Sylvie Willis
The external fuel tank of the Space Shuttle, also known as the ET, is painted orange due to the colour of the spray-on foam insulation. The ET is the largest element of the Space Shuttle and is responsible for containing the liquid hydrogen fuel and liquid oxygen oxidizer. The ET also features eight propellant-depletion sensors, four for fuel, and four for oxidizer, which constantly compute the instantaneous mass of the vehicle.
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What You'll Learn
The orange colour is from spray-on foam insulation
The orange colour of some fuel tanks is due to the spray-on foam insulation that is applied to them. The Space Shuttle external tank (ET), for example, had an orange colour due to the application of this insulation. The ET was the component of the Space Shuttle launch vehicle responsible for containing and supplying liquid hydrogen fuel and liquid oxygen oxidizer to the engines during lift-off and ascent.
The ET was the largest and heaviest element of the Space Shuttle when loaded. While these external tanks were always discarded, there were plans to reuse them in orbit for various purposes, such as incorporating them into a space station or utilising them as rocket fuel tanks for interplanetary missions. The orange colour of the ET is a distinctive feature that sets it apart, and it is directly attributed to the spray-on foam insulation used in its construction.
This insulation played a critical role in protecting the tank from extreme temperatures and ensuring the thermal stability of the fuel and oxidizer it contained. By applying this insulation, engineers could mitigate the effects of rapid temperature changes during the shuttle's journey through different atmospheric layers. The choice of colour for the insulation may have been influenced by a combination of practical and safety considerations, ensuring the tank is easily identifiable and distinguished from other components.
Additionally, the orange colour of the spray-on foam insulation on fuel tanks can serve as a visual indicator for maintenance and inspection. It provides a clear contrast against the typical colours of fuels and lubricants, making it easier to identify leaks or damage during routine checks. This visual contrast aids in quickly detecting and addressing any issues, contributing to the overall safety and maintenance of the fuel tanks and surrounding systems.
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Fuel tanks contain liquid hydrogen and liquid oxygen
Fuel tanks can contain liquid hydrogen, which is stored at very low temperatures (22 K or -251.15 °C) and requires specialised tanks that can withstand such low temperatures. Hydrogen tanks come in different shapes, with spherical forms used for some liquid hydrogen tanks, and cylindrical containers being the most common form. These tanks are part of a hydrogen storage system and are often referred to as hydrogen cylinders, cartridges, or canisters.
Liquid hydrogen is highly combustible and offers higher energy density than gaseous storage, allowing more energy to be stored in less space. This makes it an attractive fuel option for rockets and space flight. However, hydrogen is usually stored just under its –423 °F boiling point, and it often evaporates, forming bubbles as it enters the pump. This cavitation impairs the performance of the turbopump and can lead to physical degradation over time.
To address this issue, engineers have designed impellers and inducers (spiral-shaped rotors) that increase flow at the pump’s inlet to reduce the effects of cavitation. These components are tested at facilities like NASA's Liquid Hydrogen Pump Facility (A Site) to optimise their performance. Additionally, liquid-fuel propellant tanks use a heavy gas to force the fluid out towards the turbopump. Managing the amount of this pressurant gas is crucial; too much would require thicker tank walls, adding weight, while too little would slow the flow of the cryogenic fluid, resulting in boiling or cavitation.
In the 1960s, rocket designers also explored using liquid fluorine as an oxidiser paired with liquid hydrogen fuel. Fluorine is lighter and produces more energy than traditional liquid oxygen, but it is extremely toxic and volatile, and there have been no commercial fluorine rocket engines to date.
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Sensors monitor fuel and oxygen levels
Fuel tanks, particularly those in aircraft, are monitored by sensors that measure fuel and oxygen levels. Fuel level sensors are typically installed in the tank of a vehicle at the factory, with a potentiometer connected to a float that measures the fuel level. As the fuel level changes, the position of the float changes, resulting in a change in resistance and output voltage. This data is transmitted to the vehicle's dashboard.
Some fleet operators also use ultrasonic fuel sensors to monitor fuel consumption and drains. These sensors are fixed to the outside wall of the fuel tank and send ultrasonic impulses to measure the height of the fuel column. The signal reflects back to the transmitter, and the time taken is used to calculate the height of the fuel. This information is then sent to a GPS tracking platform.
To ensure safety, aircraft fuel tanks also employ oxygen sensors to monitor oxygen levels and prevent explosions. Zirconium dioxide sensors, or zirconia oxygen sensors, are commonly used to maintain oxygen levels close to zero by introducing inert gases. By monitoring oxygen concentration, these sensors can detect fires in their early stages and prevent catastrophic explosions. The data is fed to the OBIGGS controller, which adjusts the flow of nitrogen gas to maintain the desired oxygen levels.
In addition to zirconium dioxide sensors, other methods for measuring oxygen levels include electrochemical (galvanic), infrared, ultrasonic, paramagnetic, and laser techniques. Oxygen sensors are crucial for fire prevention in aviation, as well as in automotive applications, where they are known as lambda sensors. These sensors help determine the air-fuel ratio of a combustion engine and ensure optimal performance of catalytic converters.
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Fuel tanks are heavy and are not reused
The distinctive orange colour of fuel tanks, specifically those used in space shuttles, is due to the spray-on foam insulation applied to their surfaces. This insulation serves a critical function, protecting the tanks from the extreme temperatures and conditions they encounter during space travel.
Now, addressing the statement "Fuel tanks are heavy and are not reused", it is indeed true that fuel tanks, particularly those used in space missions, can be quite heavy due to the substantial amount of fuel they carry. The weight of a fuel tank can significantly impact the overall efficiency of the spacecraft, influencing factors such as fuel consumption, manoeuvrability, and launch costs. To mitigate these challenges, engineers have continuously worked on reducing the weight of fuel tanks, employing lightweight materials and innovative designs to strike a balance between fuel capacity and overall weight.
One notable example of a heavy fuel tank is the Space Shuttle External Tank (ET), which was the largest and heaviest component of the Space Shuttle when loaded with fuel. The ET consisted of three main parts: the forward liquid oxygen (LO2) tank, the aft liquid hydrogen (LH2) tank, and the interconnecting structures. While liquid hydrogen has a very low density, the substantial volume required for space missions resulted in a hefty weight for the LH2 tank.
Regarding reuse, fuel tanks, especially those in spacecraft, are often not designed for reuse due to various technical and logistical considerations. For instance, the Space Shuttle ET was jettisoned shortly after main engine cutoff and broke up upon re-entering the Earth's atmosphere, landing in the Indian or Pacific Ocean. While there were proposals for reusing these external tanks, such as incorporating them into space stations or utilising them for interplanetary missions, the reality of implementing such concepts faced practical challenges. The weight of the ET, for example, posed difficulties in terms of cargo capacity and overall efficiency, leading NASA to focus their efforts on weight reduction rather than tank recovery and reuse.
In summary, fuel tanks, particularly those used in space exploration, can be quite heavy due to the volume of fuel they carry, and their weight has a significant impact on various aspects of the mission. While there have been ideas for reusing fuel tanks, the practical challenges and the focus on weight reduction have directed efforts towards designing lighter and more efficient tanks rather than reusing them.
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Fuel tanks are the largest element of the Space Shuttle
The Space Shuttle has three major components: the orbiter, the solid rocket boosters, and the external fuel tank (also known as the ET). The ET is the largest and heaviest element of the Space Shuttle when loaded. It was the "backbone" of the shuttle during launch, providing structural support for attachment with the Space Shuttle Solid Rocket Boosters (SRBs) and the orbiter.
The ET served two purposes: it carried the propellants for the orbiter's three main rocket engines, and it was the support structure that connected the orbiter and SRBs during ascent to orbit. The ET was connected to each SRB at one forward attachment point (using a crossbeam through the intertank) and one aft bracket. It was also connected to the orbiter at one forward attachment bipod and two aft bipods.
The ET contained liquid hydrogen fuel and liquid oxygen oxidizer, which it supplied under pressure to the three main engines in the orbiter during lift-off and ascent. 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, the ET was not reused and broke up before impact in the Indian Ocean or Pacific Ocean, away from shipping lanes.
The ET had external cameras mounted in brackets attached to the shuttle, along with transmitters that could continue to send video data long after the shuttle and ET separated. It also had a range safety system to disperse tank propellants if necessary.
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Frequently asked questions
The colour orange is used to indicate that the tank contains dangerous goods.
Fuel tanks are painted orange to make them easily identifiable and to warn people of the hazardous contents inside.
Orange fuel tanks typically contain jet fuel, which is highly flammable and poses a significant safety risk.
No, not all fuel tanks are orange. The colour of a fuel tank can vary depending on the type of fuel it contains and the regulations in a given location. However, orange is a common colour for fuel tanks due to its association with caution and safety.
