# Random Thought: Lunar Launch Loop

A while ago, on aRocket, several people were discussing the concept of the launch loop.  Read the articles here and here to get caught up to speed on the details.  The idea is a non-rocket way of launching payloads to orbit.  The launch loop has a “stator” tube with an internal “rotor” comprised of ferromagnetic particles traveling at crazy speeds (faster than orbital velocity).  The momentum of the rotor is used to lift the main section of the loop to a sufficient altitude.  Payloads are accelerated using eddy currents to transfer some momentum from the rotor to the payload (and generating a lot of waste heat in the rotor which then gets dissipated).

Now, I wasn’t a huge fan of the launch loop for terrestrial applications, just due to the shear size of the system (we’re talking a cable 1000s of km long, 100s of MW of required power capacity, the cable has to hold a vacuum, and the failure modes if the vacuum system failed could be catastrophic).  I’m typically not a big fan of megaproject launch systems as a general rule.  But I realized a lunar launch loop might not be as crazy of an idea.

First off, the track length varies with the square of the velocity change.  At 3g’s of acceleration, getting up to earth orbital speeds (7400m/s after accounting for the equatorial rotational speed of the earth’s surface) requires an acceleration track of over 930km long.  However, lunar orbital velocity is much lower (something like 1850m/s), so at 3g’s you only need a track 58km long.

The actual equation is pretty simple: s = dV^2/2*a , where s is the track length in meters, dV equals the velocity change you’re trying to deliver, and a is the acceleration in m/s^2.

Now, I don’t understand all the math behind a launch loop, but it would appear that if you want to keep the same rotor density and stator density, that you’d only need a rotor speed of 4km/s.  Also, you probably don’t need as much stator weight, since you don’t need to get up to 80km to get over the atmosphere like you do with a terrestrial system, so your tethers can be much shorter/lighter.  Also you don’t need any vacuum containing systems.  So, you could probably make the rotor lower density, since it doesn’t have to support as much mass.  The original concept assumed about 10kg/m for both the rotor and stator and everything else, but it might be possible to halve that since you’re not trying to get as high of altitude.

Also, since your rotor would be a lot shorter and much slower, it would not require anywhere near as powerful of a power plant to keep it running or get it started up.  Also, the lower velocity of the rotor means that you dissipate less power providing the same accelerating force to the cargo, which means less heating, and less power has to be added back in to accelerate things.

You’re still talking a very massive system.  Even at 5kg/m, a 3g system would weigh around 580 tons, not counting the power plant.  But to put that in perspective that’s only about 30 lander loads of cargo.  Compared to a decent lunar base, it’s not entirely crazy.

And if I’m understanding things right, there are several significant benefits of such an approach over other non-rocket methods for launching lunar payloads (such as mass drivers).

1. The energy for a launch doesn’t have to be delivered rapidly.  This means no need for rapid discharge power systems such as you would need for a mass driver.
2. The majority of the system (and all of the moving parts) can be kept up far away from the lunar dust.
3. If the launch loop is located near one of the poles, you could hang solar panels off of the tethers, and could probably make an arrangement that guaranteed constant light, even if the underlying terrain turned out not to be in “eternal light”.
4. For loops away from the poles, the loop itself functions as a massive flywheel, storing lots of energy.  It might be possible to have such a loop to be “charged up” to a speed much faster than necessary to support the structure during the day, and then slowly tapping off some of that power during the night for the settlement near the loop.  So long as you did planning right, you could probably keep at least a lighter load of launches going even during the night.
5. There’s a tiny chance you might be able to use a lunar loop for “catching” payloads and soft-landing them.  This is a lot more dicey, since you need precision navigation, trying to hit a tiny target at high speeds, but it’s no crazier than other similar ideas I’ve heard over the years (for things like controlled lithobraking, eddy-current landers, rotating tethers, etc).  The good thing is that if you miss, you’re still a decent distance up off the ground, and depending on the design of the launch loop you may still have plenty of time to do an abort to orbit maneuver.

Anyhow, the idea is probably certifiable, and it’s going to be a long time before such a scheme would be even remotely possible.  But it’s an interesting approach for how to get lots of mass off the lunar surface in a hurry if we ever get to that point.  And if you can somehow make a launch loop in such a way that it can handle landings as well as takeoffs, it could really change the equation as far as lunar development is concerned.

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#### 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.

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### 25 Responses to Random Thought: Lunar Launch Loop

1. Pingback: Launch Loops? « The Four Part Land

2. Rüdiger Klaehn says:

I think a much simpler approach for a high throughput system to launch mass from the moon would be a lunar tether catapult.

Basiically, have a rotating structure with a short rigid arm on top of a solitary mountain on the moon (preferably at a pole).

Attach a payload to a tether hanging from that rigid arm, then spin the arm at a constant rate and slowly unreel the tether until the tip is traveling with lunar orbital velocity. Then release the payload and reel in the tether again.

You can take as long as you want to unreel the tether. In fact, the whole system can act as an energy storage device for the lunar night.

A tether with a tip velocity of ~1700m/s is well within the state of the art for commercially available materials like zylon or spectra.

For rugged cargo, you need only a very short tether and the tether is almost horizontal. For a longer tether, the tether slopes down a bit, so you need a mountain. But the curvature of the moon actually helps so you only need a moderately high mountain.

I admit that this is a very far-out concept, but I think it would require less mass than a launch loop. I made some detailed calculations about this, and so far I could not see why it should not work…

3. Bob Steinke says:

Is it really necessary to make a lunar launch loop up off the ground at all? You don’t need to get above any atmosphere. If you accelerate to faster than circular orbital velocity you will lift up away from the ground. Your perigee will still be in contact with the launch loop, which will require a circularization burn, but that’s the case no matter what altitude the launch loop is at.

A launch loop on the ground could have multiple sections going at different speeds so you use a more energy efficient slow loop to accelerate to a slow speed, then you coast over to a high speed loop that takes you to launch speed.

You still have the advantages of the loop being a flywheel that can be charged slowly and discharge quickly, and store energy for overnight.

Getting up above the lunar dust is nice, but even with a flying loop you can’t get all of the moving parts away form the surface. The moving loop has to touch the surface where it turns around from descending to ascending. It’s the contact forces at that point that actually hold the loop up.

4. Jonathan Goff says:

Rudiger,
It *might* work for cargo, but even at 100g, you’re talking a tether that is (1700m/s)^2/(100*9.807m/s^2)=3km long. I’m not sure I’d want to be anywhere near something whipping around at that speed. How do you slow it down or speed it up to launch cargo, etc?

It’s an interesting idea though.

~Jon

5. Jonathan Goff says:

Bob,
The main reasons I wanted to have the launch loop up above the ground is that the lunar surface isn’t smooth. Also at the poles you get a lot higher chance of keeping most of the loop in the “permanently sunlit areas” than actually on the surface.

As for having multiple loops running at different speeds in order to increase the efficiency….that’s a really interesting idea. Unfortunately, if you have the raised loop like I was talking about, that would require having at least part of the slower loop suspended by the upper loop. But that would only be over a short section at the start (it wouldn’t need to be as long).

Anyway, it’s a clever nuance, but I don’t have a grasp of enough of the numbers to tell if it saves you enough energy to be worth the added complexity.

~Jon

6. Rüdiger Klaehn says:

Actually I was thinking about considerably longer tethers. For cargo I was looking at 20g and about 15km. For launching soft cargo at 5g you need about 60km. Both still quite short for tether standards.

Note that near the center, where the tether is close to the ground, it moves quite slowly. For a 15km radius tether the rotation frequency is about once per minute, so you could easily walk out of the way. Besides, you could easily have the rotating arm at 3m or so.

Since you build the whole thing on a solitary peak that slopes down much faster than the slope of the tether, at the tip the tether is several 100m high.

To speed up or slow down the tether you need a short rigid arm so that the tether is attached a short distance (~10m) away from the center of rotation. The arm is connected to an electric motor via a reduction gear. The whole thing needs to be bolted down since there are of course significant tether forces on the hub.

I am really more a software guy, but nevertheless I built a small prototype of this concept from lego mindstorms. It works just fine, but of course on earth you are somewhat constrained by that annoying atmosphere 🙂

7. Habitat Hermit says:

It’s an interesting idea, two things I think might be (additional) serious issues:
– Heat dissipation becomes a much larger problem even though there’s less heat.
– Not sure about the lunar dust, the system might attract dust like mad all the way up and maybe even launch dust into orbit? Think lunar terminator storms only multiplied in force and stationary.

I like launch loops and orbital rings & Jacob’s ladders and think their feasibility depends on how small one can scale initial “threads” so as not have to launch it all in one go (inherent modularity is a nice bonus, another could be a vastly smaller volume of vacuum).

8. Jonathan Goff says:

HH,
Actually, heat dissipation isn’t a bigger problem, surprisingly enough. If you think about it though, it should be clear why. The rotor in both designs is what gets heated up. In both designs it is in an evacuated tube, which means the only way it can transfer heat is via radiative heat transfer. Also in the terrestrial launch loop, most of the loop is at 80km altitude, which is almost vacuum as far as heat transfer is concerned. So in reality, I don’t think the heat transfer issues would be that much worse.

You may have a point about the dust issues though. That would have to be worked on. Of course, if you have the section of sheathing near the two anchor points designed right, you can keep all the magnetic field inside the tubes. I doubt even terminator levitated lunar dust could reach high enough to get attracted to the cable if the main section of the cable is a couple km up (say 1-5km off the surface). I could be wrong though. It deserves further investigation.

~Jon

9. Ian Woollard says:

The minimum mass of an Earth launch loop is set by wind loadings… on the moon it would be minimum gauges on the building materials. Off hand 5kg/m would be more like 0.5 kg/m or less.

As others have noted, there’s probably little point in running much above ground level, although it would be desirable to launch to either lunar escape or parallel to the ground, but at an altitude somewhat above any likely-to-be-hit mountain. 😉

10. Jonathan Goff says:

Ian,
Yeah, I haven’t been following the launch loop idea enough to know all the reasons behind the various trades. If you really could drop the required mass by an order of magnitude, that would allow you to land it on only a 2-3 cargo landers. Well, I’m sure the power requirements would weigh a lot more than all that. And installing something this big would take a lot of boots and robots on the ground. But it is a very interesting idea. Let me know if you’d be willing to run some numbers on the concept.

~Jon

11. Jonathan Goff says:

Ian,
Oh yeah, the only reasons I’d have it much off the ground at all are:

1) Get it away from the dust (which happens to have particles in it that aren’t too different from the rotor material).
2) Allow you to have a 60km long stretch of flat terrain in spite of the inherent roughness of much of the lunar terrain
3) Give you enough altitude that for launch loops near the poles you can hang solar panels where they can get (more) continuous light.

But none of those require 80km. I doubt most of them even need much more than 1km.

~Jon

12. Bob Steinke says:

I see how having a flying launch loop might be easier than bulldozing 60km of lunar terrain flat. But if you are flying at only 1 km you could support the thing with rigid towers, which could hold up some of the weight and allow the loop to run at less than orbital velocity.

13. Ian Woollard says:

I think you want the loop to run above lunar escape velocity though so that anything being launched doesn’t fall down, so running it flat should be an option.

FWIW dust may not be a big issue- the speed of impact of dust is much lower than most micrometeorites, and you’d probably want protection from those anyway. You would want to keep the area swept clean though- if nothing else some dust contains iron and may stick to the track(!)

I’m also not sure what the optimum payload size would be. It might be as low as 200kg- about enough for a man, a seat and a space suit 😉 You could just arrange to pick them up in orbit afterwards.

On the flat/raised issue, making the cable fly very probably increases the costs and I think you would need a fairly flat area to permit you to start the system up anyway. You could fly over some bumps, it doesn’t have to be completely flat; but it helps to control the cable if you’re as close to the ground as possible; pushing the cable up off the ground rather than carrying a cable on the rotor greatly reduces the lift magnets and power you would need; you only need about half, and the lift magnets have significant mass.

Compared with an Earth based loop it all looks very simple, but even that technology is just a twinkle in a few people’s eyes right now.

For powering it you could do a small flying loop to hold up solar panels or a big mirror (i.e. aluminum foil) I suppose, that might be a good use for the technology.

14. John Schneider says:

While I think a lunar launch loop like this is possible, I think that even though it is within the realm of possibility it would still require some extreme (and obviously extremely expensive) engineering to create. For all the material you would need in a fifty kilometer tall tower you’d have to strip mine a large part of the moon. In addition, the power needs to keep that thing up would be enormous. You do mention that it wouldn’t have to be delivered in extreme, short bursts like a mass driver, but I think it would require a lot more power because you’re dealing with some serious mass in order to hold the tower up. I think that a lunar space catapult is a more feasible option within this century for lunar launch purposes.

15. John,
I think you have some misconceptions on what a launch loop is. Think a long thin cable, not much bigger than a power line here on earth. Also I wasn’t advocating something 50km tall. 68km long yes, but at most a couple km up, maybe not even that, depending on the local terrain it may only be a couple dozen meters off the ground.

Sure, it’s long and somewhat big, but really not that much bigger than a mass driver facility of similar g-loading. And no, the power requirements aren’t that crazy. At most we’re probably talking a couple of MW, but possibly less.

But YMMV,

~Jon

16. Habitat Hermit says:

I probably shouldn’t write this since it’s just a gut feeling but the heat issue still bugs me even though I agree that there are no differences with the rotor itself. It probably isn’t anywhere near the most difficult challenges and might just result in a lower launch rate. Anyway here’s my thinking, flawed as it might well be ^_^

The rotor radiates the heat onto the stator and the stator radiates heat both outwards and back to the rotor, in addition the rotor moves at high speed meaning every part of the rotor spends time radiating onto parts of the stator that are in significant atmosphere (or even under water) in the Earth implementation which should help dissipate heat much more effectively. Then there’s the thermal conductivity of the material(s) of the stator that might contribute to this effect even for parts that are above any significant atmosphere. All in all one implementation is connected to a massive heatsink while the other is not (I’m sure lunar regolith or at least bedrock has a vastly higher thermal conductivity than air or water but it’s static while an atmosphere and/or ocean are dynamic).

Perhaps a bit counterintuitively I’m wondering if this might not be a worse problem at the lunar launch loop end/turning points rather than at the straight parts (if the melting temperature of the material surrounding those rather than the melting/demagnetization temperature of the rotor become the limiting metric).

It’s not that I think any of this makes the lunar launch loop infeasible just how much the launch rate is affected and whether it has to be mitigated with additional systems.

17. Jonathan Goff says:

HH,
I’m not really sure. You could be right. In which case you either end up turning the flight rate down, adding more radiators, or doing the multi-track thing Bob was suggesting (if the loop is near the ground). It would be fun to work out some more of the details.

~Jon

18. Hey! I remember Keith Hedstrom at STS-1, just a college kid back then, as I recall. We had a lengthy conversation about launch loops as we waited for the Shuttle’s first launch.

19. Jardinero1 says:

Wouldn’t a moon based cannon be simpler than a rail gun or a tether? Even a launch assisted cannon? The technology is well proven on earth.

20. Jardinero1 says:

Look up Gerard Bull for history of rocket assisted cannon launch.

21. Jonathan Goff says:

Jardinero1,
Not necessarily. Maybe if your loads can take the horrendous gees of launching from a cannon. But even for such loads, it’s questionable that you’d be able to make the propellant locally, and the high strength materials needed to make the cannon work also would likely have to be shipped from earth. It isn’t impossible, but my guess is that the numbers wouldn’t come out very favorably.

~Jon

22. Jardinero1 says:

If it was workable on earth in the 1960’s it would be more workable on the moon. You need a muzzle velocity of only 1850m/s. Even less if it is a rocket assisted payload. The cannon does not have to be built to tolerate the same stress as a terrestrial cannon. Instead of a metal tube, you could drill a deep hole in the ground and line it like they line bore holes in oil wells. That would save you the trouble of hauling high strength material to the moon. You could use local material for propellant. G-forces wouldn’t matter much for hurling lunar minerals, lunar oxygen or Helium-3. Some hardware could be designed to withstand instantaneous, brief, high-g accelerations.

23. Jonathan Goff says:

Jardinero,
I’m still not really that convinced. First off, if you dig the gun into the ground, it now pretty much has to provide escape velocity, or the launched material will just crash back down. More importantly, I’m more interested in transportation systems that are more flexible. Gun based systems are only really useful for raw materials. Where a launch loop could also launch finished goods, spacecraft, people, etc. And guns like that are going to use a lot of propellants, unlike mass drivers or launch loops. It might be possible to derive the materials needed to make a safe gun propellant there on the moon, but this is hardly a trivial exercise.

I guess I just don’t see the big benefits compared to the alternatives, and it has more drawbacks.

~Jon

24. Tim says:

Personally I don’t think a gun is the best way to go, but if you were to try it, it might be possible to keep the gs down by using something similar to the Valier-Oberth Moon gun
http://www.astronautix.com/lvs/valongun.htm
Could you dig the gun into the side of a suitable mountain to get the payload “going sideways fast” instead of “going up fast”? There would still be many problems with propellant and actually building the thing.
Actually, I did just think of a couple of benefits to using a chemically driven gun. Digging the hole would give you a chance to study what’s deep inside the moon, and if you could get the thing to use the same propellants as you get from In Situ Resource Utilisation the total infrastructure you need to deliver to the moon might be reduced. A gun might be a “quick and dirty” way to put a mass driver on the moon. I still find it dubious though.

25. Jardinero1 says:

You can drill a hole at any angle you desire. Rocket assist can adjust the orbit. Rocket assisted artillery shells have already been developed and tested by Raytheon.

http://www.globalsecurity.org/military/systems/munitions/ergm.htm

Then check out “project harp”:

http://en.wikipedia.org/wiki/Project_HARP

Guns use much less propellant per mass unit than rockets. Project HARP managed to lob 180 kgs to 3.2 km/s in earth’s atmosphere. The moon’s vacuum would make the system more efficient. In the short term, say fifty years, the only useful products from the moon are going to be raw materials. Without addressing the complexity of construction, this is the simplest way to hurl those payloads into orbit.