My thinking here is that if my rigid airship body is, say, 24g, with vacuum, why would it float with helium or hydrogen inside, when they have mass and thus weight? Makes very little sense, unless it has something to do with the density?
I haven’t actually done this yet, and I’m working out ideas for one. Everyone I know tells me I’m wrong for thinking a vacuum (assuming a full one, although they are elusive) would float over helium or hydrogen.
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All other things equal, a vacuum-filled volume will in fact weigh less than the same volume filled with H2 or He, and yes, it will be more buoyant.
The trouble is, the forces on a zero-pressure container filled with gas are very small, which allows you to make it very light weight.
OTOH, a vacuum filled container must resist huge stresses caused by atmospheric pressure unbalanced by anything on the inside. So your rigid container has to be engineered much stronger, and it ends up being much, much heavier than the difference between He and vacuum.
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