Shuttle components included the Orbiter Vehicle (OV), a pair of recoverable Solid Rocket Boosters (SRB), and an expendableExternal Tank (ET) containing liquid hydrogen and liquid oxygen. The Shuttle was launched vertically like a conventional rocket with the two SRBs operating in parallel with the OV's three main engines, which were fueled from the External Tank. The SRBs were jettisoned before the vehicle reached orbit, and the ET was jettisoned just before orbit insertion using the orbiter's two Orbital Maneuvering System (OMS) engines. At the conclusion of the mission, the orbiter fired its OMS to drop out of orbit and re-enter the atmosphere. The orbiter glided to a runway landing on Rogers Dry Lake at Edwards Air Force Base in California or at theShuttle Landing Facility at the KSC. After Edwards landings, the orbiter was flown back to KSC on the Shuttle Carrier Aircraft, a specially modified Boeing 747.
The first orbiter, Enterprise, was built purely for Approach and Landing Tests and had no capability to fly into orbit. Four fully operational orbiters were initially built: Columbia, Challenger, Discovery, and Atlantis. Of these, Challenger and Columbia were lost in mission accidents in 1986 and 2003, respectively, in which a total of fourteen astronauts were killed. A fifth operational orbiter,Endeavour, was built in 1991 to replace Challenger. The Space Shuttle was retired from service upon the conclusion of Atlantis'final flight on July 21, 2011.
The formal design of what became the Space Shuttle began with "Phase A" contract design studies issued in the late 1960s. Conceptualization had begun two decades earlier, before the Apollo program of the 1960s. One of the places the concept of a spacecraft returning from space to a horizontal landing originated was within NACA, in 1954, in the form of an aeronautics research experiment later named the X-15. The NACA proposal was submitted by Walter Dornberger.
In 1958, the X-15 concept further developed into proposal to launch an X-15 into space, and another X-series spaceplane proposal, called the X-20, which was not constructed, as well as variety of aerospace plane concepts and studies. Neil Armstrong was selected to pilot both the X-15 and the X-20. Though the X-20 was not built, another spaceplane similar to the X-20 was built several years later and delivered to NASA in January 1966 called the HL-10 ("HL" indicated "horizontal landing").
In the mid-1960s, the US Air Force conducted classified studies on next-generation space transportation systems and concluded that semi-reusable designs were the cheapest choice. It proposed a development program with an immediate start on a "Class I" vehicle with expendable boosters, followed by slower development of a "Class II" semi-reusable design and perhaps a "Class III" fully reusable design later. In 1967, George Mueller held a one-day symposium at NASA headquarters to study the options. Eighty people attended and presented a wide variety of designs, including earlier Air Force designs as the Dyna-Soar (X-20).
In 1968, NASA officially began work on what was then known as the Integrated Launch and Re-entry Vehicle (ILRV). At the same time, NASA held a separate Space Shuttle Main Engine (SSME) competition. NASA offices in Houston and Huntsville jointly issued a Request for Proposal (RFP) for ILRV studies to design a spacecraft that could deliver a payload to orbit but also re-enter the atmosphere and fly back to Earth. For example, one of the responses was for a two-stage design, featuring a large booster and a small orbiter, called the DC-3, one of several Phase A Shuttle designs. After the aforementioned "Phase A" studies, B, C, and D phases progressively evaluated in-depth designs up to 1972. In the final design, the bottom stage was recoverable solid rocket boosters, and the top stage used an expendable external tank.
In 1969, President Richard Nixon decided to support proceeding with Space Shuttle development. A series of development programs and analysis refined the basic design, prior to full development and testing. In August 1973, the X-24B proved that an unpowered spaceplane could re-enter Earth's atmosphere for a horizontal landing.
Across the Atlantic, European ministers met in Belgium in 1973 to authorize Western Europe's manned orbital project and its main contribution to Space Shuttle—the Spacelabprogram. Spacelab would provide a multidisciplinary orbital space laboratory and additional space equipment for the Shuttle.
The Space Shuttle was the first operational orbital spacecraft designed for reuse. It carried different payloads to low Earth orbit, provided crew rotation and supplies for the International Space Station (ISS), and performed servicing missions. The orbiter could also recover satellites and other payloads from orbit and return them to Earth. Each Shuttle was designed for a projected lifespan of 100 launches or ten years of operational life, although this was later extended. The person in charge of designing the STS was Maxime Faget, who had also overseen the Mercury, Gemini, and Apollo spacecraft designs. The crucial factor in the size and shape of the Shuttle Orbiter was the requirement that it be able to accommodate the largest planned commercial and military satellites, and have over 1,000 mile cross-range recovery range to meet the requirement for classified USAF missions for a once-around abort from a launch to a polar orbit. This military specified 1,085 nm cross range requirement is one of the primary reasons that the Shuttle was designed with such large wings, compared to modern commercial designs with very minimal control surfaces and glide capability. Factors involved in opting for solid rockets and an expendable fuel tank included the desire of the Pentagon to obtain a high-capacity payload vehicle for satellite deployment, and the desire of the Nixon administration to reduce the costs of space exploration by developing a spacecraft with reusable components.
Each Space Shuttle is a reusable launch system that is composed of three main assemblies: the reusable Orbiter Vehicle (OV), the expendable external tank (ET), and the two reusable solid rocket boosters (SRBs). Only the orbiter entered orbit shortly after the tank and boosters are jettisoned. The vehicle was launched vertically like a conventional rocket, and the orbiter glided to a horizontal landing like an airplane, after which it was refurbished for reuse. The SRBs parachuted to splashdown in the ocean where they were towed back to shore and refurbished for later Shuttle missions.
Five operational orbiters were built: Columbia (OV-102), Challenger (OV-099), Discovery (OV-103), Atlantis (OV-104), and Endeavour (OV-105). A mock-up, Inspiration, currently stands at the entrance to the Astronaut Hall of Fame. An additional craft, Enterprise (OV-101), was not built for orbital space flight, and was used only for testing gliding and landing. Enterprise was originally intended to be outfitted for orbital operations after its use in the approach and landing test (ALT) program, but it was found more economical to upgrade the structural test article STA-099 into orbiter Challenger (OV-099). Challenger disintegrated 73 seconds after launch in 1986, and Endeavour was built as a replacement for Challenger from structural spare components. Columbia broke apart over Texas during re-entry in 2003. Building Space Shuttle Endeavour cost about US$1.7 billion. A Space Shuttle launch cost around $450 million.
Roger A. Pielke, Jr. has estimated that the Space Shuttle program cost about US$170 billion (2008 dollars) through early 2008. This works out to an average cost per flight of about US$1.5 billion. Two missions were paid for by Germany, Spacelab D1 and D2 (D forDeutschland) with a payload control center in Oberpfaffenhofen. D1 was the first time that control of a manned STS mission payload was not in U.S. hands.
At times, the orbiter itself was referred to as the Space Shuttle. This was not technically correct. The Space Shuttle was the combination of the orbiter, the external tank, and the two solid rocket boosters. These components, once assembled in the Vehicle Assembly Buildingoriginally built to assemble the Apollo Saturn V rocket, were referred to as the "stack".
Responsibility for the Shuttle components was spread among multiple NASA field centers. The Kennedy Space Center was responsible for launch, landing and turnaround operations for equatorial orbits (the only orbit profile actually used in the program), the US Air Force at the Vandenberg Air Force Base was responsible for launch, landing and turnaround operations for polar orbits (though this was never used), the Johnson Space Center served as the central point for all Shuttle operations, the Marshall Space Flight Center was responsible for the main engines, external tank, and solid rocket boosters, the John C. Stennis Space Center handled main engine testing, and the Goddard Space Flight Centermanaged the global tracking network.
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 connected the pilot's control stick to the control surfaces or reaction control system thrusters.
A concern with digital fly-by-wire systems is reliability. Considerable research went into the Shuttle computer system. The Shuttle used five identical redundant IBM 32-bit general purpose computers (GPCs), model AP-101, constituting a type of embedded system. Four computers ran specialized software called the Primary Avionics Software System (PASS). A fifth backup computer ran separate software called the Backup Flight System (BFS). Collectively they were called the Data Processing System (DPS).
Simulation of SSLV at Mach 2.46 and 66,000 ft (20,000 m). The surface of the vehicle is colored by the pressure coefficient, and the gray contours represent the density of the surrounding air, as calculated using the OVERFLOW software package.
The design goal of the Shuttle's DPS was fail-operational/fail-safe reliability. After a single failure, the Shuttle could still continue the mission. After two failures, it could still land safely.
The four general-purpose computers operated essentially in lockstep, checking each other. If one computer failed, the three functioning computers "voted" it out of the system. This isolated it from vehicle control. If a second computer of the three remaining failed, the two functioning computers voted it out. In the unlikely case that two out of four computers simultaneously failed (a two-two split), one group was to be picked at random.
The Backup Flight System (BFS) was separately developed software running on the fifth computer, used only if the entire four-computer primary system failed. The BFS was created because although the four primary computers were hardware redundant, they all ran the same software, so a generic software problem could crash all of them. Embedded system avionic software was developed under totally different conditions from public commercial software: the number of code lines was tiny compared to a public commercial software, changes were only made infrequently and with extensive testing, and many programming and test personnel worked on the small amount of computer code. However, in theory it could have still failed, and the BFS existed for that contingency. While the BFS could run in parallel with PASS, the BFS never engaged to take over control from PASS during any Shuttle mission.
The software for the Shuttle computers was written in a high-level language called HAL/S, somewhat similar to PL/I. It is specifically designed for areal 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 had no hard disk drive, and loaded software from magnetic tape cartridges.
In 1990, the original computers were replaced with an upgraded model AP-101S, which had 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.
Early Shuttle missions, starting in November 1983, took along the GRiD Compass, arguably one of the first laptop computers. The GRiD was given the name SPOC, for Shuttle Portable Onboard Computer. Use on the Shuttle required both hardware and software modifications which were incorporated into later versions of the commercial product. It was used to monitor and display the Shuttle's ground position, path of the next two orbits, show where the Shuttle had line of sight communications with ground stations, and determine points for location-specific observations of the Earth. The Compass sold poorly, as it cost at least US$8000, but it offered unmatched performance for its weight and size. NASA was one of its main customers.
The prototype orbiter Enterprise originally had a flag of the United States on the upper surface of the left wing and the letters "USA" in black on the right wing. The name "Enterprise" was painted in black on the payload bay doors just above the hinge and behind the crew module; on the aft end of the payload bay doors was the NASA "worm" logotype in gray. Underneath the rear of the payload bay doors on the side of the fuselage just above the wing is the text "United States" in black with a flag of the United States ahead of it.
The first operational orbiter, Columbia, originally had the same markings as Enterprise, although the letters "USA" on the right wing were slightly larger and spaced farther apart.Columbia also had black markings which Enterprise lacked on its forward RCS module, around the cockpit windows, and on its vertical stabilizer, and had distinctive black "chines" on the forward part of its upper wing surfaces, which none of the other orbiters had.
Challenger established a modified marking scheme for the shuttle fleet that was matched by Discovery, Atlantis and Endeavour. The letters "USA" in black above an American flag were displayed on the left wing, with the NASA "worm" logotype in gray centered above the name of the orbiter in black on the right wing. The name of the orbiter was inscribed not on the payload bay doors, but on the forward fuselage just below and behind the cockpit windows. This would make the name visible when the shuttle was photographed in orbit with the doors open.
In 1983, Enterprise had its wing markings changed to match Challenger, and the NASA "worm" logotype on the aft end of the payload bay doors was changed from gray to black. Some black markings were added to the nose, cockpit windows and vertical tail to more closely resemble the flight vehicles, but the name "Enterprise" remained on the payload bay doors as there was never any need to open them. Columbia had its name moved to the forward fuselage to match the other flight vehicles after STS-61-C, during the 1986–88 hiatus when the shuttle fleet was grounded following the loss of Challenger, but retained its original wing markings until its last overhaul (after STS-93), and its unique black wing "chines" for the remainder of its operational life.
Beginning in 1998, the flight vehicles' markings were modified to incorporate the NASA "meatball" insignia. The "worm" logotype, which the agency had phased out, was removed from the payload bay doors and the "meatball" insignia was added aft of the "United States" text on the lower aft fuselage. The "meatball" insignia was also displayed on the left wing, with the American flag above the orbiter's name, left-justified rather than centered, on the right wing. The three surviving flight vehicles, Discovery, Atlantis and Endeavour, still bear these markings as museum displays. Enterprise became the property of the Smithsonian Institution in 1985 and was no longer under NASA's control when these changes were made, hence the prototype orbiter still has its 1983 markings and still has its name on the payload bay doors.
Atlantis was the first Shuttle to fly with aglass cockpit, on STS-101.
The Space Shuttle was initially developed in the 1970s, but received many upgrades and modifications afterward to improve performance, reliability and safety. Internally, the Shuttle remained largely similar to the original design, with the exception of the improved avionics computers. In addition to the computer upgrades, the original analog primary flight instruments were replaced with modern full-color, flat-panel display screens, called a glass cockpit, which is similar to those of contemporary airliners. With the coming of the ISS, the orbiter's internal airlocks were 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) had several improvements to enhance reliability and power. This explains phrases such as "Main engines throttling up to 104 percent." This did not mean the engines were being run over a safe limit. The 100 percent figure was the original specified power level. During the lengthy development program, Rocketdyne determined the engine was capable of safe reliable operation at 104 percent of the originally specified thrust. NASA could have rescaled the output number, saying in essence 104 percent is now 100 percent. To clarify this would have required revising much previous documentation and software, so the 104 percent number was retained. SSME upgrades were denoted as "block numbers", such as block I, block II, and block IIA. The upgrades 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 was 104 percent, with 106 percent or 109 percent used for mission aborts.
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 resulted 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 aluminum-lithium alloy. It weighs 3.4 metric tons (7,500 lb) less than the last run of lightweight tanks. As the Shuttle was not flown unmanned, each of these improvements was "tested" on operational flights.
Several other SRB improvements were planned 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. The loss of the ASRB program resulted in the development of the Super LightWeight external Tank (SLWT), which provided 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 canceled.
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 had to be removed prior to launch. The delicate nature of the foam insulation had been the cause of damage to the Thermal Protection System, the tile heat shield and heat wrap of the orbiter. NASA remained confident that this damage, while it was the primary cause of the Space Shuttle Columbia disaster on February 1, 2003, would not jeopardize the completion of the International Space Station (ISS) in the projected time allotted.
A cargo-only, unmanned variant of the Shuttle was 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. Another proposal was to convert the payload bay into a passenger area, with versions ranging from 30 to 74 seats, three days in orbit, and cost US$1.5 million per seat.
On the first four Shuttle missions, astronauts wore modified US 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 Launch-Entry Suit ended its service life in late 1995, and was replaced by the full-pressure Advanced Crew Escape Suit(ACES), which resembled the Gemini space suit in design, but retained the orange color of the Launch-Entry Suit.
To extend the duration that orbiters could stay docked at the ISS, the Station-to-Shuttle Power Transfer System (SSPTS) was installed. The SSPTS allowed these orbiters to use power provided by the ISS to preserve their consumables. The SSPTS was first used successfully on STS-118.
- Length: 122.17 ft (37.237 m)
- Wingspan: 78.06 ft (23.79 m)
- Height: 56.58 ft (17.25 m)
- Empty weight: 172,000 lb (78,000 kg)
- Gross liftoff weight (Orbiter only): 240,000 lb (110,000 kg)
- Maximum landing weight: 230,000 lb (100,000 kg)
- Payload to Landing (Return Payload): 32,000 lb (14,400 kg)
- Maximum payload: 55,250 lb (25,060 kg)
- Payload to LEO: 53,600 lb (24,310 kg)
- Payload to LEO @ 51.6° inclination (ISS):
- Payload to GTO: 8,390 lb (3,806 kg)
- Payload to Polar Orbit: 28,000 lb (12,700 kg)
- (Note launch payloads modified by External Tank (ET) choice (ET, LWT, or SLWT)
- Payload bay dimensions: 15 by 59 ft (4.6 by 18 m)
- Operational altitude: 100 to 520 nmi (190 to 960 km; 120 to 600 mi)
- Speed: 7,743 m/s (27,870 km/h; 17,320 mph)
- Crossrange: 1,085 nmi (2,009 km; 1,249 mi)
- First Stage (SSME with external tank)
- Main engines: Three Rocketdyne Block II SSMEs, each with a sea level thrust of 393,800 lbf (1.752 MN) at 104% power
- Thrust (at liftoff, sea level, 104% power, all 3 engines): 1,181,400 lbf (5.255 MN)
- Specific impulse: 455 s
- Burn time: 480 s
- Fuel: Liquid Oxygen/Liquid Hydrogen
- Second Stage
- Engines: 2 Orbital Maneuvering Engines
- Thrust: 53.4 kN (12,000 lbf) combined total vacuum thrust
- Specific impulse: 316 s
- Burn time: 1250 s
- Fuel: MMH/N2O4
- Crew: Varies.
- Length: 46.9 m (154 ft)
- Diameter: 8.4 m (28 ft)
- Propellant volume: 2,025 m3 (534,900 US gal)
- Empty weight: 26,535 kg (58,500 lb)
- Gross liftoff weight (for tank): 756,000 kg (1,670,000 lb)
- Length: 45.46 m (149 ft)
- Diameter: 3.71 m (12.2 ft)
- Empty weight (per booster): 68,000 kg (150,000 lb)
- Gross liftoff weight (per booster): 571,000 kg (1,260,000 lb)
- Thrust (at liftoff, sea level, per booster): 12.5 MN (2,800,000 lbf)
- Specific impulse: 269 s
- Burn time: 124 s
- Height: 56 m (180 ft)
- Gross liftoff weight: 2,000,000 kg (4,400,000 lb)
- Total liftoff thrust: 30.16 MN (6,780,000 lbf)
All Space Shuttle missions were launched from Kennedy Space Center (KSC). The weather criteria used for launch included, but were not limited to: precipitation, temperatures, cloud cover, lightning forecast, wind, and humidity. The Shuttle was not launched under conditions where it could have been 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 was mainly constructed of conductive aluminum, which would normally shield and protect the internal systems. However, upon liftoff the Shuttle sent out a long exhaust plume as it ascended, and this plume could have triggered lightning by providing a current path to ground. The NASA Anvil Rule for a Shuttle launch stated that an anvil cloud could not appear within a distance of 10 nautical miles.The Shuttle Launch Weather Officer monitored conditions until the final decision to scrub a launch was announced. In addition, the weather conditions had to be acceptable at one of the Transatlantic Abort Landing sites (one of several Space Shuttle abort modes) to launch as well as the solid rocket booster recovery area. While the Shuttle might have safely endured a lightning strike, a similar strike caused problems on Apollo 12, so for safety NASA chose not to launch the Shuttle if lightning was possible
Historically, the Shuttle was not launched if its flight would run from December to January (a year-end rollover or YERO). Its flight software, designed in the 1970s, was not designed for this, and would require the orbiter's computers be reset through a change of year, which could cause a glitch while in orbit. In 2007, NASA engineers devised a solution so Shuttle flights could cross the year-end boundary.
On the day of a launch, after the final hold in the countdown at T-minus 9 minutes, the Shuttle went through its final preparations for launch, and the countdown was automatically controlled by the Ground Launch Sequencer (GLS), software at the Launch Control Center, which stopped the count if it sensed a critical problem with any of the Shuttle's onboard systems. The GLS handed off the count to the Shuttle's on-board computers at T minus 31 seconds, in a process called auto sequence start.
At T-minus 16 seconds, the massive sound suppression system (SPS) began to drench the Mobile Launcher Platform (MLP) and SRB trenches with 300,000 US gallons (1,100 m3) of water to protect the Orbiter from damage by acoustical energy and rocket exhaust reflected from the flame trench and MLP during lift off (NASA article).
At T-minus 10 seconds, hydrogen igniters were activated under each engine bell to quell the stagnant gas inside the cones before ignition. Failure to burn these gases could trip the onboard sensors and create the possibility of an overpressure and explosion of the vehicle during the firing phase. The main engine turbopumps also began charging the combustion chambers with liquid hydrogen and liquid oxygen at this time. The computers reciprocated this action by allowing the redundant computer systems to begin the firing phase.
Space Shuttle Main Engine ignition
The three main engines (SSMEs) started at T-minus 6.6 seconds. The main engines ignited sequentially via the Shuttle's general purpose computers (GPCs) at 120 millisecond intervals. The GPCs required that the engines reach 90 percent of their rated performance to complete the final gimbal of the main engine nozzles to liftoff configuration. When the SSMEs started, water from the sound suppression system flashed into a large volume of steam that shot southward. All three SSMEs had to reach the required 100 percent thrust within three seconds, otherwise the onboard computers would initiate an RSLS abort. If the onboard computers verified normal thrust buildup, at T minus 0 seconds, the 8 pyrotechnic nuts holding the vehicle to the pad were detonated and the SRBs were ignited. At this point the vehicle was committed to liftoff, as the SRBs could not be turned off once ignited. The plume from the solid rockets exited the flame trench in a northward direction at near the speed of sound, often causing a rippling of shockwaves along the actual flame and smoke contrails. At ignition, the GPCs mandated the firing sequences via the Master Events Controller, a computer program integrated with the Shuttle's four redundant computer systems. There were extensive emergency procedures (abort modes) to handle various failure scenarios during ascent. Many of these concerned SSME failures, since that was the most complex and highly stressed component. After the Challenger disaster, there were extensive upgrades to the abort modes.
After the main engines started, but while the solid rocket boosters were still bolted to the pad, the offset thrust from the Shuttle's three main engines caused the entire launch stack (boosters, tank and Shuttle) to pitch down about 2 m at cockpit level. This motion was called the "nod", or "twang" in NASA jargon. As the boosters flexed back into their original shape, the launch stack pitched slowly back upright. This took approximately six seconds. At the point when it was perfectly vertical, the boosters ignited and the launch commenced. The Johnson Space Center's Mission Control Center assumed control of the flight once the SRBs had cleared the launch tower.
Shortly after clearing the tower, the Shuttle began a combined roll, pitch and yaw maneuver that positioned the orbiter head down, with wings level and aligned with the launch pad. The Shuttle flew upside down during the ascent phase. This orientation allowed a trim angle of attack that was favorable for aerodynamic loads during the region of high dynamic pressure, resulting in a net positive load factor, as well as providing the flight crew with use of the ground as a visual reference. The vehicle climbed in a progressively flattening arc, accelerating as the weight of the SRBs and main tank decreased. To achieve low orbit requires much more horizontal than vertical acceleration. This was not visually obvious, since the vehicle rose vertically and was out of sight for most of the horizontal acceleration. The near circular orbital velocity at the 380 kilometers (236 mi) altitude of the International Space Station is 7.68 kilometers per second or 27,650 km/h (17,180 mph), roughly equivalent to Mach 23 at sea level. As the International Space Station orbits at an inclination of 51.6 degrees, missions going there must set orbital inclination to the same value in order to rendezvous with the station.
Around a point called Max Q, where the aerodynamic forces are at their maximum, the main engines were temporarily throttled back to 72 percent to avoid over-speeding and hence overstressing the Shuttle, particularly in vulnerable areas such as the wings. At this point, a phenomenon known as the Prandtl-Glauert singularity occurred, where condensation clouds formed during the vehicle's transition to supersonic speed.
A few seconds later, after the shuttle had gained more altitude and reached a region of lower atmospheric pressure, this dangerous point is passed. At T+70 seconds the main engines throttled up to their maximum cruise thrust of 104% rated thrust.
At T+126 seconds after launch, pyrotechnic fasteners released the SRBs and small separation rockets pushed them laterally away from the vehicle. The SRBs parachuted back to the ocean to be reused. The Shuttle then began accelerating to orbit on the main engines. The vehicle at that point in the flight had a thrust-to-weight ratio of less than one – the main engines actually had insufficient thrust to exceed the force of gravity, and the vertical speed given to it by the SRBs temporarily decreased. However, as the burn continued, the weight of the propellant decreased and the thrust-to-weight ratio exceeded 1 again and the ever-lighter vehicle then continued to accelerate towards orbit.
The vehicle continued to climb and take on a somewhat nose-up angle to the horizon – it used the main engines to gain and then maintain altitude while it accelerated horizontally towards orbit. At about five and three-quarter minutes into ascent, the orbiter's direct communication links with the ground began to fade, at which point it rolled heads up to reroute its communication links to the Tracking and Data Relay Satellite system.
Finally, in the last tens of seconds of the main engine burn, the mass of the vehicle was low enough that the engines had to be throttled back to limit vehicle acceleration to 3 g (29.34 m/s²), largely for astronaut comfort. At approximately eight minutes post launch, the main engines were shut down.
The main engines were shut down before complete depletion of propellant, as running dry would have destroyed the engines. The oxygen supply was terminated before the hydrogen supply, as the SSMEs reacted unfavorably to other shutdown modes. (Liquid oxygen has a tendency to react violently, and supports combustion when it encounters hot engine metal.) The external tank was released by firing pyrotechnic fasteners, largely burning up in the atmosphere, though some fragments fell into the ocean, in either the Indian Ocean or the Pacific Ocean depending on launch profile. The sealing action of the tank plumbing and lack of pressure relief systems on the external tank helped it break up in the lower atmosphere. After the foam burned away during re-entry, the heat caused a pressure buildup in the remaining liquid oxygen and hydrogen until the tank exploded. This ensured that any pieces that fell back to Earth were small.
To prevent the Shuttle from following the external tank back into the lower atmosphere, the Orbital maneuvering system (OMS) engines were fired to raise the perigee higher into the upper atmosphere. On some missions (e.g., missions to the ISS), the OMS engines were also used while the main engines were still firing. The reason for putting the orbiter on a path that brought it back to Earth was not just for external tank disposal but also one of safety: if the OMS malfunctioned, or the cargo bay doors could not open for some reason, the Shuttle was already on a path to return to earth for an emergency abort landing.
The Shuttle was monitored throughout its ascent for short range tracking (10 seconds before liftoff through 57 seconds after), medium range (7 seconds before liftoff through 110 seconds after) and long range (7 seconds before liftoff through 165 seconds after). Short range cameras included 22 16mm cameras on the Mobile Launch Platform and 8 16mm on the Fixed Service Structure, 4 high speed fixed cameras located on the perimeter of the launch complex plus an additional 42 fixed cameras with 16mm motion picture film. Medium range cameras included remotely operated tracking cameras at the launch complex plus 6 sites along the immediate coast north and south of the launch pad, each with 800mm lens and high speed cameras running 100 frames per second. These cameras ran for only 4–10 seconds due to limitations in the amount of film available. Long range cameras included those mounted on the External Tank, SRBs and orbiter itself which streamed live video back to the ground providing valuable information about any debris falling during ascent. Long range tracking cameras with 400-inch film and 200-inch video lenses were operated by a photographer at Playalinda Beach as well as 9 other sites from 38 miles north at the Ponce Inlet to 23 miles south to Patrick Air Force Base (PAFB) and additional mobile optical tracking camera was stationed on Merritt Island during launches. A total of 10 HD cameras were used both for ascent information for engineers and broadcast feeds to networks such as NASA TV and HDNet The number of cameras significantly increased and numerous existing cameras were upgraded at the recommendation of the Columbia Accident Investigation Board to provide better information about the debris during launch. Debris was also tracked using a pair of Weibel Continuous Pulse Doppler X-band radars, one on board the SRB recovery ship MV Liberty Star positioned north east of the launch pad and on a ship positioned south of the launch pad. Additionally, during the first 2 flights following the loss of Columbia and her crew, a pair of NASA WB-57 reconnaissance aircraft equipped with HD Video and Infrared flew at 60,000 feet (18,000 m) to provide additional views of the launch ascent. Kennedy Space Center also invested nearly $3 million in improvements to the digital video analysis systems in support of debris tracking.
Once in orbit, the Shuttle usually flew at an altitude of 200 miles (321.9 km), and occasionally as high as 400 miles. In the 1980s and 1990s, many flights involved space science missions on the NASA/ESA Spacelab, or launching various types of satellites and science probes. By the 1990s and 2000s the focus shifted more to servicing the space station, with fewer satellite launches. Most missions involved staying in orbit several days to two weeks, although longer missions were possible with the Extended Duration Orbiter add-on or when attached to a space station.
Re-entry and landing
Almost the entire Space Shuttle re-entry procedure, except for lowering the landing gear and deploying the air data probes, was normally performed under computer control. However, the re-entry could be flown entirely manually if an emergency arose. The approach and landing phase could be controlled by the autopilot, but was usually hand flown.
Glowing plasma trail from Shuttle re-entry as seen from the Space Station
The vehicle began re-entry by firing the Orbital maneuvering system engines, while flying upside down, backside first, in the opposite direction to orbital motion for approximately three minutes, which reduced the Shuttle's velocity by about 200 mph (322 km/h). The resultant slowing of the Shuttle lowered its orbital perigee down into the upper atmosphere. The Shuttle then flipped over, by pushing its nose down (which was actually "up" relative to the Earth, because it was flying upside down). This OMS firing was done roughly halfway around the globe from the landing site.
The vehicle started encountering more significant air density in the lower thermosphere at about 400,000 ft (120 km), at around Mach 25, 8,200 m/s (30,000 km/h; 18,000 mph). The vehicle was controlled by a combination of RCS thrusters and control surfaces, to fly at a 40-degree nose-up attitude, producing high drag, not only to slow it down to landing speed, but also to reduce reentry heating. As the vehicle encountered progressively denser air, it began a gradual transition from spacecraft to aircraft. In a straight line, its 40-degree nose-up attitude would cause the descent angle to flatten-out, or even rise. The vehicle therefore performed a series of four steep S-shaped banking turns, each lasting several minutes, at up to 70 degrees of bank, while still maintaining the 40-degree angle of attack. In this way it dissipated speed sideways rather than upwards. This occurred during the 'hottest' phase of re-entry, when the heat-shield glowed red and the G-forces were at their highest. By the end of the last turn, the transition to aircraft was almost complete. The vehicle leveled its wings, lowered its nose into a shallow dive and began its approach to the landing site.