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The issue:

I have set up a lunar base with around 500 people. Cargo shuttles ("Bullets") regularly fly up and down from low lunar orbit. These shuttles, unfortunately, have a very high mass ratio of around 6 because of their propulsion system. Water is the cheapest available propellant, being present in quantities up to a billion tons in the Moon's polar craters. So the Bullets use superheated steam as propellant (being much safer than hydrogen-oxygen chemical as it avoids the obvious explosion risk). Although this allows a very fast turnaround, a nuclear reactor powered steam rocket will only have a specific impulse of ~195 seconds. Thus, every time the shuttles want to land or launch to deliver, say, 10 tons of cargo, they are sending dozens of tons of water spewing out the nozzle and into unrecoverable lunar orbits.

So, the dilemma in a TL;DR: How to avoid losing many tons of water, which is vital to conserve as much as possible?

Now comes my proposed solution. Due to the low specific impulse/exhaust velocity, much of the ejected water will be in orbit around the Moon. Therefore, it would still be accessible... if, like Earth, the Moon trapped the molecules in a magnetic field. Obviously, it does not exist. So what if the lunar base crew set up an artificial magnetic field, much like a Bussard ramscoop, to capture magnetized ions of water and bring them down to the lunar surface?

With sufficient power for the generator, would the above scheme to capture water for re-use be feasible?

It does not necessarily need to be fast or short term. I am only asking if it is possible.

I appreciate your feedback and answers. Thank you!

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  • $\begingroup$ Would this field be a moving scoop on a ship like the ramscoop? Or is this a fixed field based on the ground? $\endgroup$
    – Willk
    Dec 13 '20 at 1:58
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Frame Challenge: If you're concerned about water loss, don't use water to fuel your rockets.

It seems that, if a civilization is willing to use steam rockets ($I_{sp} = 195 \mathrm{ s}$) over Hydrogen/Oxygen ($I_{sp} = ~450\mathrm{s}$) in vacuum, they're not really that concerned about water loss. A major part of getting into space is about learning how to deal with explosive substances safely, and the Rocket equation is brutal.

If the civilization is absolutely dead-set on keeping all the water for the biologicals, consider using Aluminum-Oxygen rockets instead. These can also be produced from Lunar regolith , and are expected to pull a Specific Impulse of 285 seconds.

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Different frame challenge from @notovnny's: don't use rockets at all!

Generating a magnetic field big enough to sweep space for ionized water particles is expensive, and requires a lot of construction. If you're resigned to constructing something, why not a magnetic launch system?

All of the concerns expressed in the paper are about energy requirements, which you've indicated are not a concern, and the shuttles are used for cargo, so human-tolerance acceleration isn't a concern either. Assuming that the materials being shuttled into orbit are space-tolerant (minerals, ores, metals), they don't even need to be in a "shuttle" pod, they can be launched directly into lunar orbit as they are, and gathered there. (This also removes the need to retrieve the shuttle "empty".)

Because this system is purely mechanical, aside from wear on parts, the only resource being consumed is electricity - which your proposed propellant recapture system would be consuming anyway!

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  • $\begingroup$ If energy is free the answer to can we is yes. $\endgroup$
    – Mazura
    Dec 13 '20 at 4:03

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