Making Rocket Fuel on Mars
When humans get to Mars, Elon said the first order of business is propellant production. This means producing liquid methane and oxygen, Starship’s fuel, using only the resources and raw material gathered on Mars.
Why produce propellant on Mars?
The ability to produce rocket fuel without importing it from Earth is critical if humans want to successfully build a self sustaining base on Mars:
- To ensure humans have consistent fuel supply on the red planet for return missions.
- To reduce weight by minimizing the amount of fuel carried onboard Starship. Carrying less, we reduce launch mass and enable more efficient flights.
“In-situ propellant-production is critical to the space architecture needed for a long-term human presence on Mars, future interplanetary transport, and eventually, multi-planet colonization.”
– NASA
How does rocket propellant work?
Rocket propellant needed to launch rockets from the surface of a planet into orbit and beyond. Starship uses chemical fuel like methane and oxygen, in contrast to satellites and deep space probes which use electrical propulsion such as ion thrusters.
There are a few different types of chemical based rocket fuel, but they all have a few things in common.
Rocket fuel works similarly to the way gasoline in a car works, via combustion. For any combustion reaction, you need two things: a fuel source, and an oxidizer. The oxidizer accepts electrons. Because oxygen has 6 electrons in its outer shell, it accepts 2 electrons to create 8, a full outer valence shell.
For a combustion reaction, you really just need oxygen and a fuel source. The fuel could be liquid, like gasoline, or solid, like gunpowder. Rocket fuel is complicated – over the years, chemical engineers have tried different combinations of fuels and oxidizers to try to find the optimal rocket propellant:
- Liquid Hydrogen, for example, is the most efficient. The problem with hydrogen fuel for Mars missions is that it has a boiling point of -423 degrees F, which presents a challenge keeping it in liquid state during long trips and during the friction-intensive high temperature entry burns.
- RP-1 is another type of rocket fuel similar to kerosene, but is not suitable for SpaceX’s goal of having rapidly reusable rockets because it leaves a large amount of soot residue after use, which requires extensive effort to clean.
There are a few differences between gasoline and rocket fuel: In the case of an automobile, oxygen is readily available in the environment to be used as an oxidizer. In a rocket travelling through outer space, the oxidizer must be carried along with the fuel in a separate tank. Space travel means moving through a vacuum, where you don’t have the luxury of an endless supply oxidizer in the surrounding space.
To get to and from Mars, methane and liquid oxygen will be used. Although Mars doesn’t have an abundance of liquid Methane like Saturn’s moon Titan, the good news is that methane can be readily made on Mars from material that is available in the ground and atmosphere.
Key Requirements for Mars Fuel
It will be in our best interest to implement the following items into the propellant production process:
- Minimize electrical power needs because all electric power will need to come from batteries, solar power, or nuclear power. There is no way of knowing whether or not Mars will have fossil fuels beneath the surface, and we cannot rely on this.
- The propellant production process will be heavily dependent on chemical engineering and the ability to complete multiple chemical reactions and separations sequentially. In addition to the desired products of Hydrogen and Methane, the process will produce by-products, many of which are useable for other endeavors on Mars.
- Nitrogen is one byproduct that is specifically useful because it is inert and non-reactive. The gas can be used used for flushing of tanks and lines through which other gases pass since it is reactive neutral.
- Oxygen and water byproducts are both potentially valuable feedstock for making propellant oxidizer or for life support/drinking. For this reason, Mars engineers will need to consider options, means, and costs in any facility design with business analysts to determine the costs to market value of any manufacturing byproducts.
Efficiency: one metric ton of propellant per 17 megawatt-hours energy input. Starship needs 240 tons of fuel – which will require 4.1 gigawatt hours of energy input.
How long does propellant production take?
How much time does it take to make enough fuel for launching Starship.
This brings us to outlining the process of creating fuel. There are 4 key steps.
Chemical Reactions to make Methane Rocket Fuel:
Pre-Requisites to fuel creation:
Gathering CO2
Carbon dioxide is highly available in Mars’ atmosphere, 20 times as much as on Earth. We would likely use a type of air pump to gather the CO2.
Separation and removing contaminants
To obtain pure CO2, dust filtration will be important in this step of the process as well as removing the small amounts of ambient gasses including nitrogen, argon, neon, and krypton. Carbon molecular sieves (CMS) will be used to separate the carbon dioxide from the nitrogen, and a Vortex Swirl particle separator will use used as well.
As the reaction proceeds and produces methane and water, a separator will be used to remove the water vapor, leaving pure methane. This will be done by simply allowing the products to cool, so that water goes through the process of condensation then stored in tanks.
An important consideration is: How do we make sure no contaminant gasses are present with additional harmful byproducts?
The diagram below shows the reactions that will be required for producing propellant on Mars.
The reactions get complicated, and while I started covering them below, I found this super helpful video on YouTube that covers the chemical reactions as well as a lot of other information about making rocket fuel on Mars.
1. Electrolysis of Water
Hydrogen is the critical component that is hard to get, which we must separate from water. We need to pump water from underground wells, use robotic vehicles to mine raw water ore.
– Robotic vehicles, such as the NASA KSC Regolith Advanced Surface Systems Operations Robot (RASSOR) prototype or the OffWorld Inc.5 smart robots, are likely candidates for mining the raw “water ore”.
Then, the water will go through electrolysis to break water into components Oxygen and Hydrogen.
Once this is done, we can use the hydrogen and CO2 from the atmosphere to run the Sabatier methanation reaction.
Note, we will have to do the electrolysis of water twice throughout.
2. Sabatier Reaction: Carbon Dioxide and Hydrogen to create Methane gas and Water.
The Sabatier methanation reaction has been recommended by many of the top researchers as the most likely basis for an ISRU propellant production processing plant on Mars.
This reaction leverages the abundance of CO2 in the Martian atmosphere to create both the methane fuel and water. CO2 is taken from the atmosphere by either freezing the gas into a solid, mechanical compression, and absorption pumping. Freezing will purify the gas, and may be advantageous.
Nickel or aluminum oxide transition metal catalyst is required (why?)
The reaction has to be carried out at high temperatures (why doesn’t it happen at low temp?) There is a principle in chemistry that describes the effect of temperature on reaction speed.
“Higher temperatures mean faster reaction rates; as molecules move about more quickly, reactant molecules are more likely to interact, forming products…” – sciencing.com
Since mars is cold, the reaction has to take place in an insulated container. The good news is that the reaction is exothermic, so once it starts, there isn’t much energy required to keep it going at temperature.
This is used today on the international space station to form water for astronauts to use.
3. Carbon Dioxide solid oxide electrolysis to create Oxygen and CO byproduct.
4. RWGS (reverse water gas shift)
A way to supplement producing methane and oxygen from hydrogen and carbon dioxide.
Sources:
- https://www.thespacereview.com/article/3479/1#:~:text=It%20involves%20the%20reaction%20of,oxide)
https://www.sciencedirect.com/science/article/abs/pii/0094576578900498
https://www.lpi.usra.edu/meetings/ISRU-III-99/pdf/8004.pdf
https://ntrs.nasa.gov/citations/20120001775
https://www.nasa.gov/directorates/spacetech/niac/2020_Phase_I_Phase_II/lFueling_a_Human_Mission_to_Mars/ - how does rocket fuel work: https://www.grc.nasa.gov/www/k-12/airplane/rocket.html#:~:text=In%20a%20rocket%20engine%20%2C%20fuel,the%20flow%20and%20produce%20thrust.&text=In%20a%20solid%20rocket%2C%20the,packed%20into%20a%20solid%20cylinder.
- NASA Beginner’s guide to rockets: https://www.grc.nasa.gov/www/k-12/rocket/guided.htm
- Marspedia.org
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