I recently found a fun presentation on lunar excavation technologies that I thought deserved a bit wider circulation. I’ve actually been interested in lunar excavation for over a decade now (in fact, it played a role in leading me to my thesis topic, but that’s a post for another day), and I think that this presentation summed up a lot of my thinking on the topic better than I could.
The presentation was done by a company called Honeybee Robotics, that has been doing space robotics systems for over 20 years. I found out about them through a joint project they had been doing with one of our new neighbors at the Mojave Spaceport, Firestar Engineering. They had worked together on using the mixed nitrous monoprop that Firestar is developing as a gas generator for a lunar pneumatic excavation idea (which is discussed in this presentation, and also in this paper on using it for a Mars or Lunar sample return mission). I’ve been interested in the concept of pneumatic excavation ever since I read an article titled “Foundation Slab for Lunar Base Construction” that was presented at the fourth ASCE conference on Engineering, Construction, and Operations in Space back in 1994. The concept seemed to show a way of producing very large enclosed subselenian areas for relatively small initial investments in hardware and materials. Anyhow, my curiosity on the topic led me to do some searches to see if I could dig up any more information about their Lunar Pneumatic Excavator concept, and that led me to the presentation I wanted to write about in this post.
I’d strongly suggest reading the whole thing, but here are my notes on the paper:
- Lunar regolith is highly compacted, abrasive, high surface friction, and sticks together very strongly. All of these things drive up excavation forces.
- Most terrestrial excavation equipment uses the weight of the vehicle and the friction forces in its wheels/tracks to react against the excavation forces.
- Lower lunar gravity means that for typical lunar excavators, you might actually need several times heavier equipment to do the same job.
- Methods that lower the excavation force required can greatly reduce the mass requirement for the excavation equipment.
- Percussive/Vibratory excavation systems can lower the force and excavator mass requirements dramatically for such situations (up to a 10x mass reduction).
- Pneumatic excavation can be done using a coaxial tube setup, where the inner tube blows low-pressure gas at the regolith, and the slightly longer outer tube provides a return path for the gas vent. Gas doing the turn from the inner tube to the outer annulus imparts momentum into the regolith, and then entrains the regolith particles in the gas forcing it up the outer tube (see illustration below)
- In an experiment performed on a vomit comet, they showed that a lunar pneumatic excavator system operating at 7psia, could give a regolith mass excavated to gas expended ratio of over 3000:1 in a 1/6g environment (ie each gram of gas could move over 3kg of regolith)
- Such excavation techniques can also be used for sample return or prospecting missions, using relatively simple hardware with almost no moving parts.
They didn’t go into it much, but if you could store the gas as a liquid (either inert, or as rocket propellant), and then vaporize or combust it before shooting it down the nozzle, you could excavate a pretty sizable amount of regolith using a pretty small volume of liquid. That means that a tank about the same size as the propellant tanks we’re using on XA-0.2 (36in spherical tanks) could hold enough liquid CO2 to excavate enough regolith to bury a Bigelow Nautilus module. Of course, to move the regolith back, you’d need another tank that big. Call that about 700lb of CO2 and about 100lb of tankage. Not bad if those numbers are accurate.
Anyhow, read the presentation and that other article, and post your thoughts in comments.