Katerina Swanson
The Space Shuttle external tank (ET) is an essential component of the Space Shuttle launch vehicle, housing the liquid hydrogen fuel and liquid oxygen oxidizer required for the vehicle's operation. During lift-off, the ET supplies fuel and oxidizer to the RS-25 main engines, ensuring a fuel-rich environment to prevent engine erosion. The ET is the largest and heaviest element of the Space Shuttle, and while it is not reused, its design and utilization have evolved over time. With its intricate systems and crucial role in providing propulsion, the question arises: does the external fuel tank itself have an engine?
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What You'll Learn
Space Shuttle external tank (ET)
The Space Shuttle external tank (ET) was the component of the Space Shuttle launch vehicle that contained the liquid hydrogen fuel and liquid oxygen oxidizer. It 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 "backbone" of the shuttle during launch, providing structural support for attachment with the Space Shuttle Solid Rocket Boosters (SRBs) and the orbiter. It was connected to each SRB at one forward attachment point and one aft bracket, and to the orbiter at one forward attachment bipod and two aft bipods. The ET also had two electrical umbilicals that carried electrical power from the orbiter to the tank and the SRBs, and provided information from the SRBs and ET to the orbiter.
The ET was jettisoned just over 10 seconds after main engine cut-off (MECO) and it re-entered the Earth's atmosphere. Unlike the Solid Rocket Boosters, external tanks were not re-used. They broke up before impact in the Indian Ocean (or the Pacific Ocean in the case of direct-insertion launch trajectories), away from shipping lanes, and were not recovered. The ET was the largest element of the Space Shuttle, and when loaded, it was also the heaviest. It consisted of three major components: the forward liquid oxygen (LO2) tank, the aft liquid hydrogen (LH2) tank, and the intertank that separated the two tanks and maintained their structural attachment. The liquid hydrogen tank was 2.5 times larger than the liquid oxygen tank, but the filled hydrogen tank weighed only a fifth of what the filled oxygen tank did due to liquid hydrogen's very low density.
There were eight propellant-depletion sensors, four each for fuel and oxidizer. The fuel-depletion sensors were located at the bottom of the fuel tank, while the oxidizer sensors were mounted in the orbiter liquid oxygen feed line manifold downstream of the feed line disconnect. During RS-25 thrusting, the orbiter general-purpose computers constantly computed the instantaneous mass of the vehicle due to the usage of the propellants. Normally, main engine cutoff is based on a predetermined velocity; however, if any two of the fuel or oxidizer sensors sense a dry condition, the engines will be shut down to prevent severe erosion of engine components, which could potentially lead to loss of the vehicle and crew.
Over the years, NASA worked to reduce the weight of the ET to increase overall efficiency. The weight reduction resulted in an almost equal increase in the cargo-carrying capability of the Space Shuttle. Three different types of external tanks were built over the course of the 30-year Space Shuttle Program: standard-weight tanks (SWTs), lightweight tanks (LWTs), and super lightweight tanks (SLWTs). All three types looked the same on the outside, but differed internally in construction and materials. The SLWT provided 50% of the performance increase required for the shuttle to reach the International Space Station.
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Fuel and oxidizer sensors
The Space Shuttle external tank (ET) is the component of the Space Shuttle launch vehicle that contains the liquid hydrogen fuel and liquid oxygen oxidizer. During lift-off and ascent, it supplies the fuel and oxidizer under pressure to the three RS-25 main engines in the orbiter. The ET is jettisoned just over 10 seconds after the main engine cut-off (MECO) and re-enters the Earth's atmosphere. Unlike Solid Rocket Boosters, external tanks are not reused.
The Space Shuttle external tank is equipped with eight propellant-depletion sensors, four each for fuel and oxidizer. The fuel-depletion sensors are located at the bottom of the fuel tank, while the oxidizer sensors are mounted in the orbiter liquid oxygen feed line manifold downstream of the feed line disconnect. These sensors play a critical role in ensuring the safe operation of the engines.
The fuel sensors or fuel level indicators are ingenious devices installed inside the fuel tank to provide an accurate, real-time measurement of the fuel level. This data is crucial for efficient fuel management and can be shared with telematics software for analysis and presentation to users. In the context of the Space Shuttle external tank, the fuel-depletion sensors help ensure that the engines receive an adequate supply of fuel during the mission.
The oxidizer sensors are equally important. During RS-25 thrusting, the orbiter's general-purpose computers constantly compute the instantaneous mass of the vehicle due to the usage of propellants. Typically, the main engine cutoff is based on a predetermined velocity. However, if any two of the fuel or oxidizer sensors sense a dry condition, the engines will be shut down immediately to prevent engine damage. The strategic placement of the liquid oxygen sensors allows for the maximum consumption of the oxidizer in the engines while providing sufficient time to shut down the engines before the oxidizer pumps run dry.
Additionally, the oxidizer sensors help prevent engine shutdowns that are oxidizer-rich, as this can lead to burning and severe erosion of engine components, potentially resulting in the loss of the vehicle and crew. The sensors contribute to the overall safety and success of the mission by ensuring the efficient and safe utilization of both fuel and oxidizer.
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ET's orange colour
The Space Shuttle external tank (ET) is the component that contained the liquid hydrogen fuel and liquid oxygen oxidizer. It supplied the fuel and oxidizer under pressure to the three RS-25 main engines in the orbiter. The ET was jettisoned just over 10 seconds after the main engine cut off and re-entered the Earth's atmosphere.
The orange color of the ET is due to the spray-on foam insulation. The first two tanks, used for STS-1 and STS-2, were painted white to protect the tanks from ultraviolet light during the extended time that the shuttle spent on the launch pad before launch. However, NASA engineer Farouk Huneidi informed the agency that the paint did not actually offer protection.
The color orange is a secondary color that results from mixing red and yellow in equal proportions. It has many variations depending on the proportions of the two primary colors, and its shade, tint, and tone can be adjusted with the addition of white and/or black. Orange is a vibrant, warm, and positive color that evokes feelings of playfulness, excitement, and creativity. It is associated with friendship, happiness, and prosperity, and its vibrant energy captures the spirit of enthusiasm.
In UI design, orange is often used to prompt action through call-to-action buttons, convey warnings or alerts, and emphasize critical information. Its visibility makes it useful for catching attention and signaling safety. It is also used to indicate positive progress or the completion of tasks, forms, or processes within a user interface.
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ET's re-entry and break-up
The Space Shuttle external tank (ET) is the component of the Space Shuttle launch vehicle that contains liquid hydrogen fuel and liquid oxygen oxidizer. During lift-off and ascent, it supplies the fuel and oxidizer under pressure to the three RS-25 main engines in the orbiter. The ET is jettisoned just over 10 seconds after the main engine cut-off and re-enters the Earth's atmosphere. Unlike the Solid Rocket Boosters, external tanks are not reused and break up before impact in the Indian Ocean or the Pacific Ocean in the case of direct-insertion launch trajectories.
The process of spacecraft breaking apart during re-entry is extremely complex due to the challenging thermo-mechanical environment experienced by the satellite. There are limited observations and in-situ measurements, and it is difficult, costly, and sometimes impossible to replicate re-entry conditions on Earth. To address this challenge, the European Space Agency (ESA) has initiated various activities, including the development of the SCARAB software system in collaboration with other companies and institutes. SCARAB is a multidisciplinary software system that models a re-entry object down to the subsystem level, considering geometry and mass updates during the calculation. It has been applied to projects such as ATV, ROSAT, Ariane-5, and BeppoSAX.
The understanding of spacecraft break-up during re-entry is crucial for assessing the ground risk potential associated with fragment objects reaching the Earth's surface. The calculation of destructive re-entries and the prediction of related ground risks have gained significant interest in recent years, as evident during the re-entry of the MIR space station in 2001. The complex nature of re-entry conditions, including the non-equilibrium atmospheric chemistry within the shock layer, emphasizes the importance of free-flight experiments and sophisticated software tools like SCARAB for accurate analysis and prediction.
Additionally, erroneous readings from fuel depletion sensors have delayed several shuttle launch attempts. On December 18, 2007, a tanking test identified the cause of these errors as a fault in a wiring connector. The ET's fuel-depletion sensors are located at the bottom of the fuel tank, and the oxidizer sensors are mounted in the orbiter liquid oxygen feed line manifold. During RS-25 thrusting, the computers continuously compute the instantaneous mass of the vehicle due to propellant usage, and engine cutoff is typically based on a predetermined velocity. However, if any two of the fuel or oxidizer sensors sense a dry condition, the engines will be shut down to prevent severe erosion and potential loss of the vehicle and crew.
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ET's weight reduction
The Space Shuttle external tank (ET) is the component of the Space Shuttle launch vehicle that contained the liquid hydrogen fuel and liquid oxygen oxidizer. During lift-off and ascent, it supplied the fuel and oxidizer under pressure to the three RS-25 main engines in the orbiter. The ET was jettisoned just over 10 seconds after the main engine cut-off and re-entered the Earth's atmosphere. Unlike the Solid Rocket Boosters, external tanks were not re-used and broke up before impact in the Indian Ocean or the Pacific Ocean.
Over the years, NASA worked to reduce the weight of the ET to increase overall efficiency. The weight reduction resulted in an almost equal increase in the cargo-carrying capability of the Space Shuttle. The first two tanks, used for STS-1 and STS-2, were painted white to protect the tanks from ultraviolet light during the extended time that the shuttle spent on the launch pad before launch. However, it was later determined that the paint did not actually provide protection.
The ET has external cameras mounted in brackets attached to the shuttle, along with transmitters that can continue to send video data long after the shuttle and the ET have separated. The ET also has two electrical umbilicals that carry electrical power from the orbiter to the tank and the two SRBs, and they also provide information from the SRBs and ET to the orbiter.
The ET consists of three major components: the forward liquid oxygen (LO2) tank, the aft liquid hydrogen (LH2) tank, and the intertank that connects them. The LH2 tank is the largest part but is relatively light due to liquid hydrogen's very low density. There are eight propellant-depletion sensors, four each for fuel and oxidizer. The fuel-depletion sensors are located at the bottom of the fuel tank, while the oxidizer sensors are mounted in the orbiter liquid oxygen feed line manifold downstream of the feed line disconnect.
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Frequently asked questions
The external fuel tank (ET) does not have an engine. It is a component of the Space Shuttle launch vehicle that contains liquid hydrogen fuel and liquid oxygen oxidizer.
The ET supplies fuel and oxidizer under pressure to the three RS-25 main engines in the orbiter during lift-off and ascent.
The ET is jettisoned just over 10 seconds after the main engine cut-off and re-enters the Earth's atmosphere. Unlike Solid Rocket Boosters, external tanks are not reused and break up before impacting the ocean.
The ET consists of three major components: the forward liquid oxygen tank, the aft liquid hydrogen tank, and the interconnecting structure.
Reducing the weight of the ET results in an almost equal increase in the cargo-carrying capability of the Space Shuttle.
