Space Shuttle

The space shuttle is a reusable spacecraft that takes off like a rocket, travels around the earth like a spacecraft, and then lands once again like a glider. The first space shuttle was the Columbia, whose maiden voyage took place in April 1981. Four additional shuttles were later added to the fleet: Discovery, Challenger, Atlantis, and Endeavor. The first shuttle launched by the Soviet Union (now Russia) was Buran, which made its debut in November 1988.

At one time, both the United States and the Soviet Union envisioned complex space programs that included two parts: (1) space stations orbiting around Earth and/or other planets, and (2) shuttle spacecraft that would transport humans, equipment, raw materials, and finished products to and from the space station. For economic reasons, each nation eventually ended up concentrating on only one aspect of the complete program. The Soviets built and for many years operated advanced space stations (Salyut and Mir), while Americans have focused their attention on the shuttle system.

The shuttle system has been given the name Space Transportation System (STS), of which the shuttles have been the key element. Initially lacking a space station with which to interact, the American shuttles operated with two major goals: (1) the conduct of scientific experiments in a zero-gravity environment, and (2) the launch, capture, repair, and release of satellites.

Now an international program, STS depends heavily on the contributions of other nations in the completion of its basic missions. For example, its Spacelab modules—the areas in which astronauts carry out most of their experiments—are designed and built by the European Space Agency, and the extendable arm used to capture and release satellites—the remote manipulator system or Canadarm—is constructed in Canada.

The space shuttle has four main parts: (1) the orbiter (2) the three main engines attached to the orbiter (3) two solid rocket engines, and (4) an external fuel tank. Although the Russian Buran differs in some details from the U.S. space shuttle fleet, the main features of all shuttles are similar.

The orbiter is approximately the size of a commercial DC-9 airplane with a length of 121 ft (37 m) and a wing span of 78 ft (23 m). Its net weight is about 161,000 lb (73,200 kg). It is sub-divided into two main parts: the crew cabin and the cargo bay. The upper level of the crew cabin is the flight deck from which astronauts control the spacecraft's flight in orbit and during descent. Below the flight deck are the crew's personal quarters, containing personal lockers, sleeping, eating, and toilet facilities, and other necessary living units. The crew cabin is physically isolated from the cargo bay and is provided with temperature and pressure conditions similar to those on Earth's surface. The cabin's atmosphere is maintained with a composition equivalent to that of near-Earth atmosphere, 80% nitrogen and 20% oxygen.

The cargo bay is a large space 15 ft (4.5 m) by 60 ft (18 m) in which the shuttle's payloads are stored. The cargo bay can hold up to about 65,000 lb (30,000 kg) during ascent, although it is limited to about half that amount during descent.

In 1973, an agreement was reached between NASA and the European Space Agency (ESA) for the construction by ESA of a pressurized work space that could be loaded into the shuttle's cargo bay. The workspace, designated as Spacelab, was designed for use as a science laboratory in which a wide array of experiments could be conducted. Each of these Spacelab modules is 8.9 ft (2.7 m) long and 13 ft (3.9 m) in diameter. The equipment needed to carry out experiments is arranged in racks along the walls of the Spacelab, and the whole module is then loaded into the cargo bay of the shuttle prior to take-off. When necessary, two Spacelab modules can be joined to form a single, larger work space.

The power needed to lift a space shuttle into orbit comes from two solid-fuel rockets, each 149 ft (45.5 m) in length and 12 ft (4 m) in diameter, and the shuttle's own liquid-fuel engines. The fuel used in the solid rockets is composed of finely-divided aluminum, ammonium perchlorate, and a special polymer designed to form a rubbery mixture. The mixture is molded in such a way as to produce an 11-point starred figure. This shape exposes the maximum possible surface area of fuel during ignition, making combustion as efficient as possible within the engine.

The two solid-fuel rockets carry 1.1 million lb (500,000 kg) of fuel each, and burn out completely only 125 seconds after the shuttle leaves the launch pad. At solid-engine burnout, the shuttle is at an altitude of 161,000 ft (47,000 m) and 244 nautical miles (452 km) down range from launch site. At that point, explosive charges holding the solid rockets to the main shuttle go off and detach the rockets from the shuttle. The rockets are then returned to Earth by means of a system of parachutes that drops them into the Atlantic Ocean at a speed of 55 mi (90 km) per hour. The rockets can then be collected by ships, returned to land, refilled, and re-used in a later shuttle launch.

The three liquid-fueled shuttle engines have been described as the most efficient engines ever built by humans. At maximum capacity, they achieve 99% efficiency during combustion. They are supplied by fuel (liquid hydrogen) and oxidizer (liquid oxygen) stored in the 154 ft (46.2 m) external fuel tank. The fuel tank itself is sub-divided into two parts, one of which holds the liquid oxygen and the other, the liquid hydrogen. The fuel tank is maintained at the very low temperature (less than −454°F [Ȓ270°C]) necessary to keep hydrogen and oxygen in their liquid states. The two liquids are pumped into the shuttle's three engines through 17 in (43 cm) diameter lines that carry 1,035 gal (3,900 l) of fuel per second. Upon ignition, each of the liquid-fueled engines delivers 75,000 horsepower of thrust.

The three main engines burn out after 522 seconds, when the shuttle has reached an altitude of 57 nautical miles (105 km) and is down range 770 nautical miles (1,426 km) from the launch site. At this point, the external fuel tank is also jettisoned. Its return to the earth's surface is not controlled, however, and it is not recoverable for future use.

Final orbit is achieved by means of two small engines, the Orbital Maneuvering System (OMS) Engines located on external pods at the rear of the orbiter's body. The OMS engines are fired first to insert the orbiter into an elliptical orbit with an apogee of 160 nautical miles (296 km) and a perigee of 53 nautical miles (98 km) and then again to accomplish its final circular orbit with a radius of 160 nautical miles (296 km).

Humans and machinery work together to control the movement of the shuttle in orbit and during its descent. For making fine adjustments, the spacecraft depends on six small vernier jets, two in the nose and four in the OMS pods of the spacecraft. These jets allow human or computer to make modest adjustments in the shuttle's flight path in three directions.

The computer system used aboard the shuttle is an example of the redundancy built into the spacecraft. Five discrete computers are used, four networked with each other using one computer program, and one operating independently using a different program. The four linked computers constantly communicate with each other, testing each other's decisions and deciding when one (or two or three) is not performing properly and eliminating that computer (or those computers) from the decision-making process. In case all four of the interlinked computers malfunction, decision-making is turned over automatically to the fifth computer.

This kind of redundancy is built into every essential feature of the shuttle's operation. For example, three independent hydraulic systems are available, all operating with independent power systems. The failure of one or even two of the systems does not, therefore, place the shuttle in a critical failure mode.

The space shuttles have performed a myriad of scientific and technical tasks in their nearly two decades of operation. Many of these have been military missions about which we have relatively little information. The launching of military spy satellites is an example of these.

Some examples of the kinds of activities carried out during shuttle flights include the following:

  • After the launch of the Challenger shuttle (STS-41B) on February 3, 1984, astronauts Bruce McCandless II and Robert L. Stewart conducted the first ever untethered space walks using Manned Maneuvering Unit backpacks that allowed them to propel themselves through space near the shuttle. The shuttle also released into orbit two communication satellites, the Indonesian Palapa and the American Westar satellites. Both satellites failed soon after release but were recovered and returned to Earth by the Discovery during its flight that began on November 8, 1984.
  • During the flight of Challenger (STS-51B) that began on April 29, 1985, crew members carried out a number of experiments in Spacelab 3 determining the effects of zero gravity on living organisms and on the processing of materials. They grew crystals of mercury (II) oxide over a period of more than four days, observed the behavior of two monkeys and 24 rats in a zero-gravity environment, and studied the behavior of liquid droplets held in suspension by sound waves.
  • The mission of STS-51I (Discovery) was to deposit three communications satellites in orbit. On the same flight, astronauts William F. Fisher and James D. Van Hoften left the shuttle to make repairs on a Syncom satellite that had been placed in orbit during flight STS-51D but that had then malfunctioned.

Some of the most difficult design problems faced by shuttle engineers were those created during the reentry process. When the spacecraft has completed its mission in space and is ready to leave orbit, its OMS fires just long enough to slow the shuttle by 200 mi (320 km) per hour. This modest change in speed is enough to cause the shuttle to drop out of its orbit and begin its descent to Earth.

The re-entry problems occur when the shuttle reaches the outermost regions of the upper atmosphere, where significant amounts of atmospheric gases are first encountered. Friction between the shuttle—now traveling at 17,500 mi (28,000 km) per hour—and air molecules causes the spacecraft's outer surface to begin to heat up. Eventually, it reaches a temperature of 3,000°F (1,650°C).

Most materials normally used in aircraft construction would melt and vaporize at these temperatures. It was necessary, therefore, to find a way of protecting astronauts inside the shuttle cabin from this searing heat. The solution invented was to use a variety of insulating materials on the shuttle's outer skin. Parts less severely heated during re-entry are covered with 2,300 flexible quilts of a silica-glass composite. The more sensitive belly of the shuttle is covered with 25,000 insulating tiles, each 6 in (15 cm) square and 5 in (12 cm) thick, made of a silica-borosilicate glass composite.

The portions of the shuttle most severely stressed by heat—the nose and the leading edges of the wings—are coated with an even more resistant material known as carbon-carbon. Carbon-carbon is made by attaching a carbon-fiber cloth to the body of the shuttle and then baking it to convert it to a pure carbon substance. The carbon-carbon is then coated to prevent oxidation of the material during descent.

Once the shuttle reaches Earth's atmosphere, it ceases to operate as a rocket ship and begins to function as a glider. Its movements are controlled by aerodynamic controls, such as the tail rudder, a large flap beneath the main engines, and elevons, small flaps on its wings. These devices allow the shuttle to descend to the earth traveling at speeds of 8,000 mi (13,000 km) per hour, while dropping vertically at the rate of 140 mi (225 km) per hour. When the aircraft finally touches down, it is traveling at a speed of about 190 knots (100 m per second), and requires about 1.5 mi (2.5 km) to come to a stop.

Disasters have been associated with aspects of both the Soviet and American space programs. Unfortunately, the Space Transportation System has been no different in this respect. Mission STS-51L was scheduled to take off on January 28, 1986 using the shuttle Challenger. Only 72 seconds into the flight, the shuttle's external tank exploded, and all seven astronauts on board were killed.

The Challenger disaster prompted one of the most comprehensive studies of a major accident ever conducted. On June 6, 1986, the Presidential Commission appointed to analyze the disaster published its report. The reason for the disaster, according to the commission, was the failure of an O-ring at a joint connecting two sections of one of the solid rocket engines. Flames escaping from the failed joint reached the external fuel tank, set it on fire, and then caused an explosion of the whole spacecraft.

As a result of the Challenger disaster, a number of design changes were made in the shuttle. Most of these (254 modifications in all) were made in the orbiter. Another 30 changes were made in the solid rocket booster, 13 in the external tank, and 24 in the shuttle's main engine. In addition, an escape system was developed that would allow crew members to abandon a shuttle in case of emergencies, and NASA reexamined and redesigned its launch-abort procedures. Also, NASA was instructed to reassess its ability to carry out the ambitious program of shuttle launches that it had been planning.

The U.S. Space Transportation System was essentially shut down for a period of 975 days while NASA carried out necessary changes and tested new systems. Then, on September 29, 1988, the first post-Challenger mission was launched, STS-26. On that flight, Discovery carried NASA's TDRS-C communications satellite into orbit, putting the American STS program back on schedule once more.

In December, 1988, the crew of NASA's Space Shuttle STS-88 began construction of the International Space Station (ISS). By joining the Russian-made control module Zarya with the United States-built connecting module Unity, the crew of the Endeavor became the first crew aboard the ISS. Since the STS-88 mission, twelve more U.S. shuttle missions have led the construction of the International Space Station, a permanent laboratory orbiting 220 miles above Earth.

See also Space and planetary geology; Space physiology; Space probe; Spacecraft, manned