Rockets have to produce an incredible amount of thrust to break free from Earth’s gravity and put payloads of up to 100 tons into orbit. The fuel they use plays a critical role in their capacity to produce this kind of power.
Rockets typically use liquid or solid rocket fuel combined with an oxidizer as their propellant. Liquid-fueled rockets store their fuel and oxidizer separately, which are combined in a combustion chamber during ignition. Solid rocket fuel and oxidizer are mixed together and stored in a solid state.
The spectacular display of a rocket lifting off with its thrusters blowing white-hot gases at supersonic speeds through its nozzles can largely be attributed to the high-energy fuel mixtures these launch vehicles burn to accelerate into space with their payloads.
Although different types of propellant can be used, rockets primarily use variations of two types of fuel: Liquid or solid rocket propellant. Both fuel types are combined with an oxidizer to combust and form the gases necessary to create sufficient thrust during launch.
(An oxidizer or oxidizing agent is a substance that provides oxygen to fuel, allowing it to ignite and keep burning.)
Depending on the launch vehicle, payload requirements, and mission objectives, rockets use either one of these fuel types, and in some cases, a combination of both.
How Rockets Use Fuel To Produce Thrust
All rockets use Newton’s Third Law Of Motion to move forward and maneuver in Earth’s atmosphere and space. It states that “for every action, there is an equal and opposite reaction.”
In the case of rockets, the first force created is the thrust produced by the hot gases that are accelerated out the back through the rocket thrusters. The reactionary and opposite force is the forward propulsion of the vehicle as a result of the thrust created.
To produce this thrust, a rocket needs to burn fuel. In fact, it is the energy from the fuel created through combustion and being forced out the engine’s nozzle in the form of high-velocity gases that generate the thrust that drives the vehicle forward.
Unlike conventional aircraft that use their engines to push against the air in the atmosphere to propel them forward, a rocket accelerates as a result of the pressure created by the hot gases against the combustion chamber and rocket nozzle.
(It is this principle that also allows it to operate in the vacuum of space. Learn more about exactly what a rocket is and how its different propulsion system works in this article.)
The measurement of how efficient a specific propellant burns to produce thrust is called specific impulse. Specific impulse is generally calculated by measuring the number of seconds a kilogram (or pound) of fuel lasts while producing a kilogram (or pound) of thrust.
As previously mentioned, liquid and solid rocket propellants make up the bulk of fuel used in modern rockets. To better understand how each fuel type works, one needs to take a closer look at each propellant.
Liquid Rocket Propellants
The existence of the liquid propellant rocket can be attributed to Robert Goddard (considered by many to be the father of modern rocketry), who developed and launched the first liquid propelled rocket on 16 March 1926.
Although they have different characteristics, all liquid rocket propellants generally consist of highly combustible liquids that undergo a chemical reaction when combined with an oxidizer, allowing them to combust and burn.
The fuel and oxidizer are pumped and stored into separate tanks inside the rocket’s structural system only minutes before launch. During a launch, turbopumps transport them into a combustion chamber, where they are mixed and ignited under high pressures.
This process allows for the creation of the hot gases that are accelerated through the engine’s nozzle at supersonic speeds, providing the necessary thrust for the launch vehicle to carry its payload into space.
Liquid rocket propellants have several advantages over solid rocket fuel. By controlling the flow of fuel, a liquid-fueled rocket engine can be throttled up and down, stopped, and restarted at a later stage. (Which is not possible with solid rocket boosters.)
They also have a higher specific impulse, meaning they are much more efficient at burning fuel than their solid-fuel counterparts.
There are three primary types of liquid rocket fuel currently in use by the vast majority of modern rockets. They are:
- Liquid Hydrogen
- Liquid Methane
1) RP-1 (Rocket Propellant-1)
RP-1 or rocket propellant-1 (also known as refined petroleum-1) is a highly refined form of kerosene. Also known as kerolox (when combined with oxygen), RP-1 is one of the first liquid fuels used in the space industry and combines with liquid oxygen (LOX) to combust.
RP-1 has several advantages that make it the fuel of choice for rocket manufacturers. It is stable and can be stored at room temperature. It is also cheaper than other types of liquid propellant and is less explosive, reducing the risk of uncontrolled combustion during launch.
Although it doesn’t have the high specific impulse of fuels like liquid hydrogen, it is much denser, which means it requires much smaller fuel tanks to carry the same amount of fuel. The resulting higher energy density also allows it to provide more thrust during launch.
This type of fuel was used in the first-stage boosters of the Saturn V and also used by the first stages of modern launch vehicles like the Atlas V, Falcon 9 & Heavy, Russia’s Soyuz, and China’s upcoming Long March 9 rocket.
(Learn more about RP-1 propellant, what exactly it is, how it is made, and its various advantages and disadvantages in this article.)
2) Liquid Hydrogen
Liquid Hydrogen (LH2) is a fuel with a very low density and a high specific impulse. Also known as hydrolox (when combined with oxygen), it is typically used in the upper stages of rockets and combines with liquid oxygen (LOX) to combust.
It is a cryogenic fuel as it remains a gas under room temperatures and has to be cooled to temperatures of -253° Celsius (-423° Fahrenheit) to turn into a liquid. This makes the fuel difficult to store and transport.
Liquid hydrogen’s low density also means much larger fuel tanks are needed to store the fuel than similar liquid propellants, resulting in increased rocket mass and size. Special materials also have to be used to insulate the propellant on the launch vehicle.
However, the high specific impulse properties of the fuel make it very efficient, and combined with its low weight, also make it ideal for operation in space. Since the byproduct of burnt hydrogen is mostly water vapor, it is also an environmentally friendly alternative.
Another advantage of liquid hydrogen is that, because of its extremely low storage temperatures, it can be used as a coolant and is pumped through parts of a rocket engine that gets extremely hot, like the nozzle of a rocket thruster.
The main engines of the Space Shuttle used liquid hydrogen (assisted by solid rocket boosters during launch). Currently, United Launch Alliance’s Delta IV Heavy, the Ariane 5, and the Centaur upper-stage (used on many launch vehicles) also use liquid hydrogen.
(Learn more about liquid hydrogen, what it is, as well as the different advantages and drawbacks of this fuel in this article.)
3) Liquid Methane
Liquid Methane (CH4) is a relatively new type of liquid propellant in development and hasn’t been widely used in production yet. Also known as methalox (when combined with oxygen), it has to be combined with an oxidizer (liquid oxygen) to combust and burn.
Like liquid hydrogen, liquid methane is a cryogenic fuel and has to be cooled to a temperature of -162° Celsius (-260° Fahrenheit) to turn into a liquid, which makes its storage and transport difficult.
However, it has the advantage of having a higher specific impulse than RP-1 fuel resulting in overall better engine efficiency. It is also much denser than hydrogen, meaning that smaller fuel tanks are required, saving on rocket mass and size.
Like hydrogen, liquid methane is also a clean-burning fuel, emitting little to no pollutants. In summary, it has most of the advantages of liquid hydrogen fuel, with few of its drawbacks.
One of the main attractions of liquid methane, especially as space agencies are looking toward interplanetary travel, is that it is possible to produce it on planets like Mars. This will potentially solve the problem of carrying enough fuel for return flights on spacecraft.
SpaceX’s Starship, of which several prototypes have already been tested, uses methane for its Raptor engines. Upcoming rockets like the New Glenn and United Launch Alliance’s Vulcan will also use liquid methane.
(Learn more about liquid methane, what exactly it is, how it is made, and its various advantages and disadvantages in this article.)
Liquid rocket propellants can also be divided into three separate categories according to their specific characteristics, namely:
- Petroleum-Based Fuel
- Cryogenic Fuel
- Hypergolic Fuel
1) Petroleum-Based Fuel
A petroleum-based fuel is derived from fossil fuels, specifically crude oil. It is essentially a mixture of hydrocarbons, meaning it is a form of organic compound that exclusively consists of carbon and hydrogen.
RP-1 (or rocket propellant-1), which is a form of highly refined kerosene, is the best example of this type of rocket fuel.
2) Cryogenic Fuel
Cryogenic fuels are substances that remain in their gaseous state at normal temperatures in the Earth’s atmosphere and have to be cooled to subzero temperatures to condensate into their liquid form.
Hydrogen, methane, and oxygen fall within this category. Hydrogen gas has to be cooled to -253° Celsius (-423° Fahrenheit) to turn into a liquid, methane gas to -162° Celsius (-260° Fahrenheit), while liquid oxygen needs to be cooled to -183° Celsius (-297°Fahrenheit).
As previously stated, keeping cryogenic fuels in their liquid state makes it difficult to store and transport these propellants.
3) Hypergolic Fuel
Hypergolic fuels are propellants that spontaneously combust upon contact with each other without the need for an ignition mechanism. They can safely be stored for long periods of time at normal temperatures. Their ignition can also easily be stopped and restarted.
These attributes make hypergolic fuels ideal for launch vehicles like intercontinental ballistic missiles that require long-term storage. It also makes them suitable for orbital maneuvering systems in space where quick, reliable ignitions are essential.
However, hypergolic fuels are highly toxic and should be handled with extreme care. It makes them unsuitable for most launch vehicles operating in the atmosphere since a rocket failure on the launchpad or close to the ground will have a major environmental impact.
The fuel component of hyperbolic propellants is usually a form of hydrazine like monomethylhydrazine (MMH) or unsymmetrical dimethylhydrazine (UDMH). The oxidizer component is typically nitrogen tetroxide (NTO) or nitric acid.
(All three fuel types are highly flammable, and during launch, more than 85% of a rocket’s mass consists of fuel. This explains why a launch vehicle failure on the launchpad or in the atmosphere typically results in a spectacular explosion. Learn more in the following article.)
Liquid Oxygen (LOX) – The Oxidizer Of Choice For Liquid-Fueled Orbital Rockets
All three liquid fuels, like most flammable compounds, require some form of oxidizer to supply the necessary oxygen that is the essential second component in any combustion process. As already mentioned, all three liquid fuel types use liquid oxygen as their oxidizer.
Liquid oxygen (or LOX) is also a cryogenic compound that needs to be cooled to temperatures of -183° Celsius (-297°Fahrenheit) or lower to remain in liquid form. Like the other cryogenic fuels, this makes transporting and storing the compound more challenging.
It is also stored separately and only pumped into the launch vehicle’s internal tanks a few hours before launch when the rocket is already secured on the launchpad.
Liquid Oxygen often starts to evaporate once it is in the oxidizer tanks in the rocket and needs to be vented to avoid unnecessary pressure buildup within the vehicle while it is waiting for liftoff to commence. Lost LOX also needs to “topped up” during this period.
(The “smoke” that can be seen coming off SpaceX’s Falcon 9 rockets on their launchpads is nothing more than evaporated oxygen that is vented, and upon coming into contact with the warmer atmospheric air outside the launch vehicle, it condensates.)
Solid Rocket Propellants
Besides liquid propellants, solid rocket propellants are the most commonly used rocket fuel. Unlike a liquid-propellant rocket that carries its fuel & oxidizer in separate internal tanks, the propellants and oxidizers are mixed and cured into a solid state during manufacturing.
The fuel/oxidizer mix is applied to the inside of the rocket’s shell and runs the full length of the rocket. Binding and curing agents are also added to the mixture to produce a solid rubber-like compound.
A cylindrical hole also running the entire length of the rocket acts a the combustion chamber, from where the hot gases are expelled through the rocket’s nozzle. By changing the shape of the hole, the burn rate can be adjusted to control the engine’s thrust.
There are two primary types of solid rocket propellant, namely:
- Homogeneous Mixtures
Homogeneous Mixtures can be single-, double-, or triple base mixtures, depending on the number of primary ingredients. The microscopically small ingredients are mixed as a liquid with binders and curing agents and turned into a solid compound.
Typical fuel types for homogeneous mixtures include RDX and nitrocellulose. RDX acts as both a fuel and oxidizer, while nitrocellulose acts as a fuel, oxidizer, and structural polymer.
Composites typically consist of fuel particles like powdered aluminum or beryllium mixed with solid oxidizer granules like ammonium perchlorate or potassium nitrate. Binding and curing agents are also added, and the mixture is set into a solid state.
The more simplistic design of a solid-propellant rocket allows it to be much lighter than its liquid propellant equivalent, which gives it a much greater power–to–mass ratio.
Solid rocket boosters are typically used as strapon boosters to assist with the first stage of a rocket launch when the biggest amount of thrust is required. The best example of this application is the large solid rocket boosters that flanked the Space Shuttle during liftoff.
The main disadvantage of solid propellant rockets is that once ignited, they cannot be switched off and continue to burn until all the fuel is depleted. They also cannot be throttled or restarted at a later stage.
(Learn more about solid rocket propellant, what exactly it is, how it is made, and its various advantages and disadvantages in this article.)
Other Rocket Propellants
The vast majority of rockets, past and present, use either liquid or solid rocket propellant. There are, however, other types of fuel that have either been tried in the past or are possible future alternatives currently in development. It is worth noting some of the major ones:
- Hypergolic Fuels: An earlier section in this article already covered hyperbolic fuels. They are fuels that spontaneously combust upon contact with each other and can be stored for long periods of time at normal temperatures.
- Hybrid Fuels: These propellants consist of solid and liquid components. The fuel usually forms the solid part, while the oxidizer is the liquid. The oxidizer is forced into the repository containing the fuel (which also acts as the combustion chamber) and ignited. These fuels have the advantage of being as energic as solid propellants but can also be throttled and restarted by controlling the flow of the liquid oxidizer.
- Ion Thrusters: An ion thruster or drive is a form of electric propulsion used by spacecraft in the vacuum of Space. It works by creating positively charged atoms or ions, which are accelerated at very high velocities through electrically charged grids to produce thrust. (Learn more about ion thrusters and how they work in this in-depth article.)
Other types of rocket propulsion or fuel include compressed gases, thermal propulsion, and nuclear plasma.
As this article clearly illustrated, liquid and solid rocket propellants are the most widely used fuel used by modern launch vehicles. These are not new fuel types, and both forms of propulsion have been around for more than a century.
Although newer fuel technologies are being developed and tested, it is new types of liquid rocket propellants like methane that currently show the most promise.
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