Space Shuttle

NASA's Space Shuttle, officially called Space Transportation System (STS), is the United States government's current manned launch vehicle. The winged shuttle orbiter is launched vertically, usually carrying five to seven astronauts (although eight have been carried) and up to 22,700 kg (50,000 lb) of payload into low earth orbit. When its mission is complete, it re-enters the earth's atmosphere and makes an unpowered horizontal landing.

The Shuttle is the first orbital spacecraft designed for partial reusability. It is also so far the only winged manned spacecraft to achieve orbit and land. It carries large payloads to various orbits, provides crew rotation for the International Space Station (ISS), and performs servicing missions. The orbiter can recover satellites and other payloads from orbit and return them to Earth, but this capacity has not been used often. However, it has been used to return large payloads from the International Space Station to earth, as the Russian Soyuz spacecraft has limited capacity for return payloads. Each Shuttle was designed for a projected lifespan of 100 launches or 10 years' operational life.

The program started in the late 1960s and has dominated NASA's manned operations since the mid-1970s. According to the Vision for Space Exploration, use of the Space Shuttle will be focused on completing assembly of the ISS in 2010, after which it will be replaced by the Crew Exploration Vehicle (CEV).

Description
The Shuttle is a partially reusuable launch system composed of three main assemblies: the reusable Orbiter Vehicle (OV), the expendable External Tank (ET), and the two reusable Solid Rocket Boosters (SRBs). The tank and boosters are jettisoned during ascent; only the orbiter goes into orbit. The vehicle is launched vertically like a conventional rocket, and the orbiter glides to a horizontal landing, after which it is refurbished for reuse.

The Orbiter resembles an airplane with double-delta wings, swept 81° at the inner leading edge and 45° at the outer leading edge. Its vertical stabilizer's leading edge is swept back at a 45° angle. The four elevons, mounted at the trailing edge of the wings, and the rudder/speed brake, attached at the trailing edge of the stabilizer, with the body flap, control the Orbiter during descent and landing.

The Orbiter's crew cabin consists of three levels: the flight deck, the mid-deck, and the utility area. The highest flight deck seats the commander and pilot, with two mission specialists behind them. The mid-deck has three more seats for the rest of the crew members. The galley, toilet, sleep locations, storage lockers, and the side hatch for entering/exiting the vehicle are also located there, as is the airlock hatch. The airlock has another hatch into the payload bay. It allows two astronauts, wearing their Extravehicular Mobility Unit (EMU) space suits, to depressurize before a space walk.

The Orbiter has a large 60 by 15 ft (18 m by 4.6 m) payload bay, filling most of the fuselage. The payload bay doors have heat radiators mounted on their inner surfaces, and so are kept open for thermal control while the Shuttle is in orbit. Thermal control is also maintained by adjusting the orientation of the Shuttle relative to Earth and Sun. Inside the payload bay is the Remote Manipulator System, also known as the Canadarm, a robot arm used to retrieve and deploy payloads. Until the loss of Columbia, the Canadarm had been used only on those missions where it was needed. Since the arm is a crucial part of the Thermal Protection Inspection procedures now required for Shuttle flights, it will probably be included on all future flights.


Three Space Shuttle Main Engines (SSMEs) are mounted on the Orbiter's aft fuselage in a triangular pattern. The three engines can swivel 10.5 degrees up and down and 8.5 degrees from side to side during ascent to change the direction of their thrust and steer the Shuttle as well as push.

The Orbital Maneuvering System (OMS) provides orbital maneuvers, including insertion, circularization, transfer, rendezvous, abort to orbit, and abort once around.

The Reaction Control System (RCS) provides attitude control and translation along the pitch, roll, and yaw axes during the flight phases of orbit insertion, orbit, and re-entry.

The Thermal Protection System (TPS) covers the outside of the Orbiter, protecting it from the cold soak of -121 °C (-250 °F) in space to the 1649 °C (3000 °F) heat of reentry

The orbiter structure is made primarily from aluminium alloy, although the engine thrust structure is made from titanium.

The External Tank (ET) provides 2.025 million liters (535,000 gallons) of liquid hydrogen and liquid oxygen propellant to the SSMEs. It is discarded 8.5 minutes after launch at an altitude of 60 nautical miles (111 km) then breaks up on reentry. The ET is constructed mostly of aluminium-lithium alloy about 1/8 inch thick. Many people think that the ET is painted orange. In reality, the orange color is the true color of the tank's insulation. If it were painted, it would add another 1,000 lbs. of unnecessary payload.

Two Solid Rocket Boosters (SRBs) provide about 83% of the vehicle's thrust at liftoff and during the first stage ascent. They are jettisoned two minutes after launch at a height of about 150,000 feet (45.7 km), then deploy parachutes and land in the ocean to be recovered. The SRB cases are made of steel about 1/2 inch (1.27 cm) thick.


Computerized fly-by-wire digital flight control
The shuttle was one of the earliest craft to use a computerized fly-by-wire digital flight control system. This means no mechanical or hydraulic linkages connect the pilot's control stick to the control surfaces or reaction control system thrusters.

A primary concern with digital fly-by-wire systems is reliability. Much research went into the shuttle computer system. The shuttle uses five identical redundant IBM 32-bit general purpose computers (GPCs), model AP-101, constituting a type of embedded system. Four computers run specialized software called the Primary Avionics Software System (PASS). A fifth backup computer runs separate software called the Backup Flight System (BFS). Collectively they are called the shuttle Data Processing System (DPS).
The design goal of the shuttle DPS is fail operational/fail safe reliability. After a single failure the shuttle can continue the mission. After two failures it can land safely.

The four general-purpose computers operate essentially in lockstep, checking each other. If one computer fails, the three functioning computers "vote" it out of the system. This isolates it from vehicle control. If a second computer of the three remaining fails, the two functioning computers vote it out. In the rare case of two out of four computers simultaneously failing (a two-two split), one group is picked at random.

The Backup Flight System (BFS) is separately developed software running on the fifth computer, used only if the entire four-computer primary system fails. The BFS was created because although the four primary computers are hardware redundant, they all run the same software, so a generic software problem could crash all of them. This should never happen, as embedded system avionic software is developed under totally different conditions from commercial software. For example, the number of code lines is tiny compared to a commercial operating system, changes are only made infrequently and with extensive testing, and many programming and test personnel work on the small amount of computer code. However in theory it can fail, and the BFS exists for that contingency.

The software for the shuttle computers is written in a high-level language called HAL/S, somewhat similar to PL/I. It is specifically designed for a real time embedded system environment.

The IBM AP-101 computers originally had about 424 kilobytes of magnetic core memory each. The CPU could process about 400,000 instructions per second. They have no hard disk drive, but load software from tape cartridges.

In 1990 the original computers were replaced with an upgraded model AP-101S, which has about 2.5 times the memory capacity (about 1 megabyte) and three times the processor speed (about 1.2 million instructions per second). The memory was changed from magnetic core to semiconductor with battery backup.
[edit]



Other improvements
Internally the Shuttle remains largely similar to the original design, with the exception of the improved avionics computers. In addition to the computer upgrades, the original vector graphics monochrome cockpit displays were replaced with modern full-color, flat-panel display screens, similar to contemporary airliners like the Airbus A320. This is called a "glass cockpit". In the Apollo-Soyuz Test Project tradition, programmable calculators are carried as well (originally the HP-41C). With the coming of the ISS, the Orbiter's internal airlocks are being replaced with external docking systems to allow for a greater amount of cargo to be stored on the Shuttle's mid-deck during Station resupply missions.


The Space Shuttle Main Engines have had several improvements to enhance reliability and power. This explains phrases such as "Main engines throttling up to 104%." This does not mean the engines are being run over a safe limit. The 100% figure is the original specified power level. During the lengthy development program, Rocketdyne determined the engine was capable of safe reliable operation at 104% of the originally specified thrust. They could have rescaled the output number, saying in essence 104% is now 100%. However this would have required revising much previous documentation and software, so the 104% number was retained. SSME upgrades are denoted as "block numbers", such as block I, block II, and block IIA. The upgrades have improved engine reliability, maintainability and performance. The 109% thrust level was finally reached in flight hardware with the Block II engines in 2001. The normal maximum throttle is 104%, with 106% and 109% available for abort emergencies.

For STS-1 and STS-2 the external tank was painted white to protect the insulation that covers much of the tank, but improvements and testing showed that it was not required. The weight saved by not painting the tank results in an increase in payload capability to orbit. Additional weight was saved by removing some of the internal "stringers" in the hydrogen tank that proved unnecessary. The resulting "light-weight external tank" has been used on the vast majority of Shuttle missions. STS-91 saw the first flight of the "super light-weight external tank". This version of the tank is made of the 2195 aluminium-lithium alloy. It weighs 7,500 lb (3.4 t) less than the last run of lightweight tanks. As the Shuttle cannot fly unmanned, each of these improvements has been "tested" on operational flights.

The SRBs (Solid Rocket Boosters) have undergone improvements as well. Notable is the adding of a third O-ring seal to the joints between the segments, which occurred after the Challenger accident.

Several other SRB improvements were planned in order to improve performance and safety, but never came to be. These culminated in the considerably simpler, lower cost, probably safer and better performing Advanced Solid Rocket Booster which was to have entered production in the early to mid-1990s to support the Space Station, but was later cancelled to save money after the expenditure of $2.2 billion. The loss of the ASRB program forced the development of the Super LightWeight external Tank (SLWT), which provides some of the increased payload capability, while not providing any of the safety improvements. In addition the Air Force developed their own much lighter single-piece SRB design using a filament-wound system, but this too was cancelled.

A cargo-only, unmanned variant of the Shuttle has been variously proposed and rejected since the 1980s. It is called the Shuttle-C and would trade re-usability for cargo capability with large potential savings from reusing technology developed for the Space Shuttle.

On the first four Shuttle missions, astronauts wore full-pressure Launch Entry Suit (LES) during ascent and descent. The pressured helmet was used from STS-5 until the loss of Challenger. The LES was reinstated when Shuttle flights resumed in 1988. The LES ended its service life in late 1995, replaced by the Advanced Crew Escape Suit (ACES).
[edit]

Technical data

Orbiter Specifications (for Endeavour, OV-105)

* Length: 122.17 ft (37.24 m)
* Wingspan: 78.06 ft (23.79 m)
* Height: 58.58 ft (17.25 m)
* Empty Weight: 151,205 lb (68,586.6 kg)
* Gross Liftoff Weight: 240,000 lb (109,000 kg)
* Maximum Landing Weight: 230,000 lb (104,000 kg)
* Main Engines: Three Rocketdyne Block 2 A SSMEs, each with a sea level thrust of 393,800 lbf (178,624 kgf / 1.75MN)
* Maximum Payload: 55,250 lb (25,061.4 kg)
* Payload Bay dimensions: 15 ft by 60 ft (4.6 m by 18.3 m)
* Operational Altitude: 100 to 520 nmi (185 to 1,000 km)
* Speed: 25,404 ft/s (7,743 m/s, 27,875 km/h, 17,321 mi/h)
* Crossrange: 1,085 nautical miles (2,009.4 km)
* Crew: Seven (Commander, Pilot, two Mission Specialists, and three Payload Specialists), two for minimum.
External Tank Specifications (for SLWT)

* Length: 153.8 ft (46.9 m)
* Diameter: 27.6 ft (8.4 m)
* Propellent Volume: 535,000 gallon
* Empty Weight: 58,500 lb (26,559 kg)
* Gross Liftoff Weight: 1.667 million lb (757,000 kg)

Solid Rocket Booster Specifications

* Length: 149.6 ft (45.6 m)
* Diameter: 12.17 ft (3.71 m)
* Empty Weight: 139,490 lb (63,272.7 kg)
* Gross Liftoff Weight: 1.3 million lb (590,000 kg)
* Thrust (sea level, liftoff): 2.8 million lbf (1,270,058 kgf / 12.46MN)

System Stack Specifications

* Height: 184.2 ft (56.14 m)
* Gross Liftoff Weight: 4.5 million lb (2.04 million kg)
* Total Liftoff Thrust: 6.781 million lbf (3.076 million kgf / 30.18MN)