As mentioned previously in Part 2 of this series, one of the key elements in establishing a lunar beachhead is developing ways to safely and affordably land payloads and people on the lunar surface. In Part 3, we discussed a way of hard-landing bulk raw materials on the surface, but in this post, Iâ€™ll focus on two related methods for horizontally soft-landing equipment and people on the Moon, while significantly reducing the amount of propellant required compared to traditional rocket-based soft-landing.
While the idea of horizontally landing on a planet with negligible atmosphere may sound odd at first, there is a method to my madness.
Horizontal Soft Lithobraking
Iâ€™ve mentioned this concept in the introduction a previous blog post1, but to recap for those who haven’t read the previous post, back around the time of the Apollo Program, Krafft Ehricke2 invented a concept for horizontally landing payloads and people on the lunar surface.
Ehrickeâ€™s concept involved having a lander with ski-like skids land on a very long (10s of km long) bulldozed regolith track. The lander skids would drag along the surface exchanging momentum with the lunar regolith. In this way the rocket delta-V associated with landing could be significantly diminished. Ehricke even invented a new field of engineering called â€œharenodynamicsâ€ to study the fluid-dynamics-like properties of regolith particles in that situation.
While probably doable, and while the concept would likely significantly increase the payload deliverable from Earth, it’s not without its share of drawbacks and challenges. Obviously, prepping such a landing strip would be no small feat. You would have to clear all boulders, level the ground as smoothly as possible, probably brake up and rake the regolith, and then youâ€™d probably have to redo the strip after each landing, as the landers spray regolith in every direction. Youâ€™d probably also want a really, really accurate gravimetric map of the approach trajectory, and some good navigation aids, to enable the vehicle to hit the track, and hit it with the right velocity vector.Â This concept would likely result in a lot of wear and tear to both the track and the lander. Not to mention getting dust on pretty much everything, and being on the â€œthat scares me, and Iâ€™m fearlessâ€ side of sketchy. But thereâ€™s also been a decent amount of research put into it, so it might be feasible.
Thatâ€™s all Iâ€™ll say about his concept, but you can read more about it here on page 27. For the rest of the post, Iâ€™d like to focus on a newer variation on the theme that I think has real potential.
MFAâ„¢ Hover-Brake Horizontal Landing
This second concept is a combination of a concept I heard from a friend at XCOR about a decade ago3 combined with some newer technology from a cool company in the Bay Area called Arx Pax. Arx Pax is probably most famous for their â€œHendo Hoverboardâ€, which can levitate while carrying a full-weight human about an inch over a copper sheet, without using superconductors.
Their Magnetic Field Architecture (MFAâ„¢) technology works by using four â€œStarmsâ€ or “Hover Engines” which are rotating discs with magnets arranged on them in a way that projects most of the magnetic field down from the disc into the conductive medium youâ€™re trying to hover over.
As the Hover Engines rotors rotate, they induce a circular eddy current in the conductive medium, which creates a magnetic field that pushes back on the Hover Engines. Using two pairs of counter-rotating Hover Engines at the four corners of their hoverboard, they can push against the conductive medium with enough force to lift both the hoverboard and a human rider. They can also induce lateral forces by rotating the Hover Engines relative to the surface of the conductive medium. They think there may also be ways to modify their Hover Engines to induce push, shear, and pull forces on a conductive target.4 For those of you interested in learning more, Arx Pax just announced this last week that they’re now selling both a pair of the Hover Engine 3.0 modules shown above (capable of levitating 60kg at a height of 6mm over a 12.5mm thick sheet of 6101-T6 aluminum, with a system weight of <7kg each, as per this spec sheet)5 for $9,999 for the pair, as well as a MFAâ„¢ bundle kit with smaller hover engines and all the pieces you need to make and control your own hovering device for $1,589, both of which you can now order online here.
For this lunar landing application, you would need three elements: a long conductive track, multiple Hover Engines on the horizontal lander in order to levitate the lander over the track, and a magnet array on the bottom of the vehicle to decelerate the vehicle using eddy current braking with the long conductive track. The Hover Engines can help both keep the vehicle from touching the track, or from bouncing away from it, while also keeping it centered on the track, absorbing a lot of the shocks associated with a lunar landing, and providing additional deceleration force as the vehicle slows6. As the Hover Engines approach the conductive track, the repulsive force should increase, and as the Hover Engines get too far above the track, the repulsive force should drop off, thus providing at least some natural feedback The amount of power needed to keep the Hover Engines rotating is probably modest enough to be powered by an APU like ULA is proposing for their Integrated Vehicle Fluids system7. While there still are rather demanding requirements for hitting the landing track within the right cone of velocity vectors, the non-contact nature of this landing makes it somewhat less scary. And depending on the design of the eddy current braking magnets, the braking drag may be able to be applied gradually after the vehicle has established a clean hover on the track, if so desired.
One important question is still how much up-front infrastructure this may require. Youâ€™ll still need the bulldozer to remove boulders, and level the road. But you also no need a conductive layer of some thickness. The thickness is going to be driven by the float height, gravity, number and size of Hover Engines, and the conductivity of the conductive track. For earth hovering, they used a copper track of decent thickness8. If we needed that thick of a track, that could add up relatively quickly, even if we go with lightweight aluminum instead of copper10, to get the thickness down to much more reasonable levels.
Residual Resistance Ratio
Most people are familiar with superconductors, but did you know that if you chill high purity metal conductors to cryogenic temperatures, that their resistivity also drops off dramatically? In most cases not all the way to zero, but enough to make a real difference. You need a very pure alloy (typically 99.99% or more of the base metal), fully annealed, with minimal impurities. But if you can get the metal in that condition, the conductivity can go up by 50-100x or more compared to their room temperature values. For instance, the figure below (from page 1135 of this paper) shows the conductivity vs temperature curve for high-purity aluminum. As you can see, the resistivity at room temperature is ~2.7×10^-8 Ohm*meters, but at LN2 temperatures it’s 10x lower, and LH2 temps it is ~3400x lower.
If you could chill pure Aluminum cold enough to reach ~125x the room temperature conductivity of 6101-T6 sheet (ie resistivity = .025 Ohm*m), you would need ~100Âµm of aluminum to conduct electricity as well as the 12.5mm thick sheet of aluminum at room temperature used for the Hover Engine 3.0 modules. From the chart that’s somewhere around 45-50K. 100Âµm is a lot more workable than 12.5mm. The 100Âµm thick layer could be thermally deposited onto a microwave fused regolith track to provide a strong mechanical backing, while keeping the initially imported aluminum mass quite low11.
Running Some Initial Numbers
If you say had a 7.5m wide track with 100Âµm thick aluminum, the aluminum would mass approximately 2.03mT/km of track12. So if you could hard-land a 70mT sample of such high-purity aluminum 13, and could recover approximately 2/3 of the aluminum, that would let you lay about 23km of conductive track from a single hard-landing flight. If you were using that 23km track to decelerate something from lunar orbital velocity (~1800m/s), and assumed that you were landing on ~80% of the track length14, you could get the landing Gs down to ~9Gs (over approximately 20s). Thatâ€™s only a little bit more intense than a Gemini launch on a Titan II missile and a bit gentler than a ballistic reentry in a Soyuz capsule, but probably within what a healthy human could handle safely15. With two hard landings worth of aluminum under the above assumptions, youâ€™d be able to get the G rate down to a totally survivable 4.5G deceleration (with a 46km long track).
But how much would all the Hover Engines mass to do this? Not as much as you’d think. So, if you assume that you’ve chilled the aluminum enough that 100Âµm of pure aluminum can conduct as much as 12.5mm of 6101-T6 aluminum can at room temperature (40-50K as mentioned above), so it behaves similarly to the nominal track thickness, that means that for landing a 20mT payload attached to a 6mT ACES/Xeus/HoverEngine lander, you’d need ~64 of the Hover Engine 3.0 modules (say 8 pods of 8 Hover Engines each), which without further weight optimization would mass around ~450kg. You could probably cut that down substantially with clever design16, but that’s already in a similar order of magnitude to the mass of the landing kit for a rocket-powered Xeus lander.
As an aside, can you see why I was so interested in having a way to land large amounts of bulk raw materials on the lunar surface? Even though thereâ€™s plenty of aluminum in the regolith, and you could eventually setup facilities that could produce many tonnes of aluminum per year, being able to land that early-on dramatically lowers the cost of landing those facilities in the first place.
How to Chill
Getting back to the topic, can we really get aluminum cold enough to enable that level of conductivity? Iâ€™m honestly not sure. It may be possible to have the bulldozer that makes the path intentionally sink the track into the ground with berms around it in a way to keep the aluminum in shadow as much of the time as possible. If you combine that with some sort of â€œcryogenic selective thermal coatingâ€17 that could help keep the temperature of the track passively cool for most of the day. Will it be enough to get the track down to the desired ~40-50K? I’m not sure. According to this chart from a site with data from the LRO Diviner mission, polar sites get down to around ~50K at night, so I don’t think it’s entirely crazy at least for polar locations. At more equatorial positions, even with a properly dug trench, you’ll likely need a thicker aluminum track to make things work.
One additional knob to turn is that apparently if you have a two layer track, with a highly conductive non-ferromagnetic upper layer, and a lower ferromagnetic layer, that the ferromagnetic sheet modifies the induced magnetic field in a way that actually increases the force pushing back on the Hover Engine. So theoretically, if you did a three layer design (first the microwave sintered regolith underneath, then a layer of melted NiFe material magnetically extracted from the lunar dust, and finally a layer of thermally deposited aluminum), you might be able to get a bit more bang for the imported aluminum buck.
One other nice thing about a track like this is that it can provide two other uses other than just propellantless landing of people and payloads. First, the track is actually a pretty impressive conductor. A 100Âµm thick, 7.5m wide conductor has about the cross sectional area of a 1.25in (30mm) diameter rod. Thatâ€™s a lot of conductor. And if chilled to 125x the room temperature conductivity of aluminum, youâ€™re talking about a conductor equivalent to a 14in (350mm) diameter bar of aluminum at room temperature. A conductor that big could likely conduct megawatts of power over non-trivial distances. This means that you could have solar farms and installations located along the track, which all use the track as the high-power backbone to connect themselves to the main beachhead facility. If you had a polar facility with four tracks spaced 90 degrees from each other (say earthward, anti-earthward, and along and against the moonâ€™s orbital velocity vector), you could locate solar farms along each track in a way that half of them are always in sunlight, so you wouldnâ€™t need a lot of power storage capacity.
Second, the tracks also would serve as ideal highways/railroads for moving stuff between the main facility and other facilities down the line. Without air, and without rolling friction, these tracks could basically function as maglev railways linking installations up and down the track. The same Hover Engine kits used for landing could form the backbone of a hover vehicle that could rapidly move very heavy payloads up and down the track at very little energy cost.
And so long as you locate a track along a â€œgreat circleâ€ route (ie a surface track that is in a plane that intersects with the center of the Moon), the tracks could be used for all three applications (landing, transportation, and power). While the first one or two could be built using Earth resources to accelerate their availability, once you have access to lunar aluminum, you can start putting tracks like this down wherever they are convenient, and as long and wide and thick as is convenient. I even have an idea 18 for how you could use tracks like this for propellantless launching of payloads, but that’s a post for another day.
Itâ€™s probably more complicated than this. These ideas are very conceptual, and could use a significant amount of further baking. In fact, if I get a chance to flesh this out further, I’ll probably do follow on posts as Part 4.1, 4.2, etc. when I get the time. But they at least suggest that there may be ways to eventually cut the cost of delivering payloads to the Moon in-half, by getting rid of most of the propulsive landing delta-V. And for setting up heavy infrastructure on the Moon, the sooner you can get that big of a landing-cost savings, the more your overall system cost goes down.
Next Up: Slings
[Update: I still need to dig into this more, but it may be necessary to put more thought/analysis into the eddy current damping part of this idea. All of the kinetic energy effectively has to be dissipated as thermal energy (by resistance losses by the eddy currents flowing through the metal). The problem is that as the metal heats up, it becomes less conductive. So there’s probably a mass limit that a given track thickness/length can realistically handle. The math is currently above my hand-calc levels, but I’ll do a follow-on Part 4.x post if I get a chance to dig into this more.
One other interesting idea I’d like to look at is using pure Beryllium as the conductor instead of pure aluminum. Beryllium has 2x the heat capacity of aluminum (~1800J/kg*K vs ~900J/kg*K), and has a much higher melting point (~1550K vs ~933K), so it is much better as a heat sink. It isn’t as good of an electrical conductor though, with 3/2 the room temperature resistivity of aluminum, but it is only 2/3 the density, so for a given mass/length, you get approximately the same resistance, just with 3/2 the cross-sectional area for the Beryllium. It’s RRR (ratio of resistivity near absolute zero compared to room temperature resistivity) of ~2000, which while not as high as the highest grade coppers or aluminums, may still be good enough to be quite interesting. Needs more analysis though.]
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- The lunar magneto-lithobraking concept I discussed in that blog post turns out not to work BTW. A particle that encounters a moving magnetic field gets accelerated and then decelerated by an equal amount, so no net momentum can be transferred in that way. There might still be a way to induce charge on dust particles in front of your vehicle and use a magnet to deflect them (sort of like how magnetoshell aerocapture charges and then deflects air particles to create drag), but it’s unclear if the magnitude would be enough to make it work
- A German engineer who was a key part of the program to develop the Centaur upper stage.
- I canâ€™t remember if it was Doug Jones, Dan DeLong, or Randall Clague, but it was someone at XCOR. Apparently they used to do periodic brainstorming sessions where theyâ€™d pick a far-out engineering problem and brainstorm potential solutions, and then do some BOTE analysis. I donâ€™t know if they were still doing this once they started working on Lynx, but it was an interesting way to â€œkeep the tools sharp.â€
- Altius is working with Arx Pax to find a funding sources to develop a version of our Sticky Boomâ„¢ robotic capture system that uses their MFAâ„¢ technology to allow gripping conductive targets like upper stage propellant tanks for refueling.
- These Hover Engines 3.0 modules were designed for various Hyperloop development teams to purchase for experimentation.
- Since eddy current braking force drops linear with velocity
- In fact, according to Arx Pax, the magnetic drag seen by the Hover Engines drops rapidly as the linear velocity of the Hover Engines relative to the conducting sheet passes the tip velocity of the Hover Engine rotors.
- Between 1/8in and 3/8in depending on the maximum levitated weight for the hoverboard, and 12.5mm thick of 6101-T6 aluminum for the Hover Engine 3.0
- Aluminum has about half the conductivity of copper, but about 1/3 the density, so for mass-constrained applications, aluminum is usually the better conductor[/quote]. But there may be a way to “cheat”9I think Dana Andrews of Andrews Space once said something to the effect that: “When it comes to Aerospace Engineering, if you arenâ€™t ‘cheating’ youâ€™re not trying hard enough.”
- While aluminum is actually a fairly common component of most lunar regoliths, and could eventually be extracted in large quantities, for the very earliest landings you might want to import it, just so you can speed up the beachheading process.
- And yes, I’m using mT to stand for metric tonne, just to annoy metric purists who are probably already thinking “mT means milliTesla” before they’ve even had a chance to mouseover this note.
- You probably sunshield-cool the aluminum down to cryogenic temperatures to increase its toughness/strength before impact
- I’m a fan of having some runway margin when you’re talking about decelerating from orbital speeds. How about you?
- I hear that a lot of the Gemini pilots called the Shuttle the “grandpa rocket” because it was limited to such a lower ascent G-loading
- Use of ultraconductors for the windings, chilling the motor/windings to increase conductivity and going with a higher tip speed, etc
- A coating that is highly reflective in the wavelengths that most of sunlightâ€™s energy is in, but high emissivity in the IR spectrum. This allows it to reflect sunlight, while acting like a good blackbody in most of the IR spectrum, allowing a much lower equilibrium temperature than would otherwise be possible. There is a NIAC Phase II effort about to start up on cryogenic selective coatings that can get down to below LH2 temperatures.
- Developed by the Arx Pax team and an advisor of theirs from NASA Ames