Ramscoop for a space station or lunar base?

Question

From what I've read, the Bussard ramscoop has problems which make it impractical as a means of propulsion. However, would it be possible to use it for a different purpose? Could a space station or lunar base (anything that's not protected by a magnetic field) make use of a ramscoop, one that's directed towards the Sun? This would protect the station/base from the solar wind, and also allow it to harvest the solar wind. There isn't much material in the solar wind, but it might allow a station/base to make up for losses without needing imports.

It might be especially useful for a lunar base because the main element in the solar wind is hydrogen, which is scarce on the Moon (relative to the Earth), and could be used to refine metals from their ores, producing water as a useful byproduct.

Answer 1

The issue with any use of a Bussard Ramscoop is efficiency. In this answer I go into some of the propulsion details, basically in order to make it useful you have to be going a significant proportion of the speed of light to reach the break even point, where you gain more energy than you spend on maintaining the field (Note that is theoretical, we aren't sure if the technology could ever deliver).

On the moon if you want to collect hydrogen using a magnetic field you'll need a lot of energy, and you couldn't use hydrogen power for it as you'll never collect more than you use to generate the field as the hydrogen density won't be high enough. The power would therefore have to be nuclear fission or solar, and a whole lot of it too. The cost to manufacture, transport and maintain that much of either type of generating equipment would almost certainly be much higher than the costs of just shipping up a bit of hydrogen now and then.

Answer 2

The solar wind is not very dense; the sources in your link estimate around 6 atoms/cm³, 95% protons. Assuming a "wind" speed of 700 km/s, this amounts to $3*10^{18}$ atoms hitting a km² each second, or $9.5*10^{25}$ atoms/a, which is in the order of magnitude of a chocolate bar (150g).¹ While this is a macroscopic amount, it is not enough for industrial applications like ore refining. For some surface materials like cadmium or zinc it may not even offset the evaporation losses of the ramscoop surface. A JPL paper from 1961 contains some numbers for different materials (table 2, p. 74). Logically, the evaporation rate increases with temperature; because the "scoop" surface must face the sun it cannot be kept cold.

Of course the more technical question how to "harvest" a layer of atoms from a square kilometer of aluminum foil may be a showstopper. The hydrogen will probably "dissolve" in the metal (assuming the "scoop" is a metal foil). It is also likely to evaporate again, leaving a considerably lower net gain.

Bottom line: Interesting idea, but too little matter flow from solar wind alone.

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_{¹ I cannot refrain from citing Samuel Beckett here, from Watt: "The figures given here are incorrect. The consequent calculations are therefore doubly erroneous." (https://web.csulb.edu/~bhfinney/beckett.html)}