Flight systems

Flight systems

Space Shuttle structural overview.

Early shuttle missions took along the GRiD Compass, arguably one of the first laptop computers. The Compass sold poorly, as it cost at least $8000 (USD), but offered unmatched performance for its weight and size.^[3] NASA was one of its main customers.^[4]

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 Data Processing System (DPS).^[5][6]

Atlantis deploys landing gear before landing on a selected runway just like a common aircraft.

The design goal of the shuttle's DPS is fail operational/fail safe reliability. After a single failure, the shuttle can still continue the mission. After two failures, it can still 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. Embedded system avionic software is developed under totally different conditions from public commercial software, the number of code lines is tiny compared to a public commercial software, 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 still 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, and load software from magnetic 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.

Upgrades

During STS-101, Atlantis was the first shuttle to fly with a glass cockpit.

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 those of contemporary airliners like the Airbus A380 and Boeing 777. 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 have been 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 (SSMEs) 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%. To clarify 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 the first two missions, 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.^[7] 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 aluminum-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. Design engineers added a third O-ring seal to the joints between the segments after the Space Shuttle Challenger disaster.

The three nozzles of the Main Engine cluster with the two Orbital Maneuvering System (OMS) pods, and the vertical stabilizer above.

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. These rockets entered production in the early to mid-1990s to support the Space Station, but were later canceled to save money after the expenditure of $2.2 billion.^[8] The loss of the ASRB program resulted in 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.

STS-70 was delayed in 1995, when woodpeckers bored holes in the foam insulation of Discovery's external tank. Since then, NASA has installed commercial plastic owl decoys and inflatable owl balloons which must be removed prior to launch.^[9] The delicate nature of the foam insulation has been the cause of damage to the Thermal Protection System, the tile heat shield and heat wrap of the orbiter, during recent launches. NASA remains confident that this damage, while linked to the Space Shuttle Columbia disaster on February 1, 2003, will not jeopardize the objective of NASA to complete the International Space Station (ISS) in the projected time allotted.

A cargo-only, unmanned variant of the shuttle has been variously proposed, and rejected since the 1980s. It was called the Shuttle-C, and would have traded 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 modified U.S. Air Force high-altitude full-pressure suits, which included a full-pressure helmet during ascent and descent. From the fifth flight, STS-5, until the loss of Challenger, one-piece light blue nomex flight suits and partial-pressure helmets were worn. A less-bulky, partial-pressure version of the high-altitude pressure suits with a helmet was reinstated when shuttle flights resumed in 1988. The LES ended its service life in late 1995, and was replaced by the full-pressure Advanced Crew Escape Suit (ACES), which resembles the Gemini space suit worn in the mid-1960s.

To extend the duration that orbiters can stay docked at the ISS, the Station-to-Shuttle Power Transfer System (SSPTS) was installed on Discovery and Endeavour. The SSPTS allows these orbiters to use power provided by the ISS to preserve their consumables. The SSPTS was first used successfully on STS-118.

[edit] Technical data

Space Shuttle Atlantis transported by a Boeing 747 Shuttle Carrier Aircraft (SCA), 1998 (NASA).

Space Shuttle Endeavour being transported by a Boeing 747.

Orbiter specifications^[10] (for Endeavour, OV-105)

• Length: 122.17 ft (37.24 m)
• Wingspan: 78.06 ft (23.79 m)
• Height: 58.58 ft (17.86 m)
• Empty weight: 151,205 lb (68,585 kg)
• Gross Liftoff Weight: 240,000 lb (109,000 kg)
• Maximum Landing Weight: 230,000 lb (104,000 kg)
• Main engines: Three Rocketdyne Block IIA SSMEs, each with a sea level thrust of 393,800 pounds-force (lbf) (178,600 kilograms-force (kgf) / 1.75 meganewtons (MN))
• Maximum payload: 55,250 pounds (25,061 kg)
• Payload bay dimensions: 15 ft × 60 ft (4.6 m × 18.3 m)
• Operational altitude: 100 to 520 nmi (185 to 960 km)
• Speed: 25,404 ft/s (7,743 m/s, 27,875 km/h, 17,321 mi/h)
• Crossrange: 1,085 nmi (2,009 km)
• Crew: Varies. The earliest shuttle flights had the minimum crew of two; many later missions a crew of five. Today, typically seven people fly (commander, pilot, several mission specialists, and rarely a flight engineer). On two occasions, eight astronauts have flown (STS-61-A, STS-71). Eleven people could be accommodated in an emergency mission (see STS-3xx).

External tank specifications (for SLWT)

• Length: 153.8 ft (46.9 m)
• Diameter: 27.6 ft (8.4 m)
• Propellant volume: 535,000 US gal (2,025 m³)
• Empty Weight: 58,500 lb (26,535 kg)
• Gross Liftoff Weight: 1,667,000 lb (756,000 kg)

Solid Rocket Booster Specifications

• Length: 149.6 ft (45.6 m)
• Diameter: 12.17 ft (3.71 m)
• Empty Weight (per booster): 139,490 lb (63,272 kg)
• Gross Liftoff Weight (per booster): 1.3 million lb (590,000 kg)
• Thrust (sea level, liftoff): 2.8 million lbf (12.5 MN)

System Stack Specifications

• Height: 184.2 ft (56.14 m)
• Gross Liftoff Weight: 4.5 million lb (2,040,000 kg)
• Total Liftoff Thrust: 6.781 million lbf (30.16 MN)