Space Shuttle

C

External Fuel Tank

The giant, cylindrical, external fuel tank, with a length of 47 m (154 ft) and a diameter of 8.4 m (27.5 ft), is the largest single piece of the space shuttle. It fuels the orbiter's three main engines. During launch, the external tank also acts as a support for the orbiter and SRBs to which it is attached.

Inside separate pressurized tanks, the external tank holds the liquid hydrogen fuel and liquid oxygen oxidizer (which reacts with the hydrogen to produce combustion) that runs the shuttle's three main engines. During launch, the external tank feeds the fuel under pressure through small ducts that branch off into smaller lines that feed directly into the main engines. Some 450 kg (1,000 lb) of fuel are consumed by each of the main engines each second.

The space shuttle’s external fuel tank is the only part of the launch vehicle that currently is not reused. After its 1.99 million liters (526,000 gallons) of fuel are consumed during the first 8.5 minutes of flight, the external tank is jettisoned from the orbiter and breaks up in the upper atmosphere, its pieces falling into remote ocean waters.

During the first 17 years of shuttle flights, the external fuel tanks were made of aluminum alloys. The tanks that the first five shuttle missions used weighed about 35,000 kg (about 77,000 lb) when empty. A design change in 1983 reduced the weight to 30,000 kg (66,000 lb) when empty. In 1998, anticipating the extra power that the shuttle will need to get to the International Space Station (ISS), which will orbit at a higher altitude than the space shuttle usually uses, NASA announced the introduction of a new tank design. The new tanks, first used in May 1998, are made of aluminum lithium, which is significantly lighter than the aluminum alloys used for previous tanks. The new tanks weigh about 27,000 kg (about 59,000 lb) when empty.

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III

Early Missions

In its first five years, the earliest space-shuttle missions made significant contributions, beginning with the first orbital flight tests of the Columbia orbiter in April 1981; the first launch of the second orbiter (Challenger) in April 1983; the first flight of Spacelab, with 71 scientific experiments from the United States and European countries, in November 1983; the first repair of a satellite in orbit (the Solar Maximum Satellite) in April 1984; the first retrieval of satellites from orbit (Palapa and Westar) and their return to the Earth in November 1984; and the first manually assisted launch of a satellite (Syncon IV-3) from space, after retrieval and repair in orbit of the satellite Leasat in August 1985. The shuttle program was suspended for nearly three years for evaluation and modification following the explosion of the space shuttle Challenger in January 1986.

IV

Challenger Disaster

On January 28, 1986, Challenger and its crew were destroyed shortly after launch. The failure of an O-ring seal of a joint on one of the SRBs was the primary cause of the Challenger loss. SRBs are constructed in four cylindrical sections that must be sealed together completely to prevent the escape of the intensely hot byproducts of the burning fuel during launch. O-rings are rubber rings that play a crucial part in ensuring the seal. The cold weather on the launch day made the rubber of an O-ring on the joint between the bottom two segments of the right SRB brittle, which, combined with the faulty design of the joint, allowed hot gases from the burning solid rocket fuel to escape. The gases and flames burned through the metal holding the rocket in position. When the rocket broke loose, it ruptured the side of the external fuel tank, allowing the liquid hydrogen and oxygen to mix prematurely and explode.

In early February 1986, as the nation mourned the tragic loss of the seven Challenger crew members, U.S. President Ronald Reagan announced the creation of the Presidential Commission on the Space Shuttle Challenger Accident. Chaired by William P. Rogers, former secretary of state, it became known as the Rogers Commission. NASA's Challenger Data and Design Analysis Task Force also was established at this time to support the work of the Rogers Commission.

More than 6,000 people were involved in the commission's four-month investigation of the accident, and some 15,000 transcript pages were taken during public and closed hearings. The commission's report was published and delivered to the president on June 6, 1986. Its recommendations included modifications of hardware and NASA procedures.

During the period when the space shuttle fleet was grounded, hundreds of major and minor modifications (many of which were planned before the accident) were incorporated into the shuttle system. The SRBs were completely redesigned, and a new joint design passed stringent examination and review.

The main space shuttle engines underwent the most aggressive ground-testing program in their history, equivalent in operational time to more than 36 missions. All engine improvements were certified to demonstrate improved reliability and operating safety margins, and they were incorporated into the engines used on the Discovery, Columbia, Atlantis, and Endeavour orbiters.

NASA safety programs were completely reorganized as a result of another Rogers Commission recommendation. The Office of Safety, Reliability, Maintainability, and Quality Assurance was established in 1986, and it now has direct authority for safety and related quality controls for all NASA operations. Today, more people are assigned to safety and related programs, improved communications have been initiated, and the review system for compliance to new procedures is rigorous and well-defined. The new Office of Safety ensures that the highest levels of NASA's management team are aware of safety issues.

V

Later Missions

After the Challenger accident in 1986, more than 80 shuttle missions were completed with no serious mishaps. The most notable of these were the scientific missions that launched these exploratory spacecraft: Magellan (launched May 1989), the probe designed for radar mapping of the planet Venus; Galileo (launched October 1989), the unpiloted spacecraft that reached Jupiter in December 1995; Ulysses (launched October 1990), a probe designed for study of the Sun; and the Hubble Space Telescope (launched April 1990), a high-powered telescope designed to make astronomical observations from space, away from the interference of Earth's atmosphere. In December 1993 the first Hubble Telescope Servicing Mission was successfully completed, correcting the telescope's optics and improving the electronic systems.

In July 1995 the shuttle Atlantis linked up with the Russian space station Mir. This mission was the first of nine shuttle/Mir linkups between 1995 and 1998. These flights were the precursors to assembly of the International Space Station that began to be constructed in orbit in late 1998. The first docking with Mir was perhaps the most significant event in the history of spaceflight since the symbolic joining of Apollo and Soyuz spacecraft 20 years earlier (see Apollo program). It signaled a new age of cooperation in space, where exploration of the universe would be measured more in terms of what a coalition of nations had accomplished rather than what a single nation had achieved. See also Space Station.

After the ceremonies following the rendezvous and docking of Atlantis to Mir, the two groups of astronauts undertook several days of joint scientific investigations inside the Spacelab module tucked in Atlantis's large cargo bay. Research in seven different medical and scientific disciplines, begun previously at Mir, also was concluded on the July 1995 mission. All of these experiments took advantage of the unique microgravity environment present on the spacecraft. Scientists hope to learn more about changes in the human body caused by spaceflight; the data collected in these experiments also may advance understanding of conditions such as anemia, high blood pressure, osteoporosis, kidney stones, balance disorders, and immune deficiencies that often occur on Earth.

In March 1996 Atlantis docked again with Mir, carrying 860 kg (1,900 lb) of supplies to the space station. Atlantis also left Shannon Lucid, an American astronaut, on Mir for a planned stay of five months. Because of delays caused by problems with Atlantis, Lucid stayed aboard Mir for 188 days (more than 6 months), breaking the U.S. record for long duration spaceflight. Five more U.S. astronauts stayed aboard Mir on extended stays before shuttle/Mir missions ended in 1998, when both the United States and Russia began concentrating on International Space Station plans. Spacelab missions also ended in 1998, in hopes that the ISS will provide a new and more permanent laboratory in space.

The majority of space shuttle missions in the early 2000s were devoted to construction of the ISS. In 1998 the orbiter Atlantis was overhauled to make it more compatible with the ISS. Atlantis received new displays, navigation equipment, and an airlock with which to connect to the station. Its power and cooling systems were also improved.

In February 2000 Endeavour completed a mission that focused on mapping Earth’s terrain. Scientists used two antennas—one located at the end of a long mast and the other in the shuttle’s payload bay—to obtain high-quality, three-dimensional images that give information about topography (features such as mountains and rivers).

VI

Columbia Disaster and Return to Space

The space shuttle Columbia broke apart and burned up while reentering Earth’s atmosphere over Texas on February 1, 2003. The entire seven-member crew was killed as they returned to Earth after completing a series of scientific experiments. Investigation of the disaster pointed to structural failure of the heat-shielding system for the shuttle’s left wing. Sensors inside the wing recorded unusually high temperatures just before NASA lost contact with the shuttle. The investigation determined that the wing was damaged during liftoff when it was struck by a piece of insulation foam from the external fuel tank, opening a hole in the left wing. On reentry, superheated gases in the atmosphere penetrated the left wing, dooming the craft and the crew.

The space shuttle fleet was grounded until July 2005, when the shuttle Discovery returned to space. However, during the launch of Discovery a chunk of insulation foam broke off again from the external fuel tank, despite a more than two-year and nearly $1-billion effort to prevent a recurrence of the problem. Although the Discovery appeared to be undamaged, NASA suspended further shuttle flights until the foam problem could be studied further. Discovery returned to orbit in July 2006, when it docked with the International Space Station.

Shuttle missions have concentrated on adding modules and equipment to the International Space Station, aiming for completion in 2010 when the shuttle is scheduled to be retired. A planned service mission to the Hubble Space Telescope (HST) calls for shuttle astronauts to upgrade, repair, or replace a number of working parts and scientific instruments, and to give the HST a boost to a higher orbit. The efforts should help extend the HST’s scientific working life until the new James Webb Space Telescope is operational.

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