Encyclopedia
NASA's
Space Shuttle, officially called
Space Transportation System , is the
United States government's current
manned launch vehicle. The winged shuttle orbiter is launched vertically, usually carrying five to seven
astronauts and up to 50,000 lb of payload into low earth orbit. When its mission is complete, it fires its maneuvering thrusters to drop out of orbit and
re-enters the
Earth's atmosphere. During the descent and landing, the shuttle orbiter acts as a
glider and makes a completely unpowered 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 , and performs servicing missions. The orbiter can also 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 ISS 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.
Description
The Shuttle is a
partially reusable launch system composed of three main assemblies: the reusable
Orbiter Vehicle , the expendable
External Tank , and the two reusable
Solid Rocket Boosters . 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.
Orbiter Vehicle
The Orbiter resembles an aircraft 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 has a large 60 by 15 ft payload bay, filling most of the fuselage.
Three
Space Shuttle Main Engines 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 orbiter structure is made primarily from
aluminium alloy, although the engine thrust structure is made from
titanium .
External Tank
The External Tank provides approximately 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 , which then burns up on re-entry. The ET is constructed mostly of ? inch thick aluminium-
lithium alloy.
The external tanks of the first two missions were painted white, which added an extra 600 pounds of weight to each ET. Subsequent missions have had unpainted tanks showing the natural orange-brown color of the spray-on foam insulation. The lighter, unpainted tanks have increased the
payload capacity by almost the entire weight savings of 600 pounds.
Solid Rocket Boosters
Two Solid Rocket Boosters 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 , then deploy parachutes and land in the ocean to be recovered. The SRB cases are made of steel about ½ inch thick.
Flight systems
Early Shuttle missions took along the
GRiD Compass, arguably one of the first
laptop computers. The Compass sold poorly, because it cost at least
$8000, but offered unmatched performance for its weight and size. NASA was one of its main customers.
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 , model AP-101, constituting a type of
embedded system. Four computers run specialized software called the Primary Avionics Software System . A fifth backup computer runs separate software called the Backup Flight System . Collectively they are called the shuttle Data Processing System .
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 , one group is picked at random.
The Backup Flight System 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 is unlikely to ever 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 and three times the processor speed . The memory was changed from magnetic core to semiconductor with battery backup.
Upgrades
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 . 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 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 the two first 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. 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 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 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 disaster.
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 , 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 holed the foam insulation of Discoverys external tank. Since then, NASA has installed commercial plastic owl decoys and inflatable owl balloons which must be removed prior to launch.
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 including a helmet during ascent and descent. From the fifth flight,
STS-5, until the loss of
Challenger only helmets were worn without a suit. The LES with a helmet was reinstated when Shuttle flights resumed in 1988. The LES ended its service life in late 1995, replaced by the
Advanced Crew Escape Suit .
Technical data
Orbiter Specifications- Length: 122.17 ft
- Wingspan: 78.06 ft
- Height: 58.58 ft
- Empty Weight: 151,205 lb
- Gross Liftoff Weight: 240,000 lb
- Maximum Landing Weight: 230,000 lb
- Main Engines: Three Rocketdyne Block 2 A SSMEs, each with a sea level thrust of 393,800 lbf
- Maximum Payload: 55,250 lb
- Payload Bay dimensions: 15 ft by 60 ft
- Operational Altitude: 100 to 520 nmi
- Speed: 25,404 ft/s
- Crossrange: 1,085 nautical miles
- Crew: Seven , two for minimum.
External Tank Specifications- Length: 153.8 ft
- Diameter: 27.6 ft
- Propellant Volume: 535,000 gallon
- Empty Weight: 58,500 lb
- Gross Liftoff Weight: 1.667 million lb
Solid Rocket Booster Specifications- Length: 149.6 ft
- Diameter: 12.17 ft
- Empty Weight: 139,490 lb
- Gross Liftoff Weight: 1.3 million lb
- Thrust : 2.8 million lbf
System Stack Specifications- Height: 184.2 ft
- Gross Liftoff Weight: 4.5 million lb
- Total Liftoff Thrust: 6.781 million lbf
Launch
The shuttle will not be launched under conditions where it could be struck by
lightning. Aircraft are often struck by lightning with no adverse effects because the
electricity of the strike is dissipated through its conductive structure and the aircraft is not electrically grounded. Like most jet airliners, the shuttle is mainly constructed of conductive aluminium, which would normally protect the internal systems. However, upon takeoff the shuttle sends out a long exhaust plume as it ascends, and this plume can trigger lightning by providing a current path to ground. While the shuttle might safely endure a lightning strike, a similar strike caused problems on
Apollo 12, so for safety
NASA chooses not to launch the shuttle if lightning is possible.
At T minus 16 seconds, the massive sound suppression system begins to drench the
Mobile Launcher Platform and SRB trenches with 300,000 U.S. gallons of water to protect the Orbiter from damage by acoustical energy and rocket exhaust reflected from the flame trench and MLP during liftoff.
At T-minus ten seconds, hydrogen ignitors are activated beneath each engine bell to quell the stagnant gas inside the cones before ignition. Failure to burn excess gasses before ignition can trip the onboard sensors and create the possibility of overpressure and explosion during the firing phase.
The three
Space Shuttle Main Engines start at T minus 6.6 seconds. All three SSMEs must reach the required 100% thrust within three seconds. If the onboard computers verify normal thrust buildup, at T minus 0 the
SRBs are ignited. At that point the vehicle is committed to takeoff, as the SRBs cannot be turned off once ignited. After the SRBs reach a stable thrust pyrotechnic fasteners, large nuts that split in half, are detonated to release the craft. There are extensive emergency procedures to handle various failure scenarios during ascent. Many of these concern SSME failures, since that is the most complex and highly stressed component. After the
Challenger disaster, there were extensive upgrades to the abort modes.
When watching a launch, look for the "nod" . After the main engines start, but while the solid rocket boosters are still clamped to the pad, the offset thrust from the Shuttle's three main engines causes the entire launch stack to flex forwards about 2m at cockpit level. As the boosters flex back into their original shape, the launch stack springs slowly back upright. This takes approximately 6 seconds. At the point when it is perfectly vertical, the boosters ignite and the launch commences.
Shortly after clearing the tower the Shuttle begins a roll and pitch program so that the vehicle is below the external tank and SRBs. The vehicle climbs in a progressively flattening arc, accelerating as the weight of the SRBs and main tank decrease. To achieve low orbit requires much more horizontal than vertical acceleration. This is not visually obvious since the vehicle rises vertically and is out of sight for most of the horizontal acceleration. Orbital velocity at the 380 km altitude of the
International Space Station is 7.68 km per second , roughly equivalent to Mach 23. For missions towards the International Space Station, the shuttle must reach an azimuth of 51.6 degrees inclination to rendezvous with the station.
Around a point called "
Max Q", where the aerodynamic forces are at their maximum, the main engines are temporarily throttled back to avoid overspeeding and hence overstressing the Shuttle, particularly in vulnerable areas such as the wings. At this point, a phenomenon known as the "
Prandtl-Glauert Singularity" occurs, where condensation clouds form during the vehicle's transition to supersonic speed.
126 seconds after launch, explosive bolts release the SRBs and small separation rockets push them laterally away from the vehicle. The SRBs parachute back to the ocean to be reused. The Shuttle then begins accelerating to orbit on the
Space Shuttle main engines. The vehicle at that point in the flight has a thrust to weight ratio of less than one — the main engines actually have insufficient thrust to exceed the force of gravity, and the vertical speed given to it by the SRBs temporarily decreases. However, as the burn continues, the weight of the propellant reduces and the ever-lighter vehicle produces more and more acceleration until the thrust-to-weight ratio exceeds 1 again and the vehicle can hold itself up.
The vehicle continues to climb and takes on a somewhat nose-up angle to the horizon — it uses the main engines to gain and then maintains altitude whilst it accelerates horizontally towards orbit. At about five and three-quarter minutes into ascent, the orbiter rolls heads up to switch communication links from ground stations to
Tracking and Data Relay Satellites.
Finally, in the last tens of seconds of the main engine burn, the mass of the vehicle is low enough that the engines must be throttled back to limit vehicle acceleration to 3 g, largely for astronaut comfort.
Before complete depletion of propellant, as running dry would destroy the engines, the main engines are shut down and the external tank is released by firing explosive bolts. The tank then falls, largely to burn up in the atmosphere, with some fragments falling into the Indian Ocean.
To keep the shuttle from following the external tank back into the atmosphere, the
OMS engines are fired to raise the perigee out of the atmosphere. On some missions , the OMS engines are also used while the main engines are still firing.
Landing
The vehicle begins reentry by firing the
OMS engines in the opposite direction to orbital motion for about three minutes. The resulting deceleration of the Shuttle lowers its orbit
perigee down into the atmosphere. This OMS firing is done roughly halfway around the globe from the landing site. The entire reentry, except for lowering the landing gear and deploying the air data probes, is then under computer control. However the reentry can be and has been flown manually. The final landing can be done on autopilot, but is usually hand flown.
The vehicle starts significantly entering the atmosphere at about 400,000 ft at around
Mach 25 . The vehicle is controlled by a combination of RCS thrusters and control surfaces, to fly at a 40 degrees nose-up attitude producing high drag, not only to slow it down to landing speed, but also to reduce reentry heating. In addition, the vehicle needs to bleed off extra speed before reaching the landing site. This is achieved by performing s-curves at up to a 70 degree roll angle.
In the lower atmosphere the Orbiter flies much like a conventional glider, except for a much higher descent rate, over 10,000 feet per minute. It glides with a ratio of 4:1. At approximately Mach 3, two air data probes, located on the left and right sides of the Orbiter's forward lower fuselage, are deployed to sense air pressure related to vehicle's movement in the atmosphere.
When the approach and landing phase begins, the Orbiter is at 10,000 ft altitude, 7.5 miles to the runway. The pilots apply aerodynamic braking to help slow down the vehicle. The Orbiter's speed is reduced from 424 mph to approximately 215 mph , , at touch-down. The landing gear is deployed while the Orbiter is flying at 267 mph . To assist the speed brakes, a 40 ft drag chute is deployed once the nose gear touches down at about 213 mph . It is jettisoned as the Orbiter slows through 69 mph .
After landing, the vehicle stands on the runway for several minutes to permit the fumes from poisonous
hydrazine, used as propellant for attitude
control, to dissipate, and for the shuttle fuselage to cool before the astronauts disembark.
Conditions permitting, the Space Shuttle will always land at Kennedy Space Center. However, if the conditions make landing there unfavorable, the Shuttle can touch down at
Edwards Airforce Base in
California or at other sites. A landing at Edwards means that the shuttle must be mated to the
Shuttle Carrier Aircraft and returned to
Cape Canaveral, costing NASA roughly an additional million dollars.
See also
...
the early laptop carried aboard the shuttle.
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This is a
list of missions flown by space shuttles....
Fiction
DOS-based shuttle simulator from the 1990s.
- Orbiter a freeware simulator that allows users to fly various spacecraft including the Shuttle.
Physics
Similar spacecraft
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...
Notes
Further reading
External links
- : Current status of Shuttle missions
- NASA TV: View live streaming of launch and mission coverage
- [news:sci.space.shuttle Space Shuttle Newsgroup - sci.space.shuttle]
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- Official NASA system
- with Google Maps
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