Ameer Randolph
The amount of fuel used by a space shuttle depends on the variant of the external tank (ET) used. The ET is the component of the space shuttle launch vehicle that contains the fuel and oxidizer. There are three variants of the ET, with progressively reducing tank weight, and the Super Lightweight Tank (SLWT) is the most advanced. The ET supplies the fuel and oxidizer to the three RS-25 main engines in the orbiter, and it is the largest and heaviest element of the space shuttle when loaded. The two solid rocket boosters (SRBs) use a solid propellant cake of Ammonium Perchlorate Composite Propellant (APCP), which provides 124 seconds of burn time. The external tank capacity is 629,340 kg (1,387,457 lb) of liquid oxygen (LOx) and 106,261 kg (234,265 lb) of liquid hydrogen (LH2) as fuel components, providing 480 seconds of burn time.
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What You'll Learn
The external tank (ET)
The Space Shuttle external tank (ET) was an integral component of the Space Shuttle launch vehicle. It was the largest and heaviest element of the Space Shuttle when loaded. The ET provided the liquid hydrogen fuel and liquid oxygen oxidizer to the three RS-25 main engines during lift-off and ascent. The ET was jettisoned just over ten seconds after the main engine cut-off and re-entered the Earth's atmosphere.
The external tank was not reused and broke up before impact in either the Indian Ocean or the Pacific Ocean, away from shipping lanes. The ET's orange colour is due to the spray-on foam insulation, which was left unpainted from STS-3 onwards to reduce weight. Over the years, NASA worked to further reduce the weight of the ET, resulting in an almost equal increase in the cargo-carrying capability of the Space Shuttle.
The ET had three major components: the forward liquid oxygen (LOX) tank, the aft liquid hydrogen (LH2) tank, and the interconnecting structure that joined the two tanks. The LH2 tank was the largest part but was relatively light due to liquid hydrogen's low density. The ET also had two electrical umbilicals that carried electrical power from the orbiter to the tank and the two SRBs, as well as providing information from the SRBs and ET to the orbiter.
The ET was also the "backbone" of the shuttle during launch, providing structural support for attachment to the Space Shuttle Solid Rocket Boosters (SRBs) and the orbiter. It was connected to each SRB and the orbiter through various attachment points and bipods.
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Liquid hydrogen fuel
The Space Shuttle's external tank (ET) was the component of the launch vehicle that contained the liquid hydrogen fuel and liquid oxygen oxidizer. The ET acted as the "backbone" of the space shuttle, supplying the fuel and oxidizer to the orbiter's three RS-25 main engines during lift-off and ascent. The ET was the largest and heaviest element of the Space Shuttle when loaded. The tank consisted of three major components: the aft liquid hydrogen (LH2) tank, which was the largest part; the liquid oxygen (LOX) tank; and the forward structure.
Preparing liquid hydrogen for use as a rocket propellant presented several challenges. Firstly, oxygen and hydrogen can only be liquefied at extremely low temperatures, requiring the use of cooling systems that take up space and add weight. Secondly, liquid hydrogen must be insulated from other heat sources, such as rocket engine exhaust and air friction during flight, to prevent evaporation. Finally, liquid hydrogen expands rapidly when it absorbs heat, so venting is necessary to prevent tank explosions.
The external tank's liquid hydrogen fuel and liquid oxygen oxidizer were consumed in a specific ratio of 6:1. To ensure a fuel-rich cutoff, an additional 1,100 lb (500 kg) of liquid hydrogen was loaded. This was crucial to prevent oxidizer-rich engine shutdowns, which could lead to severe erosion of engine components and potential loss of the vehicle and crew. The tanks were equipped with propellant-depletion sensors to monitor fuel and oxidizer levels and enable timely engine shutdowns.
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Liquid oxygen oxidizer
The Space Shuttle's external tank (ET) was the component that contained the liquid hydrogen fuel and liquid oxygen oxidizer. The ET supplied the fuel and oxidizer under pressure to the three RS-25 main engines in the orbiter during lift-off and ascent. The ET was the largest and heaviest element of the Space Shuttle when loaded.
The liquid oxygen oxidizer played a critical role in the Space Shuttle's propulsion system. It reacted with the liquid hydrogen fuel in the RS-25 engines to produce thrust, enabling the shuttle to overcome Earth's gravity and reach orbit. The oxidizer-fuel mixture ratio was carefully maintained at 6:1 to ensure efficient combustion and engine performance.
The liquid oxygen oxidizer was stored in a separate tank within the ET, known as the oxygen tank. This tank had a vent to release oxygen vapour and prevent ice accumulation during the countdown. The oxygen tank also featured liquid oxygen sensors and depletion sensors to monitor the oxidizer levels and ensure the engines shut down before the oxidizer pumps ran dry.
The oxidizer sensors were strategically located in the orbiter liquid oxygen feed line manifold, downstream of the feed line disconnect. This positioning allowed for the maximum consumption of oxidizer in the engines while providing sufficient time for a safe shutdown. The fuel depletion sensors were located at the bottom of the fuel tank.
The Space Shuttle's propulsion system also included a helium system, used to purge the engines during reentry and provide pressure for actuating engine valves within the propellant management system. Additionally, the main oxidizer and fuel bleed valves were used to dump any residual propellant after engine shutdown, with residual liquid oxygen venting through the engine.
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Solid Rocket Boosters (SRB)
Solid Rocket Boosters (SRBs) are a key component of the Space Shuttle. Each shuttle uses a matched pair of SRBs, which provide 71.4% to 85% of the thrust required to lift the shuttle off the launchpad. Each SRB can provide a maximum of 14.7 MN (3,300,000 lbf) of thrust. The propellant for each solid rocket motor weighs approximately 500,000 kg (1.1 million lb) to 1,100,000 lb. SRBs are the first solid-propellant rockets to be used for primary propulsion in human spaceflight.
The SRBs are jettisoned from the shuttle at an altitude of about 146,000 ft (45 km). After burnout, they are ejected and parachuted into the Atlantic Ocean, where they are recovered, examined, refurbished, and reused. SRBs are cheaper to design, test, and produce than liquid propellant boosters, and their reusability decreases hardware costs.
The SRB assembly process is complex and involves several components. United Space Boosters Inc. (USBI) was the original SRB prime contractor for SRB assembly, checkout, and refurbishment. SRBs are transported from Utah to the Kennedy Space Center in Florida via rail, covering 2,000 miles (3,200 km) and eight states. Each segment and its custom-built rail car weigh approximately 300,000 lb (140,000 kg).
SRB failure rates have been estimated to range from 1 in 1,000 to 1 in 100,000. Nozzle blocking, deformation, and defects in the booster's casing can lead to overpressure or a reduction in thrust, and even cause the assembly to break apart. The failure of an O-ring seal on the Challenger space shuttle's right SRB caused the shuttle to disintegrate shortly after liftoff. Solid rocket motors also present a handling risk on the ground, as they can accidentally ignite.
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Propellant weight reduction
The Space Shuttle used solid-propellant boosters because of their lower costs and ease of refurbishment for reuse. The Solid Rocket Boosters (SRBs) provided 71.4% of the Space Shuttle's thrust during liftoff and ascent, and were the largest solid-propellant motors ever flown. Each SRB was 45 metres tall and 3.7 metres wide, weighing 68,000 kg.
The two SRBs used roughly 500,000 kg of an 11-star perforated solid propellant cake of Ammonium Perchlorate Composite Propellant (APCP) each, providing 124 seconds of burn time.
In 1998, a super lightweight external tank (SLWT) was first flown on STS-91. The SLWT used 2195 aluminium-lithium alloy, which was 40% stronger and 10% less dense than its predecessor, 2219 aluminium-lithium alloy. The SLWT weighed 3,400 kg less than the LWT, which allowed the Space Shuttle to deliver heavy elements to the ISS's high inclination orbit.
The weight reduction in the SLWT was achieved by eliminating portions of stringers (structural stiffeners running the length of the hydrogen tank), using fewer stiffener rings, and modifying major frames in the hydrogen tank. The SLWT provided 50% of the performance increase required for the shuttle to reach the International Space Station.
The use of lightweight materials and propellant weight reduction allowed the Space Shuttle to carry more payload and reach higher orbits.
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Frequently asked questions
A space shuttle uses a mixture of liquid hydrogen and oxygen as fuel. The amount of fuel used depends on the variant of the external tank, but the Super Lightweight Tank (SLWT) can carry 629,340 kg of liquid oxygen and 106,261 kg of liquid hydrogen.
The fuel economy of a space shuttle is about 9.5 miles per gallon. This is based on an average shuttle mission going 5 million miles and using a medium external tank with 500,000 gallons of fuel.
Space shuttles use liquid hydrogen and liquid oxygen as fuel.
The fuel in a space shuttle is mostly used up during the first 15 minutes of a mission.
The SRBs are solid-propellant motors that provide 85% of the shuttle's thrust during liftoff and ascent. Each SRB is about 45 meters long and 12 meters in diameter, weighing approximately 590 tonnes at launch.
