Microwaving the moon may be best for landing spacecraft, propose scientists

The NASA-funded study, titled ”The Cost of Lunar Landing Pads with a Trade Study of Construction Methods,” published recently in the journal New Space, analyzed the challenges of construction in conditions where lunar dust would swirl around at high speeds during launches or landings, as there’s no air to slow down the rocket’s plume. Another difficulty of building on the Moon is getting materials and the necessary equipment up there, given the exorbitant expense of transportation. 

How to build a lunar landing pad

The study, carried out by the defense and space manufacturing company Cislune along with researchers from the University of Florida (UCF) and Arizona State University, concluded that the easiest and most economical way of building the lunar landing pads may be by utilizing sintering - a method that uses microwaves to melt the soil, while also engaging beneficiation, or sorting, technology.

To arrive at this approach, the researchers studied four construction methods, poring over various combinations of inner and outer landing pad rings. If the cost of transportation to the Moon stays above $100,000 per kilogram (or around $45,000 a pound), sintering was found to be the cheapest method. Savings would increase even more if the UCF-developed beneficiation technology is also employed, as it can use magnetic fields to pull the most microwavable minerals to the surface, according to the study. Beneficiation works by utilizing the fact that UCF scientists discovered that many of the most microwavable minerals happen to be the most magnetic. Sorting particles based on “magnetic susceptibility” could improve microwave absorption by 70 percent to 80 percent, shared Dr. Philip Metzger, co-author of the research paper and planetary scientist at UCF’s Florida Space Institute, in a press release.

The construction process would entail having rovers scoop up lunar soil, sort it using magnetic fields, then put the soil back on the surface and melt it with microwaves.

Interesting Engineering (IE) reached out to Dr. Metzger for more details on their work. The following conversation has been lightly edited for clarity and flow.

Interesting Engineering: What kind of technology is required to carry out the microwave sintering?

Professor Metzger: For the microwave sintering, you first need the ability to do site preparation, including bulldozing and grading the surface. Second, you need power generation equipment so you can have enough power for the microwaving process. You can get power either from solar or nuclear systems delivered to the Moon. Third, for highly efficient sintering, you need to beneficiate the soil at the lunar surface first, using magnetic fields to sort the sand grains according to how magnetic they are.

Magnetic sorting of the soil before microwaving is a patented invention we innovated here at the University of Central Florida. It can reduce energy needs by 70 to 80 percent, which is huge. The device that does that process would need to scoop up a layer of soil, maybe 20 cm thick, and run it over a large magnet, so the soil grains fall into different bins according to their magnetic susceptibility. Then the grains flow back out and are layered onto the lunar surface, with the less magnetic grains going down first and the more magnetic ones going on top of them. That ensures that most of the microwave energy will be absorbed in the top layer that we are trying to sinter.

Fourth, you will probably need to roll a device over the soil to compact it more, but further research is needed to see if this step is needed. Fifth, you use a simple microwave device, which could be very similar to the one you have in your kitchen, to direct microwaves down onto the soil until it slightly melts. And that is it! 

IE:  How large of an area of the Moon would need to be "microwaved"? What area is best for sintering?

The size of a landing pad depends on the size of the spacecraft you plan to land on it. Heavier spacecraft require more thrust during descent to counteract lunar gravity, and the amount of thrust determines how dense the rocket exhaust is while it blows across the pad, out over the edge of the pad, and over the untreated lunar soil surrounding the pad. If the pad is not wide enough, then that gas will still be able to lift up soil around the pad’s edges and blow it at high velocity onto surrounding hardware. That is what we are trying to prevent since the blowing soil and dust can travel much faster than a bullet and can damage surrounding hardware. So we need the pad to be wide enough so that all the gas that flows past the edge of the pad will have spread out into the lunar vacuum enough that it is no longer capable of lifting and entraining the soil particles. We estimate that a 40-ton lunar lander would require a pad that is about 27 meters from the center to the edge. 

IE: Are there plans to test this approach here on Earth or on the Moon?

We are still developing the technology, so we are applying for grant funding from NASA and other government agencies, and our partner on this project, the Cislune Company, is seeking funds to advance the technology. Eventually, a full-size landing pad should be built in some desert here on Earth so we can demonstrate the full operation and then have a small rocket land on it as a final demonstration. We should also send a smaller payload to the lunar surface to test the beneficiation and microwave processes using real lunar soil in the lunar environment. Those tests should be enough so that then we can build the full-scale versions to go to the Moon.

Abstract: 

This study estimates the cost of building lunar landing pads and examines whether any construction methods are economically superior to others. Some proposed methods require large amounts of mass transported from the Earth, others require high energy consumption on the lunar surface, and others have a long construction time. Each of these factors contributes direct and indirect costs to lunar activities. The most important economic variables turn out to be the transportation cost to the lunar surface and the magnitude of the program delay cost imposed by a construction method. The cost of a landing pad depends sensitively on the optimization of the mass and speed of the construction equipment, so a minimum-cost set of equipment exists for each construction method within a specified economic scenario. Several scenarios have been analyzed across a range of transportation costs with both high and low program delay costs. It is found that microwave sintering is currently the most favorable method to build the inner, high temperature zone of a lunar landing pad, although other methods are within the range of uncertainty. The most favorable method to build the outer, low temperature zone of the landing pad is also sintering when transportation costs are high, but it switches to polymer infusion when transportation costs drop below about $110K/kg to the lunar surface. It is estimated that the Artemis Basecamp could build a landing pad with a budgeted line-item cost of $229M assuming that transportation costs will be reduced modestly from the current rate $1M/kg to the lunar surface to $300K/kg. A landing pad drops to $130M when the transportation cost drops further to $100K/kg, or to $47M if transportation costs fall below $10K/kg. Ultimately, landing pads can be built around the Moon at very low cost, due to economies of scale.