Modeling lunar in-situ resource utilization can help plan future prototypes

by Andy Tomaswick, Universe Today

In-situ resource utilization will likely play a major role in any future long-term settlement of the moon. However, designing such a system in advance with our current level of knowledge will prove difficult, mainly because there's so much uncertainty around both the availability of those resources and the efficacy of the processes used to extract them.

Luckily, researchers have tools that can try to deal with both of those uncertainties—statistical modeling. A team from Imperial College London, the University of Munich, and the Luxembourg Institute of Science and Technology recently released a pre-print paper on arXiv that uses a well-known statistical modeling method known as Monte Carlo simulation to try to assess what type of ISRU plan would be best for use on the moon.

First, the researchers had to define what type of ISRU methodology they would use. But why would you use one when you could potentially use two methodologies? The paper proposes a "hybrid" system that combines two commonly studied ISRU architectures: carbothermal reduction and water extraction from icy regolith.

Carbothermal reduction is a standard technology for oxygen extraction on the moon. It uses common regolith and a reducing agent like methane (which could be shipped from Earth) to extract oxygen from the lunar dust. It is widely used in industrial processes on Earth and, therefore, well understood, but it requires a large amount of power and does require the input of a carbon source (i.e., methane).

Water extraction is more familiar to most ISRU enthusiasts. This technology melts "icy" regolith from the permanently shadowed regions (PSR) at the lunar poles, extracting the water that has laid dormant there for billions of years. The result is water, which can be further split by electrolysis into hydrogen and oxygen (useful for jet fuel), and also "dry" regolith, which can undergo the same carbothermal reduction process described above.

The hybrid architectures described in the paper utilize both these techniques but in different orders. They detail a "parallel" architecture and a "series" architecture and discuss the advantages and disadvantages of each.

In this case, parallel architecture mines and processes the regolith directly in a PSR and the opposing "peak of eternal light" (PEL), which is rarely out of direct sunlight. Water is extracted from the regolith at the PSR and transported to the processing plant at the PEL to be split into hydrogen and oxygen, two of the main feedstocks needed for any future long-term human presence.

On the other hand, the series architecture does all the mining in the PSR, processes both icy and dry regolith directly in the PSR itself, and then transports it to an additional hydrogen/oxygen storage facility on the neighboring PEL. Each architecture requires a mobile hauler to transport the materials from one site to another, with an estimated distance of about 5km between them.

To calculate which architecture is better, the authors considered several distinct variables. One was the speed with which regolith can be processed—i.e., how much regolith an architecture had to excavate to meet an intended production target. Another was the power consumption of the system, which included considerations like whether or not local power would be available to it.

But one of the most important was the system's total mass required to achieve the production objective—since mass is equivalent to cost in terms of landing things on the moon, which was the equivalent of how expensive it would be to operate these systems fully.

The Monte Carlo simulations were used to calculate those values, given the uncertainty in process efficiency and availability of those resources. The authors ran a series of different scenarios with different input values for the important but little-understood features of the system, such as the amount of water available in the regolith and the efficacy of the carbothermal reduction method in low gravity.

Their simulations include results for the stand-alone architectures and the hybrid ones. Carbothermal reduction was the lightest architecture if the operational period was only one year. Still, its reliance on continual inputs of carbon sources from Earth made its weight cost rise significantly over a long time.

Water extraction was the architecture most affected by input variability. If the estimated water content in the PSRs is as high as recently stated, it would require very low power and mass, but given the uncertainty of those measurements, it's a risk to base the system wholly on those water content estimates.

Hybrid architectures try to take the best of both worlds. The parallel architecture ended up between the carbothermal reduction and water extraction methods regarding power consumption and regolith excavation requirements. Its main advantage seems to be that it is a potential test bed to prove both technologies before fully committing to either.

On the other hand, the series architecture is very efficient and doesn't vary much with differences in water content. However, it is very heavy compared to the other architectures, which is only exacerbated by the requirement to get power down into a PSR from the surrounding sunlit peaks.

Overall, there is no definitive winner regarding what architecture will be best for future lunar ISRU missions. More information is needed, especially about the overall availability of water and how the processes run. With further information, future mission architects and decision-makers can use the framework developed in the paper to help decide what architecture to use.

More information: Kosuke Ikeya et al, Hybrid lunar ISRU plant: a comparative analysis with carbothermal reduction and water extraction, arXiv (2024). DOI: 10.48550/arxiv.2408.04936

Journal information: arXiv

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