I wonder if there is a sweet spot between the two. Have a rigid structure with an internal pressure less than one, but more than zero atmospheres.

There is not. The costs of dealing with pressure differentials are extreme, and there are a lot of small thorny issues.

For one, if you have a pressure differential, you need to care very much about even the tiniest leaks. A surprising fact about airships is that they don't actually have to be very gas-tight to work just fine. As people found to their surprise in both world wars, you can shoot an airship full of small holes and it only very slowly degrades in performance.

Yes, if you just shoot a hydrogen airship, not much happened, only after they introduced incendiary rounds, strong damage was done.

No idea why this was downvoted, but in case anyone believed it was imaginary:

https://en.wikipedia.org/wiki/Pomeroy_bullet

At the halfway point you get 7% more lift but still need to withstand ~50kPa of external pressure. I can't see how you'll ever build a rigid structure that can withstand 5 tonnes per square meter and only weighs 7% more than a gas bag which operates only under a tiny fraction of that force and only under tension.

What was that based on?

> I don't see how the vacuum ever wins.

3 ways.

- Vacuums aren't combustible.

- Vacuums can be created without sourcing a gas.

- Vacuums allow you adjust buoyancy without requiring you to carry extra gases.

The major downside of a vacuum is finding something ridged enough, light enough, and cheap enough to maintain a vacuum.

Helium isn't combustible either.

It's also finite and much more expensive.

"vacuums aren't combustible" oh I love it idk what you did here, but the assumption of hydrogen still being used is awesome. reminds me of a great Archer episode.

Its a little disingenuous to imply that a vacuum is safer than Hydrogen just because it doesn't combust. Have it spring a leak near the passenger cabin and watch what happens: https://www.youtube.com/watch?v=cPoVuFtWs_Y&t=7s