If lunar transportation is based on oxygen/hydrogen, and if LULOX is used, the price for this advantage is the cost of importing H₂, from Earth. It is therefore of vital importance to minimize this import. Fortunately,this can be done.

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     Conventional lunar descent and ascent are prime consumers of H₂. But for ascent, there is a way to capture and reuse the H₂ (see next section).And for descent, the amount of H₂ required can be reduced to a fraction of that used by conventional systems.

     A new concept, the Lunar Slide Lander (LSL), is proposed. It lands on a long, flat area and transfers its momentum to the lunar surface material, thus requiring very little propellant to land. This concept is not only decisively cost-saving, it avoids release of increasingexhaust gases as lunar traffic grows,preserving the Moon's valuable high vacuum for cost-effective ascent and for industrial uses.

     In the DS-3 discussion, it was suggested that Cynthia, the Central Lunar Processing Complex, be established in the far western part of Oceanus Procellarum because, among other reasons, a flat area for slide landing exists. Covered with dust and sand, it can be turned into a suitable landing stip with little preparation. I envision a landing strip sweeper (Fig. 5) that can easily cleanse the area of larger stones down to do a depth of 20-50 cm. My calculations show that the LSL braking surfaces go down only a few centimeters.

     Touchdown at velocities of up to 5500 km/h, bringing the LSL to a halt along an 80-km-long landing strip through interaction with glassy-sandy material, introduces a new branch of spaceflight dynamics, for which I propose the term harenodynamics (from harenosus, Latin for sandy). Harenodynamics encompasses the dynamics of flow, boundary layer formation,and pressure, temperature,and gas (O₂) release conditions in the boundary layer, at high speed flow of sand along harenodynamic brakes. The LSL's most critical component is its braking assembly, particularly the linings. It is desirable to manufacture the linings on the Moon from lunar materials. A harenomechanical and harenothermo-dynamic data base must be established to define target characteristics.

     The lunar environment provides many advantages for slide landing. Vacuum permits high-speed LSL approach without temporary communication blackout due to ionized boundary layer formation. The absence of atmospheric effects, and superb sky and ground visibility (including optical signals at night) permit high predictability and automation of approach navigation. The LSL body as a whole is not subject to aerodynamic heating, which probably simplifies the design and results in lower structural mass. Possibilities for aborting the landing exist practically to touchdown.

     The LSL descends from low CLO along an elliptic path. For elliptic descent from 10, 20, and 40 km, the supercircular velocity excess at perilune is about 5, 10, and 20 m/s. Therefore, a retromaneuver of only a few meters per second reduces the speed to subcircular and causes the LSL to approach the surface at a shallow downward path angle.

     The approach phase is followed by a supporting vertical thrust phase, whose purpose is threefold: control of the touchdown point; fine adustment of the vertical velocity component for smooth touchdown; and initial support of most of the lander weight, to control deceleration and stability during the high-velocity phase.

     The vertical thrust, which replaces the aerodynamic lift of a landing aircraft, is eventually terminated in the third and final phase. The slide time is about two minutes, of which supporting thrust is needed for at most 90 seconds. Average LSL weight during this period is about 0.8 of its full weight due to the centrifugal effect during the high-speed phase. The supporting thrust of the LSL at 16 tons full lunar weight requires, under these conditions, a propellant consumption equivalent to a velocity change of less than 120 m/s—much less than the almost 1700 m/s that must be eliminated at conventional landing. LSL propellant consumption is therefore reduced to less than 10% of that required for conventional landing, corresponding to a hydrogen expenditure per unit mass of payload of P_H~0.01.

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     Figure 6 shows one of my LSL concepts. A very controlled touchdown begins with the aft edge which, together with the supporting thrust, stabilizes the vehicle for careful ground contact of the main drag vanes. These are spring supported, inclined, and in adjustable yaw position, in order to hurl the lunar material away from the vehicle. I have examined a multitude of other aspects for the slide landing process-navigational, harenodynamic, emergency options, etc. Suffice it to say that none appears to pose serious problems.

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 Moon, 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. 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.

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.

So here’s the crazy thought I’ve been noodling for the past few weeks (spoiler alert: it doesn't work.). 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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Addendum:

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.

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, I’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
I’ve mentioned this concept in the introduction a previous blog post, but to recap for those who haven’t read the previous post, back around the time of the Apollo Program, Krafft Ehricke invented a concept for horizontally landing payloads and people on the lunar surface.

Ehricke’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 “harenodynamics” 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 you’d probably have to redo the strip after each landing, as the landers spray regolith in every direction. You’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. 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 “that scares me, and I’m fearless” side of sketchy. But there’s also been a decent amount of research put into it, so it might be feasible.

That’s all I’ll say about his concept. For the rest of the post, I’d like to focus on a newer variation on the theme that I think has real potential.

MFA™ Hover-Brake Horizontal Landing
This second concept is a combination of a concept I heard from a friend at XCOR about a decade ago combined with some newer technology from a cool company in the Bay Area called Arx Pax. Arx Pax is probably most famous for their “Hendo Hoverboard”, which can levitate while carrying a full-weight human about an inch over a copper sheet, without using superconductors.

Their Magnetic Field Architecture (MFA™) technology works by using four “Starms” 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 you’re trying to hover over.

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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. 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) for $9,999 for the pair, as well as a MFA™ 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 slows. 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 system. 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. You’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 thickness. If we needed that thick of a track, that could add up relatively quickly, even if we go with lightweight aluminum instead of copper, 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.

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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 ~100µm 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. 100µm is a lot more workable than 12.5mm. The 100µm 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 low.

Running Some Initial Numbers
If you say had a 7.5m wide track with 100µm thick aluminum, the aluminum would mass approximately 2.03mT/km of track. So if you could hard-land a 70mT sample of such high-purity aluminum , 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 length, you could get the landing Gs down to ~9Gs (over approximately 20s). That’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 safely. With two hard landings worth of aluminum under the above assumptions, you’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 100µm 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 design, 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 there’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? I’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 “cryogenic selective thermal coating” 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.

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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.

Multi-Use Track
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 100µm thick, 7.5m wide conductor has about the cross sectional area of a 1.25in (30mm) diameter rod. That’s a lot of conductor. And if chilled to 125x the room temperature conductivity of aluminum, you’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 moon’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 wouldn’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 “great circle” 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 for how you could use tracks like this for propellantless launching of payloads, but that’s a post for another day.

Conclusions
It’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.