Moving Regolith

 

Quite often some form of bulldozer on the moon or Mars to dig ores or cover habitats is suggested. There seems to be a meme out there that the best way to move sandy gravely material is similar to the way it is done here on Earth. It is sometimes suggested that this moondozer will have to be weighted down to get sufficient traction to dig up the packed lunar regolith for mining or radiation protection.  A better approach might be to design a more limited machine for a specific type of task. Such a design effort might profit by looking at older technology that was in use before the availability of large reliable diesel engines. People that had to do things with a ten horsepower engine that weighed a ton* or more had to find ways of applying that power.            *On display at Florida Flywheelers park in Ft Meade FL.

One reason for looking at other techniques are shipment costs from Earth at  a hundred grand or so a pound, depending on assumptions. My small Caterpillar rubber track loader weighs about seven thousand pounds.  Seven hundred million dollars to deliver a machine that won’t even work properly when it gets there is a show stopper. It would weigh about twelve hundred pounds on the moon which wouldn’t provide enough traction to dig up packed soil.

The second main reason is the problems of a mobile power plant in an airless environment. Even if you put an electric motor with the same power in my loader, it would need a massive extension cord to deliver voltage for a sixty horsepower engine. That or a massive battery pack that would be even heavier and more shipment cost. Heat dissipation from both the electric motor and extension cord would be a problem with no air to get rid of the excess heat.

The traction issue would be as large a problem as the heat dissipation unless a few tons of ballast were added to the loader. The view of most equipment designers I’ve talked to seems to be that ballast is a sign of poor engineering. Whether that is totally true or not, it remains that carrying excess mass costs energy and causes equipment wear.

My excavator weighs about the same as the loader and would have many of the same problems. Anyone that has ever tried to dig hard ground with an excavator or backhoe will understand that the machine needs weight to hold it down. Then the material has to be transported to the ore processing facility  or habitat being covered. Excavators do poorly at material transport.

Robots of smaller dimensions will certainly be used in the early stages. Moving a hundred tons of material with a hundred pound machine is certainly feasible in a lunar day. The problem is that even the smaller robots have the same drawbacks as the larger machines.

An older tech that is still in use is the pan type machine. Still in use today hauling millions of tons of material around mines and construction sites, they started out much smaller behind horses. In the simple description, a pan is a box with four wheels and an open end. The horses, oxen, or tractor drag the box along the ground with the open end first. As the box fills, it adds weight to the pan, which both makes it harder to pull and adds ability to dig deeper. When the pan is filled, the box is raised just a bit to clear the ground and the material is transported to the material dump site. At the dump site the bottom of the box is opened so that the material falls out in the right location. Different techniques like shovels or bulldozers are used to push the material the last few feet when desired.

A different old tech is a stationary traction engine pair. A steam engine, windmill, or early internal combustion engine at a fixed location pulled a cable across a field. The plow or other farm implement attached to the cable was pulled across without the need for wheels or tracks that could supply the traction. The best current mind picture would be a couple of fast winches on opposite sides of the field to drag the implement back and forth. As clumsy as this method is to us, it was an advance on feeding and tending livestock year around in some parts of the world when they could only work a few months a year at best.

For a Lunar regolith mover, I suggest a single traction engine at the mine or habitat site that pulls a simple pan to gather regolith and pulls it right up to the stationary location. The traction engine can be powered by solar or nuclear power that also serves the base. A tiny electric motor on the pan just sufficient to move the empty pan reverses to the back of the digging area before the next regolith pull. The traction engine can serve two or more pans by pulling in one while the other reverses for its’ next load.

 

regolith pan

The first landing with telerobotic pans and traction engines could well total under a ton. A few pans that weigh ten pounds or so each with a similar mass of cable could be used to test gather material to the stationary lander. A ten pound pan could gather fifty pounds or so of material on each pass. At ten miles an hour and a 300 foot radius, each pan could make seven trips an hour. Two pans could gather seven hundred pounds of material an hour for a total of about a hundred tons during one Lunar day.

The stationary power source could be shielded by the first lunar night with regolith which, sun heated, could keep the machinery warm until sunrise. If nuclear powered instead of solar, it could run an active base on a constant basis. A pre-landed human base could be radiation protected by a very small system in a fairly short time. Given a few cycles of small development, a large mining operation could be assured of reliable large scale gathering on the first try.

After development and test on the moon, it should be considerably easier to engineer soil movers on Mars and other low gravity, no oxygen bodies in the solar system.

 

 

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17 Responses to Moving Regolith

  1. Andrew W says:

    What you’re proposing is a cross between a motor scraper and a drag-line excavator.

    My concerns would be wear on the 100 metre cable being pulled over regolith – a notoriously abrasive material, and control over the scraper on its way in. If it gets jammed against a larger rock on its way in, or hits an immovable object at 10 mph, it could easily break or flip.

    I’m not sure I agree with some of your other points, traction can be increased with a greater footprint area and if it’s going to and from a single point as your scraper is, any power storage only has to get it over that small distance between recharges.

  2. Lars says:

    Drills.

    Drills are more efficient at excavating under low forces than any other method, especially when using rotary percussive drilling.

  3. john hare says:

    Andrew W,
    I agree that cable abrasion would be a concern. A little steering can avoid visible rocks and the cut is shallow enough to bounce over hidden ones. Greater footprint can only make up for a certain amount of missing down pressure mass. I’ve spun a fair number of tracks when there wasn’t enough oomph to hold the machine down. Tracks are far higher maintenance than wheels. On board power has the temperature problems I mentioned. Charging during the few seconds that the machine is dumping could get entertaining. The attempted point of the idea is to simplify and lighten the machine that is getting the wear and tear.

    If you are right about being able to charge in the dump phase and handle the temperature, the winch could be aboard the pan which would reduce or eliminate the cable abrasion while getting a little more reasonable range.

    Lars,
    This one is about gathering loose regolith, not digging and pounding down through the lunar rock. The packed regolith, though hard, is still in discrete particles for the material of interest to us. It is entirely possible that my traction concerns are irrelevant to a gathering machine that just uses a magnet to pull the charged grains of the right size into the hopper.

  4. Chas Becht says:

    Reading this immediately made me think of a pneumatic excavation system I had read about previously. It’s an elegant concept and does quite a lot with limited mass. Googling about, it turns out I read about it here. Here’s the link:

    http://selenianboondocks.com/2009/06/random-thoughts-lunar-excavation-technologies

    Other related pneumatic excavation research:
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110008766_2011009461.pdf
    http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20110014001_2011014280.pdf

  5. john hare says:

    Chas,
    I had forgotten that one until I reread it just now. With a good directional tube, the regolith could simply be shot to the collection site ballistically with the energy of the excavating gas. Thanks.

  6. Andrew Swallow says:

    Fast charging suggests use of supercapacitors instead of batteries.
    http://en.wikipedia.org/wiki/Electric_double-layer_capacitor

  7. ken anthony says:

    I love articles like this because it requires thinking like a lunatic or martian! The problem with the pneumatic approach it requires a lot of consumables. Drills would certainly work, but they’re overkill and slow making them inefficient compared to a more common approach.

    A pan is a good idea but requires a certain consistency of dirt (but does happen to match the consistency found commonly on mars, although not the moon.) Combine a pan with a rotary tiller and the type of dirt doesn’t matter as much. This would be my recommendation.

    This could work in conjunction with a traction engine and cables of course, but I think the rotary tiller solves the lack of mass problem by itself.

    The tiller itself would just be a cylinder with bolt holes in it. When the bolts have worn down you just replace them.

  8. Andrew Swallow says:

    If you can get the weight down to ~1/3 tonne NASA may be able to land the equipment on the Moon in 4-5 years time.

  9. Chas Becht says:

    Also of interest is “Parameters Governing Regolith Site Work by Small Robots”
    http://www.ri.cmu.edu/pub_files/2010/3/Skonieczny_E&S10.pdf
    which takes an interesting task-based approach to comparing different methods. They consider two tasks, digging a trench and building a berm. Each task is broken down into individual operations: excavating, driving some distance, dumping, returning, periodically driving to and from a charging station. They then look to see how dependent the performance is on various metrics. Cargo mass ratio, driving speed, soil friction and so on. The result is a heavy sensitivity to non-digging factors. To me, this would seem to imply that separate excavator and “dump truck” vehicles would be of use. Also, the ratio of these vehicles could be adjusted for different tasks.

    There was also this gem:
    “Past trade studies have examined the applicability of various excavation
    robot options to lunar outpost site work. Boles et al [Boles 1993] compared the
    probable required launch mass of several construction machine suites. They
    concluded that typical terrestrial excavation machines would not be as effective as tripod cranes, sweeper leveller/excavators, and other innovative vehicles.”
    I’d love to hear more about “other innovative vehicles,” but unfortunately that article is behind a paywall:
    http://ascelibrary.org/doi/pdf/10.1061/%28ASCE%290893-1321%281993%296%3A3%28217%29
    Although the citeseer page for it has several other promising leads:
    http://130.203.133.150/showciting;jsessionid=189DDF6C19919ECA1012C5C33013AA8C?cid=10684906
    The literature on this subject is more extensive and fascinating than I realized. Thanks for killing my afternoon. =)

  10. Paul451 says:

    “That or a massive battery pack that would be even heavier […]
    [traction would be a problem] unless a few tons of ballast were added to the loader.”

    Ahem. :)

    Okay, so weighing it down with batteries doesn’t solve the launch-mass problem. An alternative would be power beamed from a central source. Vastly shorter distance than any SPS proposal, a nice early proof-of-concept. It does limit you to line-of-sight, but so does a cable.

    That said, something about a drag-line system does seem wonderfully low-tech for a lunar base. (And we need low-tech solutions if ISRU is going to be useful.)

    As an alternative configuration… What about putting the cart out at 100m with a small A-frame tower on it, at the top of the tower a pulley-wheel. The cable runs from the central motor (also elevated) out to the outer cart and back. The now-separate pan (or simple drag-bucket) is attached to the main cable by a short arm.

    During the drag, all pressure is on the well-secured central hub. During the outward leg, only the much lower pressure of an empty bucket is on the outer pulley-cart. The cable would be held above the regolith at all times.

    This outer pulley-cart would be at right-angles to the cable, so it could slowly move the cable/digger in an arc around the hub to reach fresh regolith as required. The bucket (or pan) wouldn’t need to be on wheels, it would just be a single solid piece. The power required for the outer-cart would be tiny (it only moves a single drag-width every few drags, the pulley is passive) and easily managed by solar panels and a small electric motor.

  11. Chas Becht says:

    @ #7
    Pneumatics use less consumables than you might think. The main reason that article stuck with me was that the mass ratios were pretty surprising. (To me at least). Quoting from the previous selenian boondocks article:

    “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)”

    I also seem to remember reading at some point (can’t remember where now) about clever cyclonic dust separation shenanigans that allowed you to recover the working gas. In that case you’d just be re-pumping the stuff rather than blowing through consumables. You’d presumably still lose some to leakage, but not much. Although that would rule out our host’s ballistic delivery suggestion from #5. It’d be interesting to see a trade study of dump truck robot vs big gas cylinder and regolith fountain. Especially if you can bake volatiles out of regolith to generate your own working gas in-situ.

    The pneumatic method also has the handy characteristic of being able to scale down more without running into traction problems. So it’s theoretically easier to try it out with some small tech demo vehicle.

  12. Chas Becht says:

    @ #10
    Ooh. The elevated cable is a nice touch. The cable abrasion issue was the biggest problem I could see with the plan. I also *really* like that this means that the pan itself is basically inert. No wheels, no motors, no batteries, no solar panels, nothing but dumb metal. Just needs something to move the excavated regolith from the hub to wherever you want it.

    Unless you want it in the hub. When do you need a moat-around-a-mesa? Build the hub winch on top of a habitat to bury it? Reflective solar concentrators in the moat pointing to a power tower on the mesa? I wonder to what extent you can cheat by designing your base architecture to match the kinds of shapes and structures your excavators naturally make?

  13. johnhare says:

    When people can both redesign the ideas to eliminate the obvious problems, and show how much better options are out there than my concepts, I call it a win. Thanks!!!

  14. ech says:

    Long ago, I worked on lunar base concepts under one of the support contracts at JSC. One of the two major problems with lunar excavation has been mentioned above: regolith is very, very abrasive. It sticks to almost anything and gums it up quite rapidly. Bulldozers are probably not going to work without lots of maintenance.

    The second is that regolith, when you get past the fluffy top layer is very densely packed. It’s packed much, much denser than wet beach sand, and it’s going to be really hard to excavate. Remember all the problems that Apollo had with their drills.

    The favored concept for ISRU regolith collection was a dragline. Fewer moving parts, can be moved, can be made light by having a compartment in it that is filled with regolith after arrival.

    There are also unanswered questions for all ISRU planners. How thick is the regolith? How many rocks are mixed in? What is the size distribution of the rocks?

  15. Warren Platts says:

    Electric motors may not be necessary. Hydrogen/oxygen internal combustion engines may prove to be better. ULA is working on using them in their upper stages already.

    As for the regolith being too tightly packed, that’s a problem easily cured with explosives. Blast and clear, baby!

    As for the size of the excavators, we really want to go as big as possible as soon as possible. We need to move 100 tonnes per hour, not every two weeks! After all, 100 mT is nothing! Assuming 50 mT/week, you’d be lucky to get 2500 mT in a year. If you were going for water for rocket fuel and the content was 10%, you would only wind up with ~166 mT of LH2/LO2 (mass ratio 5). Not game changing. In rough, order of magnitude terms, annual production levels will get you the following:

    10 mT —> will launch an ascender or two
    100 mT —> will begin to make a dent in base fuel requirements
    1000 mT —> will render base self-sufficient in propellant
    10,000 mT —> will enable abundant chemical Mars architecture

    So if the Moon is to be more than a self-licking ice cream cone, to borrow a phrase from Kirk Sorensen, we really need to be looking at producing 10,000 mT of propellant per year. This is not on the scale of the Anaconda copper mine, but it is non-trivial nevertheless, requiring excavation of roughly 150,000 mT (a pit of about 4 hectares down to a depth of 2 meters per year). Little, mini-excavators aren’t going to cut it. We’ll want machines massing 20 to 40 tonnes–as heavy as can practically be landed in one or two pieces, which of course would require the sort of beefy landers that were once proposed by the ULA engineers…

  16. john hare says:

    For the early development, starting small can usefully work out the kinks before committing gigabucks to development. It is entirely possible to commit tens of billions to a project and then find out that the baseline method is not feasible in economic or technical terms. Large scale will happen in any scenario, like heavy lift to LEO, but pushing it ahead of its’ proper time is a road to bankruptcy. I agree with you on the desirability of large scale, just not on the timing.

  17. Andrew Swallow says:

    We are currently building lunar landers with a half tonne payload. We are investigating landers with 5 to 14 tonne payloads. Build any lunar architecture around those. In about 10 years time we can start work on bigger ones, if a need can be shown.

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