Space Shuttle program: Meaning (information, definition, explanation, facts)

NASA's Space Shuttle, officially called Space Transportation System (STS), is the United States government's sole manned launch vehicle currently in service. The Space Shuttle orbiter was manufactured by North American Rockwell, now part of the Boeing Company. Martin Marietta (now part of Lockheed Martin) designed the external fuel tank and Morton Thiokol (now the Thiokol corporation) designed the solid rocket boosters.

The Shuttle is the first spacecraft designed for partial reusability. It carries large payloads to various orbits, provides crew rotation for the International Space Station (ISS), and performs servicing missions. While the vehicle was designed with the capacity to recover satellites and other payloads from orbit and return them to Earth, this capacity has not been used often; it is, however, an important use of the Space Shuttle in the context of the ISS program, as only very small amounts of experimental material, hardware needing to be repaired, and trash can be returned by Soyuz.

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.

The shuttle Discovery launched at 10:39 AM EDT on July 26, 2005 and is currently on mission STS-114, referred to as "Return to Flight". Discovery docked with the ISS at 07:18 AM EDT on July 28, 2005. This is the first mission since the Space Shuttle Columbia disaster at the end of STS-107 in 2003. See the STS-114 article for more information as news becomes available.

After a post-disaster review of the program, NASA, under the direction of administrator Sean O'Keefe, decided that the Orbiter must be inspected externally each mission in orbit before re-entry, a task that the agency had decided is too expensive to be done without the facilities of the ISS. Therefore, for example, further service missions to the Hubble Space Telescope (HST) were determined to be impractical, because the Orbiter is incapable of reaching both HST and ISS during the same mission. Since Michael Griffin took Sean O'Keefe's place, however, a method was developed to examine the Orbiter using the new Orbital Boom Sensor System (OBSS) attached to the Remote Manipulator System, also known as Canadarm, allowing the Shuttle to operate free of the ISS.

Once Flight STS-114 has been completed, the Space Shuttle will be grounded once again. A piece of debris broke off the external fuel tank on Discovery upon its July 26, 2005 launch; a similar problem is blamed for setting off the chain of events that led to Columbias disintegration. Though debris is not thought to have damaged the Discovery orbiter (the astronauts on board used the afore-mentioned robotic arm to check for damage and none was found), NASA announced on July 27 that the shuttle fleet will be grounded until investigations are complete and the problem of foam debris is solved.

NASA currently has a tentative plan for a Hubble service mission sometime in 2007 to replace batteries, gyroscopes, and other instruments. In addition, a hypergolic retrorocket will be affixed to the telescope to allow a controlled, safe reentry once it is retired. This mission, however, rides on the success of the Return to Flight mission. Should there be extensive delays in a Hubble service mission, NASA estimates that the telescope will remain serviceable until 2009 or 2010.

According to the Vision for Space Exploration, use of the Space Shuttle will be focused on completing assembly of the ISS in 2010, after which it will be replaced by the yet-to-be-developed Crew Exploration Vehicle (CEV).

History

The Shuttle decision

NASA had conducted a series of paper-projects throughout the 1960s on the topic of reusable spacecraft to replace their expedient "one-off" systems like Mercury, Gemini, and Apollo. Meanwhile, the U.S. Air Force had a continuing interest in smaller systems with more rapid turn-around times, and were involved in their own spaceplane project, the X-20 Dyna-Soar. In several instances groups from both worked together to investigate the state of the art.

With the major Apollo development effort winding down in the second half of the 1960s, NASA started looking to the future of the space program. They envisioned an ambitious program consisting of a large space station being launched on huge boosters, served by a reusable logistics "space shuttle," both providing services for a permanently manned Lunar colony and eventual manned missions to Mars.

However, reality interceded, and NASA found itself with a rapidly plunging budget. Rather than stepping back and looking at their future as a whole given their new financial situation, they attempted to save as many of the individual projects as possible. The mission to Mars was quickly eliminated, but the Space Station and Shuttle continued on. Eventually only one of them could be saved, so it stood to reason that a low-cost Shuttle system would be the better bet, because without it a large station would never be affordable.

A number of designs were proposed, but many of them were complex and varied widely in their systems. An attempt to re-simplify was made in the form of the "DC-3" by one of the few people left in NASA with the political clout to pull it off, Maxime Faget, who had designed the Mercury capsule, among others. The DC-3 was a small craft with a 20,000-pound (9 tonne) (or less) payload, a four-man crew, and limited maneuverability. At a minimum, the DC-3 provided a baseline "workable" (but not terribly advanced) system by which other systems could be compared for price/performance trade-offs.

The final defining moment was when NASA, in desperation to see their only remaining project saved, went to the Air Force for its blessing. NASA asked that the USAF place all of their future launches on the Shuttle instead of their current expendable launchers (like the Titan II), in return for which they would no longer have to continue spending money upgrading those designs — the Shuttle would provide more than enough capability.

The Air Force reluctantly agreed, but only after demanding a large increase in capability to allow for launching their projected spy satellites (mirrors are heavy). These were quite large, weighing an estimated 40,000 pounds (18 tonnes), and needed to be put into polar orbits, which require higher energies than lower inclination orbits. And since the Air Force also wanted to be able to abort after a single orbit (as did NASA), and land at the launch site (unlike NASA), the spacecraft would also require the ability to maneuver significantly to either side of its orbital track to adjust for launch point rotational drift while in polar orbit — for example, in a 90 minute orbit, Vandenberg AFB would drift over 1,000 miles (1,600 km), whereas in more equatorially aligned orbits, the required cross-range would be less than 250 mi / 400 km. This large 'cross-range' capability for polar orbits meant the craft had to have a greater lift-to-drag ratio than originally planned, requiring the addition of bigger, heavier wings.

The result was that the simple "DC-3" was clearly out of the picture because it had neither the cargo capacity nor the cross-range the Air Force demanded. In fact all existing designs were far too small, as a 40,000-pound (18 tonnes) delivery to polar orbit equates to a 65,000-pound (29-tonne) delivery to an eastwardly launched orbit with typical 28-degree inclination. In fact, any design using simple straight or fold-out wings was not going to meet the cross-range requirements, so any future design would require a more complex, heavier delta wing.

Of further concern, any increase in the weight of the upper portion of a launch vehicle, which had just occurred, required an even bigger increase in the capability of the lower stage used to launch it. Suddenly, the two-stage system had grown in size to something larger than the Saturn V, and the complexity and costs to develop it skyrocketed.

While all of this was going on, others were suggesting a completely different approach to the future. They stated that NASA was better off using the existing Saturn to launch their space station, supplied and manned using modified Gemini capsules on top of the Air Force's newer Titan II-M. The cost of development for this looked to be considerably less than the Shuttle alone, and would have a large space station in orbit earlier.

In reply, Shuttle advocates answered that given enough launches, a reusable system would more than pay for the cost of development, when compared with the launch costs of throwaway rockets. Another factor in the cost benefit analysis was inflation, and in the 1970s this was high enough that the payback from the development had to happen very quickly to see a positive return. Hence, a high launch rate was needed to make the system economically feasible.

But there was no way that a space station or Air Force payloads could demand such rates (roughly one or two a week), so they went further and suggested that all future US launches would take place on the Shuttle, once built. In order to do this, the cost of launching the Shuttle would have to be lower than any other system, with the exception of very small rockets, ignored for practical reasons, and very large boosters, which were rare and excessively expensive in any case.

With a baseline project now gelling, NASA started to work through the process of obtaining stable funding for the five years the project would take to develop. Here, too, they found themselves increasingly backed into a corner.

With the budgets being pressed by inflation at home and the Vietnam War abroad, Congress and the Administration were generally uninterested about anything as long-term as space exploration. Some members were therefore looking to further cut NASA's budget. But with a single long term project on the books, there wasn't much they could do in terms of cutting whole projects — the Shuttle was all that was left, and its cancellation meant that there would be no US manned space program by 1980.

Instead, they looked to reduce the year-to-year costs of development to a stable figure. That is, they wished to see the development budgets spread out over several more years. This is somewhat difficult to do—you can't build half a rocket. The result was another intense series of redesigns in which the re-usable booster was eventually abandoned as impossible to pay for. Not surprising, in that some designs for reusable boosters amounted to vehicles the size of the then-new Boeing 747 that would have to fly faster than the record-holding — and quite small — X-15 rocket plane. Instead a series of simpler rockets would launch the system, and then drop away for recovery. Another change was that the fuel for the Shuttle itself was placed in an external tank instead of internal tanks from the previous designs. This allowed a larger payload bay in an otherwise much smaller craft, although it also meant throwing away the tankage after each launch.

The last remaining debate was over the nature of the boosters. NASA had been looking at no less than four solutions to this problem, one a development of the existing Saturn lower stage, another using "dumb" pressure-fed liquid fuel engines of a new design, and finally either a large single solid rocket, or two (or more) smaller ones. The decision was eventually made on the smaller solids due to their lower development costs (a decision that had been echoed throughout the whole Shuttle program). While the liquid fueled systems provided better performance and enhanced safety, delivery capability to orbit is much more a function of the upper-stage performance and weight than the lower. The money was simply better spent elsewhere.

Development

The Shuttle program was launched on January 5, 1972, when President Richard M. Nixon announced that NASA would proceed with the development of a reusable low cost Space Shuttle system.

The project was already to take longer than originally anticipated due to the year-to-year funding caps. Nevertheless, work started quickly and several test articles were available within a few years.

Most notable among these was the first complete Orbiter, originally to be known as Constitution. However, 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, built in Palmdale, California, was the Columbia, which was delivered to Kennedy Space Center on March 25, 1979 and was first launched on April 12, 1981 with a crew of two. Challenger was delivered to KSC in July 1982, Discovery was delivered in November 1983, and Atlantis was delivered in April 1985. The Shuttle was meant to visit Space Station Freedom, announced in 1984, an ambitious and much-delayed project later downsized and merged into the International Space Station program. Challenger was destroyed in an explosion during launch on January 28, 1986 with the loss of all seven astronauts onboard. The Endeavour was built to replace it (using spare parts originally intended for the other Orbiters) and delivered in May 1991. Columbia was lost, with all seven crew, during re-entry on February 1, 2003, and has not been replaced.

The Space Shuttle consists of three main components: the reusable Orbiter itself, a large expendable external fuel tank, and a pair of reusable solid-fuel booster rockets. The fuel tank and booster rockets are jettisoned during ascent. The longest the Shuttle has stayed in orbit in a single mission is 17.5 days, on mission STS-80 in November 1996.

The Shuttle has a large 60 x 15 ft / 18 x 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.

The Space Shuttle system has undergone numerous improvements over the years.

The Orbiter has changed its thermal protection system several times in order to save weight and ease workload. The original silica-based ceramic tiles need to be inspected for damage after every flight, and they also soak up water and thus need to be protected from the rain. The latter problem was initially fixed by spraying the tiles with Scotchgard, but a custom solution was adopted. Later, many of the tiles on the cooler portions of the Shuttle were replaced by large blankets of insulating felt-like material, which means huge areas (notably the cargo bay area) no longer have to be inspected as much.

Internally the Shuttle remains largely similar to the original design, with the exception that the avionics continues to be improved. The original systems were "hardened" IBM 360 computers connected to analog displays in the cockpit similar to contemporary airliners like the DC-10. Today the cockpits are being replaced with "all glass" systems and the computers themselves are many times faster. The computers use the HAL/S programming language. In the Apollo-Soyuz Test Project tradition, programmable calculators are carried as well (originally the HP-41C). In addition to the "glass cockpit," several improvements have been made for safety reasons after the Challenger explosion, including a crew escape system for use in situations that require the Orbiter to "ditch." With the coming of the Space Station, the Orbiter's internal airlocks are being 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 is why during launch you may hear curious phrases such as "Go to throttle-up at 106%." This does not mean the engines are being run over-limit. The 100% figure is the power level for the original main engines. The actual engine contract requirement was for 109%. The original flight engines could handle 102%. The 109% number was finally reached in flight hardware with the Block II engines in 2001.

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. This saved considerable weight, and thereby increases the payload the Orbiter can carry into 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 Aluminum-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 have been "tested" on operational flights.

And, of course, 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.

A number of 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 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 design using a filament-wound system, but this too was cancelled.

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 re-using technology developed for the Space Shuttle.

Components

The Space Shuttle consists of three main components:

• The reusable Orbiter Vehicle (OV), with a large payload bay and three main engines (fed from the external tank) and an orbital maneuvering system with two smaller engines (used after jettisoning the external tank)
• A large expendable external fuel tank (ET) containing liquid oxygen and liquid hydrogen (at the forward and aft ends, respectively) for the three main engines of the Orbiter; it is discarded 8.5 minutes after launch at an altitude of 60 nautical miles (111 km) and breaks up in the atmosphere upon re-entry; the pieces fall in the ocean and are not recovered
• A pair of reusable solid-fuel rocket boosters (SRB); the propellant consists mainly of ammonium perchlorate (oxidizer, 70% by weight) and aluminum (fuel, 16 %); they are separated two minutes after launch at a height of 36 nautical miles (67 km) and are recovered after landing in the ocean, their fall slowed by parachutes

Initial plans for the so-called Space Transportation System included space tugs and extra fuel tanks for the orbital maneuvering system engines, among many other concepts. None of this hardware has actually ever been built.

Technical data

• System Stack Height: 184.2 ft / 56.14 m
• Orbiter Length: 122.17 ft / 37.236 m
  • Wingspan: 78.06 ft / 23.79 m
• Gross Liftoff: 4.5 million lb / 2,040,000 kg
  • ET: 1.7 million lb / 751,000 kg
  • SRBs: 1.3 million lb / 590,000 kg each (x 2)
  • Orbiter: 240,000 lb / 109,000 kg
• Total Liftoff Thrust: 7.82 million lb / 34.8 MN
  • SSMEs: 400,000 lbf / 1.8 MN each (x 3) = 1.2 million lbf / 5.3 MN
  • SRBs: 3.30 million lbf / 14.7 MN each (x 2) = 6.61 million lbf / 29.4 MN
• Maximum Landing: 230,000 lb / 104,000 kg
• Maximum Launch Payload: 63,500 lb / 28,800 kg
• Operational Altitude: 100 to 520 naut mi / 115 to 600 mi / 185 to 1000 km
• Speed: 25,404 ft/s / 17,321 mph / 4.811 mi/s / 27,875 km/h / 7.743 km/s
• Passenger Capacity: 10 Astronauts (crews other than 5 to 7 are uncommon, 8 was largest crew)

Abort modes

In the event of problems during launch, the operation of the SRBs cannot be stopped. After their ignition, the choice of abort mode depends on the situation of the Orbiter when the burn completes.

The abort modes are the following:

• Return To Launch Site (RTLS) — has never been tried, but would involve turning the Shuttle around while continuing to burn the SSMEs, jettisoning the ET, and gliding to a landing at KSC.
• East Coast Abort Landing (ECAL) — has never occurred
• Transoceanic Abort Landing (TAL) — has never occurred
• Abort Once Around (AOA) — has never occurred
• Abort to Orbit (ATO) — happened on STS-51-F mission; required mission replanning, but the mission was nevertheless declared a success.

To the extent that the hydrogen and oxygen are not needed, they are used up deliberately to allow the ET to be discarded safely.

The designated sites for ECAL are Wilmington, North Carolina, MCAS Cherry Point, North Carolina, NAS Oceana, Wallops Flight Facility, Dover Air Force Base, Atlantic City, New Jersey, Gabreski, New York, Otis ANGB, Pease International, (all USA), Halifax, Stephenville, St John's, Gander and Goose Bay, (all Canada).

A TAL would be declared between roughly T+2:30 minutes (liftoff plus 2 minutes, 30 seconds) and Main Engine Cutoff (MECO), about T+8:30 minutes. The Shuttle would then land at a predesignated friendly airstrip in Africa or Europe. Potential sites include Istres Air Base in France; Banjul International Airport in Gambia; and Zaragoza Air Base and Morón Air Base in Spain. Prior to a Shuttle launch, two of them are selected depending on the flight plan, and staffed with standby personnel in case they are used. The list of TAL sites has changed over time; most recently Ben Guerir Air Base in Morocco was eliminated due to terrorism concerns. Past TAL sites have included Kano, Nigeria, Easter Island (for Vandenberg launches), and Rota, Spain.

Emergency landing sites for the Orbiter include Lajes, Beja, (both Portugal), Keflavik (Iceland), Shannon International Airport (Ireland), RAF Fairford (UK), Köln Bonn Airport (Germany), Ankara (Turkey), Riyadh (Saudi Arabia), Diego Garcia (Indian Ocean).

Were the Orbiter unable to reach a runway, it could ditch in water, or could land on terrain other than a landing site. It would be unlikely for the flight crew still on board to survive. However, if the Orbiter were aborted during a controlled gliding flight, the In-flight Crew Escape System would allow the crew to escape with parachutes. A special Escape Pole would take each crewmember on a trajectory beneath the Orbiter's left wing.

In the two disasters, things went wrong so fast that little could be done. In the case of Challenger, the SRBs were still burning as they tore free from the rest of the stack. The orbiter disintegrated almost instantly because of aerodynamic stresses as the stack broke up. The Columbia disaster occurred high in the atmosphere during re-entry. Even if the crew had been able to bail out, they would have been killed by the heat generated by the friction of the air.

Shuttles

From left to right: Columbia, Challenger, Discovery, Atlantis and Endeavour. Not illustrated: Enterprise and Pathfinder.

Individual Shuttles are both named, in a manner similar to ships, and numbered, using the NASA Orbiter Vehicle Designation system.

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

• Main propulsion test article, with no spaceflight capability whatsoever:
  • 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 before refit:
  • Enterprise (OV-101)

• Test vehicle suitable only for glide/landing tests, with no spaceflight capability without major refit:
  • STA-099 which became Challenger

• Lost in accidents (see below):
  • Challenger (OV-099, ex-STA-099) - destroyed after liftoff - January 28, 1986
  • Columbia (OV-102) - destroyed during re-entry February 1, 2003

• In use:
  • Atlantis (OV-104)
  • Discovery (OV-103)
  • Endeavour (OV-105)

Applications

• Crew rotation of 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:
  • A higher Earth orbit; these have included:
    • Chandra X-ray Observatory
    • Many TDRS satellites
    • Two DSCS-III (Defense Satellite Communications System) communications satellites in one mission
    • A Defense Support Program satellite
  • An interplanetary orbit; these have included:
    • Magellan probe
    • Galileo spacecraft
    • Ulysses probe

Flight statistics (as of February 3, 2003)

† Satellites deployed

• (STS-80)

Accidents

Two Shuttles have been destroyed, both with the loss of all astronauts on board:

• Challenger — lost 73 seconds after lift-off, January 28, 1986; see STS-51-L.
• Columbia — lost during re-entry, February 1, 2003; see Space Shuttle Columbia disaster.

Retrospect

Costs

While the Shuttle has been a reasonably successful launch vehicle, it has been unable to meet its goal of radically reducing flight launch costs, as the average launch expenditures during its operations up to 2005 accumulates to $1.3 billion [1], a rather large figure compared to the initial projections of $10 to $20 million. The total cost of the program has been $145 billion as of early 2005 ($112 billion of which was incurred while the program was operational) and is estimated at $174 billion when the Shuttle retires in 2010. NASA's budget for 2005 allocates 30% or $5 billion to Space Shuttle operations. [2]

The original mission of the Shuttle was to operate at a high flight rate, at low cost, and with high reliability. It was intended to improve greatly on the previous generation of single-use manned and unmanned vehicles. Although it did operate as the world's first reusable crew-carrying spacecraft, it did not improve on those parameters in any meaningful way, and is considered by some to have failed in its original purpose.

Although the design is radically different from the original concept, the project was still supposed to meet the upgraded USAF goals, and to be much cheaper to fly in general. One reason behind this apparent failure appears to be inflation. During the 1970s the US suffered from severe inflation, driving up costs about 200% by 1980. In contrast, the rate between 1990 and 2000 was only 34% in total. This magnified the development costs of the Shuttle. The original process by which contractors bid for Shuttle work has also inflated overall project costs as there were political and industrial pressures to spread Shuttle work around. For instance, the need for a single piece SRB design was dismissed as only one company was located close enough to the launch site to make this viable. The company that secured the SRB contract, Morton Thiokol, is based in Utah, necessitating the modular design that contributed to the Challenger loss. Ironically, the US aerospace mergers of the 1990s mean that the vast majority of the STS contracts are now held by a single company (Boeing).

However, this does not explain the high costs of the continued operations of the Shuttle. Even accounting for inflation, the launch costs on the original estimates should be about $100 million today. The remaining $400 million arises from the operational details of maintaining and servicing the Shuttle fleet, which have turned out to be tremendously more expensive than anticipated. Some of this can be attributed to operating beyond the 10 year anticipated lifespan of each Shuttle.

Shuttle operations

When originally conceived, the Shuttle was to operate similarly to 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 takes months. Decisions to cut short-term development costs have resulted in a continued high-cost maintenance schedule. The documentation requirements have become extremely thorough. Dramatically increasing the number of support personnel needed to launch also caused a significant increase in costs. This was exacerbated in the aftermath of the Challenger disaster. Even simple changes require significant amounts of documentation. This paperwork results from the fact that, unlike current expendable launch vehicles, the Space Shuttle is manned and has no escape systems mode for most of the flight regime, and therefore any accident which would result in the loss of a booster would also result in the loss of the crew. 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 there are cases where there are no abort modes — no potential way to prevent failure from becoming critical — many pieces of hardware simply must function perfectly and so must be carefully inspected before each flight. The result is a massively inflated labor cost, with around 25,000 workers in Shuttle operations and labor costs of about $1 billon per year.

Initially NASA hoped the Shuttle's manned capacity would be justified as a 'space taxi' to a revived Skylab or a Saturn V launched 'Skylab 2'. With the go-ahead for the large modular "Freedom" Space Station proposal the Shuttle appeared to have a continued justification with the prospect of a 6 to 10 crew outpost only being serviceable by the Shuttle. The scale-back of the Space Station concept in the 1990s ultimately made the utility of the Shuttle as a manned ferry obsolete.

NASA's justification of the STS for its own unmanned science missions has also declined. Following the Challenger disaster, use of the powerful Centaur upper stages required for interplanetary probes was ruled out. The Shuttle's history of unexpected delays also makes it liable to miss the narrow launch windows. Advances in technology over the last decade have made probes smaller and lighter and as a result it is possible to reach Mars using a relatively cheap and reliable Delta launcher.

Another possible impediment to the Shuttle system was the politically required participation of the United States Air Force. To receive the funding required, Congress mandated that the Shuttle replace all other launch vehicles in the national inventory as a cost cutting measure. This requirement dramatically altered the size and scope of the program as the Air Force needed significant capabilities to allow it to meet national defense objectives. Ironically, neither NASA nor the Air Force got the system they wanted or needed, and the Air Force eventually returned to their older launch systems and abandoned their Vandenberg shuttle launch plans. The capabilities that most seriously hobbled the Shuttle system — namely the 65,000 pound (29 t) payload, large payload bay, and 1000 mile (1,600 km) cross-range — have in fact, except for the payload bay, never been used.

Opinions differ on the lessons of the Shuttle. In general, however, future designers look to systems with only one stage, automated checkout, and in some cases, over designed (more durable) low-tech systems.

See: Space Shuttles in fiction