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In my book which is set in the near future (some hundreds years from today) the mankind has mastered interplanetary travel. Although the travel to let's say Pluto is possible, it still takes several months and is not available for ordinary people.

Would a spaceport on the Moon give a considerable advantage for the interplanetary travel at this level of technology?

I've looked at this question https://space.stackexchange.com/questions/10466/why-dont-we-launch-spacecraft-from-the-moon and basically the answer is not really. But than I watched Ad Astra some time ago, and there they go to the Moon first and only after that go to Mars. Is it pure science-fiction without any scientific background?

What I have in mind is a huge moon base shared by different Earth nations to launch their spacecrafts further. This basically impies that the benefit of starting from the Moon is so big, that different countries can even overcome their political differences and otherwise hostile bahavior for the sake of maintaining this expensive project.

Can I plausibly pull it off or is it too far fetched?

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  • $\begingroup$ You have a funny notion of "near future". $\endgroup$ – Starfish Prime Jan 22 at 15:31
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    $\begingroup$ It's not far-fetched, it's just not useful. There's no advantage to be gained by launching your interplanetary flights from the moon as opposed to earth's orbit. $\endgroup$ – Morris The Cat Jan 22 at 15:55
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    $\begingroup$ Ad Astra also involved a race against moon buggy pirates and an unplanned stop in the middle of an interplanetary transit to rendezvous with another ship on a completely different transit, in defiance of anything vaguely resembling orbital mechanics, to find said ship is full of murderous space monkeys. It should not be taken as even vaguely scientifically accurate. $\endgroup$ – Christopher James Huff Jan 22 at 23:33
  • $\begingroup$ As for the moon, no, it doesn't make sense as a stopping point. Craft traveling between Earth orbit and the rest of the system would be making their trips harder by stopping there, while getting little in return. At most, it might be a source of propellant exported to Earth orbit, but the people who actually live there and mine those scarce volatile resources might have other things they'd prefer to use them for. $\endgroup$ – Christopher James Huff Jan 22 at 23:46
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The best place for spacecraft is in space. Anything designed for efficient and/or fast long distance travel would be optimised for its job, and that job is flying from orbit to orbit. Landing and taking off from a planet or moon is a completely different job, requiring different kinds of engine and load bearing structures and indeed a much more strictly defined notion of "up" and "down" than a non-landing spaceship really needs.

That's not to say that a moon base wouldn't be useful for other things... there's plenty of useful minerals down there that can be refined to supply shipyards, and a facility buried under the lunar regolith will be much better protected against radiation and meteorites than one in orbit would be.

What you really need is a pair of Lunar Space Elevators.

Lunar L1 Space Elevator

This shows a nearside space elevator terminating at the Earth-Moon L1 Lagrangian point, which you'd use for travel between the Moon and Earth. The farside elevator would look pretty similar, but would be used for travel out of the Earth-Moon system to elsewhere in the Solar System.

Your actual "moon base" would be at the south pole, where it can take advantage of Peaks of Eternal Light for convenient solar power, but most of the actual business interplanetary and cislunar travel would be conducted entirely from space. Cargo and spacecraft can be hauled out to the ends of the elevator cables past the counterweights where they can simply fall under gravity away from the moon for a sort of free launch.

The space elevators would be difficult to construct, but vastly easier than making similar constructions on Earth, by some orders of magnitude. I'd expect a spacefaring civilisation with a couple of hundred years of experience would be entirely capable of building and maintaining such things.

It isn't entirely clear to me that building this sort of lunar infrastructure is useful in and of itself, especially if you can travel further afield to more convenient places for harvesting resources (like the asteroid belt) but you can certainly see how such a place could be useful once you had put it together. You have authorial fiat, so handwaving it in is not unreasonable.

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A re-tanking depot on the Moon is not ideal. In fact it would suffer from a lot of problems. First it is necessary to land on the surface which requires considerable propellant both LOX and presumably methane or hydrogen. It is then necessary to launch back into orbit again which uses even more propellant.

Another issue is the availability of hydrogen on the Moon. Some is available in the form of water ice at the poles but that means diverting to a polar orbit and back again also using more propellant. Alternatively hydrogen could be imported as it is only a small fraction by weight of the total, but that removes some advantages of landing in the first place.

IMO A better option for an advanced civilization would be to find a low gravity source of water ice perhaps in the asteroid belt and ship it back sunward in large tankers via a slow transfer orbit using ion drive or solar sails or similar. This water could then be processed in solar powered orbital refineries to produce propellants.

If the Moon were to be used bear in mind that the most efficient way would be just to fly a minimalist propellant tanker into lunar orbit rather than to land the entire interplanetary space craft on the Moon’s surface.

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  • $\begingroup$ There's a problem with shipping water around: chemical engines burn fuel-rich for various reasons, while nuclear engines would only be interested in the hydrogen. Water contains more oxygen than you have a use for, and you'd probably just dump it as a waste product. You'd save a lot of propellant by splitting the water first, so you can dump the oxygen early instead of transporting it across the solar system. $\endgroup$ – Christopher James Huff Jan 27 at 3:28
  • $\begingroup$ @James Huff SpaceX's Raptor full flow staged combustion engine (that will power Starship and Superheavy for Musks Mars ambitions), burns 3.8 times as much oxygen as methane. So the reverse is the case for this engine at least. Which particular rocket engine do you think burns fuel rich? $\endgroup$ – Slarty Jan 27 at 9:44
  • $\begingroup$ Every engine apart from perhaps some lab experiments, Raptor's no exception. For methane, a stoichiometric ratio would be 2 O2 per CH4, or 4:1 by mass. If your propellant plant produces oxygen and methane in stoichiometric ratio for consumption by Raptor engines, 1 t out of every 20 t of oxygen it produces is going to be waste product. It's worse for hydrogen fuel: the RS-25 as an example hydrogen engine burns propellant with an O:F ratio of 6.03:1. The stoichiometric ratio is 8:1, so you'll be throwing away about a quarter of the oxygen you get from ice. $\endgroup$ – Christopher James Huff Jan 27 at 13:14
  • $\begingroup$ @Christopher James Huff - OK you're right - my bad, I have it back to front, there would be an excess of oxygen in both cases for rocket propulsion purposes. So maybe it would pay to process the propellants at the source. But it gets a bit complex because the oxygen might be used for other purposes and there might well be considerable boil off from a hydrogen tank on a long low energy voyage so a number of trade-offs to consider. $\endgroup$ – Slarty Jan 27 at 21:38
  • $\begingroup$ Some of the oxygen can be used for other purposes...but you're going to need a lot of propellant, and smelting is also going to produce oxygen. It's going to be a common waste product of space industry. Boiloff...if you can liquefy it, you can build a tanker that keeps it liquid. However, hydrogen will diffuse straight through most materials you'd build tanks from. Lots of tradeoffs to consider...double-walled tanks with recovery systems, vs. converting it to methane (needs a carbon source, and means moving a bunch of carbon as well as the hydrogen/oxygen, but much easier to store)... $\endgroup$ – Christopher James Huff Jan 27 at 22:42

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