Sylvie Willis
The amount of rocket fuel used by NASA varies depending on the mission and the type of rocket. For example, the successful Artemis I mission required 2.75 million gallons of super-cooled liquid oxygen and hydrogen, as well as various hypergolics. The combustion rate of the fuel is largely determined by the oxidizer flux and exposed fuel surface area. The combustion of LOx and kerosene produces hot gas that is expanded out of the nozzle to create thrust. NASA's engineers in the Propellants and Life Support Branch ensure that the organization and its commercial partners have the necessary commodities to fuel their rockets and spacecraft.
| Characteristics | Values |
|---|---|
| Percentage of rocket that is fuel | 90% |
| Propellants used | Liquid oxygen (LOx), liquid hydrogen (LH2), kerosene, hydrazine, mono-methyl hydrazine, dinitrogen tetroxide, helium |
| Combustion rate determining factors | Oxidizer flux, exposed fuel surface area |
| Hypergolics | Hydrazine, mono-methyl hydrazine, dinitrogen tetroxide |
| Amount of fuel used for Artemis I mission | 2.75 million gallons of super-cooled liquid oxygen and hydrogen |
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What You'll Learn
Liquid oxygen and hydrogen propellants
Liquid oxygen (LOX) and liquid hydrogen are used as propellants in the high-efficiency main engines of rockets, including the Space Shuttle, the Centaur upper stage, the Delta IV rocket, the H-IIA rocket, the European Ariane 5, and the Space Launch System core and upper stages. LOX and liquid hydrogen offer the highest specific impulse for conventional rockets, delivering excellent performance that offsets the disadvantage of low density, which requires larger fuel tanks.
Liquid oxygen is the only flown cryogenic oxidizer and has been used in combination with other fuels since the early days of rocketry. Robert H. Goddard used liquid oxygen and gasoline as propellants for his first partially successful liquid-propellant rocket launch in 1926. In the late 1920s, German engineers and scientists began building and testing liquid propulsion rockets, including the V-2 missile, which used an alcohol/LOX liquid-propellant engine.
Liquid hydrogen is well-suited for upper-stage use where specific impulse is at a premium, and it has been used in various rockets, including the Space Shuttle, the Centaur upper stage, and the Delta IV rocket. It has an excellent safety record and performance that surpasses other practical chemical rocket propellants. Liquid hydrogen is also considered the cleanest when oxidized with oxygen, as the only byproduct is water. However, it requires special engine designs, such as running propellant lines horizontally, to prevent pipe ruptures due to boiling in confined spaces.
Liquid hydrogen has a very low density, which means it requires a storage volume much larger than other fuels. It is typically stored as a cryogenic liquid, using helium as a cooling refrigerant to prevent boil-off. The use of liquid hydrogen also presents challenges in terms of storage temperature, as it must be kept at extremely low temperatures (-253 °C or -423 °F) to remain in a liquid state.
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Hypergolics and their vapours
Hypergolic propellants are combinations of two materials—a fuel and an oxidizer—that spontaneously self-ignite at room temperature because they are highly reactive. Hypergolic fuels, such as hydrazine, monomethylhydrazine, and unsymmetrical dimethylhydrazine (UDMH), are liquids at room temperature and pressure, making them suitable for long-duration spacecraft missions. Hydrazine-based hypergolic propellants were among the first to be used and remain popular due to their ease of storage compared to cryogenic fuels. However, hydrazine and its derivatives are toxic and hazardous, requiring strict safety precautions.
The Apollo Lunar Module and the Space Shuttle, for example, employed hypergolic fuels in their rocket engines and reaction control systems. Hypergolic engines are usually simple and reliable, as they require no ignition system. They can fire multiple times by simply opening and closing the propellant valves, making them well-suited for spacecraft manoeuvring. However, larger hypergolic engines may use turbopumps to feed the propellants into the combustion chamber.
The oxidizer in hypergolic propellants is typically nitrogen tetroxide, which is also a liquid under pressure. When hydrazine reacts with nitrogen tetroxide, it produces red fumes. However, when hydrazine reacts with nitric acid, it creates a whitish-yellow fog. Nitrogen tetroxide itself is highly toxic and corrosive, forming nitric acid and nitrous acid upon contact with the eyes, skin, and respiratory system, which can cause tissue destruction and immediate symptoms like headaches, coughing, nausea, and chest and stomach pain. Delayed pulmonary edema can also occur within 5 to 72 hours, leading to death.
The high toxicity and reactivity of hypergolics pose explosion risks and have resulted in unintentional spills, explosions, and fires in NASA programs like Apollo, Space Shuttle, and Titan. Investigations have revealed safety and operational shortcomings, including improper configuration control, ageing hardware, inadequate cleanliness, and improper procedural oversight. The corrosivity, toxicity, and carcinogenicity of hypergolics necessitate expensive safety measures. As a result, Western space agencies are trending away from large hypergolic rocket engines towards hydrogen/oxygen or methane/oxygen engines.
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Solid fuel hydroxy-terminated polybutadiene
Hydroxyl-terminated polybutadiene (HTPB) is a low-viscosity, colourless liquid with a number-average molecular weight (Mn) of 1500–10,000 g/mol. HTPB is an important component in solid rocket propellant, acting as a binder for the oxidizing agent, fuel, and other ingredients in a solid but elastic mass. It is used in all 3/4 stages of the Japanese M-5 launch vehicles and in 1/3 stages of the Indian PSLV launch vehicle. Similar propellants, often referred to as APCP (ammonium perchlorate composite propellant), are used in larger model rockets and produce 2–3 times the specific impulse of the black powder propellant used in smaller rocket motors.
HTPB has a range of applications beyond rocket propellant. It can be used to create ABA triblock copolymers, toughened resins, cross-linked elastomers, PB vitrimers, and supermolecular polymer gels. HTPB-based polyurethanes (PU) are particularly useful as binders for solid propellants, explosives, adhesives, foams, coatings, sealants, elastomers, unsaturated polyesters, toughened polymers, and gas separation membrane applications.
The mechanical strength and swelling characteristics of HTPB-based copolyurethanes have been studied to develop improved solid propellant binders. By varying the ratios of the hydroxy pre-polymers, chain extenders, and crosslinkers, copolyurethanes with a wide range of tensile strength and elongation can be obtained. These systems are desirable for their use as propellant binders due to their unique possibility of tailored polymer topology and adjustable formulations.
HTPB elastomers have been the focus of recent studies, which have analysed the effects of different isocyanates and plasticizers on their thermomechanical and tensile properties. One challenge with HTPB elastomers is their intrinsic self-healing problem, which can result in permanent damage. To address this, a strategy involving hydroxy-carboxy-terminated polybutadiene (HCTPB) and Fe3+ has been proposed to facilitate ionic bond formation and enhance the self-repair rate.
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Nitrous oxide and paraffin wax
Nitrous oxide/paraffin (N2OP) hybrid rocket engines have been invented as alternatives to rocket engines that burn granular, rubbery solid fuels. These engines were originally intended for use in launching spacecraft, but they would also be suitable for terrestrial use in rocket-assisted takeoff of small airplanes.
The main features of these engines are the use of reinforced paraffin as the fuel and nitrous oxide as the oxidizer. Paraffin has a regression rate about 3 to 4 times that of conventional hybrid fuels, making it suitable for large-scale applications. However, pure paraffin fuel grains soften when heated, so paraffin fuel grains can potentially slump during firing. To prevent this, the paraffin is moulded into a 3-volume-percent graphite sponge or similar carbon matrix, which supports the paraffin against slumping during firing.
Nitrous oxide is non-toxic, can be stored safely for long periods under non-cryogenic conditions, and can serve as its own autogenous pressurant gas. This eliminates the need for high-pressure helium tanks, increasing reliability while reducing mass and cost. Nitrous oxide can also be used as a monopropellant, in place of hydrazine, to reduce cost and enhance safety by eliminating the toxic hazard posed by hydrazine.
The use of nitrous oxide and paraffin wax in hybrid rocket engines offers advantages of safety, simplicity, reliability, and reduced cost compared to other rocket engines.
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Liquid fuel vs solid fuel
Rocket propellant is used as a reaction mass ejected from a rocket engine to produce thrust. The energy required for this can either come from the propellants themselves or from an external source. Rockets create thrust by expelling mass rearward at high velocity.
Liquid-fuelled rockets are more versatile than solid-fuelled rockets. You can adjust the throttle in-flight once you hit your desired cruising speed, so they can be more fuel-efficient. However, the oxidiser tends to be highly corrosive and toxic, so orbital velocity rockets are usually only fuelled just before launch. Liquid-fuelled rockets are also less safe, as a single wrench falling and punching a hole in the tank can cause an explosion.
Solid-fuelled rockets, on the other hand, cannot be adjusted or turned off once they are lit. They will continue burning at the pace determined by their geometry. Solid-fuelled rockets have more thrust and shorter burn times. They are also more stable and can stay fuelled for longer, which is important for ICBMs, which may need to be launched without taking the time to refuel. Solid-fuelled rockets are also much easier to store and handle than liquid-fuelled rockets, and their simplicity and low cost make them ideal for military applications.
Solid rocket propellant was first developed during the 7th century under the Chinese Song dynasty. During the 1950s and 60s, researchers in the United States developed ammonium perchlorate composite propellant (APCP), a mixture of finely ground ammonium perchlorate (an oxidiser), aluminium powder (a fuel), and polybutadiene acrylonitrile (PBAN) or hydroxyl-terminated polybutadiene (a rubber fuel). This mixture is formed as a thickened liquid and then cast into the correct shape and cured into a firm but flexible load-bearing solid.
Upper stages of rockets, which mostly or only operate in the vacuum of space, tend to use high-energy, high-performance, low-density liquid hydrogen fuel. For future planetary missions, the use of local resources and solar energy for in-situ propellant production is considered.
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Frequently asked questions
The amount of rocket fuel NASA uses depends on the mission and the type of rocket. For example, the successful Artemis I mission required 2.75 million gallons of super-cooled liquid oxygen and hydrogen.
Rocket fuel, or propellant, is made up of an oxidizing agent and a reducing agent (fuel). The type of propellant used depends on the rocket. For example, the Apollo mission used liquid oxygen and kerosene, while the Artemis I mission used liquid oxygen and hydrogen.
Rocket fuels create thrust by expelling mass rearward at high velocity. The thrust produced can be calculated by multiplying the mass flow rate of the propellants by their exhaust velocity relative to the rocket (specific impulse).
Solid-propellant rockets are much easier to store and handle than liquid-propellant rockets. They have a high propellant density, making them compact in size. Solid propellants are also simpler and more cost-effective, making them ideal for military applications. Liquid propellants, on the other hand, can provide more power but require more complex systems for storage and handling.
The oxidizer is a crucial component of rocket fuel as it provides the oxygen necessary for combustion. The combustion rate of the fuel is largely determined by the oxidizer flux and exposed fuel surface area.
