6 ms·

Probably a very stupid question, but can someone help me understand the following: once you have airships that have a rigid shell, why isn't using a vacuum bett

Probably a very stupid question, but can someone help me understand the following: once you have airships that have a rigid shell, why isn't using a vacuum better than a lifting gas? Isn't the buoyant force simply the result of displacing some volume of air with something less dense? (i.e. any excess lifting force comes from the fact that the mass of the hydrogen or whatever is less than the mass of the displaced air) A vacuum (or near vacuum) would provide more lifting force per liter, would not have the scarcity problem of helium nor the safety problem of hydrogen, and assuming the thing that generates the vacuum is transportable, it'd eliminate the need for separate ballast. Edit: the wikipedia article cited by slibhb has all sorts of good info - thank you for sharing that!

lucideer 4y ago
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.
- bagels 4y ago
  It's an understatement. It would not be buoyant, and would not fly.
vel0city 4y ago
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.
oliveshell 4y ago
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.
- morpheos137 4y ago
  You could make many small vacuums in a honeycomb structure but at that point is just cheaper to use a lifting gas...
  - kilianinbox 4y ago
    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.
- mLuby 4y ago
  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.
slibhb 4y ago
That's a fascinating idea. I googled it and came upon https://en.wikipedia.org/wiki/Vacuum_airship https://en.wikipedia.org/wiki/Vacuum_airship and https://scholarsbank.uoregon.edu/xmlui/handle/1794/22370#:~:text=The%20Vacuum%20Airship%2C%20or%20Null,in%20nearly%20all%20conventional%20airships https://scholarsbank.uoregon.edu/xmlui/handle/1794/22370#:~:.... Sounds like it's feasible but a materials/engineering challenge.
- dredmorbius 4y ago
  s/challenge/impossibility/
cdot2 4y ago
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.
lordnacho 4y ago
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.
- Asooka 4y ago
  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.
  - Tuna-Fish 4y ago
    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.
    - hutzlibu 4y ago
      Yes, if you just shoot a hydrogen airship, not much happened, only after they introduced incendiary rounds, strong damage was done.
      - hutzlibu 4y ago
        No idea why this was downvoted, but in case anyone believed it was imaginary: https://en.wikipedia.org/wiki/Pomeroy_bullet https://en.wikipedia.org/wiki/Pomeroy_bullet
  - taneq 4y ago
    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.
WorldMaker 4y ago
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.

lucideer 4y ago

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.

bagels 4y ago
It's an understatement. It would not be buoyant, and would not fly.

vel0city 4y ago

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.

oliveshell 4y ago

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.

morpheos137 4y ago
You could make many small vacuums in a honeycomb structure but at that point is just cheaper to use a lifting gas...
- kilianinbox 4y ago
  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.
mLuby 4y ago
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.

slibhb 4y ago

That's a fascinating idea. I googled it and came upon https://en.wikipedia.org/wiki/Vacuum_airship https://en.wikipedia.org/wiki/Vacuum_airship and https://scholarsbank.uoregon.edu/xmlui/handle/1794/22370#:~:text=The%20Vacuum%20Airship%2C%20or%20Null,in%20nearly%20all%20conventional%20airships https://scholarsbank.uoregon.edu/xmlui/handle/1794/22370#:~:.... Sounds like it's feasible but a materials/engineering challenge.

dredmorbius 4y ago
s/challenge/impossibility/

cdot2 4y ago

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.

lordnacho 4y ago

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.

Asooka 4y ago
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.
- Tuna-Fish 4y ago
  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.
  - hutzlibu 4y ago
    Yes, if you just shoot a hydrogen airship, not much happened, only after they introduced incendiary rounds, strong damage was done.
    - hutzlibu 4y ago
      No idea why this was downvoted, but in case anyone believed it was imaginary: https://en.wikipedia.org/wiki/Pomeroy_bullet https://en.wikipedia.org/wiki/Pomeroy_bullet
- taneq 4y ago
  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.

WorldMaker 4y ago

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.