Using in-situ propellant has been a central pillar of the plan to explore much of the solar system. The logic is simple – the less mass (especially in the form of propellant) we have to take out of Earth’s gravity well, the less expensive, and therefore more plausible, the missions requiring that propellant will be. However, a new paper from Donald Rapp, the a former Division Chief Technologist at NASA’s JPL and a Co-Investigator of the successful MOXIE project on Mars, argues that, despite the allure of creating our own fuel on the Moon, it might not be worth it to develop the systems to do so. Mars, on the other hand, is a different story.
Let’s be up front about something – many organizations, but NASA especially, are currently struggling with their lunar exploration programs. A perfect example is the cancellation last year of the VIPER rover, which was originally intended to scout for water ice in the Moon’s south polar regions. Its cancellation highlighted a simple fact – we have never successfully created propellant on the Moon from resources gathered there. And it doesn’t look like it will be easy to do so.
There are two main techniques put forward when discussing how to create propellant on the Moon. One is the carbothermal reduction process, and the other is mining polar ice. Both have severe logistical disadvantages and limited de-risking of their technology.
Fraser interviews Michael Hecht, one of the other researchers on the successful MOXIE demonstrator.
Methane is a key ingredient to the carbothermal reduction process, and it isn’t available on the Moon and must be shipped from Earth. In this process, regolith is heated to over 1650℃ where it creates a melt pool. Methane is then introduced to reduce the oxides present in the regolith, releasing the oxygen stored within. Not only does this require an external feedstock of an explosive gas, it requires significant power to get a reactor up to that temperature. According ot Dr. Rapp, it also requires a 14-step production cycle which will have to include autonomous excavators, vibratory inclines, and waste dumpers. None of those have yet been tested in a real lunar environment, though some have been preliminary tested in vacuum chambers.
While we know the general chemical makeup and form of regolith, we have much less data about the ice in the polar caps on the Moon. We know it’s there, but is it snow or rock hard permafrost? No one really knows, and that would dramatically change the processing technique used to extract it. VIPER was supposed to provide some ground-truths to that question, but its cancellation leaves a gaping hole in our knowledge of the water resources available there. But even if we understood what was available, there are still logistical nightmares for extracting it, including the fact that many of the Permanently Shadows Regions where the ice would exist literally lack any sunlight that could be used to power the processing systems needed to create the oxygen.
Contrast the massive derisking required by those propellant processing methods with that for Mars. MOXIE, which admittedly Dr. Rapp is partial to given his key role in the project, has already been operated successfully on Mars, separating oxygen out of the Martian atmosphere. Using the atmosphere, which the Moon lacks, is one of the key advantages of the technology. It doesn’t require complex mining, sorting, and waste disposal technology. You simply turn a pump on, and oxygen and carbon dioxide come out the other side. Scaling is a relatively simple engineering challenge, compared to the massive uphill technological one that faces propellant production on the Moon.
JPL Video showing how MOXIE was used on Mars. Credit – NASA Jet Propulsion Laboratory YouTube Channel
One final consideration about where to put investment resources is the amount of fuel needed to transport propellant from Low Earth Orbit (LEO) to these celestial bodies. By Dr. Rapp’s calculation, delivering 1 kg of propellant to the Moon requires 2.5 kg of spent propellant, whereas getting 1 kg to Mars requires between 8 and 10 kg of spent propellant. So even if engineers did develop a way to efficiently pull oxygen out of lunar soil, it still only has ¼ the return on fuel saved that investing in a technology to do so on Mars would.
That’s under the assumption, though, that Mars is our next target for a return mission. As of right now, the budget problems for the Mars Sample Return mission, which could use a technology like MOXIE, are putting that mission on a knife’s edge. While it might be the most costly option, getting in-situ propellant production up and running on a world where we’re actually planning on getting things off of it in the next few decades might be more valuable that developing a technology that might be able to scale simply, but would sit unused for even more decades. Resources for space exploration are limited, and sometimes deciding where they are best used isn’t based exclusively on making sure the technology works.
Learn More:
EurekaAlert / Beijing Institute of Technology Press – Near-term NASA Mars and Lunar in situ propellant production: Complexity versus simplicity
D. Rapp – Near-Term NASA Mars and Lunar In Situ Propellant Production: Complexity versus Simplicity
UT – This New Robot Has A Clever Spin On Lunar Mining
UT – Blue Alchemist Is One Step Closer to Creating Sustainable Infrastructure on the Moon