The Space Shuttle was a partially reusable low Earth orbital spacecraft system that was operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program name was Space Transportation System (STS), taken from a 1969 plan for a system of reusable spacecraft of which it was the only item funded for development. The first of four orbital test flights occurred in 1981, leading to operational flights beginning in 1982. In addition to the prototype whose completion was cancelled, five complete Shuttle systems were built and used on a total of 135 missions from 1981 to 2011, launched from the Kennedy Space Center (KSC) in Florida. Operational missions launched numerous satellites, interplanetary probes, and the Hubble Space Telescope (HST); conducted science experiments in orbit; and participated in construction and servicing of the International Space Station. The Shuttle fleet’s total mission time was 1322 days, 19 hours, 21 minutes and 23 seconds.
The Space Shuttle was the first operational orbital spacecraft designed for reuse. Each Space Shuttle orbiter was designed for a projected lifespan of 100 launches or ten years of operational life, although this was later extended. At launch, it consisted of the orbiter, which contained the crew and payload, the external tank (ET), and the two solid rocket boosters (SRBs).
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 U.S. 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 Center managed the global tracking network.
Shuttle components include the Orbiter Vehicle (OV) with three clustered Rocketdyne RS-25 main engines, a pair of recoverable solid rocket boosters (SRBs), and the expendable external tank (ET) containing liquid hydrogen and liquid oxygen. The Space 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 ET. The SRBs were jettisoned before the vehicle reached orbit, and the ET was jettisoned just before orbit insertion, which used the orbiter’s two Orbital Maneuvering System (OMS) engines. At the conclusion of the mission, the orbiter fired its OMS to deorbit and reenter the atmosphere. The orbiter then glided as a spaceplane to a runway landing, usually to the Shuttle Landing Facility at Kennedy Space Center, Florida or Rogers Dry Lake in Edwards Air Force Base, California. After landing at Edwards, the orbiter was flown back to the KSC on the Shuttle Carrier Aircraft, a specially modified Boeing 747.
The first orbiter, Enterprise, was built in 1976, used in Approach and Landing Tests and has no orbital capability. Four fully operational orbiters were initially built: Columbia, Challenger, Discovery, and Atlantis. Of these, two were lost in mission accidents: Challenger in 1986 and Columbia in 2003, with a total of fourteen astronauts killed. A fifth operational (and sixth in total) orbiter, Endeavour, was built in 1991 to replace Challenger. The Space Shuttle was retired from service upon the conclusion of Atlantis‘s final flight on July 21, 2011. The U.S. has since relied on the Russian Soyuz spacecraft to transport astronauts to the International Space Station, pending the Commercial Crew Development and Space Launch System programs on schedule for first flights in 2019 and 2020.
Atlantis and Endeavour on launch pads. This particular occasion is due to the final Hubble servicing mission, where the International Space Station is unreachable, which necessitates having a Shuttle on standby for a possible rescue mission.
During the 1950s, the United States Air Force proposed using a reusable piloted glider to perform military operations such as reconnaissance, satellite attack, and employing air-to-ground weapons. In the late-1950s, the Air Force began developing the partially reusable X-20 Dyna-Soar. The Air Force collaborated with NASA on the Dyna-Soar, and began training 6 pilots in June 1961. The rising costs of development and the prioritization of Project Gemini led to the cancellation of the Dyna-Soar program in December 1963. In addition to the Dyna-Soar, the Air Force conducted a study in 1957 to test the feasibility of reusable boosters. This became the basis for the Aerospaceplane, a fully reusable spacecraft that was never developed beyond the initial design phase in 1962–1963.
Beginning in the early 1950s, NASA and the Air Force collaborated on developing lifting bodies to test aircraft that primarily generated lift from their fuselages instead of wings, and tested the M2-F1, M2-F2, M2-F3, HL-10, X-24A, and the X-24B. The program tested aerodynamic characteristics that would later be applied to the Space Shuttle, including unpowered landing from a high altitude and speed.
The X-24A, M2-F3, and HL-10 lifting bodies
After it arrived at Edwards AFB, Enterprise underwent flight testing with the Shuttle Carrier Aircraft, a Boeing 747 that was modified to carry the orbiter. In February 1977, Enterprise began the Approach and Landing Tests and underwent captive flights, where it remained attached to the Shuttle Carrier Aircraft for the duration of the flight. On August 12, 1977, Enterprise conducted its first glide test, where it detached from the Shuttle Carrier Aircraft and landed at Edwards AFB. After four additional flights, Enterprise was moved to the Marshall Space Flight Center on March 13, 1978. Enterprise underwent shake tests in the Mated Vertical Ground Vibration Test, where it was attached to an external tank and solid rocket boosters, and underwent vibrations to simulate the stresses of launch. In April 1979, Enterprise was taken to the Kennedy Space Center, where it was attached to an external tank and solid rocket boosters, and moved to LC-39. Once installed at the launch pad, the Space Shuttle was used to verify the proper positioning of launch complex hardware. Enterprise was taken back to California in August 1979, and later served in the development of the SLC-6 at Vandenberg AFB in 1984.
Enterprise during the Approach and Landing Tests
On November 26, 1980, Columbia was mated with its external tank and solid-rocket boosters, and was moved to LC-39 on December 29, 1980. The first Space Shuttle mission, STS-1, would be the first time NASA performed a crewed first-flight of a spacecraft. On April 12, 1981, the Space Shuttle launched for the first time, and was piloted by John Young and Robert Crippen. During the two-day mission, Young and Crippen tested equipment on board the shuttle, and found several of the ceramic tiles had fallen off the top side of the Columbia. NASA coordinated with the Air Force to use satellites to image the underside of Columbia, and determined there was no damage. Columbia reentered the atmosphere on April 14, and landed at Edwards AFB.
NASA conducted three additional test flights with Columbia in 1981 and 1982. On July 4, 1982, STS-4, flown by Ken Mattingly and Henry Hartsfield, landed at Edwards AFB. President Ronald Reagan and his wife Nancy met the crew, and delivered a speech. After STS-4, NASA declared the Space Shuttle operational.
Six orbiters were built for flight: Enterprise, Columbia, Challenger, Discovery, Atlantis, and Endeavour. All were built in Palmdale, California, by the Pittsburgh, Pennsylvania-based Rockwell International company. The first orbiter, Enterprise, made its maiden flight in 1977. An unpowered glider, it was carried by a modified Boeing 747 airliner called the Shuttle Carrier Aircraft and released for a series of atmospheric test flights and landings. Enterprise was partially disassembled and retired after completion of critical testing. The remaining orbiters were fully operational spacecraft, and were launched vertically as part of the Space Shuttle stack.
Attitude control system
The Reaction Control System (RCS) was composed of 44 small liquid-fueled rocket thrusters and their very sophisticated fly-by-wire flight control system, which utilized computationally intensive digital Kalman filtering. This control system carried out the usual attitude control along the pitch, roll, and yaw axes during all of the flight phases of launching, orbiting, and re-entry. This system also executed any needed orbital maneuvers, including all changes in the orbit’s altitude, orbital plane, and eccentricity. These were all operations that required a lot more power and energy than mere attitude control.
Space Shuttle forward reaction control thrusters
The forward rockets of the Reaction Control System, located near the nose of the Space Shuttle orbiter, included 14 primary and two vernier RCS rockets. The aft RCS engines were located in the two Orbital Maneuvering System (OMS) pods at the rear of the orbiter, and these included 12 primary (PRCS) and two vernier (VRCS) engines in each pod. The PRCS system provided the pointing control of the Orbiter, and the VRCS was used for fine maneuvering during the rendezvous, docking, and undocking maneuvers with the International Space Station, or formerly with the Russian Mir space station. The RCS also controlled the attitude of the orbiter during most of its re-entry into the Earth’s atmosphere – until the air became dense enough that the rudder, elevons and body flap became effective. During the early design process of the orbiter, the forward RCS thrusters were to be hidden underneath retractable doors, which would open once the orbiter reached space. These were omitted in favor of flush-mounted thrusters for fear that the RCS doors would remain stuck open and endanger the crew and orbiter during re-entry.
The galley, toilet, sleep locations, storage lockers, and the side hatch for entering and exiting the orbiter were also located on the mid-deck, as well as the airlock. The airlock had an additional hatch into the payload bay. This airlock allowed two or three astronauts, wearing their Extravehicular Mobility Unit (EMU) space suits, to depressurize before a walk in space (EVA), and also to repressurize and re-enter the orbiter at the conclusion of the EVA.
The utility area was located under the floor of the mid-deck and contained air and water tanks in addition to the carbon dioxide scrubbing system.
Three Space Shuttle Main Engines (SSMEs) were mounted on the orbiter’s aft fuselage in the pattern of an equilateral triangle. These three liquid-fueled engines could be swiveled 10.5 degrees vertically and 8.5 degrees horizontally during the rocket-powered ascent of the orbiter in order to change the direction of their thrust. Hence, they steered the entire Space Shuttle, as well as providing rocket thrust towards orbit. The aft fuselage also housed three auxiliary power units (APU). The APUs chemically converted hydrazine fuel from a liquid state to a gas state, powering a hydraulic pump which supplied pressure for all of the hydraulic system, including the hydraulic sub-system that pointed the three main liquid-fueled rocket engines, under computerized flight control. The hydraulic pressure generated was also used to control all of the orbiter’s “flight control surfaces” (the elevons, rudder, speed brake, etc.), to deploy the landing gear of the orbiter, and to retract the umbilical hose connection doors located near the rear landing gear, which supplied the orbiter’s SSMEs with liquid hydrogen and oxygen from the external tank.
Atlantis‘s main engines during launch
Two Orbital Maneuvering System (OMS) thrusters were mounted in two separate removable pods on the orbiter’s aft fuselage, located between the SSMEs and the vertical stabilizer. The OMS engines provided significant thrust for coarse orbital maneuvers, including insertion, circularization, transfer, rendezvous, deorbit, abort to orbit, and to abort once around. At lift-off, two solid rocket boosters (SRBs) were used to take the vehicle to an altitude of roughly 140,000 feet.
The orbiters were protected by Thermal Protection System (TPS) materials (developed by Rockwell Space Systems) inside and out, from the orbiter’s outer surface to the payload bay. The TPS protected it from the cold soak of −121 °C (−186 °F) in space to the 1,649 °C (3,000 °F) heat of re-entry.
Discovery‘s ventral thermal protection system
The orbiter’s structure was made primarily from aluminium alloy, although the engine thrust structure was made from titanium alloy. The later orbiters (Discovery, Atlantis and Endeavour) substituted graphite epoxy for aluminum in some structural elements in order to reduce weight. The windows were made of aluminum silicate glass and fused silica glass, and comprised an internal pressure pane, a 1.3-inch-thick (33 mm) optical pane, and an external thermal pane. The windows were tinted with the same ink used to make American banknotes.
Discovery approaches the ISS on STS-121
Space Shuttle Main Engines
Three Space Shuttle Main Engines (SSMEs) were mounted on the orbiter’s aft fuselage in a triangular pattern. The engine nozzles could gimbal 10.5 degrees up and down, and 8.5 degrees from side to side during ascent to change the direction of their thrust to steer the Shuttle. The orbiter structure was made primarily from aluminum alloy, although the engine structure was made primarily from titanium alloy.
The Space Shuttle Main Engines (SSMEs) had several improvements to enhance reliability and power. During the development program, Rocketdyne determined that the engine was capable of safe reliable operation at 104% of the originally specified thrust. To keep the engine thrust values consistent with previous documentation and software, NASA kept the original specified thrust as 100%, but had the SSME operate at higher thrust. SSME upgrade versions were denoted as block I and block II. 109% thrust level was achieved with the Block II engines in 2001. The normal maximum throttle was 104 percent, with 106% or 109% used for mission aborts.
The Space Shuttle Main Engine with the two Orbital Maneuvering System (OMS) pods.
The main function of the Space Shuttle external tank was to supply the liquid oxygen and hydrogen fuel to the main engines. It was also the backbone of the launch vehicle, providing attachment points for the two solid rocket boosters and the orbiter. The external tank was the only part of the Shuttle system that was not reused. Although the external tanks were always discarded, it would have been possible to take them into orbit and re-use them (such as a wet workshop for incorporation into a space station).
The external tank after separation on STS-29.
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” was first flown on STS-6 and used on the majority of Shuttle missions. STS-91 saw the first flight of the “super light-weight external tank”. This version of the tank was made of the 2195 aluminum-lithium alloy. It weighed 3.4 metric tons (7,500 lb) less than the last run of lightweight tanks, allowing the Shuttle to deliver heavy elements to ISS’s high inclination orbit.
Interior of an External Tank
Solid rocket boosters
At launch, the external tank was connected to two solid rocket boosters (SRBs) that provided over 70% of the Space Shuttle’s thrust. They were the largest solid rocket motors ever flown, and the first solid rocket motors used on a crewed spacecraft. Each SRBs was 45 m (149 ft) tall and 3.7 m (12 ft) wide, weighed 68,000 kg (150,000 lb), and had a steel exterior approximately 13 mm (.5 in) thick. The SRB contained four sections that comprised the solid rocket motor, a nose cone, and the rocket nozzle. The four sections of the rocket motor were filled with a total 1.25 million pounds of solid rocket propellant, and joined together at the Vehicle Assembly Building at the Kennedy Space Center. For the first two minutes of flight, they each provided 13,300 kN (3,000,000 lbf) of thrust. After expending their fuel, the SRBs were jettisoned two minutes after launch at an altitude of approximately 46 km (150,000 ft). Following separation, they deployed parachutes, landed in the ocean, and were recovered by the crews aboard the ships MV Freedom Star and MV Liberty Star.
Two SRBs on the crawler prior to mating with the Shuttle
The SRB cases were made of steel about ½ inch (13 mm) thick. The solid rocket boosters were re-used many times; the casing used in Ares I engine testing in 2009 consisted of motor cases that had been flown, collectively, on 48 Shuttle missions, including STS-1. Astronauts who have flown on multiple spacecraft report that Shuttle delivers a rougher ride than Apollo or Soyuz. The additional vibration is caused by the solid rocket boosters, as solid fuel does not burn as evenly as liquid fuel. The vibration dampens down after the solid rocket boosters have been jettisoned. The solid rocket boosters underwent improvements as well. Design engineers added a third O-ring seal to the joints between the segments after the 1986 Space Shuttle Challenger disaster. 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 U.S. Air Force developed their own much lighter single-piece SRB design using a filament-wound system, but this too was canceled.
- 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 204 kilometers (110 nmi) @ 28.5° inclination: 27,500 kilograms (60,600 lb)
- Payload to LEO (407 kilometers (220 nmi) @ 51.6° to ISS): 16,050 kilograms (35,380 lb)
- 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) (diameter by length)
- 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)
- Main Stage (SSME with external tank)
- Engines: Three Rocketdyne Block II SSMEs, each with a sea level thrust of 393,800 lbf (1,752 kN) at 104% power
- Thrust (at liftoff, sea level, 104% power, all 3 engines): 1,181,400 lbf (5,255 kN)
- Specific impulse: 455 seconds (4.46 km/s)
- Burn time: 480 s
- Fuel: Liquid Hydrogen/Liquid Oxygen
- Orbital Maneuvering System
- Engines: 2 OMS Engines
- Thrust: 53.4 kN (12,000 lbf) combined total vacuum thrust
- Specific impulse: 316 seconds (3.10 km/s)
- Burn time: 150–250 s typical burn; 1250 s deorbit burn
- Fuel: MMH/N2O4
- Crew: Varies
- The earliest Shuttle flights had the minimum crew of two; later missions used a crew of five. By program end, typically seven people would fly: (commander, pilot, several mission specialists, one of whom (MS-2) acted as the flight engineer starting with STS-9 in 1983). On two occasions, eight astronauts have flown (STS-61-A, STS-71). Eleven people could be accommodated in an emergency mission (see STS-3xx).
- Length: 46.9 m (153.8 ft)
- Diameter: 8.4 m (27.6 ft)
- Propellant volume: 2,025 m3 (534,900 U.S. gal)
- Empty weight: 26,535 kg (58,500 lb)
- Gross liftoff weight (for tank): 756,000 kg (1,670,000 lb)
Solid Rocket Boosters
- Length: 45.46 m (149 ft)
- Diameter: 3.71 m (12.2 ft)
- Empty weight (each): 68,000 kg (150,000 lb)
- Gross liftoff weight (each): 571,000 kg (1,260,000 lb)
- Thrust (at liftoff, sea level, each): 12,500 kN (2,800,000 lbf)
- Specific impulse: 269 seconds (2.64 km/s)
- Burn time: 124 s
- Height: 56 m (180 ft)
- Gross liftoff weight: 2,000,000 kg (4,400,000 lb)
- Total liftoff thrust: 30,160 kN (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 (19 km). 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 (NPR8715.5).
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.
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 (1,100,000 L) of water to protect the Orbiter from damage by acoustical energy and rocket exhaust reflected from the flame trench and MLP during lift off. 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.The three main engines (SSMEs) started at T-6.6 seconds. The main engines ignited sequentially via the Shuttle’s general purpose computers (GPCs) at 120 millisecond intervals. All three SSMEs were required to reach 90% rated thrust within three seconds, otherwise the onboard computers would initiate an RSLS abort. If all three engines indicated nominal performance by T-3 seconds, they were commanded to gimbal to liftoff configuration and the command would be issued to arm the SRBs for ignition at T-0. Between T-6.6 seconds and T-3 seconds, while the SSMEs were firing but the SRBs were still bolted to the pad, the offset thrust caused the entire launch stack (boosters, tank and orbiter) to pitch down 650 mm (25.5 in) measured at the tip of the external tank. The three second delay after confirmation of SSME operation was to allow the stack to return to nearly vertical. At T-0 seconds, the 8 frangible nuts holding the SRBs to the pad were detonated, the SSMEs were commanded to 100% throttle, and the SRBs were ignited. By T+0.23 seconds, the SRBs built up enough thrust for liftoff to commence, and reached maximum chamber pressure by T+0.6 seconds. The Johnson Space Center’s Mission Control Center assumed control of the flight once the SRBs had cleared the launch tower.
STS-98 launch in February 2001
Shortly after liftoff, the Shuttle’s main engines were throttled up to 104.5% and the vehicle began a combined roll, pitch and yaw maneuver that placed it onto the correct heading (azimuth) for the planned orbital inclination and in a heads down attitude with wings level. 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 a view of the horizon as a visual reference. The vehicle climbed in a progressively flattening arc, accelerating as the mass 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 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 30 seconds into ascent, the SSMEs were throttled down—usually to 72%, though this varied—to reduce the maximum aerodynamic forces acting on the Shuttle at a point called Max Q. Additionally, the propellant grain design of the SRBs caused their thrust to drop by about 30% by 50 seconds into ascent. Once the Orbiter’s guidance verified that Max Q would be within Shuttle structural limits, the main engines were throttled back up to 104.5%; this throttling down and back up was called the “thrust bucket”. To maximize performance, the throttle level and timing of the thrust bucket was shaped to bring the Shuttle as close to aerodynamic limits as possible.
At around T+126 seconds, 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. Acceleration at this point would typically fall to 0.9 g (8.829 m/s2, 28.97 ft/s2), and the vehicle would 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.
Solid Rocket Booster (SRB) separation during STS-1. The white external tank pictured was used on STS-1 and STS-2.
At about seven and a half minutes into ascent, the mass of the vehicle was low enough that the engines had to be throttled back to limit vehicle acceleration to 3 g (29.43 m/s2, 96.6 ft/s2). The Shuttle would maintain this acceleration for the next minute, and MECO occurred at about eight and a half minutes after launch. 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.) A few seconds after MECO, the external tank was released by firing its two frangible nuts. At this point the Shuttle and external tank were on a slightly suborbital trajectory, coasting up towards apogee. Once at apogee, about half an hour after MECO, the Shuttle’s Orbital Maneuvering System (OMS) engines were fired to raise its perigee and achieve orbit, while the external tank fell back into the atmosphere and burned up over 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.
Once in orbit, the Shuttle usually flew at an altitude of 320 km (170 nmi), although the STS-82 mission reached 620 km (330 nmi). In the 1980s and 1990s, many flights involved space science missions on the 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. STS-80 was the longest at almost 17 days and 16 hours.
Endeavour docked at ISS
Re-entry and landing
Simulation of the outside of the Shuttle as it heats up to over 1,500 °C (2,730 °F) during re-entry.
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. 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. 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.
Flight deck view of the Space Shuttle Discovery during STS-42 re-entry
The vehicle started encountering more significant air density in the lower thermosphere at about 400,000 ft (120 km), at around Mach 25 (30,626 km/h; 19,030 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 accomplished aerobraking with 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. The orbiter’s maximum glide ratio/lift-to-drag ratio varied considerably with speed, ranging from 1:1 at hypersonic speeds, 2:1 at supersonic speeds and reaching 4.5:1 at subsonic speeds during approach and landing In the lower atmosphere, the orbiter flew much like a conventional glider, except for a much higher descent rate, over 9,800 ft/min (50 m/s). At approximately Mach 3 (3,675 km/h; 2,284 mph), two air data probes, located on the left and right sides of the orbiter’s forward lower fuselage, were deployed to sense air pressure related to the vehicle’s movement in the atmosphere.
Final approach and landing phase
When the approach and landing phase began, the orbiter was at a 3,000 m (9,800 ft) altitude, 12 km (7.5 mi) from the runway. The pilots applied aerodynamic braking to help slow down the vehicle. The orbiter’s speed was reduced from 682 to 346 km/h (424 to 215 mph), approximately, at touch-down (compared to 260 km/h or 160 mph for a jet airliner). The landing gear was deployed while the Orbiter was flying at 430 km/h (270 mph). To assist the speed brakes, a 12 m (39 ft) drag chute was deployed either after main gear or nose gear touchdown (depending on selected chute deploy mode) at about 343 km/h (213 mph). The chute was jettisoned once the orbiter slowed to 110 km/h (68.4 mph).
Atlantis landing at KSC after STS-122.
Discovery deploying its brake parachute after landing on STS-124.
The primary Space Shuttle landing site was the Shuttle Landing Facility to be at Kennedy Space Center. In the event of unfavorable landing conditions, the Shuttle could delay its landing or land at an alternate location. The primary alternate was Edwards AFB, which was used for over 50 shuttle landings. Space Shuttle Columbia landed at the White Sands Space Harbor after STS-3, and required extensive post-processing after exposure to the gypsum-rich sand. Landings at alternate airfields required the Shuttle Carrier Aircraft to transport the orbiter back to Cape Canaveral. In addition to the pre-planned landing airfields, there were emergency landing sites to be used in different abort scenarios. Facilities on the east coast of the US were planned for East Coast Abort Landings, while several sites in Europe and Africa were planned in the event of a Transoceanic Abort Landing. The facilities were prepared with equipment and personnel in the event of an emergency shuttle landing, but were never used.
After landing, the vehicle stayed on the runway for several hours for the orbiter to cool. Teams at the front and rear of the orbiter tested for presence of hydrogen, hydrazine, monomethylhydrazine, nitrogen tetroxide and ammonia (fuels and by-products of the reaction control system and the orbiter’s three APUs). If hydrogen was detected, an emergency would be declared, the orbiter powered down and teams would evacuate the area. A convoy of 25 specially designed vehicles and 150 trained engineers and technicians approached the orbiter. Purge and vent lines were attached to remove toxic gases from fuel lines and the cargo bay about 45–60 minutes after landing. A flight surgeon boarded the orbiter for initial medical checks of the crew before disembarking. Once the crew left the orbiter, responsibility for the vehicle was handed from the Johnson Space Center back to the Kennedy Space Center.
Discovery after landing on Earth for crew disembarkment
If the mission ended at Edwards Air Force Base in California, White Sands Space Harbor in New Mexico, or any of the runways the orbiter might use in an emergency, the orbiter was loaded atop the Shuttle Carrier Aircraft, a modified 747, for transport back to the Kennedy Space Center, landing at the Shuttle Landing Facility. Once at the Shuttle Landing Facility, the orbiter was then towed 2 miles (3.2 km) along a tow-way and access roads normally used by tour buses and KSC employees to the Orbiter Processing Facility where it began a months-long preparation process for the next mission.