere.

When the approach and landing phase begins, the orbiter is at a 3,000 m (9,800 ft) altitude, 12 km (7.5 mi) from the runway. The pilots apply aerodynamic braking to help slow down the vehicle. The orbiter's speed is reduced from 682 to 346 km/h (424 to 215 mph), approximately, at touch-down (compared to 260 km/h (160 mph) for a jet airliner). The landing gear is deployed while the Orbiter is flying at 430 km/h (270 mph). To assist the speed brakes, a 12 m (39 ft) drag chute is deployed either after main gear or nose gear touchdown (depending on selected chute deploy mode) at about 343 km/h (213 mph). The chute is jettisoned once the orbiter slows to 110 km/h (68.4 mph).

After landing, the vehicle stands on the runway for several minutes to permit the fumes from poisonous hydrazine (which is used as a fuel for attitude control, and the orbiter's three APUs) to dissipate, and for the shuttle fuselage to cool before the astronauts disembark.

Discovery touches down at the end of STS-95.

Columbia lands at Kennedy Space Center at the end of STS-73.

Endeavour brake chute deploys after touching down

Discovery after landing on Earth for crew disembarkment

Landing sites

See also: List of space shuttle landing runways

Space Shuttle landings are always planned for Kennedy Space Center.^{[citation needed]} If weather conditions make landing there unfavorable, the shuttle can delay its landing until conditions are favorable, touch down at Edwards Air Force Base, California, or use one of the multiple alternate landing sites around the world. A landing at any site other than Kennedy Space Center means that after touchdown the shuttle must be mated to the Shuttle Carrier Aircraft and returned to Cape Canaveral. Space Shuttle Columbia (STS-3) landed at the White Sands Space Harbor, New Mexico; this is viewed as a last resort as NASA scientists believe that the sand could potentially damage the shuttle's exterior.

There are many alternative landing sites that have never been used.

Technical data

Space Shuttle orbiter illustration

Space Shuttle drawing

Space Shuttle wing cutaway

Space Shuttle Atlantis transported by a Boeing 747 Shuttle Carrier Aircraft (SCA), 1998 (NASA).

Space Shuttle Endeavour being transported by a Boeing 747.

Space Shuttle Orbiter and Soyuz-TM (drawn to scale).

An overhead view of Atlantis as it sits atop the Mobile Launcher Platform (MLP) before STS-79. Two Tail Service Masts (TSMs) to either side of the orbiter's tail provide umbilical connections for propellant loading and electrical power.

Water is released onto the mobile launcher platform on Launch Pad 39A at the start of a rare sound suppression system test in 2004. During launch, 300,000 US gallons (1,100 m³) are poured onto the pad in only 41 seconds.

Two Space Shuttles sit at launch pads. This particular occasion is due to the final Hubble servicing mission, where the International Space Station is unreachable, necessitating having a Shuttle on standby for a possible rescue mission.

Orbiter specifications^[23] (for Endeavour, OV-105)

• Length: 122.17 ft (37.237 m)
• Wingspan: 78.06 ft (23.79 m)
• Height: 58.58 ft (17.86 m)
• Empty weight: 172,000 lb (78,000 kg)^[24]
• Gross liftoff weight: 240,000 lb (110,000 kg)
• Maximum landing weight: 230,000 lb (100,000 kg)
• Main engines: Three Rocketdyne Block IIA SSMEs, each with a sea level thrust of 393,800 lbf (1.752 MN) at 104% power
• Maximum payload: 55,250 lb (25,060 kg)
• Payload bay dimensions: 15 by 59 ft (4.6 by 18 m)
• Operational altitude: 100 to 520 nmi (190 to 960 km; 120 to 600 mi)
• Speed: 7,743 m/s (27,870 km/h; 17,320 mph)
• Crossrange: 1,085 nmi (2,009 km; 1,249 mi)
• Crew: Varies. The earliest shuttle flights had the minimum crew of two; many later missions a crew of five. Today, typically seven people fly (commander, pilot, several mission specialists, and rarely a flight engineer). On two occasions, eight astronauts have flown (STS-61-A, STS-71). Eleven people could be accommodated in an emergency mission (see STS-3xx).

External tank specifications (for SLWT)

• Length: 46.9 m (154 ft)
• Diameter: 8.4 m (28 ft)
• Propellant volume: 2,025 m³ (534,900 US gal)
• Empty weight: 26,535 kg (58,500 lb)
• Gross liftoff weight: 756,000 kg (1,670,000 lb)

Solid Rocket Booster specifications

• Length: 45.46 m (149 ft)^[25]
• Diameter: 3.71 m (12.2 ft)^[25]
• Empty weight (per booster): 68,000 kg (150,000 lb)^[25]
• Gross liftoff weight (per booster): 571,000 kg (1,260,000 lb)^[26]
• Thrust (at liftoff, sea level): 12.5 MN (2,800,000 lbf)^[11]

System Stack specifications

• Height: 56 m (180 ft)
• Gross liftoff weight: 2,000,000 kg (4,400,000 lb)
• Total liftoff thrust: 30.16 MN (6,780,000 lbf)

Description

The Space Shuttle is the first orbital spacecraft designed for reusability. It carries payloads to low Earth orbit, provides crew rotation for the International Space Station (ISS), and performs servicing missions. The orbiter can also recover satellites and other payloads from orbit and return them to Earth. Each Shuttle was designed for a projected lifespan of 100 launches or 10 years' operational life, although this was later extended. The person in charge of designing the STS was Maxime Faget, who had also overseen the Mercury, Gemini and Apollo spacecraft designs. The crucial factor in the size and shape of the Shuttle Orbiter was the requirement that it be able to accommodate the largest planned commercial and classified satellites, and have the cross-range recovery range to meet the requirement for classified USAF missions for a once-around abort from a launch to a polar orbit. Factors involved in opting for solid rockets and an expendable fuel tank included the desire of the Pentagon to obtain a high-capacity payload vehicle for satellite deployment, and the desire of the Nixon administration to reduce the costs of space exploration by developing a spacecraft with reusable components.

Six airworthy Space Shuttle orbiters have been built; the first, Enterprise, was not built for orbital space flight, and was used only for testing purposes. Five space-worthy orbiters were built: Columbia, Challenger, Discovery, Atlantis, and Endeavour. Enterprise was originally intended to be made fully space-worthy after use for the approach and landing test (ALT) program, but it was found more economical to upgrade the structural test article STA-099 into orbiter Challenger (OV-099). Challenger disintegrated 73 seconds after launch in 1986, and Endeavour was built as a replacement from structural spare components. Columbia broke apart during re-entry in 2003.

Each Space Shuttle is a reusable launch system that is composed of three main assemblies: the reusable Orbiter Vehicle (OV), the external tank (ET), and the two reusable solid rocket boosters (SRBs).^[5] The tank and boosters are jettisoned during ascent; only the orbiter enters orbit. The vehicle is launched vertically like a conventional rocket, and the orbiter glides to a horizontal landing, after which it is refurbished for reuse. The SRBs parachute back to earth, where they are collected from the ocean and refilled for another use. Although the external tanks have always been discarded, it is possible to take them into orbit and re-use them (such as for incorporation into a space station).^[3][6]

Roger A. Pielke, Jr. has estimated that the Space Shuttle program has cost about US$170 billion (2008 dollars) through early 2008. This works out to an average cost per flight of about US$1.5 billion.^[7] However, two missions were paid for by Germany, Spacelab D-1 and D-2 (for Deutschland) with a mission control in Oberpfaffenhofen, Germany.^[8][9]

At times, the orbiter itself is referred to as the Space Shuttle. Technically, this is a slight misnomer, as the actual "Space Transportation System" (Space Shuttle) is the combination of the orbiter, the external tank, and the two solid rocket boosters. Combined, these are referred to as the "Stack"; the components are assembled in the Vehicle Assembly Building, which was originally built to assemble the Apollo Saturn V rocket stacks.

Orbiter vehicle

Main article: Space Shuttle orbiter

The orbiter resembles a conventional aircraft, with double-delta wings swept 81° at the inner leading edge and 45° at the outer leading edge. Its vertical stabilizer's leading edge is swept back at a 50° angle. The four elevons, mounted at the trailing edge of the wings, and the rudder/speed brake, attached at the trailing edge of the stabilizer, with the body flap, control the orbiter during descent and landing.

The orbiter has a large payload bay measuring 15 by 60 feet (4.6 by 18 m) comprising most of the fuselage. Two mostly symmetrical lengthwise payload bay doors hinged on either side of the bay comprise its entire top. Payloads are generally loaded horizontally into the bay while the orbiter is oriented vertically on the launch pad and unloaded vertically in the near-weightless orbital environment by the orbiter's robotic remote manipulator arm (under astronaut control), EVA astronauts, or under the payloads' own power (as for satellites attached to a rocket "upper stage" for deployment.)

Three Space Shuttle main engines (SSMEs) are mounted on the orbiter's aft fuselage in a triangular pattern. The three engines can swivel 10.5 degrees up and down, and 8.5 degrees from side to side during ascent to change the direction of their thrust and steer the shuttle as well as push. The orbiter structure is made primarily from aluminum alloy, although the engine thrust structure is made primarily from titanium alloy.

The orbiter can be used in conjunction with a variety of add-ons depending on the mission. This has included orbital laboratories (Spacelab, Spacehab), boosters for launching payloads farther into space (Inertial Upper Stage, Payload Assist Module), and other add-ons like the Extended Duration Orbiter, Multi-Purpose Logistics Modules, and Canadarm (RMS).

The space-capable orbiters built are OV-099 Challenger, OV-102 Columbia, OV-103 Discovery, OV-104 Atlantis, and OV-105 Endeavour.^[10]

Orbiter add-ons:

MPLM Leonardo	IUS deploying with Galileo	PAM-D with satellite	EDO being installed
Spacelab in orbit	RMS (Canadarm)

Solid Rocket Boosters

Main article: Space Shuttle Solid Rocket Booster

Two solid rocket boosters (SRBs) each provide 12.5 million newtons (2.8 million lbf) of thrust at liftoff,^[11] which is 83% of the total thrust needed for liftoff. The SRBs are jettisoned two minutes after launch at a height of about 45.7 kilometers (150,000 ft), and then deploy parachutes and land in the ocean to be recovered.^[12] The SRB cases are made of steel about 1.3 centimeters (0.51 in) thick.^[13] The Solid Rocket Boosters are re-used many times; the casing used in Ares I engine testing in 2009 consisted of motor cases that have been flown, collectively, on 48 shuttle missions, including STS-1.^[14]

Flight systems

Early shuttle missions took along the GRiD Compass, arguably one of the first laptop computers. The Compass sold poorly, as it cost at least US$8000, but it offered unmatched performance for its weight and size.^[15] NASA was one of its main customers.^[16]

The shuttle was one of the earliest craft to use a computerized fly-by-wire digital flight control system. This means no mechanical or hydraulic linkages connect the pilot's control stick to the control surfaces or reaction control system thrusters.

A primary concern with digital fly-by-wire systems is reliability. Much research went into the shuttle computer system. The shuttle uses five identical redundant IBM 32-bit general purpose computers (GPCs), model AP-101, constituting a type of embedded system. Four computers run specialized software called the Primary Avionics Software System (PASS). A fifth backup computer runs separate software called the Backup Flight System (BFS). Collectively they are called the Data Processing System (DPS).^[17][18]

The design goal of the shuttle's DPS is fail-operational/fail-safe reliability. After a single failure, the shuttle can still continue the mission. After two failures, it can still land safely.

The four general-purpose computers operate essentially in lockstep, checking each other. If one computer fails, the three functioning computers "vote" it out of the system. This isolates it from vehicle control. If a second computer of the three remaining fails, the two functioning computers vote it out. In the rare case of two out of four computers simultaneously failing (a two-two split), one group is picked at random.

Atlantis deploys landing gear before landing on a selected runway just like a common aircraft.

The Backup Flight System (BFS) is separately developed software running on the fifth computer, used only if the entire four-computer primary system fails. The BFS was created because although the four primary computers are hardware redundant, they all run the same software, so a generic software problem could crash all of them. Embedded system avionic software is developed under totally different conditions from public commercial software: the number of code lines is tiny compared to a public commercial software, changes are only made infrequently and with extensive testing, and many programming and test personnel work on the small amount of computer code. However, in theory it can still fail, and the BFS exists for that contingency. While BFS will run in parallel with PASS, to date, BFS has never been engaged to take over control from PASS during any shuttle mission.

The software for the shuttle computers is written in a high-level language called HAL/S, somewhat similar to PL/I. It is specifically designed for a real time embedded system environment.

The IBM AP-101 computers originally had about 424 kilobytes of magnetic core memory each. The CPU could process about 400,000 instructions per second. They have no hard disk drive, and load software from magnetic tape cartridges.

In 1990, the original computers were replaced with an upgraded model AP-101S, which has about 2.5 times the memory capacity (about 1 megabyte) and three times the processor speed (about 1.2 million instructions per second). The memory was changed from magnetic core to semiconductor with battery backup.

Space Shuttle program insignia

Markings and insignia

The typeface used on the Space Shuttle Orbiter is Helvetica.^[19] On the side of the shuttle between the cockpit windows and the cargo bay doors is the name of the orbiter. Underneath the rear of the cargo bay doors is the NASA insignia, the text 'United States' and a flag of the United States. Another United States flag appears on the right wing.

Upgrades

During STS-101, Atlantis was the first shuttle to fly with a glass cockpit.

Internally, the shuttle remains largely similar to the original design, with the exception of the improved avionics computers. In addition to the computer upgrades, the original analog primary flight instruments were replaced with modern full-color, flat-panel display screens, similar to those of contemporary airliners like the Airbus A380 and Boeing 777. This is called a glass cockpit. Programmable calculators are carried as well (originally the HP-41C). With the coming of the ISS, the orbiter's internal airlocks have been replaced with external docking systems to allow for a greater amount of cargo to be stored on the shuttle's mid-deck during station resupply missions.

The Space Shuttle Main Engines (SSMEs) have had several improvements to enhance reliability and power. This explains phrases such as "Main engines throttling up to 104%." This does not mean the engines are being run over a safe limit. The 100% figure is the original specified power level. During the lengthy development program, Rocketdyne determined the engine was capable of safe reliable operation at 104% of the originally specified thrust. They could have rescaled the output number, saying in essence 104% is now 100%. To clarify this would have required revising much previous documentation and software, so the 104% number was retained. SSME upgrades are denoted as "block numbers", such as block I, block II, and block IIA. The upgrades have improved engine reliability, maintainability and performance. The 109% thrust level was finally reached in flight hardware with the Block II engines in 2001. The normal maximum throttle is 104%, with 106% or 109% used for mission aborts.

For the first two missions, STS-1 and STS-2, the external tank was painted white to protect the insulation that covers much of the tank, but improvements and testing showed that it was not required. The weight saved by not painting the tank results in an increase in payload capability to orbit.^[20] Additional weight was saved by removing some of the internal "stringers" in the hydrogen tank that proved unnecessary. The resulting "light-weight external tank" has been used on the vast majority of shuttle missions. STS-91 saw the first flight of the "super light-weight external tank". This version of the tank is made of the 2195 aluminum-lithium alloy. It weighs 3.4 tonnes (7,500 lb) less than the last run of lightweight tanks. As the shuttle cannot fly unmanned, each of these improvements has been "tested" on operational flights.

The SRBs (Solid Rocket Boosters) have undergone improvements as well. Design engineers added a third O-ring seal to the joints between the segments after the Space Shuttle Challenger disaster.

The three nozzles of the Main Engine cluster with the two Orbital Maneuvering System (OMS) pods, and the vertical stabilizer above.

Several other SRB improvements were planned in order to improve performance and safety, but never came to be. These culminated in the considerably simpler, lower cost, probably safer and better performing Advanced Solid Rocket Booster. These rockets entered production in the early to mid-1990s to support the Space Station, but were later canceled to save money after the expenditure of $2.2 billion.^[21] The loss of the ASRB program resulted in the development of the Super LightWeight external Tank (SLWT), which provides some of the increased payload capability, while not providing any of the safety improvements. In addition, the Air Force developed their own much lighter single-piece SRB design using a filament-wound system, but this too was canceled.

STS-70 was delayed in 1995, when woodpeckers bored holes in the foam insulation of Discovery's external tank. Since then, NASA has installed commercial plastic owl decoys and inflatable owl balloons which must be removed prior to launch.^[22] The delicate nature of the foam insulation has been the cause of damage to the Thermal Protection System, the tile heat shield and heat wrap of the orbiter, during recent launches. NASA remains confident that this damage, while it was the primary cause of the Space Shuttle Columbia disaster on February 1, 2003, will not jeopardize the objective of NASA to complete the International Space Station (ISS) in the projected time allotted.

A cargo-only, unmanned variant of the shuttle has been variously proposed, and rejected since the 1980s. It was called the Shuttle-C, and would have traded re-usability for cargo capability, with large potential savings from reusing technology developed for the Space Shuttle.

On the first four shuttle missions, astronauts wore modified U.S. Air Force high-altitude full-pressure suits, which included a full-pressure helmet during ascent and descent. From the fifth flight, STS-5, until the loss of Challenger, one-piece light blue nomex flight suits and partial-pressure helmets were worn. A less-bulky, partial-pressure version of the high-altitude pressure suits with a helmet was reinstated when shuttle flights resumed in 1988. The Launch-Entry Suit ended its service life in late 1995, and was replaced by the full-pressure Advanced Crew Escape Suit (ACES), which resembles the Gemini space suit worn in the mid-1960s.

To extend the duration that orbiters can stay docked at the ISS, the Station-to-Shuttle Power Transfer System (SSPTS) was installed. The SSPTS allows these orbiters to use power provided by the ISS to preserve their consumables. The SSPTS was first used successfully on STS-118.