The Space Shuttle is one of the most complex machines ever devised. Its main elements ｭ the Orbiter, Space Shuttle Main Engines, External Tank, and Solid Rocket Boosters ｭ are assembled from more than 2.5 million parts, 230 miles of wire, 1,060 valves, and 1,440 circuit breakers. Weighing approximately 4.5 million-pounds at launch, the Space Shuttle accelerates to an orbital velocity of 17,500 miles per hour ｭ 25 times faster than the speed of sound ｭ in just over eight minutes. Once on orbit, the Orbiter must protect its crew from the vacuum of space while enabling astronauts to conduct scientific research, deploy and service satellites, and assemble the International Space Station. At the end of its mission, the Shuttle uses the Earths atmosphere as a brake to decelerate from orbital velocity to a safe landing at 220 miles per hour, dissipating in the process all the energy it gained on its way into orbit.

THE ORBITER

The Orbiter is what is popularly referred to as "the Space Shuttle." About the size of a small commercial airliner, the Orbiter normally carries a crew of seven, including a Commander, Pilot, and five Mission or Payload Specialists. The Orbiter can accommodate a payload the size of a school bus weighing between 38,000 and 56,300 pounds depending on what orbit it is launched into. The Orbiters upper flight deck is filled with equipment for flying and maneuvering the vehicle and controlling its remote manipulator arm. The mid-deck contains stowage lockers for food, equipment, supplies, and experiments, as well as a toilet, a hatch for entering and exiting the vehicle on the ground, and - in some instances - an airlock for doing so in orbit. During liftoff and landing, four crew members sit on the flight deck and the rest on the mid-deck.

Different parts of the Orbiter are subjected to dramatically different temperatures during re-entry. The nose and leading edges of the wings are exposed to superheated air temperatures of 2,800 to 3,000 degrees Fahrenheit, depending upon re-entry profile. Other portions of the wing and fuselage can reach 2,300 degrees Fahrenheit. Still other areas on top of the fuselage are sufficiently shielded from superheated air that ice sometimes survives through landing.

To protect its thin aluminum structure during re-entry, the Orbiter is covered with various materials collectively referred to as the Thermal Protection System. The three major components of the system are various types of heat-resistant tiles, blankets, and the Reinforced Carbon-Carbon (RCC) panels on the leading edge of the wing and nose cap. The RCC panels most closely resemble a hi-tech fiberglass ｭ layers of special graphite cloth that are molded to the desired shape at very high temperatures. The tiles, which protect most other areas of the Orbiter exposed to medium and high heating, are 90 percent air and 10 percent silica (similar to common sand). One-tenth the weight of ablative heat shields, which are designed to erode during re-entry and therefore can only be used once, the Shuttles tiles are reusable. They come in varying strengths and sizes, depending on which area of the Orbiter they protect, and are designed to withstand either 1,200 or 2,300 degrees Fahrenheit. In a dramatic demonstration of how little heat the tiles transfer, one can place a blowtorch on one side of a tile and a bare hand on the other. The blankets, capable of withstanding either 700 or 1,200 degrees Fahrenheit, cover regions of the Orbiter that experience only moderate heating.

SPACE SHUTTLE MAIN ENGINES

Each Orbiter has three main engines mounted at the aft fuselage. These engines use the most efficient propellants in the world - oxygen and hydrogen - at a rate of half a ton per second. At 100 percent power, each engine produces 375,000 pounds of thrust, four times that of the largest engine on commercial jets. The large bell-shaped nozzle on each engine can swivel 10.5 degrees up and down and 8.5 degrees left and right to provide steering control during ascent.

EXTERNAL TANK

The three main engines burn propellant at a rate that would drain an average-size swimming pool in 20 seconds. The External Tank accommodates up to 143,351 gallons of liquid oxygen and 385,265 gallons of liquid hydrogen. In order to keep the super-cold propellants from boiling and to prevent ice from forming on the outside of the tank while it is sitting on the launch pad, the External Tank is covered with a one-inch-thick coating of insulating foam. This insulation is so effective that the surface of the External Tank feels only slightly cool to the touch, even though the liquid oxygen is stored at minus 297 degrees Fahrenheit and liquid hydrogen at minus 423 degrees Fahrenheit. This insulating foam also protects the tanks aluminum structure from aerodynamic heating during ascent. Although generally considered the least complex of the Shuttles main components, in fact the External Tank is a remarkable engineering achievement. In addition to holding over 1.5 million pounds of cryogenic propellants, the 153.8-foot long tank must support the weight of the Orbiter while on the launch pad and absorb the 7.3 million pounds of thrust generated by the Solid Rocket Boosters and Space Shuttle Main Engines during launch and ascent. The External Tanks are manufactured in a plant near New Orleans and are transported by barge to the Kennedy Space Center in Florida. Unlike the Solid Rocket Boosters, which are reused, the External Tank is discarded during each mission, burning up in the Earths atmosphere after being jettisoned from the Orbiter.

SOLID ROCKET BOOSTERS

Despite their power, the Space Shuttle Main Engines alone are not sufficient to boost the vehicle to orbit ｭ in fact, they provide only 15 percent of the necessary thrust. Two Solid Rocket Boosters attached to the External Tank generate the remaining 85 percent. Together, these two 149-foot long motors produce over six million pounds of thrust. The largest solid propellant rockets ever flown, these motors use an aluminum powder fuel and ammonium perchlorate oxidizer in a binder that has the feel and consistency of a pencil eraser.

Each of the Solid Rocket Boosters consists of 11 separate segments joined together. The joints between the segments were extensively redesigned after the Challenger accident, which occurred when hot gases burned through an O-ring and seal in the aft joint on the left Solid Rocket Booster. The motor segments are shipped from their manufacturer in Utah and assembled at the Kennedy Space Center. Once assembled, each Solid Rocket Booster is connected to the External Tank by bolts weighing 65 pounds each. After the Solid Rocket Boosters burn for just over two minutes, these bolts are separated by pyrotechnic charges and small rockets then push the Solid Rocket Boosters safely away from the rest of the vehicle. As the boosters fall back to Earth, parachutes in their nosecones deploy. After splashing down into the ocean 120 miles downrange from the launch pad, they are recovered for refurbishment and reuse.

THE SHUTTLE STACK

The first step in assembling a Space Shuttle for launch is stacking the Solid Rocket Booster segments on the Mobile Launch Platform. Eight large hold-down bolts at the base of the Solid Rocket Boosters will bear the weight of the entire Space Shuttle stack while it awaits launch. The External Tank is attached to the Solid Rocket Boosters, and the Orbiter is then attached to the External Tank at three points ｭ two at its bottom and a "bipod" attachment near the nose. When the vehicle is ready to move out of the Vehicle Assembly Building, a Crawler-Transporter picks up the entire Mobile Launch Platform and carries it ｭ at one mile per hour ｭ to one of the two launch pads.