Wild wild speculation: I'd imagine "rigidity" is a trade-off between weight and ability to withstand the given pressure differential for any two gases (outside & inside). A vacuum would undoubtedly give extra buoyancy but the additional weight required to achieve rigidity might not make the trade off worthwhile.

It's an understatement. It would not be buoyant, and would not fly.

I'd imagine its because its one thing to make a basic rigid shell (able to hold its own weight without collapsing) and another thing to make a giant pressure vessel able to withstand immense pressures. Imagine a storage tank made out of glued together popsicle sticks with a plastic bag around it versus a CO2 cylinder. One is going to weigh quite a bit more than the other, all because one is trying to fight some massive pressure differentials while the other can accept near ambient pressures on both sides.

I looked into this a while back, and there's simply no feasible way to construct an airship from known materials that could sustain a vacuum of the necessary volume without being crushed by atmospheric pressure.

You can keep reinforcing the vacuum chamber, but by the time it's strong enough, it'll be too heavy for the buoyant forces to lift it.

You could make many small vacuums in a honeycomb structure but at that point is just cheaper to use a lifting gas...

Honeycomb structures; may be dynamic and expanded as a sort of umbrella since pressure is lower higher up above the clouds. I guess a nifty construction with tiny small mirrors can direct lots of solar energy and work as sails, controlled by smart bending compliant mechanisms. Also - regarding flying objects, I've always wondered if a rotating disc, ie frisbee falls slower to thee ground because of atmosphere interaction, or if it's still falls slower in a vacuum with other forces at work. Well, still rails and boats are good stuff. I really like that honeycomb for an -n dimensional space construction idea.

Instead of additional mass, you could add additional energy.

For example, a small maglev running around the inside of the vacuum sphere would steady outward pressure on that part of the sphere. Add more trains at different angles to even out the forces.

As a bonus, this kind of "active structure" can dynamically vary its structural strength as atmospheric pressure changes with altitude or weather.

See Orbital Rings for more info.

I suspect that the materials and engineering required to maintain a vaccum would be so much heavier than the engineering required to hold hydrogen that it would literally outweight the benefit.

That's a fascinating idea. I googled it and came upon https://en.wikipedia.org/wiki/Vacuum_airship and https://scholarsbank.uoregon.edu/xmlui/handle/1794/22370#:~:....

Sounds like it's feasible but a materials/engineering challenge.

s/challenge/impossibility/

I think the key is in the wikipedia article: You only gain 14% lift with a vacuum vs helium. The tradeoff is either:

- Build a pressure vessel to withstand the differential to the atmosphere

- Let the helium do the pushing from the inside but make the thing 14% bigger

I don't see how the vacuum ever wins.

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

One interesting modern wrinkle versus hydrogen is the idea of using an H2 fuel cell for easy access to electricity (versus lift) without needing to carry another electricity source.