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This article is about the NASA Space Shuttle. For information on the Soviet space shuttle, see the article Shuttle Buran.

NASA's Space Shuttle, officially called Space Transportation System (STS), is the United States government's current manned launch vehicle. The winged shuttle orbiter is launched vertically, usually carrying five to seven astronauts (although eight have been carried) and up to 22,700 kg (50,000 lb) of payload into low earth orbit. When its mission is complete, it fires its maneouvering thrusters to drop out of orbit and re-enters the earth's atmosphere. During the descent and landing, the shuttle orbiter acts as a glider and makes a completely unpowered landing.

The Shuttle is the first orbital spacecraft designed for partial reusability. It is also so far the only winged manned spacecraft to achieve orbit and land. It carries large payloads to various orbits, provides crew rotation for the International Space Station (ISS), and performs servicing missions. The orbiter can recover satellites and other payloads from orbit and return them to Earth, but this capacity has not been used often. However, it has been used to return large payloads from the ISS to earth, as the Russian Soyuz spacecraft has limited capacity for return payloads. Each Shuttle was designed for a projected lifespan of 100 launches or 10 years' operational life.

The program started in the late 1960s and has dominated NASA's manned operations since the mid-1970s. According to the Vision for Space Exploration, use of the Space Shuttle will be focused on completing assembly of the ISS in 2010 (more specifically, the construction completion of the ISS), after which it will be replaced by the Crew Exploration Vehicle (CEV).

Development

Even before the Apollo moon landing in 1969, in October 1968 NASA began early studies of space shuttle designs. The early studies were denoted "Phase A", and in June 1970, "Phase B", which were more detailed and specific.

In 1969 President Richard Nixon formed the Space Task Group, chaired by vice president Spiro T. Agnew. They evaluated the shuttle studies to date, and recommended a national space strategy including building a space shuttle.Heppenheimer, T.A. The Space Shuttle Decision: NASA's Search for a Reuseble Space Vehicle. Washington, DC: National Aeronautics and Space Administration, 1999.

During early shuttle development there was great debate about the optimal shuttle design that best balanced capability, development cost and operating cost. Ultimately the current design was chosen, using a reusable winged orbiter, solid rocket boosters, and expendable external tank.

The Shuttle program was formally launched on January 5, 1972, when President Nixon announced that NASA would proceed with the development of a reusable Space Shuttle system. The final design was less costly to build and less technically ambitious than earlier fully reusable designs.

The prime contractor for the program was North American Aviation (later Rockwell International), the same company responsible for the Apollo Command/Service Module. The contractor for the Space Shuttle Solid Rocket Boosters was Morton Thiokol (now part of Alliant Techsystems), for the external tank, Martin Marietta (now Lockheed Martin), and for the Space shuttle main engines, Rocketdyne.

The first complete orbiter was originally named Constitution, but a massive write-in campaign from fans of the Star Trek television series convinced the White House to change the name to Enterprise. Amid great fanfare, the Enterprise was rolled out on September 17, 1976, and later conducted a successful series of glide-approach and landing tests that were the first real validation of the design.

The first fully functional Shuttle Orbiter was the Columbia, built in Palmdale, California. It was delivered to Kennedy Space Center on March 25, 1979, and was first launched on April 12, 1981—the 20th anniversary of Yuri Gagarin's space flight—with a crew of two. Challenger was delivered to KSC in July 1982, Discovery in November 1983, and Atlantis in April 1985. Challenger was destroyed when it disintegrated during ascent due to O-Ring failure on the right SRB on January 28, 1986, with the loss of all seven astronauts on board. Endeavour was built to replace Challenger (using spare parts originally intended for the other Orbiters) and delivered in May 1991; it was first launched a year later. Seventeen years after Challenger, Columbia was lost, with all seven crew members, during reentry on February 1, 2003, and has not been replaced. Out of five functional shuttle orbiters only three remain for use.

Description

The Shuttle is a partially reusable launch system composed of three main assemblies: the reusable Orbiter Vehicle (OV), the expendable External Tank (ET), and the two reusable Solid Rocket Boosters (SRBs). The tank and boosters are jettisoned during ascent; only the orbiter goes into 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 Orbiter resembles an airplane 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 45° 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's crew cabin consists of three levels: the flight deck, the mid-deck, and the utility area. The upper-most is the flight deck which seats the commander and pilot, with two mission specialists behind them. The mid-deck, which is below the flight deck, has three more seats for the rest of the crew members. The galley, toilet, sleep locations, storage lockers, and the side hatch for entering/exiting the vehicle are also located on the mid-deck, as is the airlock hatch. The airlock has another hatch into the payload bay. It allows two astronauts, wearing their Extravehicular Mobility Unit (EMU) space suits, to depressurize before a space walk.

The Orbiter has a large 60 by 15 ft (18 m by 4.6 m) payload bay, filling most of the fuselage. The payload bay doors have heat radiators mounted on their inner surfaces, and so are kept open for thermal control while the Shuttle is in orbit. Thermal control is also maintained by adjusting the orientation of the Shuttle relative to Earth and Sun. Inside the payload bay is the Remote Manipulator System, also known as the Canadarm, a robot arm used to retrieve and deploy payloads. Until the loss of Columbia, the Canadarm had been used only on those missions where it was needed. Since the arm is a crucial part of the Thermal Protection Inspection procedures now required for Shuttle flights, it will probably be included on all future flights. 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 Orbital Maneuvering System (OMS) provides orbital maneuvers, including insertion, circularization, transfer, rendezvous, abort to orbit, and abort once around.

The Reaction Control System (RCS) provides attitude control and translation along the pitch, roll, and yaw axes during the flight phases of orbit insertion, orbit, and re-entry.

The Thermal Protection System (TPS) covers the outside of the Orbiter, protecting it from the cold soak of -121 °C (-250 °F) in space to the 1649 °C (3000 °F) heat of reentry

The orbiter structure is made primarily from aluminium alloy, although the engine thrust structure is made from titanium.

The External Tank (ET) provides 2.025 million liters (535,000 gallons) of liquid hydrogen and liquid oxygen propellant to the SSMEs. It is discarded 8.5 minutes after launch at an altitude of 60 nautical miles (111 km), which then burns up on re-entry. The ET is constructed mostly of 1/8 inch thick aluminium-lithium alloy.

The external tanks of the first two missions were painted white, which unnecessarily added an extra 600 pounds (273 kg) of weight to each ET. Subsequent missions have had unpainted tanks showing the natural orange-brown color of the spray-on foam insulation. The lighter, unpainted tanks have increased the payload capacity by almost the entire weight savings of 600 pounds.National Aeronautics and Space Administration "NASA Takes Delivery of 100th Space Shuttle External Tank." Press Release 99-193. 16 Aug 1999.

Two Solid Rocket Boosters (SRBs) provide about 83% of the vehicle's thrust at liftoff and during the first stage ascent. They are jettisoned two minutes after launch at a height of about 150,000 feet (45.7 km), then deploy parachutes and land in the ocean to be recovered. The SRB cases are made of steel about 1/2 inch (1.27 cm) thick.

Early Shuttle missions took along the GRiD Compass, arguably one of the first laptop computers. The Compass sold poorly, because it cost at least $8000, but offered unmatched performance for its weight and size. NASA was one of its main customers.

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 shuttle Data Processing System (DPS). The design goal of the shuttle DPS is fail operational/fail safe reliability. After a single failure the shuttle can continue the mission. After two failures it can 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.

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. This should never happen, as embedded system avionic software is developed under totally different conditions from commercial software. For example, the number of code lines is tiny compared to a commercial operating system, 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 fail, and the BFS exists for that contingency.

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, but load software from 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.

Upgrades

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 vector graphics monochrome cockpit displays were replaced with modern full-color, flat-panel display screens, similar to contemporary airliners like the Airbus A320. This is called a "glass cockpit". In the Apollo-Soyuz Test Project tradition, 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 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%. However 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% and 109% available for abort emergencies.

For 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. 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 aluminium-lithium alloy. It weighs 7,500 lb (3.4 t) 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. Notable is the adding of a third O-ring seal to the joints between the segments, which occurred after the Challenger accident.

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 which was to have entered production in the early to mid-1990s to support the Space Station, but was later cancelled to save money after the expenditure of $2.2 billion. The loss of the ASRB program forced 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 cancelled.

STS-70 was delayed in 1995 when woodpeckers holed 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. *

A cargo-only, unmanned variant of the Shuttle has been variously proposed and rejected since the 1980s. It is called the Shuttle-C and would trade 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 full-pressure Launch Entry Suit (LES) including a helmet during ascent and descent. From the fifth flight, STS-5, until the loss of Challenger only helmets were worn without a suit. The LES with a helmet was reinstated when Shuttle flights resumed in 1988. The LES ended its service life in late 1995, replaced by the Advanced Crew Escape Suit (ACES).

Technical data

Orbiter Specifications (for Endeavour, OV-105)
  • Length: 122.17 ft (37.24 m)
  • Wingspan: 78.06 ft (23.79 m)
  • Height: 58.58 ft (17.25 m)
  • Empty Weight: 151,205 lb (68,586.6 kg)
  • Gross Liftoff Weight: 240,000 lb (109,000 kg)
  • Maximum Landing Weight: 230,000 lb (104,000 kg)
  • Main Engines: Three Rocketdyne Block 2 A SSMEs, each with a sea level thrust of 393,800 lbf (178,624 kgf / 1.75MN)
  • Maximum Payload: 55,250 lb (25,061.4 kg)
  • Payload Bay dimensions: 15 ft by 60 ft (4.6 m by 18.3 m)
  • Operational Altitude: 100 to 520 nmi (185 to 1,000 km)
  • Speed: 25,404 ft/s (7,743 m/s, 27,875 km/h, 17,321 mi/h)
  • Crossrange: 1,085 nautical miles (2,009.4 km)
  • Crew: Seven (Commander, Pilot, two Mission Specialists, and three Payload Specialists), two for minimum. External Tank Specifications (for SLWT)
    • Length: 153.8 ft (46.9 m)
    • Diameter: 27.6 ft (8.4 m)
    • Propellant Volume: 535,000 gallon
    • Empty Weight: 58,500 lb (26,559 kg)
    • Gross Liftoff Weight: 1.667 million lb (757,000 kg)

    Solid Rocket Booster Specifications

    • Length: 149.6 ft (45.6 m)
    • Diameter: 12.17 ft (3.71 m)
    • Empty Weight: 139,490 lb (63,272.7 kg)
    • Gross Liftoff Weight: 1.3 million lb (590,000 kg)
    • Thrust (sea level, liftoff): 2.8 million lbf (1,270,058 kgf / 12.46MN)

    System Stack Specifications

    • Height: 184.2 ft (56.14 m)
    • Gross Liftoff Weight: 4.5 million lb (2.04 million kg)
    • Total Liftoff Thrust: 6.781 million lbf (3.076 million kgf / 30.18MN)

    Launch

    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 aluminium, which would normally 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. 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.

    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 U.S. gallons (1,135,623 L) of water to protect the Orbiter from damage by acoustical energy and rocket exhaust reflected from the flame trench and MLP during liftoff. National Aeronautics and Space Administration. "Sound Suppression Water System" Revised 2000-08-28. Accessed 2006-07-09. The three Space Shuttle Main Engines (SSMEs) start at T minus 6.6 seconds. National Aeronautics and Space Administration. "NASA - Countdown 101" Accessed 2006-07-10. All three SSMEs must reach the required 100% thrust within three seconds. If the onboard computers verify normal thrust buildup, at T minus 0 the SRBs are ignited. At that point the vehicle is committed to takeoff, as the SRBs cannot be turned off once ignited. After the SRB's reach a stable thrust pyrotechnic fasteners, large nuts that split in half, are detonated to release the craft. 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.

    When watching a launch, look for the "nod" ("twang" in "NASAese"). 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 flex forwards about 2m at cockpit level. As the boosters flex back into their original shape, the launch stack springs slowly back upright. This takes approximately 6 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 so that the vehicle is below the external tank and SRBs. 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. Orbital velocity at the 380 km (236 miles) altitude of the International Space Station is 7.68 km per second (27,648 km/h, 17,180 mph), roughly equivalent to Mach 23. For missions towards the International Space Station, the shuttle must reach an azimuth of 51.6 degrees inclination to rendezvous with the station.

    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 vulnerable parts 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 reduces and the ever-lighter vehicle produces more and more acceleration until the thrust to weight ratio exceeds 1 again and the vehicle can hold itself up.

    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 maintains altitude whilst it accelerates horizontally towards orbit.

    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, largely for astronaut comfort.

    Before complete depletion of propellant (running dry would destroy the engines) the main engines are shut down and the external tank is released by firing explosive bolts. The tank then falls, largely to burn up in the atmosphere, with some fragments falling into the Indian Ocean.

    To keep the shuttle from following the external tank back into the atmosphere, the OMS engines are fired to raise the perigee out of the atmosphere. On some missions (e.g., STS-107 and missions to the ISS), the OMS engines are also used while the main engines are still firing.

    Landing

    The vehicle begins reentry by firing the OMS engines in the opposite direction to orbital motion for about three minutes. The resulting deceleration of the Shuttle lowers its orbit perigee down into the atmosphere. This OMS firing is done roughly halfway around the globe from the landing site. The entire reentry, except for the lowering the landing gear and deploying the air data probes, is then under computer control. However the reentry can be and has (once) been flown manually. The final landing can be done on autopilot, but is usually hand flown.

    The vehicle starts significantly entering the atmosphere at about 400,000 ft (120 km) at around Mach 25 (8.2 km/s). The vehicle is controlled by a combination of RCS thrusters and control surfaces, to fly at a 40 degrees nose-up attitude producing high drag, not only to slow it down to landing speed, but also to reduce reentry heating. In addition, the vehicle needs to bleed off extra speed before reaching the landing site. This is achieved by performing s-curves at up to a 70 degree roll angle.

    In the lower atmosphere the Orbiter flies much like a conventional glider, except for a much higher descent rate, over 10,000 feet (3 km) per minute. It glides with a glide angle of 4:1. 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 vehicle's movement in the atmosphere.

    When the approach and landing phase begins, the Orbiter is at 10,000 ft (3048 m) altitude, 7.5 miles (12.1 km) to the runway. The pilots apply aerodynamic braking to help slow down the vehicle. The Orbiter's speed is reduced from 424 mph (682 km/h) to approximately 215 mph (346 km/h), (compared to 160 mph for a jet airliner), at touch-down. The landing gear is deployed while the Orbiter is flying at 267 mph (430 km/h). To assist the speed brakes, a 40 ft (12.2 m) drag chute is deployed once the nose gear touches down at about 213 mph (343 km/h). It is jettisoned as the Orbiter slows through 69 mph (111 km/h).

    After landing the vehicle stands on the runway for several minutes to permit the fumes from poisonous hydrazine, used as propellant for attitude control, to dissipate.

    Operations, applications and accidents

    Individual Orbiters are both named, in a manner similar to ships, and numbered, using the NASA Orbiter Vehicle Designation system. While all Orbiters are externally very similar, they have minor internal differences; new equipment is fitted on a rotating basis as they are maintained, and the newer Orbiters tend to be structurally lighter.

    • Handling test article designed with no spaceflight capability:
      • Pathfinder (Orbiter Simulator, no series number)

    • Main propulsion test article, with no spaceflight capability:
      • MPTA-ET (External Tank) which is now attached to Pathfinder
      • MPTA-098 suffered major damage due to engine failure.

    • Structural test article, with no spaceflight capability:
      • STA-099 which became Challenger, which then destroyed after liftoff

    • Test vehicle suitable only for glide/landing tests, with no spaceflight capability without major refit:

    The Space Shuttle's applications include:

    • Crew rotation and servicing of Mir and the ISS
    • Manned servicing missions, such as to the Hubble Space Telescope (HST)
    • Manned experiments in LEO
    • Carry to LEO:
      • Large satellites — these have included the HST
      • Components for the construction of the ISS
      • Supplies
    • Carry satellites with a booster, the Payload Assist Module (PAM-D) or the Inertial Upper Stage (IUS), to the point where the booster sends the satellite to:

    Flight statistics

    Accidents

    As of 2006, two Shuttles have been destroyed in 114 missions, both with the loss of the entire crew (14 astronauts total):

    This gives a 2% death rate per astronaut per flight, and a failure rate of almost 1 every 60 missions.

    While the technical details of the accidents are different, the organizational problems show similarities. In both cases events happened which were not planned for or anticipated. In both cases, engineers were greatly concerned about possible problems but these concerns were not properly communicated to or understood by senior NASA managers. The vehicle gave ample warning beforehand of abnormal problems. A heavily layered, procedure-oriented bureaucratic structure inhibited necessary communication and action. A mind set among senior managers developed that concerns had to be objectively proven rather than simply suspected.

    With Challenger an O-ring which should not have eroded at all did erode on earlier shuttle launches. Yet managers felt because it had not previously eroded by more than 30%, that this was not a hazard as there was "a factor of three safety margin". Morton Thiokol designed and manufactured the SRBs, and during a pre-launch conference call with NASA, the Thiokol engineer most experienced with the O-rings pleaded with management repeatedly to cancel or reschedule the launch. He raised concerns that the unusually cold temperatures would stiffen the O-rings, preventing a complete seal, which was exactly what happened on the fatal flight. However, Thiokol's senior managers overruled him, dismissing his safety concerns and allowed the launch to proceed. Challenger's O-rings eroded completely through as predicted, resulting in the complete destruction of the spacecraft and the loss of all seven astronauts on board.

    Columbia was destroyed because of damaged thermal protection from foam debris that broke off the external tank during ascent. The foam had not been designed or expected to break off, but had been observed in the past to do so without incident. The original shuttle operational specification said the orbiter thermal protection tiles were designed to withstand virtually no debris hits at all. Over time NASA managers gradually accepted more tile damage, similar to how O-ring damage was accepted. The Columbia Accident Investigation Board called this tendency the "normalization of deviance" — a gradual acceptance of events outside the design tolerances of the craft simply because they had not been catastrophic to date.

    The subject of missing or damaged thermal tiles on the Shuttle fleet only became an issue following the loss of Columbia in 2003 as it broke up on re-entry. In fact Shuttles had previously come back missing as many as 20 tiles without any problem. STS-1, STS-16 and STS-41 have all flown with missing thermal tiles from the orbital maneuvering system pods (visible to the crew). This image from the NASA archives shows many missing tiles on the STS-1 OMS pods : The problem on Columbia was that the damage was sustained to the carbon-carbon leading edge panel of the wing, not the heat tiles. On the same subject, a little-publicised detail about the first Shuttle mission, STS-1, was that it had a protruding gapfiller that ducted hot gas into the right wheel well on re-entry, buckling the right main gear on landing as a result.[http://www.thespacereview.com/article/15/1

    Current status

    Since the Space Shuttle Columbia disaster in 2003, the ISS has been operating on a skeleton crew of two and is currently being serviced primarily by Russian space vehicles. While the "return to flight" mission STS-114 in 2005 was successful, a similar piece of foam from a different portion of the tank was shed. Although the debris did not strike the Orbiter, the program was grounded once again.

    Mission STS-121, considered a return-to-flight test mission, launched on July 4, 2006, at 2:38 pm local time (18:38 GMT), after two previous launches were scrubbed because of lingering thunderstorms and high winds around the launch pad and the launch took place despite objections from its chief engineer and safety head. This mission increased the ISS crew to three. A five-inch crack in the foam insulation of the external tank gave reason for concern; however, the Mission Management Team gave the go for launch. Chien, Philip (June 27, 2006) "NASA wants shuttle to fly despite safety misgivings." The Washington Times Space Shuttle Discovery touched down successfully at 9:14:43 am Eastern Daylight time on Runway 15 at the Kennedy Space Center on July 17th, 2006.

    Following the success of STS-121, it appears the Shuttle will now start flying regularly again. The next mission, STS-115, is scheduled for launch on August 28, 2006.

    The Shuttle program is scheduled to be retired in 2010. The Shuttle's planned succesor is Project Constellation with its Ares I and Ares V launch vehicles and Crew Exploration Vehicle. NASA hopes to launch 16 more shuttle flights by then. National Aeronautics and Space Administration. "NASA Names New Rockets, Saluting the Future, Honoring the Past" Press Release 06-270. 30 June 2006.

    Design limitations

    Although prepared slowly and checked many times, all systems as complex as the space shuttle inevitably have some design issues:
    • Placing the heat shield where it could be damaged by foam and ice falling off the external tank.
    • A design using the ceramic heat shield, as the tiles are susceptible to damage, and cost millions to replace.
    • The O-ring/joint design on the SRB was originally quickly thought out and this in combination with operation at low temperatures led to the loss of Challenger.
    • The SSMEs originally didn't use hydrostatic bearings, and this added to the maintenance issues.
    • It was originally thought that the SRBs could be safely switched off before burn out. However, this could not be achieved, reducing safety.

    Costs

    The shuttle has not met the goal of greatly reducing launch costs. There are various ways to measure per-launch costs. One way is dividing the total cost over the life of the program (including buildings, facilities, training, salaries, etc) by the number of launches. This method gives about $1.3 billion per launchAnother method is calculating the incremental (or marginal) cost differential to add or subtract one flight — just the immediate resources expended/saved/involved in that one flight. This is about $55 million [http://sciencepolicy.colorado.edu/admin/publication_files/resource-100-1993.01.pdf. Neither figure is right or wrong; they are simply different ways to examine the picture. However, the original cost justifications used flight rates far higher than any operational rate that has ever been achieved.

    The total cost of the program has been $145 billion as of early 2005, and is estimated to be $174 billion when the Shuttle retires in 2010. NASA's budget for 2005 allocates 30%, or $5 billion, to Space Shuttle operations. *

    Original goals of the Shuttle included operating at a fairly high flight rate (roughly 12 flights per year *), at low cost, and with high reliability. Improving in these areas over the previous generation of single-use and unmanned launchers was a motivation. Although it did operate as the world's first reusable crew-carrying spacecraft, it did not greatly improve on those parameters, and is considered by some to have failed in its original purpose.

    Although the final design differs from the original concept, the project was still supposed to meet USAF goals and be much cheaper to fly in general. One reason behind this apparent failure is inflation. During the 1970s the U.S. suffered from severe inflation. Between when the program began in 1972, and first flight in April 1981, inflation increased prices over 200%. When evaluating shuttle development costs in later-year dollars, this superficially appeared to be a large cost overrun in the program. In fact, discounting inflation, the shuttle development program was within the initial cost estimate given to President Richard M. Nixon in 1971 *.

    The more than anticipated, if counting all associated support resources (total expenditures, including development costs, divided by number of flights). Some of this can be attributed to a lower flight rate, operating beyond the 10-year anticipated lifespan of each Shuttle, and higher than anticipated maintenance costs. The marginal or incremental per launch costs have been about 50% more than early projections.

    It takes about 20 pounds (9.1 kg) of fuel to lift 1 pound of cargo into orbit. Astronomy Answers

    Some reasons for higher than expected operational costs can be ascribed to:

  • Maintenance of thermal protection tiles turned out to be very labor-intensive, averaging about a week's work for one person to replace a tile, with hundreds damaged with each launch.
  • The Space shuttle main engines were highly complex and maintenance-intensive, needing removal and extensive inspection after each flight. Before the current "Block II" engines, the turbopumps (a primary engine component) had to be removed, dissembled, and totally overhauled after each flight.
  • Launch rate is significantly lower than initially expected. This does not reduce actual operating costs, but if dividing total program costs by number of launches, more launches per year produces a lower per-launch cost figure. Some early hypothetical studies examined 55 launches per year, but the maximum possible launch rate was limited to 24 per year, based on manufacturing capacity of the external tank. Early in the shuttle development, the expected launch rate was about 12 per year *. Launch rates reached 9 per year in 1985 but averaged less thereafter.
  • Early cost estimates of $118 per pound of payload were based on marginal or incremental launch costs, and based on 1972 dollars and assuming a 65,000 pound payload capacity. Correcting for inflation and other factors, this equates to roughly $36 million incremental costs per launch. Compared to this, today's actual incremental per launch costs are about 50% more, or $55 million per launch *.

    Shuttle operations

    The Shuttle was originally conceived to operate somewhat like an airliner. After landing, the orbiter would be checked out and start "mating" to the rest of the system (the ET and SRBs), and be ready for launch in as little as two weeks. Instead, this turnaround process usually takes months; Columbia was once launched twice within 56 days. Because loss of crew is unacceptable, the primary focus of the Shuttle program is to return the crew to Earth safely, which can conflict with other goals, namely to launch payloads cheaply. Furthermore, because in many cases there are no survivable abort modes, many pieces of hardware simply must function perfectly and so must be carefully inspected before each flight. The result is high labor cost, with around 25,000 workers in Shuttle operations and labor costs of about $1 billon per year.

    During development, shuttle features were primarily chosen based on capability required to service the future space station. Even though the initially planned Space Station Freedom was significantly scaled back, the shuttle was still vital to service it. No other launch vehicle had the Shuttle's payload capability or could return large items from the space station to Earth.

    NASA's plan for using the Shuttle to launch all unmanned payloads declined, then was discontinued. Following the Challenger disaster, use of the Shuttle to carry the powerful liquid fueled Centaur upper stages planned for interplanetary probes was ruled out. The Shuttle's history of unexpected delays also makes it liable to miss narrow launch windows. Advances in technology over the last decade have made probes smaller and lighter, and as a result unmanned probes and communications satellites can use cheaper and more reliable expendable rockets, including Delta launcher, and Atlas V.

    Looking back and ahead

    Opinions differ on the lessons of the Shuttle. While it was developed within the original development cost and time estimates given to President Richard M. Nixon in 1971 *, the operational costs, flight rate, payload capacity, and reliability have been worse than anticipated.

    In general, future designers need to look to less complex, more reliable launch systems with lower maintenance costs. One approach is Single Stage To Orbit (SSTO), which would be 100% reusable and use a single stage. NASA evaluated several concepts in the 1990s, and selected the X-33, which would eventually have been the VentureStar. During design that program increased in complexity and development cost, encountered problems and was finally cancelled.

    Another variant of SSTO is a hypersonic, scramjet-powered, airbreathing vehicle. This would be launched and landed horizontally like an airliner. It would achieve much of orbital velocity while still within the upper atmosphere. It was originally investigated by the U.S. Department of Defense, but passenger-carrying civilian versions were planned, sometimes called the "New Orient Express". The official name was the Rockwell X-30. Like the X-33, the X-30 encountered major technical difficulties, primarily due to the system complexity and materials required for hypersonic flight, and was also cancelled.

    Another approach is lower cost expendable launch vehicles. NASA currently uses these for unmanned launches, and plans to use them for future manned launches. NASA plans on using modified shuttle components to build an expendable Shuttle Derived Launch Vehicle. This technology would be used to develop two separate launchers, one for manned missions and the other for unmanned heavy cargo. This contrasts with the current shuttle where astronauts and heavy cargo are launched in a single vehicle. Unlike the shuttle, this future launcher and associated crew exploration vehicle will have a launch escape system to greatly improve the chances that the crew can be saved in the event of a disaster.

    Cultural issues and problems

    Some researchers have identified a cultural issue in the design and maintenance of the Space Shuttle in particular and in overall NASA operations in general. Anthropologist Diane Vaughan ("The Challenger Launch Decision: Risky Technology, Culture and Deviance at NASA", University of Chicago press, 1997) examined in detail the engineering and managerial processes used in launching the shuttle.

    She found that in recommendations such as "think like a manager and not an engineer", aired at a videoconference before the 1986 launch of Challenger, a "normalized deviance", which can be best described as a decrease in s/r, where s is the amount of resources given to safety and r is the emphasis on ontime launches. That is, s/r's values in the 1980s started to decrease, with full buy-in from NASA upper level managers but some resistance from engineers, relative to their values in the first decade of the Shuttle.

    In addition, the late physicist Richard Feynman, appointed to the official enquiry on Challenger, published a personal statement as an appendix to the official report in which Feynman said that in some ways, NASA was trying to repeal the laws of nature in its aggressive launch schedules.

    Aggressive launch schedules, according to Vaughan, started in the Reagan years as attention turned to the space program in general and the shuttle in particular (as America's only manned spaceflight after the final Apollo missions) not so much for scientific reasons but instead as a way to enhance America's prestige post-Vietnam.

    Despite Feynman's warnings, and despite the fact that Ms. Vaughan served on safety boards and committees at NASA, follow-ups in the general and technical press have found that the NASA culture, described charitably as aggressive, mathematically as smaller s/r, and uncharitably as normalized deviance, persists to this day. Evidence for these claims exists in the disregard of small foam chunk breakage and the assumption that the lack of damage from past breakages made a larger and more serious incident less rather than more probable.

    Terrestrial transportation vehicles

    • The Crawler-Transporter carries the Mobile Launcher Platform and the Space Shuttle from the Vehicle Assembly Building to Launch Complex 39.
    • The Shuttle Carrier Aircraft are two modified Boeing 747s. Either can fly an Orbiter from alternative landing sites back to Cape Canaveral.
    • A 36-wheeled transport trailer, originally built for the U.S. Air Force's launch facility at Vandenberg Air Force Base in California (since then converted for Delta IV rockets) that would transport the Orbiter from the landing facility to the launch pad, which allowed both "stacking" and launch without utilizing a separate VAB-style building and crawler-transporter roadway. Prior to the closing of the Vandenberg facility, Orbiters were transported from the OPF to the VAB on its undercarriage, only to be raised when the Orbiter was being lifted for attachment to the SRB/ET stack. The trailer allows the transportation of the Orbiter from the OPF to either the SCA-747 "Mate-Demate" stand or the VAB without placing any additional stress on the undercarriage.

    See also

    Notes

    Further reading

    External links

    Reusable launch vehicles | Rocket-powered aircraft | Space launch vehicles | Spaceplanes | Space Shuttle program

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This article is licensed under the GNU Free Documentation License. It uses material from the "Space Shuttle program".

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