Katerina Swanson
Satellites are fascinating pieces of technology that orbit Earth and perform a variety of functions, from photography to transmitting and receiving information from Earth. While a satellite in orbit doesn't generally require power to maintain its orbit, it does need fuel for other purposes. So, how much fuel do satellites carry, and what is it used for? Well, the amount of fuel a satellite carries depends on its mission requirements and the type of propulsion system it uses. Modern satellites typically use hydrazine-based fuel for manoeuvring and electric propulsion with xenon propellant for station-keeping.
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What You'll Learn
- Satellites use hypergolic fuel, not liquid hydrogen, kerosene or oxygen
- Electric propulsion with xenon propellant is used for station-keeping
- Hydrazine-based fuel is toxic and requires protective gear to handle
- Refuelling satellites is challenging as they weren't designed for this
- Solar energy can power satellites for day-to-day tasks, but fuel is needed for movement
Satellites use hypergolic fuel, not liquid hydrogen, kerosene or oxygen
A satellite in orbit doesn't generally require power to keep orbiting. They use their power supplies to maintain their electronic systems, which enable them to carry out tasks such as photography and transmission to and from Earth.
In the normal course of orbit, a satellite doesn't need to burn fuel; it is kept moving by gravity and the lack of friction in space. However, satellites are generally launched with some fuel, which can be used to operate thrusters in a variety of situations. For example, fuel is used to move the satellite into the intended orbit.
The amount of fuel a satellite carries depends on its mission requirements. Satellites in orbit use hypergolic fuel, not liquid hydrogen, kerosene, or oxygen. Hypergolic fuel is a type of rocket propellant that spontaneously ignites when the fuel and oxidizer come into contact with each other. It is commonly used in satellite propulsion systems because it is highly reliable and can be stored for long periods without degradation.
Liquid hydrogen, liquid oxygen, and kerosene are used in rocket propulsion systems, but they are not typically used as fuel for satellites in orbit. Liquid hydrogen, for instance, has a very low density, which makes it impractical for use in satellites. It also has a high cost and causes difficulties in the design, manufacture, and operation of the vehicle. Kerosene, on the other hand, delivers a lower specific impulse than cryogenic fuels like liquid hydrogen. While it is generally considered better than hypergolic propellants, it still produces residues that limit the operational lifetime of the engines.
Oxygen is avoided in satellite fuel because it is difficult to store long-term due to boil-off. It is also not necessary for the electric propulsion systems that satellites use.
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Electric propulsion with xenon propellant is used for station-keeping
The amount of fuel a satellite carries depends on its purpose and the type of fuel used. For example, satellites that require movement, such as moving into the intended orbit, generally carry fuel for operating thrusters. On the other hand, a satellite in orbit doesn't usually require fuel to keep orbiting, as it is kept moving by gravity and the absence of friction in space.
Electric propulsion with xenon propellant is a highly effective method for station-keeping. This technique offers several advantages over traditional chemical propulsion systems. Firstly, it consumes far less propellant due to its ability to generate continuous thrust over extended periods. Secondly, the xenon propellant is ejected at a much higher velocity compared to chemical thrusters, resulting in significantly enhanced propulsive performance. This increased ejection speed contributes to the overall system's superior mass efficiency.
The NASA Evolutionary Xenon Thruster (NEXT) project serves as a testament to the durability and efficiency of xenon-based electric propulsion. During a test spanning more than 48,000 hours, the NEXT engine consumed approximately 870 kilograms of xenon propellant. To produce an equivalent impulse using conventional rocket propellant, more than 10,000 kilograms of fuel would have been necessary. This striking comparison underscores the efficiency of electric propulsion with xenon propellant.
The benefits of electric propulsion extend beyond propellant efficiency. By requiring less mass to accelerate a spacecraft, electric propulsion systems facilitate significant reductions in launch mass. This advantage translates directly into substantial cost savings in the hundreds of millions of dollars for satellite manufacturers and purchasers.
The versatility of electric propulsion systems, compatible with various propellants like rare gases (xenon or argon), liquid metals, or traditional propellants, further enhances their appeal. This adaptability ensures their suitability for a broad range of missions, including telecommunications, space transportation, and space exploration.
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Hydrazine-based fuel is toxic and requires protective gear to handle
The fuel that satellites use in the present day is generally hydrazine-based. Hydrazine is colourless and has a similar appearance and behaviour to water. It has a comparable freezing point, surface tension, density, and viscosity. However, hydrazine is toxic and requires protective gear to handle. Exposure to high levels of hydrazine can cause a host of health problems, including damage to the liver, kidneys, and central nervous system. If hydrazine spills while a satellite is still on the ground, its volatile and explosive nature can become a public safety issue.
Preparing a hydrazine-fuelled satellite for space is a hazardous task that requires special precautions for anyone involved. This includes wearing space suit-like clothing to ensure that, should something go wrong, the people handling the fuel would not breathe in the gas. In 2011, the European Chemicals Agency added hydrazine to its list of "substances of very high concern", meaning its usage could soon be restricted.
Due to the health and safety concerns associated with hydrazine, researchers are developing safer alternatives. For example, HyproGEO, a project coordinated by aerospace company Airbus, has developed a non-toxic propulsion system for satellites in geostationary orbit around Earth. This system uses a hybrid propulsion system that relies on hydrogen peroxide instead of hydrazine. Hydrogen peroxide is highly acidic but still less risky to work with than hydrazine, as it breaks down into oxygen and water without releasing harmful fumes.
Despite the hazards of hydrazine, it remains a popular fuel choice for satellites due to its stability during long missions and its ability to provide controlled thrust using small amounts of fuel. Iridium NEXT satellites, for instance, use hydrazine as their fuel of choice. Their engines use only fuel and heat, with no oxygen, which makes the propellant system much simpler and lighter.
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Refuelling satellites is challenging as they weren't designed for this
The amount of fuel carried by satellites depends on their use case. Satellites don't require much fuel to stay in orbit, as they are kept moving by gravity and the lack of friction in space. However, they do need fuel for thrusters to move them into their intended orbit and to avoid debris. As a result of the high cost of launching additional weight into space, satellites carry a limited supply of fuel.
Refuelling satellites is challenging because most are not designed to be refuelled. Once their initial fuel reserve is depleted, they become space junk. This design flaw could be addressed in future satellites by mandating support for refuelling and developing tanker spacecraft to deliver fuel. However, refuelling satellites that weren't designed for it would require bespoke engineering, and some are too complicated to be worth the effort.
Some companies and organisations are working on projects to refuel satellites in orbit. Orbit Fab, for example, aims to provide an all-inclusive satellite refuelling service by 2025, with fuel available for pre-order at $20 million for 100 kilograms. They plan to use a network of fuel depots and shuttles to refuel client satellites, employing their proprietary Rapidly Attachable Fluid Transfer Interface (RAFTI) to dock with satellites. However, a significant challenge is that many satellites in orbit are not compatible with RAFTI. The UK Space Agency is investigating a range of interfaces to find the best solution and encourage the adoption of an international standard.
NASA is also developing technology to refuel satellites in space through its On-orbit Servicing, Assembly, and Manufacturing 1 (OSAM-1) mission. OSAM-1 is designed to refuel satellites that weren't built to be refuelled. The mission was planned to launch in 2026 and will showcase advanced systems for reliable autonomous docking.
The ability to refuel satellites in orbit would extend their lives and revolutionise the way they are operated, enabling operators to "maneuver without regret" to avoid potential conflicts with adversary satellites.
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Solar energy can power satellites for day-to-day tasks, but fuel is needed for movement
Solar energy is a clean and efficient way to power satellites for their day-to-day tasks, but fuel is still needed for movement and other functions. Satellites have been powered by solar energy since Vanguard I in 1958, when solar panels were used to power one of its radio transmitters.
Satellites primarily use their power supplies to maintain their electronic systems, which enables them to carry out tasks such as photography and transmitting or receiving communications from Earth. Solar panels on satellites are charged by solar energy when the sun is visible, and they can continue to power the satellite when eclipsed by the Earth until they are recharged.
However, solar energy alone cannot provide all the power a satellite needs. Satellites are generally launched with some fuel, which is used for movement and other functions. For example, fuel is used to operate thrusters to move the satellite into its intended orbit. The fuel that satellites use today is typically hydrazine-based, although this is toxic and the industry is researching safer alternatives.
Other types of fuel have been used in the past, such as hypergolic fuel, but these are less common now due to the technical challenges of refuelling in orbit. In addition, electric propulsion with xenon propellant is often used for station-keeping, as it is more efficient than fuel with an O2 oxidizer.
Space-based solar power (SBSP) is a concept that involves collecting solar power in space and distributing it to Earth. This technology has the potential to provide a significant amount of clean energy, with a single solar power satellite generating around 2 gigawatts of power, equivalent to a conventional nuclear power station. However, as of 2014, none of the proposed SBSP systems have been economically viable due to the high cost of launching satellites into space.
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Frequently asked questions
Satellites don't need to carry much fuel because they don't need fuel to stay in orbit. Gravity and the lack of friction in space keep them moving. However, they do need fuel for certain functions, such as moving into the intended orbit.
Satellites use hypergolic fuel, which doesn't require oxygen. This is because oxygen is difficult to store long-term due to boil-off. The fuel that satellites use today is generally hydrazine-based.
Oxygen is avoided because it is difficult to store long-term due to boil-off.
Satellites don't consume much fuel at all for station-keeping. They use electric propulsion with xenon propellant for station-keeping instead.
