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).
- 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.
- The majority of the system (and all of the moving parts) can be kept up far away from the lunar dust.
- 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”.
- 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.
- 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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