Protecting it are thousands of insulating tiles and thermal blankets that cover the underbelly, nose and bottom of the wings. The insulation protects the shuttle during its fiery return into Earth's atmosphere.
The tiles are made primarily of pure-sand silicate fibers, mixed with a ceramic binder. About as dense as balsa wood, the tiles dissipate the heat so quickly that a white-hot tile at a temperature of 2,300 degrees Fahrenheit (1,260 degrees Celsius) can be taken from an oven and safely held with bare hands.
Since the Challenger accident, NASA incorporated more than 200 design changes to the orbiter. Among the changes are an improved braking system, nosewheel steering and an escape system in which astronauts can bail out through the side hatch and parachute to a water landing.
The crew cabin
The crew cabin contains the pressurized area where the astronauts work and live while the shuttle is in orbit. It normally carries about seven people but can hold up to 10 in an emergency.
There are three levels to the crew cabin, where astronauts live and work in a "shirt-sleeve" environment.
Uppermost is the flight deck that holds the shuttle commander, pilot and, behind them, two mission specialists. Three other astronauts sit below the flight deck, in the mid-deck area, during launch and landing.
The flight deck has both a forward cockpit where the commander or pilot can maneuver the shuttle and an aft console that the astronauts use for operating the 50-foot (15-meter) robot arm in the cargo bay.
The new $9 million "glass cockpit," flying for the first time on Atlantis on mission STS 101, features 11 full-color, flat-panel display screens that replace 32 gauges and four cathode-ray tube displays.
It is 75 pounds (34 kilograms) lighter and uses less power than the old model. Its color displays make it easier for shuttle pilots to recognize key functions of the equipment. All shuttles will have the new cockpit by 2002.
The crew cabin has 11 windows and three hatches, two of which are in the airlock. That's where astronauts go to don their spacesuits for extravehicular activities (EVAs), known as "spacewalks."
The mid-deck contains the galley, toilet, sleep stations and storage and experiment lockers for the basic needs of living in weightlessness.
It also contains the hatch leading to the airlock in the cargo bay from which astronauts take their spacewalks and the side hatch where astronauts enter the orbiter before launch or leave it after landing.
Below the mid-deck's floor is storage space for air and water tanks.
The cargo bay
The shuttle's cargo bay can be used for many jobs. At 60 feet (18.2 meters) long and 15 feet (4.5 meters) wide, it is big enough to hold a city bus.
But instead it carries satellites, science experiments and pieces of the International Space Station to orbit. It is a workstation for astronauts to repair satellites, a foundation from which to erect space structures and a hold for captured satellites to be returned to Earth.
Mounted on the port side of the cargo bay, behind the crew cabin, is the remote manipulator system (RMS). The RMS is a 50-foot- (15.2-meter-) long robot arm and hand with three joints, much like those on a human shoulder, elbow and wrist.
Operated from the aft flight deck by an astronaut inside the crew cabin, the Canadian-built arm can move anything from spacewalkers to satellites in or out of the cargo bay.
The airlock, which used to be located inside the crew cabin on earlier shuttles, is now located in the cargo bay and connected to the cabin by a tunnel. That allows shuttles to dock at the International Space Station and afford easy entrance through the airlock into the station.
The Defense Department in 1970s, which envisioned using the shuttle to launch large spy satellites into orbit, dictated the size of the cargo bay. The Pentagon backed away from the shuttle after the 1986 Challenger disaster grounded the fleet for more than two years.
Only two NASA payloads -- the Hubble Space Telescope in 1990 and the Chandra X-ray Observatory in 1999 -- have filled the length of the cargo bay.
Chandra, which carried an attached inertial upper stage, was the heaviest single payload carried by a shuttle at 43,512 pounds (19,737 kilograms).
The main engines
Crucial to the space shuttle getting into orbit are its three liquid-fuel powered main engines, which provide the necessary thrust after the twin solid-fuel rocket boosters have spent their fuel and separated from the shuttle.
The main engines are high-performance, liquid-propellant rocket engines -- and the world's first reusable ones. Clustered at the aft end of the orbiter, they have a combined thrust of almost 1.2 million pounds at sea level.
Built by Boeing's Rocketdyne division, they roar to life six seconds before the shuttle lifts off the launch pad and burn for the entire eight and a half-minute climb to orbit.
Each is about 14 feet (4.3 meters) long, 8 feet (2.4 meters) in diameter at the nozzle end and weighs about 7,000 pounds (3,175 kilograms). A main engine is designed to operate for 55 flights.
Technological marvels, the thrust of each powerhouse can be varied over a range of 65 to 109 percent of its rated output level. Together, the three engines produce the power equivalent of 23 Hoover Dams.
If the engines pumped water instead of fuel, they could drain an average-size swimming pool in 25 seconds.
Most of the exhaust they produce is water since liquid hydrogen and liquid oxygen are the fuel.
Temperatures inside the engines reach more than 6,000 degrees Fahrenheit (3,315 degrees Celsius) -- hotter than iron's boiling point -- yet the liquid hydrogen fuel they use is the second coldest liquid on Earth at minus 423 degrees Fahrenheit (minus 252.8 degrees Celsius).
At the heart of the engine is its high-pressure turbopump -- about the size of a car engine -- which regulates and pumps fuel to the combustion chamber. A main-engine turbopump's discharge pressure is high enough to squirt a column of liquid fuel 36 miles (58 kilometers) into the air.
The turbo's main shaft rotates at a head-spinning 37,000 rotations per minute (r.p.m.), compared to about 3,000 r.p.m. for a car traveling 60 m.p.h. (97 kilometers per hour).
Since the first shuttle flew in 1981, three overhauls to the main engines have more than tripled estimates of their safety. A fourth overhaul is planned that will make them safer still by 2005.
The improvements will include the addition of a high-tech optical and vibration sensor system and computer to the engines that will "see" trouble coming and shut down the system a fraction of a second before any harm can be done.
Other planned improvements: The engines' main combustion chamber will be enlarged to reduce pressure on internal parts without reducing thrust. And a new, simplified engine-nozzle design will eliminate the need for more than 500 welds; each weld is a weak link that could become the source of a potential leak.
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