Making Rocket Fuel Out of Lunar Regolith

In the coming years, NASA and other space agencies plan to extend the reach of human exploration. This will include creating infrastructure on the Moon that will allow for crewed missions on a regular basis. This infrastructure will allow NASA and its international partners to make the next great leap by sending crewed missions to Mars (by 2039 at the earliest). Having missions operate this far from Earth for extended periods means that opportunities for resupply will be few and far between. As a result, crews will need to rely on In-Situ Resource Utilization (ISRU), where local resources are leveraged to provide for basic needs.

In addition to air, water, and building materials, the ability to create propellant from local resources is essential. According to current mission architectures, this would consist of harvesting water ice in the polar regions and breaking it down to create liquid oxygen (LOX) and liquid hydrogen (LH₂). However, according to a new study led by engineers from McGill University, rocket propellant could be fashioned from lunar regolith as well. Their findings could present new opportunities for future missions to the Moon, which would no longer be restricted to the polar regions.

The research team was led by Sebastian K. Hampl, a M.Sc. Candidate in Mechanical Engineering at McGill University and part of the Alternative Fuels Laboratory. He was joined by multiple colleagues from McGill’s Department of Mechanical Engineering, as well as researchers from the Department of Aerospace and Mechanical Engineering at the University of Texas at El Paso, the Research Institute of Advanced Materials in Seoul, and the Eindhoven University of Technology in the Netherlands. Their paper, “Conceptual design of rocket engines using regolith-derived propellants,” recently appeared in Acta Astronautica.

Producing propellant from lunar resources is one of several measures designed to reduce the cost of missions to deep space. Whereas resupply missions to the International Space Station (ISS) can be mounted within a few hours, sending one to the Moon would take about three days. Based on current launch costs, sending one to the Moon would cost over $35,000 per kg ($15,909 per lb). When you factor in the time it takes to make a one-way transit to Mars using current propulsion technology – 6 to 9 months – the importance of ISRU becomes all the more apparent.

The need to produce propellant in situ will also reduce the mass and payload requirements of ships. As the Rocket Equation establishes, rockets generate thrust by expelling some of their mass (i.e. propellant). The amount of propellant is directly related to the spacecraft’s full mass and payload, which makes propellant the single greatest source of spacecraft mass. Consider the Block 1 variant of NASA’s Space Launch System (SLS) – the rocket sent the uncrewed Artemis I spacecraft beyond the Moon and farther from Earth than any crew-capable vehicle in history.

While the SLS weighs 1,588 metric tons (3.5 million lbs) when unfueled (aka. dry mass), it weighs up to 2,603 metric tons (5.74 million lbs) fully-fueled. The Starship and Super Heavy, the most powerful launch system in the world, has a total dry mass of 285 metric tons (~630,000 lbs) but weighs a whopping 4,885 metric tons (10.77 million lbs) fully fueled. In short, propellant mass makes up 64% and 94% of these spacecraft launch masses, respectively. As Hampl explained to Universe Today via email:

“We need to produce resources locally as they take up a lot of space in terms of payload on the rocket. That limits the amount of resources we can carry to the lunar surface. Without refueling, the range of the missions is very limited as every drop of propellant needs to be budgeted and if something goes wrong that uses extra propellant, the astronauts might not be able to return back to Earth. The system we currently have could be compared to a car infrastructure where you could only fuel up in one place on the whole globe and any “exploration mission” you want to do would have to be planned meticulously and every mistake could leave you stranded.”

The concept of ISRU is time-honored, though no attempts were made during the Apollo Era when astronauts last stood on the lunar surface. Currently, the main ISRU concept calls for harvested water ice from surface regolith and subjecting it to electrolysis to produce hydrogen and oxygen. But as Hampl indicated, surface water is localized on the Moon, existing in Permanently Shadowed Regions (PSRs) around the poles. In the South Pole-Aitken Basin, craters like Shoemaker, Shackleton, and Faustini all act as “cold traps,” ensuring that water ice does not sublimate from exposure to the Sun.

Furthermore, extraction is a challenge, and hydrogen storage for longer periods of time is very problematic. This imposes many limits, which is why Hampl and his colleagues began investigating an alternative that NASA investigated back in the 80s (but never developed). As Hampl explained:

“We proposed to use lunar regolith to derive propellants that are ubiquitous. From regolith, you can extract metallic components (which will be the fuel) and oxygen (which will be used as the oxidizer). We also investigate how extracting sulfur (which is abundant enough, albeit, not as abundant as the metallic components) to expand our options for rocket engine configurations. As oxygen production from regolith is vital for sustaining the lunar habitat, the reduction technology to extract oxygen from the regolith is being developed. The metallic powder will be a byproduct of the process and we conveniently propose to use it as the rocket fuel.”

A benefit of this process is that it will rely on space mining technologies developed by startups hoping to take advantage of the commercialization of Low Earth Orbit (LEO) and Cis-Lunar space in the coming decades. The process is also “fuel lean,” which refers to having more oxidizer than fuel in a rocket engine. “In our case, a small amount of metallic powder and a large amount of oxygen,” said Hampl. “The ratio of oxidizer and fuel can be adjusted and greatly influences combustion parameters such as temperatures and performance.”

The advantages of their proposed system are numerous. For starters, it would allow future missions to produce propellant anywhere on the lunar surface with electricity. “The only things one would need, obviously, are the production facility and an electrolyte, which probably will have to be brought from Earth (but the quantities are manageable),” said Hampl. “There are reduction methods only requiring electricity but they are less efficient and do not seem to work as well (research ongoing). Additionally, the propellant is easier to store, more dense than hydrogen, and could be transported more easily.”

Moreover, engines that rely on metallic powder propellant are currently being developed, especially with ramjets and applications for air-breathing propulsion. The one trade-off is that the predicted performance of a rocket using this propellant is less than what a rocket relying on LH₂/LOX can deliver. However, the “fuel lean” nature of their propellant results in much lower combustion temperatures, causing less material strain and reducing the cost of repair and refurbishment. In addition, the performance decrease compared to LH₂/LOX at lower combustion temperatures is not as pronounced.

This proposed method could open new doors for ISRU on the Moon and greater flexibility when it comes to refueling missions. “Our work focused on the thermodynamic calculations and proposing ways how this could be implemented as well as making the case where the advantages of this technology lie,” said Hampl. “We hope that someone will pick up the idea and start developing and testing such an engine since we strongly believe that this would be a better concept than using hydrogen/oxygen and should get more attention.”

It is fitting that in their plans to return to the Moon (this time, to stay), space agencies like NASA are reexamining concepts that were proposed during the Apollo Era but never developed. These concepts, which include everything from metallic propellants, ISRU, closed-loop habitats, and nuclear propulsion, will also be vital in exploring Mars and beyond. They will also be vital in our efforts to extend humanity’s presence beyond Earth and the Earth-Moon system.

Further Reading: Acta Astronautica