Thursday, April 29, 2010

Space Shuttle, the world's first reusable spacecraft


The US space shuttle, first launched in 1981, was the world's first reusable space craft. Of its three components - the orbiter space plane, rocket boosters, and external fuel tank - only the fuel tank is not recovered after a mission.
Special heat resistant tiles prevent the orbiter from burning up when it re-enters the Earth's atmosphere. The remote manipulator arm in the orbiter's payload bay can put satellites into space, recapture them from space, and act as a stable platform for astronauts working in the bay.
The space shuttle was initially used to deploy satellites in orbit; to carry scientific experiments such as Spacelab, a modular arrangement of experiments installed in the shuttle's cargo bay; and to carry out military missions. As the program has matured, the space shuttle also has been used to service and repair orbiting satellites and to retrieve and return to the earth previously deployed spacecraft.
The space shuttle carries a wide range of equipment, known as the payload, into space, ranging from communication, military, and astronomical satellites; space experiments for studying the apparent weightlessness (called 'microgravity') experienced aboard a shuttle flight; and human experimental facilities. Often, NASA collaborates with other countries by allowing them to use shuttle cargo space for special projects.
The space shuttle is designed to leave the earth as a vertically launched rocket weighing up to 2.0 million kg (4.5 million lb.) with 3 million kg (7 million lb.) of thrust from its multiple propulsion systems. The orbiter segment returns from space-withstanding the intense heat when entering the earth's atmosphere. Flown by the shuttle crew much like an aircraft, the shuttle lands horizontally on a conventional airport runway.
The crew of the shuttle is an integral part of the system and is critical to the success of each mission. The flight crew is led by the commander and backed up by the pilot-both are professional astronauts and proven pilots with extensive space systems and operations training. Their primary responsibility is to fly the shuttle as a launch vehicle, spacecraft, and aircraft.
The remaining crew members-up to five more people-are responsible for the unique aspects of a particular space mission. The mission specialist is the lead astronaut and ensures that the mission meets all the objectives. Payload specialists are experts in that mission's objectives and cargo, which are usually space experiments or artificial satellites. Often the payload specialists are astronauts from other countries on board to help with a project in which their country has an interest.

Spacecraft and Supporting Systems

The space-shuttle system, called the Space Transportation System (STS), remains the most technologically advanced and complex machine on earth. It consists of the orbiter, propulsion systems-two solid rocket boosters (SRBs) and three main engines, and an external fuel tank.

Space-Shuttle Orbiter

The orbiter is both the brains and heart of the STS, and it contains the latest advances in flight control, thermal protection, and liquid-rocket propulsion. About the same size and weight as a DC-9 aircraft (a fairly small two-engine jet aeroplane), the orbiter is composed of the pressurised crew compartment (which can carry up to seven crew members), the huge cargo bay, and the three main engines mounted on its aft, or rear, end.
The crew cabin has three levels: the flight deck, the mid-deck, and the utility area. Uppermost is the flight deck where the commander and pilot control the craft, surrounded by an array of switches and controls. During launch of a seven-member crew, two additional astronauts are positioned on the flight deck behind the commander and pilot. The three other crew members are in launch positions in the mid-deck, which is below the flight deck.
The galley, toilet, sleep stations, and storage and experiment lockers are found in the mid-deck. Also located in the mid-deck are the side hatch for passage to and from the vehicle before and after landing, and the airlock hatch into the cargo bay and space beyond.
Astronauts pass through this hatch to don their space suits and manoeuvring units (called Simplified Aid for EVA Rescue, or SAFER, these units strap on an astronaut's back over the space suit and allow an astronaut to move about in space without being tethered to the shuttle). This equipment prepares astronauts for extravehicular activities (EVAs), more popularly known as spacewalks. Below the mid-deck's floor is a utility area for air and water tanks.
The space shuttle's cargo bay is adaptable to hundreds of tasks. Large enough to accommodate a tour bus at 18 by 4.6 m (60 by 15 ft), the cargo bay carries satellites, spacecraft, and scientific laboratories for the modular Spacelab system to and from the earth's orbit. It also is a workstation for astronauts to repair satellites, a foundation from which to erect space structures, and a storage area for satellites retrieved from space to be returned to the earth.
Mounted on the port (left, as seen while facing the nose of the shuttle) side of the cargo bay behind the crew quarters is the remote manipulator system (RMS), developed and funded by the Canadian government. The RMS (about 15 m [50 ft] in length) is a robot arm and hand with three joints analogous to those of the human shoulder, elbow, and wrist. Two television cameras mounted near its elbow and wrist provide visual cues to the crew member who operates it from the rear station of the orbiter's flight deck. The RMS can move anything from satellites to astronauts to and from the cargo bay or to different points in nearby space. It has been used on many missions, deploying and retrieving various scientific and communications satellites.
Thermal tile insulation and larger flexible sheets of insulating material (also known as the thermal protection system or TPS) cover the underbelly, bottom of the wings, and other heat-bearing surfaces of the orbiter and protect it during its fiery re-entry into the earth's atmosphere.
In contrast to earlier manned spacecraft such as the Apollo command module, which used material that burned and melted off in layers during re-entry and could never be used again, the shuttle's silicate fibre tiles were designed to be used for 100 missions before requiring replacement.
Some 24,000 individual tiles must be installed by hand on the orbiter's surfaces. These tiles are incredibly lightweight, about the density of balsa wood, and dissipate heat so quickly that a white-hot tile with a temperature of 1260° C (2300° F) can be taken from an oven and held in bare hands without injury.

Propulsion Systems

The two SRBs, with their combined thrust of some 2.6 million kg (about 5.8 million lb), provide most of the power for the first two minutes of flight. The SRBs take the space shuttle to an altitude of 45 km (28 mi) and a speed of 4973 km/hr (3094 mph) before they separate and fall back into the ocean to be retrieved, refurbished, and prepared for another flight.
After the boosters fall away, the three main engines continue to provide thrust. These engines are clustered at the rear end of the orbiter and have a combined thrust of almost 540,000 kg (almost 1.2 million lb). The space shuttle's liquid-propellant engines are the world's first reusable rocket engines. They fire for only eight minutes for each flight, just until the shuttle reaches orbit, and are designed to operate for 55 flights. The engines are very large-4.2 m (14 ft) long and 2.4 m (8 ft) in diameter at the wide end of the cone-shaped nozzle at the rear of the orbiter.
Another propulsion system takes over once the space shuttle's main engines shut down as the ship approaches the altitude at which it will begin orbiting around the earth, known as the orbital insertion point. Two orbital manoeuvring system (OMS) engines, mounted on either side of the aft fuselage, provide thrust for major orbital changes. For more exacting maneuvers in orbit, 44 small rocket engines (known as the reaction control system), clustered on the shuttle's nose and on either side of the tail, are used. They have proven indispensable in performing the shuttle's important work of retrieving, launching, and repairing satellites in orbit.

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 pressurised tanks, the external tank holds the liquid hydrogen fuel and liquid oxygen oxidiser (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 (1000 lb.) of fuel are consumed by each of the main engines each second.
external fuel tank is the only part of the launch vehicle that currently is not reused. After its 1.99 million litres (526,000 gal) 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

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