Katerina Swanson
Hydrogen is the lightest molecule and the first element on the periodic table. It is a popular rocket fuel because of how intensely it burns when combined with oxygen. Hydrogen is highly efficient, allowing it to lift a lot of mass relative to its own mass. It is also readily available and can be produced as a petroleum byproduct, through electrolysis, or through other innovative technologies. Liquid hydrogen has a higher specific impulse, meaning it can produce more thrust for longer and has better fuel efficiency. However, it has a lower boiling point, is harder to store, and is more expensive to produce and transport.
| Characteristics | Values |
|---|---|
| How hydrogen is used as rocket fuel | Hydrogen is combined with oxygen to produce energy, with the main byproduct being water vapour. |
| How much hydrogen is burned | The amount of hydrogen burned depends on the mixture ratio, which can be dynamic during launch. |
| Advantages of hydrogen rocket fuel | Hydrogen is the most efficient propellant by weight, has a higher specific impulse, and burns cleaner. |
| Disadvantages of hydrogen rocket fuel | Hydrogen has a lower boiling point, is more difficult to store, and is more expensive to produce and transport. |
| Examples of hydrogen rocket fuel use | The main engine of the Space Shuttle, the Atlas Centaur stage rocket, Delta III and IV rockets, the H-IIA rocket, and the European Ariane 5 and 6. |
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What You'll Learn
Hydrogen's high specific impulse
Hydrogen is the lightest molecule and the first element on the periodic table. It is a popular rocket fuel because of how intensely it burns when combined with oxygen. Hydrogen is highly efficient, allowing it to lift a lot of mass relative to its own mass. The bipropellant liquid oxygen and hydrogen offer the highest specific impulse for conventional rockets. This extra performance offsets the disadvantage of low density, which requires larger fuel tanks.
Liquid hydrogen has a high specific impulse, which means it can produce more thrust for longer and has better fuel efficiency. It can make rockets go farther and faster while using less fuel. Hydrogen's high specific impulse is due to the availability of high-performance oxidizers. Its high specific impulse is also related to the thrust per fuel flow. The interpretation of "thrust per fuel flow" is traditional in the industry because of its focus on cost efficiency.
The highest specific impulse for a chemical propellant ever test-fired in a rocket engine was 542 seconds (5.32 km/s) with a tripropellant of lithium, fluorine, and hydrogen. This combination is impractical because of the extreme corrosiveness of lithium and fluorine, the ease with which they ignite, and the toxicity of their exhaust.
Nuclear electric systems, where nuclear reactors are a heat source for electric ion drives, can also provide a high specific impulse. The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) uses radiofrequency energy to ionize and heat gas into a hot plasma. The heated plasma is then directed using magnetic fields to generate thrust. VASIMR can use hydrogen as a propellant and has demonstrated a very high specific impulse of 4000 to over 10,000 s, indicating that 90% or more of the propellant weight could be saved.
Advanced space propulsion systems that use hydrogen do not combust the hydrogen. Instead, they heat it to high temperatures, and the hydrogen exits the engine at a high velocity. The specific impulse from these engines depends on the temperature to which the hydrogen is heated, but it is generally much higher than that of conventional hydrogen/oxygen engines.
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Hydrogen's clean combustion
Hydrogen is the most efficient propellant by weight, with a higher specific impulse, allowing rockets to go further and faster while using less fuel. It also has a higher thrust-to-weight ratio than other fuels, making it a popular choice for upper-stage rockets. Its high specific impulse is due to its small molecular size and highly energetic nature. When hydrogen combines with oxygen, it produces energy and the main byproduct is pure water vapour. This clean combustion of hydrogen reduces the environmental impact of space missions by producing no harmful gases or pollutants, carbon emissions, coking, soot, or other types of residue buildup in rocket engines.
Liquid hydrogen is a powerful rocket fuel that has been used for many decades, including in NASA's Apollo program. Its clean-burning nature and environmentally friendly exhaust plumes make it a highly sought-after fuel. However, it has some disadvantages, including a lower boiling point, which makes it more difficult and expensive to store and transport. Hydrogen fuel is produced as a petroleum byproduct, through electrolysis, or through other innovative technologies. It is also quick to refuel, taking only a short time compared to the long charge times of batteries.
Hydrogen is the lightest molecule and the first element on the periodic table. Its intense combustion when combined with oxygen makes it highly efficient, allowing it to lift a lot of mass relative to its own mass. However, hydrogen and oxygen fuels need to be kept as liquids, which requires very high pressures and/or extremely cold temperatures. This, in turn, necessitates specialised equipment for storage and transportation, such as expertly crafted, highly insulated pressure vessels.
Liquid hydrogen also has a tendency to leak due to the small size of its molecules, which can result in unintended combustion and the destruction of the entire engine. Despite these challenges, hydrogen continues to show promise as a rocket propellant for both government and private industry launch systems and vehicles. Engineers have recognised its potential as a clean energy option for new aerospace applications, and it has been used in rocket fuel and hydrogen fuel cells since the early years of space exploration.
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Hydrogen's low density
Hydrogen is the lightest molecule and the first element on the periodic table. Its low density is a disadvantage when used as rocket fuel as it occupies about seven times more volume per kilogram than dense fuels such as kerosene. This means that the fuel tank, plumbing, and pump must be correspondingly larger, increasing the vehicle's dry mass and reducing performance.
Liquid hydrogen has a low volumetric energy density, which means that the hydrogen volumes needed for combustion are large. This makes the storage, transportation, and application of hydrogen more difficult. Hydrogen requires very cold temperatures and/or high pressures to be in a liquid state, and it needs to be stored in expertly crafted, highly insulated pressure vessels to keep it at high pressures.
Despite its low density, hydrogen is still a popular choice for rocket fuel due to its high specific impulse, which allows it to produce more thrust for a longer time with better fuel efficiency. It also burns cleanly, producing only water vapour when combined with oxygen, making it environmentally friendly.
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Liquid hydrogen's storage
Hydrogen is an essential element for many industries, including the automotive and aerospace sectors. Liquid hydrogen storage is crucial for harnessing its potential, but it comes with challenges due to hydrogen's characteristics as the lightest gas in the universe. Achieving liquifying conditions requires a significant amount of energy, and maintaining cryogenic temperatures is essential for safe storage.
Liquid hydrogen storage requires extremely low temperatures, typically around -253°C, which can be challenging to maintain. This low temperature is necessary because hydrogen has a very low boiling point of -252.8°C at atmospheric pressure. The low ambient temperature density of hydrogen results in low energy per unit volume, necessitating storage solutions with higher energy density.
One storage option is to keep hydrogen as a compressed gas in high-pressure tanks ranging from 350-700 bar tank pressure (5,000-10,000 psi). This method increases storage density and reduces volume, making it the preferred choice for some car manufacturers. However, it may be less suitable for light-duty vehicles due to the large storage volumes required.
Liquid hydrogen storage is the preferred choice for the space industry and bulk transportation. It is used as a powerful rocket fuel and has served this purpose for many decades. Liquid hydrogen's high specific impulse allows rockets to go farther and faster while using less fuel. It also burns cleanly, producing only water vapour when combined with oxygen, reducing the environmental impact of space missions.
Additionally, liquid hydrogen faces challenges such as the significant energy consumption during liquefaction and the loss of hydrogen through boil-off during storage. The development of safe and optimised liquid hydrogen storage methods is crucial for harnessing hydrogen's potential across various industries.
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Hydrogen's production
Hydrogen can be produced using a variety of processes, and nearly all of the world's current supply of hydrogen is created from fossil fuels. The two most common methods for producing hydrogen are steam-methane reforming and electrolysis. Steam-methane reforming, also known as natural gas reforming, involves using high-temperature steam and methane (the main component of natural gas) in the presence of a catalyst to produce hydrogen. This process can emit a significant amount of carbon dioxide, but carbon capture and storage can be employed to reduce these emissions, resulting in what is known as blue hydrogen.
Electrolysis is another widely used method for hydrogen production, where water is split into hydrogen and oxygen using electricity. This process is considered clean and environmentally friendly, and the hydrogen produced from renewable electricity is often referred to as green hydrogen. However, producing green hydrogen is currently more expensive than grey hydrogen from fossil fuels. Electrolysis has attracted interest due to its use of water, a readily available resource, and its high electrical efficiency of 70-80%.
Other methods of hydrogen production include biomass gasification, methane pyrolysis, and extraction of underground hydrogen. Hydrogen can also be produced through thermochemical processes using heat and chemical reactions to release hydrogen from organic materials, such as fossil fuels, biomass, or water. Direct solar water splitting, or photolytic processes, use light energy to split water into hydrogen and oxygen and are currently being researched for their potential for low-impact hydrogen production. Additionally, microorganisms like bacteria and algae can produce hydrogen through biological processes.
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Frequently asked questions
Hydrogen rockets burn liquid hydrogen, which has a very low density of 0.071 g/ml. This means that it requires a storage volume several times greater than other fuels. Hydrogen rockets are typically used in the upper stages of a rocket, after solid rocket fuels have been used for liftoff.
Liquid hydrogen is used in rockets because it has the lowest mass possible for a rocket fuel. It also has a higher specific impulse, meaning it can produce more thrust for longer and has better fuel efficiency. It burns cleanly, producing only water vapour when combined with oxygen.
Hydrogen has a lower boiling point and is more difficult to store and transport than other fuels. It is also more expensive to produce. Liquid hydrogen also has a tendency to leak due to the small size of its molecules.
