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.
Jonathan Goff
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The one gas that can be made on the Moon is oxygen. Inflammability puts limits on the material that the drill can be made from, for instance stainless steel rather than iron.
CO2 is available on Mars.
To move the regolith “back”, one might “only” need a conveyor belt.
Also, I wonder whether there would be advantages to rapidly pulsed gas jets. The scoop benefits from controlled vibrations (tuned to regolith characteristics). The addition of either strong acoustics or mechanical pulsing might complicate things a lot, though.
I understand the basic concept here, my only concern is that it may be difficult for a robot to keep the nozzle positioned just right so that the path of least resistance for the gas is back up the tube rather than through the regolith and out into space.
I also understand that they are concerned with the mass of the excavator (and this offers a nifty way to get around some of that), but have they considered a design as simple as a street cleaner? Or for that matter, what about using a light-weight frame for the main excavator, and then load it down with regolith (a la dump truck mode), or a manned crew cabin for jobs that require human presence?
Eric,
I’ve also been a fan of the street-sweeper approach, but I think the pneumatic excavator has its place, just because it looks like it is very fast, and solves both the excavation and moving steps at the same time. But I do agree, that there’s probably some work in there on making sure that the dust and gas mostly go up the outer tube instead of just blowing all over the place.
~Jon
MG,
Pulsed fluid jets do cut better than steady-state ones (that was what my thesis was about), though with 7psi air, I’m not sure how much better. If the central jet was some liquid that only flashed to gas after hitting the lunar regolith, then you could probably do something like the pulsed waterjet work I did for my thesis.
~Jon
Interesting article. Thanks for summarizing it.
My favorite way of moving regolith is something like the magnetic conveyor belt ( see http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1662.pdf ).
After I read about lunar dust hovering due to electrostatic forces I imagined something similar with electrostatic instead of magnetic forces. Electron cannon charges dust. The dust is then moved by a peristaltic modulation of electrostatic fields.
This probably moves only very small, loose particles, but for most tasks it is more important to move regolith than to dig deep. Covering a habitat with regolith is sufficient, no need to dig it into the ground.
Such methods need no carrier gas, but they need vacuum, and therefore won’t work at the NASA excavation challenge 🙁 Which, by the way and IIRC, also does not allow use of gas for excavating.
(Haven’t checked sources yet so this is a “drive-by” 😉
Have panels that “deflect” the regolith upwards and use extra gas to get a vertical velocity of “x”…
Simple math now tells you that the regolith will reach peak altitude an then fall back into place in “x” amount of time…
You now have “x” amount of time to emplace your “habitat” to be buried…
We now return you to your regularly scheduled comments…
(Without Randy’s “smart-axed” comments 😉
Randy
Definitely an interesting concept, essentially a reduction to the differential scale of the shot drills long used in quarrying. I do have a concern, though, as to its effectiveness against the cohesion of the regolith.
My suspicion is that it will pay to separate the function of breaking up, in place, the soil to be excavated from that of moving it to another place. See : http://www.lunarcc.org/papers/0207.html