Northrop Grumman recently won a contract to study a rail network on the Moon. The grant was part of the 10-Year Lunar Architecture (LunA-10) capability study, meaning the grants max out at $1 million. This is pocket change to Northrop Grumman, meaning that it’s not a serious effort to actually develop such a system. The most likely outcome is a study writeup (which may or may not become public) that sits on a server somewhere forever. But anything involving space and trains is up my alley, so I wanted to give my take.
As much as I love trains, it’s not clear to me that any kind of lunar train network will pencil for the foreseeable future. What kinds of goods will even be shipped, and in what quantities, and over what distance? It’s a bit hard to say.
But for starters, there will be spaceports at least some distance from each base, iron mines, aluminum mines, silicon mines, water mines, carbon mines, nitrogen mines (though water, carbon, and nitrogen will all likely come from permanently shadowed craters they may not come from the same permanently shadowed craters).
On Earth the case for rail over road is that the rails give you better fuel economy, longer-lived equipment, and the ability to use less labor to ship more stuff. But building a rail line costs money—more than a road, despite having a narrower use case—so it doesn’t pencil unless there’s high volume (coal and oil are often shipped by train, along with other stuff) or someone’s already built the rail line.
On the moon I think the driving factor towards something like a railroad is different: Electric power.
In my view solar photovoltaics are the path of least resistance for the Moon if you can make them work. The biggest challenge there is the two-week long lunar night, which would mean very large storage requirements.
But there is another way: Since bases will be clustered around the South Pole, you can build a grid that circumnavigates the pole with solar sites spaced at, say, 120 degrees. I have suggested something similar, but much more technically challenging, for Earth. At near-polar latitude the panels will cast long shadows, meaning your array will use a lot of land, but the moon has as much land as all of South America and I don’t think this is a big deal. You can also build your power stations significantly north and run lines down to the base locations. China has power lines that go 3200km (diameter of the Moon is 3400km) and they’re not at the physical limit, meanwhile the low gravity, stable environment, and vacuum make it even easier to build power lines on the Moon.
There is a point to this digression, which is that we should expect the Moon to have a pretty well-built out power network early on. This ends up being pretty important for the idea of long range transport, because burning hydrocarbons is a waste of hydrogen and carbon unless you recapture, which comes with difficulties, nuclear is poorly suited to the size of transport, batteries are inconveniently heavy—especially if you want really good ones brought from Earth, etc. The most logical way to build out a transportation network is to piggyback on your electricity transmission network.
So what you end up with is roads—paved in the busiest corridors (with what I’m not sure, possibly just sintered) or cleared and leveled elsewhere, under an electrical catenary which provides power to the vehicles running underneath it.
I asked Bing’s AI image generator to make this, just to see what I could get. Didn’t capture everything I’d want (had a tough time getting it to put Earth in the sky) but it’s a cool look anyway:
If for some reason a rail system is built (it’ll make sense eventually), thermal expansion/contraction problems are way worse on the Moon than anywhere on Earth. It’s a fun design question though. I’d do something like this (probably occurring over a much longer distance than shown here). Rails in the top of the picture allowed to slide against the rail on the bottom but shall always overlap. Ties, probably stiffer at the joint, keep everything aligned left to right. Rails are probably made stiffer also at the joint to partially compensate for tapered shape.
The ability to use regular steel or maybe aluminum (price difference much smaller if both are smelted electrically) is going to be pretty critical for rail to make sense. If you’re using fancy temperature insensitive alloys you might as well just use trucks.
A cool thing you can do on the Moon, especially near the poles, is to use shade and insulation to get your cables really cold. You can probably get cold enough to use currently-existing superconductors, but even regular conductors have ~half the resistance at 150K.
It’ll be a while before we get there, but power and transportation are going to be key infrastructure for a lunar civilization and I think the things shown here are things we can build.
Gamma Factor 