For more than half a century, RP-1, hydrogen, solid rocket & hypergolic fuel were the propellant of choice for launch providers. In recent years, though, liquid methane has become an attractive alternative for rocket propulsion.
In rocket propulsion, liquid methane is a cryogenic fuel used to power orbital rockets. Its high Specific Impulse, availability as natural gas, small carbon footprint, and ability to be manufactured on other celestial bodies make it an attractive alternative to traditional rocket propellants.
For more than half a century, rocket propellants like RP-1 and hydrogen were mixed and combusted with liquid oxygen (LOX) in combustion chambers of orbital launch vehicles to power their first and upper stages.
Solid rocket propellants are commonly used to launch and allow these large rockets to push through Earth’s thick atmosphere and escape its gravity, while hypergolic fuels are primarily used to power the Reaction Control Systems of spacecraft.
(Learn more about the different types of fuel orbital rockets use and their various advantages and drawbacks in this article.)
However, with the establishment of private space agencies after the turn of the century and a strong push to establish interplanetary travel and reach Mars within the next few decades, liquid methane is gaining popularity as the primary propellant to achieve this goal.
Private companies like SpaceX and Blue Origin are already developing rocket engines that run exclusively on this fuel type with their Raptor and BE-4 engines, respectively. As the following sections will describe, there is a good reason for this focus on liquid methane.
What is Liquid Methane?
In its normal state, methane (CH4) is a naturally occurring gas found in abundance within soil and rock sediments below the Earth’s surface. The majority of it is created by the decay and breakdown of organic matter beneath the Earth’s surface at high temperatures.
Like RP-1 propellant (which is a highly refined form of kerosene), methane is a hydrocarbon, meaning it is an organic compound consisting entirely of hydrogen and carbon. It is the simplest type of hydrocarbon, consisting of one carbon and four hydrogen molecules.
(It is its simple makeup that gives methane so many advantages over kerosene, which can consist of multiple chains of carbon and hydrogen combinations, which will be illustrated in upcoming sections of this article.)
Liquid methane is a cryogenic fuel, which means the gas has to be cooled to temperatures of -162° Celsius (-260° Fahrenheit) or below to turn into a liquid.
In rocket propulsion, liquid methane is used as fuel to power orbital launch vehicles. Its high Specific Impulse, low carbon footprint, and ability to be manufactured on other celestial bodies make it an attractive alternative to traditional rocket propellants.
Like all other fuels used in orbital rockets, liquid methane needs an oxidizer to combust. This comes in the form of liquid oxygen (LOX). The fuel and oxidizer are mixed in the combustion chamber, where they combust to form the hot gases that propel the spacecraft.
The following sections will highlight the specific advantages of using liquid methane as rocket propellant, as well as some of the drawbacks. However, one first needs to establish how this fuel is produced in the first place.
How Liquid Methane Is Made
Although natural gas can be found close to underground crude oil or coal reserves, deeper deposits often contain a much purer form of methane (CH4) that does not require as much refinement to remove unwanted compounds.
The gas that is captured underground in natural gas fields and below the ocean floor in pockets of sediment and rock formations is extracted by drilling vertical and horizontal wells to allow the gas to escape, after which it is brought to the surface.
From the wells, the gas is typically transported via a network of pipelines to processing plants, where water vapor and non-hydrocarbon compounds like helium, nitrogen, and carbon dioxide are removed to produce a pure form of natural gas (methane).
To turn it into a liquid, the methane gas is cooled to temperatures of -162° Celsius (-260° Fahrenheit) and below, which is the boiling point of methane.
Advantages Of Using Liquid Methane In Orbital Rockets
Compared to more traditional rocket propellants, liquid methane offers several advantages but also a few drawbacks. Some of the main advantages of using liquid methane include:
- Simpler And Cheaper To Produce
- Little To No Coking And Other Forms Of Residue Buildup
- Environmentally Friendly
- Higher Specific Impulse Than RP-1
- Can Be Produced On Other Celestial Bodies
- Smaller Fuel Tanks Required Compared To Hydrogen
- No Additional Compounds Needed To Keep Fuel Tanks Pressurized
- Allows Rocket Engines To Run At Higher Pressures
1) Simpler And Cheaper To Produce
As described in the previous section, liquid methane does require a certain degree of refinement to remove any unwanted compounds and refrigeration to produce the final cryogenic propellant.
However, the process is far simpler and cheaper than the numerous complex steps involved in the production of RP-1 propellant or liquid hydrogen, which are also both more expensive.
(Learn more about RP-1 propellant, what it is, and its different advantages and drawbacks in this article.)
In a typical orbital rocket, of which at least 85% of its mass consists of liquid propellant, keeping the cost of the fuel down is crucial and always has to be taken into consideration.
2) Little To No Coking And Other Forms Of Residue Buildup
In the description of liquid methane, it was highlighted that it is a hydrocarbon, but not just any hydrocarbon. Methane is the simplest type of hydrocarbon, consisting of only one carbon atom bonded by four hydrogen atoms.
This is in sharp contrast with the long chains of carbon and surrounding hydrogen molecules that make up RP-1 propellant, the fuel still used in the first stages of the majority of modern launch vehicles. (One chain can be up to 20 carbons in length.)
These complex long chains of molecules mean RP-1 never burns completely. Instead, it breaks down and produces soot and other residue buildups, commonly referred to as coking, in rocket engines. This has a number of adverse effects on orbital rockets.
Residue buildup within rocket engines can clog up rocket engines and reduce performance and reliability. It also makes reusing an orbital launch vehicle much more difficult since the coking and resulting damage result in a more complex and expensive refurbishing process.
Methane’s simple makeup means that when it burns, it burns completely and leaves no residue buildup within the rocket engine. This not only makes the engine perform more efficiently and reliably but also makes refurbishing the craft easier and less expensive.
3) Environmentally Friendly
RP-1 propellant does not only cause coking and other types of residue buildup. Its exhaust plumes also contain large amounts of carbon dioxide, soot, nitrogen oxides, sulfur compounds, and carbon monoxide. All of which contribute to air pollution.
In contrast, due to its ability to burn almost completely and its high hydrogen content, the exhaust plumes produced by the combustion of liquid methane primarily consist of water, some carbon dioxide, and small amounts of nitrogen oxides.
This makes methane one of the cleanest burning rocket propellants currently available, with only hydrogen capable of producing more environmentally friendly exhaust products.
4) Higher Specific Impulse Than RP-1
More than 85% of an orbital rocket’s mass consists of fuel since it takes an incredible amount of propellant to provide enough thrust to allow a large launch vehicle to push through Earth’s thick atmosphere and break free from its gravity to reach orbit.
As a result, one of the Holy Grails of rocket propulsion is how efficiently a rocket can burn its fuel. Specific Impulse is the term used to describe this efficiency and is typically measured in seconds. It is essentially the rocket equivalent of the automotive “miles per gallon.”
(Going into a detailed discussion about Specific Impulse falls beyond the scope of this article, but you can learn more about it in the following article about nuclear propulsion.)
The Specific Impulse of any rocket engine is determined to a large degree by the type of fuel it uses. To date, liquid hydrogen has proven to be the most fuel-efficient propellant for launch vehicles and is commonly used in the upper stages of many orbital rockets.
However, its low density means hydrogen requires much larger fuel tanks than other liquid propellants. This adds to the mass and size of the rocket, something rocket engineers are always trying to avoid or keep to a minimum.
Methane does not have the same efficiency (Specific Impulse) as hydrogen, but it has a greater density, requiring smaller fuel tanks for essentially the same amount of fuel. What also counts in its favor is that liquid methane has a higher Specific Impulse than RP-1.
To illustrate this point, one can look at the Specific Impulse generated by modern examples of rocket engines running on each fuel type:
- Liquid Hydrogen: 366 – 452 seconds (Space Shuttle/SLS RS-25 engine)
- Liquid Methane: 330 – 350 seconds (SpaceX Raptor engine)
- RP-1 Propellant: 282 – 311 seconds (SpaceX Merlin engine)
(Credit: Everyday Astronaut)
From this comparison, it is clear that liquid methane is not as energy-efficient as liquid hydrogen but significantly more efficient than RP-1. Combined with the smaller fuel tank requirements than hydrogen, one can start to see part of the appeal of using this fuel type.
However, as the following section will illustrate, the advantages of using liquid methane as a rocket propellant go far beyond its fuel efficiency and volume.
5) Can Be Produced On Other Celestial Bodies
In 2017, NASA launched its Artemis Program with the aim of returning humans to the Moon and establishing a base for further exploration of the Solar System, including Mars, a project that is also the main focus of private aerospace companies like SpaceX.
Taking all the fuel required for such a long trip will be impossible for any orbital rocket. Instead, scientists are looking to produce the fuel needed for the spacecraft on the planned destinations themselves, which is where the real advantage of liquid methane comes in.
Theoretically, methane can be produced on Mars. The planet’s atmosphere consists of 95% carbon dioxide and a substantial amount of water below its surface and on its poles. Through a process called the Sabatier Reaction, they can be used to produce methane.
If a production facility generating methane can be established on Mars, it will not only help to make interplanetary travel a more realistic endeavor but also make it sustainable. If successfully implemented, Mars can also be used as a base for further exploration.
To do this, spacecraft need to be powered by liquid methane, which is why companies like SpaceX and Blue Origin are investing so many resources in developing methane-powered rockets that can take advantage of the possibility of off-planet fuel production.
6) Smaller Fuel Tanks Required Compared To Hydrogen
As mentioned in the section on Specific Impulse, it was highlighted that liquid methane does not have hydrogen’s high Specific Impulse and, as a result, is not as energy-efficient.
However, it is also much denser, which means it requires smaller fuel tanks than hydrogen for the same amount of fuel, which not only brings the launch vehicle’s overall mass down but also allows it to be smaller.
(Combined with the fact that it also has a higher Specific Impulse than RP-1 propellant makes liquid methane an attractive option, even though RP-1’s high density means the latter requires even smaller fuel tanks.)
7) No Additional Compounds Needed To Keep Fuel Tanks Pressurized
All propellant tanks in an orbital rocket need to be pressurized and stay pressurized to allow continuous and consistent propellant flow and maintain the structural integrity of the tanks.
Typically, a light gas like helium placed in smaller tanks is used, which is released in the fuel tanks in a controlled manner to maintain the correct pressure. However, this adds to the complexity of the propellant tank structure and, again, adds to the mass of the rocket.
However, methane tanks can be pressurized by a gaseous version of the same fuel by warming up the liquid methane in the launch vehicle’s engine and using the methane gas to keep the tanks pressurized through a process called autogenous pressurization.
This makes the pressurization of propellant tanks in methane-fueled rockets a lot simpler, reducing possible complications and bringing the overall mass of the vehicle down.
8) Allows Rocket Engines To Run At Higher Pressures
As already stated, liquid methane has a higher Specific Impulse than RP-1 propellant due to its lower density. When they are burned at the same pressure inside a combustion chamber, methane offers around a five percent increase in performance compared to RP-1.
However, liquid methane can be burned at much higher pressures than its kerosene-based counterpart. (SpaceX’s Raptor engine is designed to run at pressures of up to 300 bar.) The increased pressure can result in a performance gain of approximately twenty percent.
Disadvantages Of Using Liquid Methane In Orbital Rockets
Despite the numerous advantages of liquid methane as a rocket propellant, it also has several drawbacks. The most notable disadvantages of using methane include:
- Do Not Produce The Same Amount Of Thrust As RP-1
- Larger Fuel Tanks Required Due To Lower Density Than RP-1
- Lower Specific Impulse Than Hydrogen
1) Do Not Produce The Same Amount Of Thrust As RP-1
Methane has a lower density than RP-1 propellant, allowing rocket engines to achieve higher exhaust velocities, which increases their Specific Impulse. However, due to the smaller molecular mass of the fuel, it does not create the same amount of thrust as RP-1.
RP-1’s higher thrust (as a result of the fuel’s larger mass per volume and resulting increased density) is crucial in a launch vehicle’s first-stage boosters to allow the rocket to push through Earth’s thick atmosphere & escape the planet’s gravitational forces to reach space.
2) Larger Fuel Tanks Required Than RP-1
One of the advantages methane has over hydrogen is that it is much denser, which means it requires smaller fuel tanks for the same amount of fuel, bringing the launch vehicle’s overall mass down and allowing it to be smaller.
However, the fuel is not nearly as dense as RP-1 propellant, which means the latter still has a clear advantage when it comes to tank size and can use smaller fuel tanks for the same amount of fuel (in mass). This is an important consideration when a rocket’s mass is crucial.
(This was one of the reasons why engineers only used hydrogen, which is even less dense than methane, for the 2nd and 3rd stages of the Saturn V rocket while using RP-1 for the first stage. They simply couldn’t practically make the first stage’s fuel tanks any larger.)
3) Lower Specific Impulse Than Hydrogen
As mentioned in an earlier section, liquid methane has a significantly higher Specific Impulse than RP-1 propellant, which makes it a more energy-efficient fuel. However, when it comes to fuel efficiency, no liquid propellant can match or even come close to liquid hydrogen.
Liquid methane has a Specific Impulse of 330 – 350 seconds compared to hydrogen’s 366 – 452 seconds, clearly showing hydrogen’s superiority in this regard.
This helps to explain why most launch vehicles like the Saturn V, Atlas, Delta, and Ariane V rockets use hydrogen to propel their upper stages. Liquid methane simply does not provide any clear advantage when it comes to fuel efficiency.
(Learn more about liquid hydrogen, what it is, as well as the different advantages and drawbacks of this fuel in this article.)
Conclusion
Until recently, liquid methane provided no clear advantage over Liquid hydrogen or RP-1 propellant as rocket fuel. It is more energy-efficient than RP-1 but not as much hydrogen. It requires smaller fuel tanks than hydrogen but is still larger than the ones used by RP-1.
However, ever since aerospace companies like NASA and SpaceX indicated their intent to make a concerted effort to reach Mars within the next few decades, methane has become a very attractive proposition as the fuel of choice for this objective.
Combined with numerous other advantages the fuel provides, it is clear why liquid methane is receiving so much attention and why several new rocket engines are being developed running on this fuel type.
This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.
