Random Thoughts: Lunar Magneto-Lithobraking

One of the big challenges in lunar exploration and development is the amount of delta-V needed get down to the surface and back again. On planets like Earth, and to a lesser extent Mars, spacecraft can dump momentum into the atmosphere for aerocapture, aerobraking, or aeroentry. The atmosphere does add some cost to the launch from those planets, but it provides a lot of benefit for landing. While the Moon’s gravity well isn’t that deep, it is enough that the rocket equation makes landing and return from the moon costly enough to be worth looking at alternatives. A purely rocket propulsion method for getting material to and from the Moon is a lot harder to close economically than when you can “cheat” and do a large chunk of that ascent/descent propellantlessly. While there are a lot of great ideas for propellantless launch from the Moon1, there are a lot fewer options for propellantless soft-landing on the Moon. And very few of those are completely non-crazy.

One of the earliest such propellantless landing ideas I’ve seen was from space visionary Krafft Ehricke, where he suggested landing spacecraft horizontally using skids on a long, pre-cleared but unpaved landing strips.2 Momentum would be dumped via friction with the lunar regolith. Ehricke invented the field of “harenodynamics” to study the fluid-dynamics-like properties of regolith particles in that situation. More details on the concept can be found here on page 27.

Lithobraking Slide Landers -- Not Quite Crazy Enough...

Lithobraking Slide Landers — Not Quite Crazy Enough…

While this is an interesting idea, it also requires pre-landing pretty large construction equipment to clear the 10s of km long landing strip for landing. And it’s still pretty sporty from a controls standpoint.3

So here’s the crazy thought I’ve been noodling for the past few weeks. The fine portion of lunar regolith has a surprisingly high magnetic susceptibility–I’ve seen a demo where Dr Larry Taylor of University of Tennessee where he picked up actual lunar regolith samples inside a test tube using a magnet. What if you took a horizontal lander like ULA’s DTAL/Masten’s Xeus, and wrapped a really powerful magnet around it, and then flew really close to the lunar surface? As you fly over, you’d attract particles, and dump momentum into them. You’d have to cancel out the vertical forces (gravity minus centrifugal acceleration plus the vertical component of the momentum you impart as you pick up the lunar regolith), but there’s a decent chance that would drastically lower the propellant cost of a landing. Yes, this is crazy, since you’d be flying just above the lunar surface at ridiculous speeds (starting at the earth equivalence of Mach 5 horizontal velocity) for 1-2 minutes as you decelerate. What I’m curious about is if the ideas is just crazy, or if it’s crazy and also stupid.

Key questions I’d like to answer:

  1. How much horizontal versus vertical force would such a system impart into the spacecraft–if too much of the magnetic force ends up pulling the spacecraft down into the regolith (compared with accelerating the regolith horizontally), then the idea won’t save you any propellant.
  2. How close do you need to fly to the lunar surface on average for this to work. Are we talking 1m? 5m? 50cm?
  3. How much horizontal deceleration force can you generate realistically? How it it effected by speed? I would think that at higher speeds you pass the particle too quickly to accelerate it all the way to your velocity, but as the speed gets lower you have more time to accelerate the particle.
  4. How much “hovering” delta-V do you need to expend during the deceleration? If you can decelerate at 1G horizontally, the hovering delta-V just to cancel out lunar gravity would be less than 300m/s, much less than decelerating all the way from orbit.
  5. Are there smooth enough stretches on the moon to realistically do this on an unprepared stretch of regolith? If you have to pop up to dodge a boulder (we’ve got good enough maps now that I’d think you’d be able to know in advance when you had to do such a maneuver), how much deceleration time do you lose? How much does that increase the “track length” you need to work with, if you assume a certain number of boulder hops per linear distance?
  6. How powerful of a magnet do you need to make this work? Are we talking 0.5 Teslas? 1 Tesla? 10 Teslas? Does the magnetic hardware outweigh the propellant you’d save?

A few weeks ago, before I got sucked into proposal writing purgatory, I started making some physics models for the system. I found a good model for estimating the force on a magnetically susceptible regolith particle due to a magnetic field. I think my next analysis would be to model the trajectory of a particle as the lander passes by at various relative heights, speeds, and magnetic field strengths (I wonder if there’s some dimensionless number I can use to scale things?) Once I’ve done that I’ll have a better idea of how much momentum I can impart per particle, and how much additional vertical force I’ll need to null out. After that, the next step would be to take those drag numbers at various vehicle states, and use it to create a 1DOF landing simulation.

The cool thing is that if this works, it could theoretically work on first missions to certain sites, possibly allowing you to greatly decrease the cost of landing robotic cargo on the Moon in preparation for manned landings.

The idea is probably both crazy and stupid, but I figured it was worth sharing, in case there’s someone who likes the idea and has both the physics background to help me analyze this, more spare time than I do.

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
  1. The primary topic of my “Slings and Arrows of Outrageous Lunar Transportation Architectures” series that I badly need to knuckle down and finish one of these days…
  2. He was also the guy who came up with the brilliant quote “If God wanted man to become a spacefaring species, he would have given man a moon.”
  3. Plus, it isn’t crazy enough.
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21 Responses to Random Thoughts: Lunar Magneto-Lithobraking

  1. If you’re going to “hover” anyway, why not point your exhaust a little forward too and kick up that dust? That should reduce how close you need to be to the surface, some.

  2. Trent,

    It’s a good idea. I was thinking along somewhat similar lines, but hadn’t taken it quite that far. Harder to model, but probably a lot more effective than trying to kick stuff up off the ground. Would also probably reduce the amount of downward magnetic force the thrusters would need to cancel out, since you’re not as close to the ground, and the thrusters are providing some of the upward loft to get the particles close enough for you to decelerate.

    Now I just need to figure out how I’d model that. Maybe I could talk with Phil Metzger about his plume impingement/excavation modeling…

    ~Jon

  3. Robert Clark says:

    Don’t have a comment about the lunar landing proposal, but the “magneto-” part reminded me of the magnetoshell method of aerobraking Altius is working on. The major problem is producing the large amount of power required. Is it now being investigated only using stored onboard power? Could you instead use the heat produced during reentry to produce the needed power?
    By thermodynamics, the higher temperatures means you can get more efficient conversion to other forms of energy. For instance a rocket engine operating at thousands of degrees gets above 90% efficient conversion to kinetic energy, as measured by exhaust velocity.
    Also electric generators get above 85% efficient conversion of mechanical, i.e., kinetic energy to electrical energy. So we should be able to get above 75% efficient conversion of the reentry heat to electrical power.
    But how to use the reentry heat? Note that with a capsule undergoing reentry the hypersonic air flow is directed around the sides of the capsule. Then we could have annular shaped de Laval nozzle around the circumference of the capsule. The exiting airflow would then be directed towards turbines to produce the power.

    Bob Clark

  4. DougSpace says:

    Why is the lander landing on the Moon? If to bring cargo then how much of that cargo could be produced on the Moon thereby negating the need to ship it to the lunar surface. If to deliver crew then could we solve the problems that make it necessary to ship the crew back as in some sort of 6-month rotation schedule. I’m not arguing against the idea but only pointing out that there are systems approaches which reduce the need for flights and hence saves money which is the goal of this exotic landing approach.

  5. Doug,
    This is for landing initial payloads on the moon to get the bootstrapping process started. I’m totally a fan of ISRU, and doing longer tours of duty for crew landings (I’m on the record as being in favor of lunar one-way missions) but in reality you’re going to need to land a *lot* of payload on the lunar surface to get ISRU up and running. If this idea can be done in a way that significantly increases your landed mass per lander, it may make future steps a lot easier.

    ~Jon

  6. NA says:

    Like the magnetoshell reentry concept, would you want to effectively trail a drag anchor device, or put the main field around the lander proper? How would this compare to a lander with a hovercraft plenum plate on the underside fed by vernier exhaust, rather than just skids?

    What’s the would be minimum sizes for a dozer/grader to prep the landing strip? Are we looking at something on the order of a snow grooming machine?

  7. Eric Collins says:

    It’s an interesting idea. I’ve been considering something similar for a few years now, but my version requires a bit more ground infrastructure. The analogy I was thinking of at the time was something like a cross between regenerative braking (where you could actually recover some of the energy of the landing craft as electricity) and just simply dumping kinetic energy as resistive heating into some large metal plates along the landing zone. I hadn’t actually considered dumping the energy into the regolith itself. Your version is certainly much better for early missions when there is little or no infrastructure available on the moon.

    One variation on this theme that I considered was to use something like a Lorentz force to provide lift to the vehicle as it dumps it’s horizontal velocity. As a refresher, a charged object traveling through a magnetic field experiences a force that is perpendicular to the plane defined by the object’s velocity and the magnetic field. So if the lander is given an electric charge, and you have some infrastructure on the ground providing a magnetic field oriented horizontal to the surface and perpendicular to the direction of the lander approach, then the Lorentz force would be directed upwards, perhaps enough to cancel out most of the downward gravity force. The only uncertainty with this variation is if it would be possible to generate the drag force at the same time as the lift force.

  8. born01930 says:

    The KE is too high for a cable capture like on an aircraft carrier. How much savings could be had if you combined rocket deceleration to the point where you could cable capture? Feasible? or just laughable?

  9. ken anthony says:

    …C)argo R)etro A)utomated S)hip H)ard landing?

  10. David says:

    What scale of payloads are you talking? I think that is probably something that needs to be part of the parametric analysis here.

    Another option – ISRU – already mentioned also has a 1/2 way point… pay the price to land what you need to make fuel on the surface of the moon from ice – and then just keep refueling your landers after they touch down… they go pack up, grab another payload from some transfer ship that could be shot from earth into orbit around the moon – or if you are feeling sporty – a free return trajectory around the moon. Either way, the fuel to land the payload is made on the moon and you reuse the lander.

  11. Jeff Greason says:

    I’ve never forgotten KE’s paper about lithobraking and I continue to look it over from time to time. I think it might really be practical. I encourage you to keep running the numbers on this one; you never know….

  12. Jonathan Goff Jonathan Goff says:

    Jeff,

    Yeah, I’m intrigued by the idea, especially Trent’s suggestion of using the hovering rockets to kick up the regolith so you don’t have to lift it all the way off the surface using the magnet. If magnetoshell aerocapture ends up working, you’re going to want a big electromagnet on your upper stage anyway, so being able to use it for both aerobraking *and* lithobraking seems like a great win. 🙂

    Maybe I should propose it to next year’s NIAC solicitation so I can get enough money to be able to focus on the idea…

    ~Jon

  13. DougSpace says:

    @Jon, Re: ISRU requiring a large infrastructure

    Maybe I just don’t understand but it is not obvious to me that lunar ISRU would need a large amount of infrastructure requiring a large amount of mass to be delivered from the Earth to the Moon.

    I take it as a given that the first material ISRU on the Moon will be lunar polar ice. LCROSS found that in Cabeus Crater it is at one part per 18 (5.6%). How much infrastructure is needed to harvest and process it into propellant sufficient to refuel a medium-sized lander such as the ULA-DTAL or Masten-Xeus?

    The lander can land directly on the crater floor and the water can be loaded onto it directly meaning that there wouldn’t need to be separate propellant storage tanks. Steaming of the volatiles from the regolith could be done either in the ground or within the body of the ice harvester so there is no need for a separate oven. Propellant could be stored temporarily in tanks on the body of the ice harvester which could then transport it to the lander for processing so there would be no need for a separate hauler. Electrolyzing and distillation equipment could be relatively small.

    The whole thing could look like this which I don’t consider to be a large infrastructure:

    http://cislunarone.com/Arrangement2.png

  14. Jonathan Goff Jonathan Goff says:

    Doug,

    It all depends on the flight rate you want to enable. If you’re fine with enabling a round trip every several years, sure you don’t need much infrastructure. Almost every realistic proposal I’ve seen for setting up an ISRU situation with reasonable production rates needed tens of tons of materials on the lunar surface. Especially if you’re doing launch and landing purely propulsively.

    Can you do a lunar architecture without this? Sure, just like your question about doing one without depots on my Space Show interview. Don’t let me stop you.

    ~Jon

  15. Peterh says:

    One though about this breaking scheme: depending on speed, altitude, and magnetic field strength the magnetic dust might be pulled free of the surface but not accelerated enough to catch the magnet. If the dust does reach the incoming vehicle while it is still at high speed it may hit with enough speed to greatly erode the surface.

    Short of pulling magnetic dust from the surface I’d expect the down force from the magnet to be greater than horizontal drag.

    It may take more infrastructure, but an aluminum trough for a magnetic bobsled landing would seem to be a nicer option.

  16. Axel says:

    Such bold and crazy concepts, perfect for a rainy sick day. Googling harenodynamics finds me some interesting texts, but not many and only abstracts or summaries. There seems to be the sled lander, and something like a barrel or shell lander, spinning at up to 50 Hz and braking pretty rapidly (obviously not for human use). Has anyone checked how much abrasion the sled lander skids would suffer? How hot they would become? Are there any plausible guestimates if the lander needs to be made out of unobtanium or can really be manufactured? Any examples which make this believable? Like maybe someone landed a mach 5 rocket horizontally in a sandy desert without completely shredding it?

    Jon, could you describe your magnetic idea setup a bit more in detail? You intend to use magnetic adhesion to add a protective sand layer to the skids? Or you intend to have a contact free eddy current brake? Or both?

    The eddy current brake seems to depend a lot on geometry and material properties. At least there seem to be no formulas on Wikipedia. How do you plan to orient your magnets? Single coil or multiple coils? Permanent magnets?

    Are the conductive and magnetic properties of regolith consistent enough to be relied on? Which range of particle sizes are to be expected? Besides the ferromagnetic metals, how much other conductive metal is in the regolith?

    Can’t really answer any of your questions. They all depend. 1) consider multiple coils/magnets to reduce downwards pull? 2) a fraction of the coil diameter, or else effects become very week 3) no idea, but eddy current brakes are said to work better at high speeds; is there a relevant upper limit? 4) your answer plus forces from answer to question 1 and 3. 5) depends a lot on answer to question 2; if you have to hop for every pebble, it will be hard. If you just hop over ditches between sand dunes it may be more likely. 6) depends on answer to question 2; magnetic fields decay very fast. If you have to fly high, your magnets need to become insanely strong.

    I’ve seen some demos for MR medical devices. I believe they have fields in the range of 1 to 3 teslas. They say you should take off all metals for the demo, but that is mostly because once some metal gets really close to the coils inside the machine, it may suddenly jump inside, do damage on impact, and may be impossible to remove. Outside of the machine effects are effectively imperceptible.

    Also those MR coils are massive, need constant cooling and take days to power up.

    The demo also included an aluminium plate placed near the entrance of the MR tube – it falls over quite slowly due to the eddy currents.

    Maybe you should design your lander so it stirrs up a little dust ahead and somehow funnels it into a magnetic coil system…? 😉

  17. Tom D says:

    Can useful amounts of magnetic dust be kicked up by thrusters? As best I can recall from “A Man on the Moon” and Mythbusters, the surface of the Moon has a consistency more similar to concrete than the fluid-like particles of “A Fall of Moondust”.

  18. Jonathan Goff Jonathan Goff says:

    Tom D,

    Yes the dust can be kicked up (the lunar landers kicked up a lot of dust). The dust may be packed tight enough that it is hard to compress, but that doesn’t mean it can’t be blown off. Can enough be blown off with the right trajectory to do any good? That’s what analysis is for. 🙂

    ~Jon

  19. D. F. Linton says:

    Here’s a variant on your idea: instead of shear drag via magnetic coupling to the regolith (or via dragging an anchor), how about somehow kicking up a cloud of dust in front of the vehicle and allowing impacts against a forward shield to slow the vehicle. The dust cloud would function as a dense aero brake with Newtonian rather than continuum dynamics. Two ways to raise the cloud spring to mind: ground infrastructure to electrodynamically throw regolith into the path of the vehicle or shooting small impacters down from orbit to splash the regolith upwards.

  20. Andy Clark says:

    Jon,

    The idea is not crazy but the implementation may be a bit problematic in terms of the dust getting into the vehicles systems. If you have to land a lot of equipment to produce a 10kM runway then why not also costruct a series of magnetic rings just above but anchored to the surface and fly the vehicle through them. Basically an electromagnetic catapult operating in reverse. If that is possible then a complete launch and landing system would exist and could be used both on bodies that have significant atmospheres as well as places like the moon.

    Thoughts?

    Andy.

  21. Pingback: The Slings and Arrows of Outrageous Lunar Transportation Schemes: Part 4–Propellantless Horizontal Soft-Landing Methods | Selenian Boondocks

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