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.
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:
- 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.
- How close do you need to fly to the lunar surface on average for this to work. Are we talking 1m? 5m? 50cm?
- 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.
- 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.
- 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?
- 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.