Might Astronauts Dive In And Out Of A Lunar Cave Habitat Through A Liquid Airlock?

By Robert Walker | October 8th 2018 06:45 PM

This is a rather fun idea originally suggested in the Moon Miner's Manifest Classics - 1987-1988 (see page 31). It's not so likely in the early stages, because of the large amounts of water needed to construct it, but it may perhaps be of great value at a later stage, especially for bases that have a lot of traffic in and out. If the liquid is water, it has to be over sixty meters deep (62.3 meters), to equalize the pressure inside and outside the habitat. The depth can be much less, if it is a denser fluid. You then don't need any doors but can just dive through it and come out on the surface of the Moon.

It's like the way that In a barometer, the weight of the mercury counterbalances the pressure of the atmosphere outiside, with a vacuum above the mercury. It's like an "inside out" barometer with the vacuum on the outside. The weight of the water in the column counteracts the pressure of the air in the habitat.

You might think that as for a barometer, mercury is the best fluid of all, because of its high density, at least, if it weren't for its toxicity. Especially for a surface base, the lower column height the better. Mercury would give a column height of only 76 centimeters on Earth, so on the Moon, (9.807/1.622)×0.76 = 4.6 meters. Should we look for the densist liquid we could send to the Moon?

Well, if supplied from Earth, the actual mass of the mercury would be the same as the mass of water. So in that sense, a denser fluid is no saving at all. If you can source the liquid in situ it's also a considerable saving, so if there is ice on the Moon that might well swing it in favour of water rather than some denser liquid from Earth. As for the column height, that's less of a consideration if the base is constructed inside a lunar cave, as many lava tube caves are likely to be at least 60 meters below the surface of the Moon. It might be useful to be able to float up 60 meters to the surface. A denser fluid would help with a surface base.

More:
https://www.science20.com/robert_walker/might_astronauts_dive_in_and_out_of_a_lunar_cave_habitat_through_a_liquid_airlock-234621