# why would a rigid airship body with helium or hydrogen would float but not one with a vacuum inside?

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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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A vacuum airship would work, as it would have the best buoyancy characteristics compared to other gasses.

However it presents a hurdle in that while you can fill a thin bag with helium, how do you get your vacuum chamber to retain its shape under atmospheric pressure? Any container we can build would need to be far heavier in order to retain its shape than its buoyancy can lift.

A balloon filled with vacuum would indeed float, and be more buoyant than the same volume filled with a gas. However, it’s much harder to maintain a vacuum than it is to fill the space with gas. A gas filled balloon has roughly equal pressure with the atmosphere, and as such has less force on its body. It also takes up the most space, since a balloon filled with vacuum would pull in everywhere it can.

It would. It’s just really hard to make something that can hold a vacuum that’s also lightweight.

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.