Methane as Rocket Fuel

Methane as Rocket Fuel

The highest specific impulse chemical rockets use liquid propellants (liquid-propellant rockets). They can consist of a single chemical, which is called a monopropellant, or a mix of two chemicals, which is called bipropellants. Bipropellants can be further divided into two sub-categories: hypergolic propellant, which ignites when the fuel and oxidizer make contact, and non-hypergolic propellant which requires a source of ignition.

For a chemical rocket, the highest specific impulse means the most propellant-efficient engine, producing either the most thrust for a given amount of propellant, or providing the desired thrust with less propellant. A higher specific impulse equals to a higher effective exhaust velocity, allowing the rocket to achieve a greater change in momentum from the same propellant mass. This efficiency is primarily determined by the propellant chemistry. For the best result, the usual combination is liquid hydrogen and liquid oxygen. Until now.

SpaceX’s Starship is one of the first orbital rockets to run exclusively on liquid methane; source SpaceX

In recent years liquid methane has become an alternative for rocket propulsion. Using liquid methane and liquid oxygen as propellants is sometimes called as a methalox propulsion.

We have to mention, that Methane (CH₄) is a naturally occurring gas, primarily formed by the decomposition of organic matter at shallow depths, such as in swamps and landfills, and also through thermogenic processes at high temperatures and pressures deep underground, which creates fossil fuels like coal and natural gas. It is found in sediments, rock, and as gas hydrates below the Earth's surface. 

Methane is the simplest type of hydrocarbon, consisting of one carbon and four hydrogen molecules.

Methane (CH₄) is the simplest hydrocarbon, an organic compound composed solely of one carbon atom and four hydrogen atoms. As the most basic hydrocarbon, it consists of a central carbon atom bonded to four hydrogen atoms, forming a stable, three-dimensional tetrahedral shape. 

As we already mentioned, Methane is naturally occurring gas. Liquid methane is a cryogenic fuel, which means the gas has to be cooled to temperatures of -162° Celsius or below to turn into a liquid. This extremely low temperature shrinks the volume of the methane by about 600 times, making it easier to store and transport as a clear, colorless liquid. On top of this, the purification processes also remove harmful or undesirable components, resulting in a cleaner fuel. 

As indicated, methane does require a certain amount of refinement to remove any unwanted compounds and refrigeration to produce the final cryogenic propellant. However, this process is much simpler and cheaper than the numerous complex steps involved in producing RP-1 propellant or liquid hydrogen. Both are also more expensive.

RP-1 propellant combustion produces pollutants including soot (black carbon), carbon dioxide, nitrogen oxides, and sulfur compounds, all of which significantly contribute to air pollution and climate change. 

On the other hand, methane is considered one of the cleanest-burning rocket propellants because its simple molecular structure leads to less soot and residue compared to kerosene-based fuels, making it a great choice for reusable rocket engines. Methane burns more completely, leaving less carbon buildup and reducing the need for engine cleaning and maintenance, than kerosene's complex hydrocarbon molecules. This property, combined with its potential to be produced on Mars, makes methane a key fuel for future reusable rocket systems like SpaceX's Starship. 

Why Methane Burns Cleaner

1. Complete Combustion: Methane, with its single carbon atom (CH₄), burns more efficiently and completely than complex hydrocarbon fuels like kerosene. 

2. Less Soot: The combustion process produces significantly less carbon soot, which is a major source of buildup and wear in rocket engines. 

3. Reduced Maintenance: The minimal soot production translates to engines that remain cleaner, requiring less maintenance and allowing for more frequent reusability. 

More than 85% of an orbital rocket’s mass goes to fuel since it takes an incredible amount of propellant to provide enough thrust to allow a large launch vehicle to push through Earth’s atmosphere and break from the gravity to reach orbit.

So, the question here is, how efficiently a rocket can burn its fuel. Specific Impulse is the term which is used to describe the efficiency and it is typically measured in seconds. The Specific Impulse of rocket engine is determined by the type of fuel it uses. Up to date, liquid hydrogen has the most fuel-efficient propellant for launch vehicles and is commonly used in the upper stages of many orbital rockets. However, hydrogen's low density means it requires much larger fuel tanks than other liquid propellants. Methane doesn't have the same specific impulse as hydrogen but it has a greater density, leading to smaller fuel tanks. But another thing is that methane has a higher specific impulse than RP-1.

Mars, the goal

Methane can be used as rocket fuel as it has been proposed as a fuel source for planets like Mars by utilizing local resources.

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. The Sabatier reaction is a process where carbon dioxide (CO2) and hydrogen (H2) react to form methane (CH₄) and water (H2O) in the presence of a catalyst, usually nickel, at elevated temperatures and pressures. 

Methane fuel can be created on Mars by combining hydrogen, sourced from the electrolysis of water, with the carbon dioxide from the Martian atmosphere using mentioned process called the Sabatier reaction. This in-situ resource utilization aims to produce fuel for return trips. Various methods, such as electrochemical systems, photoelectrochemical cells, proton-conducting ceramics, and synthetic enzymes, are being researched to efficiently carry out these processes on the Martian surface. 

There is another disadvantage of Methane.  Unlike some other propellants, methane and oxygen do not ignite spontaneously and require an ignition source to initiate combustion.

Zhuque-2 is a methane-powered orbital launch vehicle developed by the Chinese company LandSpace, and it was the first of its kind to successfully reach orbit. The rocket utilizes a liquid oxygen and liquid methane (methalox) propellant combination. The successful launch of the Zhuque-2 in July 2023 marked a significant achievement for LandSpace, placing it ahead of competitors in the development of methane-fueled rocket technology. 

Zhuque-2 [LandSpace]