# Forum > Discussion > Mad Science and Grumpy Technology >  Would a vacuum airship be viable if it launched and landed from high altitude?

## Bohandas

The main issue with vacuum airships seems to be keeping them from collapsing from the external air pressure. Adding the necessary structural components cancels out the gain from using a vacuum chamber instead of helium of hydrogen. However, if the ship were intended to fly between mountain peaks, much less reinforcement would be required because the atmosphheric pressure is much lower. Some cursory googling tells me that at the top of Everest the air pressure is only 30% what it is at sea level, would that reduce the necessary reinforcement enough to see a gain in lift

Similarly if your concern was reaching higher altitudes rather than maximizing lifting force would a vacuum airship be viable then? A normal high altitude balloon can already reach about six times the height of everest, what if such a balloon were equipped with an extremely lightweight internal frame and a pump that started evacuating the lifting gas after the balloon was a certain number of miles above the ground, could that ever be helpful in attaining a higher altitude or would any pump powerful enough to be helpful in this regard also be too heavy?

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## halfeye

> The main issue with vacuum airships seems to be keeping them from collapsing from the external air pressure. Adding the necessary structural components cancels out the gain from using a vacuum chamber instead of helium of hydrogen. However, if the ship were intended to fly between mountain peaks, much less reinforcement would be required because the atmosphheric pressure is much lower. Some cursory googling tells me that at the top of Everest the air pressure is only 30% what it is at sea level, would that reduce the necessary reinforcement enough to see a gain in lift


That's a nope.

Air pressure is ridiculously high. At sea level it's 15 pounds per square inch, your 30% would take that down to about 5 psi, that's still ridiculous. 5 lb per square inch is 5 * 36^2 per square yard, calculating that on paper I get 6,480 lb. The thing about balloons is that the pressure is outward, so tensile stress on the material keeps the shape of the gasbag. With a vacuum the pressure is inward, so you need to keep the shape against the pressure, and that's much harder.

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## Chronos

When the air is less dense, you also get less lift from any given volume, and so the weight of the envelope hurts you more.

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## Fat Rooster

Nope, if you halve the air pressure, you halve the air density, meaning it needs to be bigger. The only thing helping you is that temperature drops, but not nearly enough to make it work well. If anything, at higher pressures your sphere is smaller, so it is easier. The biggest problem is that the thin walls are likely to buckle, so the thicker walls required from higher pressure actually makes it easier.

The biggest problem with vacuum airships is that hydrogen is cheap* and already so much lighter than air that a vacuum isn't much better. You only get ~8% improvement in lift per unit volume. A vacuum isn't worth the effort.
The biggest problem with hydrogen airships is that helium exists, and everybody gets mad when you suggest using it... "Why are you cheaping out on safety** by not using helium".
The biggest problem with helium is cost, so every now and again somebody goes back to suggesting vacuum because it is cheaper.

* By volume. It sort of sucks as a fuel because of its low density, but when the whole point is the volume this is great.
**We absolutely can build safe enough hydrogen airships now. It bugs me that we don't, especially when people are suggesting vacuum airships. If a vacuum airship starts to fail, it implodes spectacularly and almost instantly, releasing as much energy as if 1 part in ~1000 of the mass of the vehicle were TNT. They are vastly more dangerous than anything you could build with hydrogen.

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## Mechalich

> We absolutely can build safe enough hydrogen airships now. It bugs me that we don't, especially when people are suggesting vacuum airships.


This is mostly because our other transit infrastructure is already too good. There are relatively few places that are inaccessible to ships, roads, and rail where airships would be useful. They aren't non-existent, for example an airship would be an ideal base for certain kinds of scientist research projects such as those in remote jungles, but they are limited.

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## Bohandas

What if it launched from somewhere that was already vacuum and the interior was merely a more complete vacuum. Like taking off from the moon but the density in the interior chamber is closer to that of the interstellar medium than that of the lunar medium? Is there any way to make that work in theory?

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## Khedrac

> What if it launched from somewhere that was already vacuum and the interior was merely a more complete vacuum. Like taking off from the moon but the density in the interior chamber is closer to that of the interstellar medium than that of the lunar medium? Is there any way to make that work in theory?


Even worse as to get any noticable lift the "vacuum envelope" now has to be absolutely huge - I suggest going back and re-reading what people wrote about the change in density and the weight of the baloon.  Ask questions if you don't fully understand..
At a rough guess, a vacuum balloon for the moon might be too big to fit in the "atmosphere", it will definitely be big enough that it will now act as a far better solar sail that lifting mechanism.

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## Chronos

Buoyancy doesn't care at all about the _ratio_ of densities, only about their _difference_.  With a high-density medium like water, buoyancy is easy:  Wood, for instance, is about 70%-80% as dense as water, but 20%-30% the density of water is still a pretty large difference, so you can easily make a raft out of wood.  But when you're on the Moon, the "atmosphere" has a density that's just a sliver above zero, and so even with an absolutely perfect vacuum, that tiny sliver is all the difference in density you can ever get.

Incidentally, this also means that, for an airship (in ordinary air), even though hydrogen is half the density of helium, it's only slightly more efficient as a lifting gas.  Air has an average molecular mass of about 29, helium 4, and hydrogen 2, so hydrogen's difference is 27, compared to helium's 25.  In other words, being half the density of helium only gives it about 8% more lift.  Vacuum, meanwhile, even if you had some magically strong and lightweight material to make the envelope out of, would have a difference of 29 from air, still not all that much more than helium's 25.

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## Mastikator

> What if it launched from somewhere that was already vacuum and the interior was merely a more complete vacuum. Like taking off from the moon but the density in the interior chamber is closer to that of the interstellar medium than that of the lunar medium? Is there any way to make that work in theory?


If you put an empty ball on the surface of the moon then there'd be nothing to prop up the interior. It would lie flat. I mean, what we're talking about here is an empty container in empty space. There's no buoyancy to lift it and no internal pressure to expand it. It just... sits there.

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## Bohandas

> If you put an empty ball on the surface of the moon then there'd be nothing to prop up the interior. It would lie flat. I mean, what we're talking about here is an empty container in empty space. There's no buoyancy to lift it and no internal pressure to expand it. It just... sits there.


I'm imagining it as having a very simple frame made of some very light material. Something like three perpendicular hoops going all the way around

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## halfeye

> I'm imagining it as having a very simple frame made of some very light material. Something like three perpendicular hoops going all the way around


They still weigh something, and you've got almost no lift. Your material has to be light enough to lift itself, and strong enough to hold out the external pressure, that combination just isn't happening. The less the external pressure, the less the lifting power, so the problem stays the same as the external pressure drops.

Did you see the demonstration with the steel can full of steam that collapses when the steam turns back to water? That's a partial vacuum, there's still the pressure in there from water vapour, but that pressure inside is nothing like enough to stop air pressure from crushing the can, and that can is steel, it's not weak at all. You can make a can that's strong enough that it won't collapse, but it will be very heavy.

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## Radar

> I'm imagining it as having a very simple frame made of some very light material. Something like three perpendicular hoops going all the way around


Which means that the container would have some significant mass having an incomparably higher impact on lift cost than what you could possibly gain from buoyancy. And that assumes you can actually get the vacuum inside rarer than that of the Moon atmosphere, which is of order of 10^(-15) bar, which is beyond standard high throughput vacuum pumps and would require a very sophisticated system to achieve along with nontrivial container materials that would not evaporate. To put things in perspective: even LHC and LIGO have higher pressure inside and those facilities need the best vacuum we can get to operate properly.

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## Hyoi

Your first sentence captures the problem:




> The main issue with vacuum airships seems to be keeping them from collapsing from the external air pressure


The various answers all boil down to "the best way to keep the airship from collapsing is to fill the empty space with something super lightweight that will push back against the pressure (aka hydrogen/helium)".

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## Bohandas

Here's another thought. What if the balloon were somehow given a positive electrical charge? This would cause the sides of the balloon to repel each other. 

There would probably be severe limitations to the size of balloon that could be inflated in this manner and to what pressures it would be viable in, but is there a sweet spot where it could work?

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## georgie_leech

> Here's another thought. What if the balloon were somehow given a positive electrical charge? This would cause the sides of the balloon to repel each other. 
> 
> There would probably be severe limitations to the size of balloon that could be inflated in this manner and to what pressures it would be viable in, but is there a sweet spot where it could work?


It would also, you know, have a good chance of electrocuting things, as you would need an extremely strong charge to balance out air pressure. Since the whole point is to get a big balloon to lift things, you're talking about opposing charges balancing out air pressure over a great distance; anything closer than the the other side of the balloon is going to be experiencing a great deal of stress from the positive charge doing its best to equalize. To say nothing about what regularly happens to strong charges in the atmosphere. And as awesome as it sounds for a weird sci-fi version of D&D or possibly the Red Alert franchise, an airship regularly causing lightning strikes seems like the kind of thing that would get sent back to the drawing board.

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## Chronos

> Quoth *Bohandas*:
> 
> I'm imagining it as having a very simple frame made of some very light material. Something like three perpendicular hoops going all the way around


It's almost amusing how, every time anyone ever suggests vacuum airships, this is always the envelope design they come up with.  Whatever material you're making your hoops out of, it would always be more effective to make the whole envelope out of that same material.  Spheres are stronger than hoops.

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## Manga Shoggoth

> Whatever material you're making your hoops out of, it would always be more effective to make the whole envelope out of that same material.  Spheres are stronger than hoops.


Nope - the hoops are stronger because they are thicker. As a result they are also heavier. The envelope as a whole is lighter because you have a thin membrane for most of its surface given support by the hoops. You make the hoops as small as possible (to reduce the weight) and the membrane area as large as possible (to increase the volume).

This is great when using something like Hydrogen or Helium (or even hot air), because the internal gas also gives internal support. It is useless if you are trying to use vaccuum, because air pressure at one atmosphere will crush things very effectively. If you can put up with the music, there's a nice demonstration here. What appears to be the slo-mo version is here.

By the time you have sufficient internal strength, you also have far too much weight.

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## Fat Rooster

> Nope - the hoops are stronger because they are thicker. As a result they are also heavier. The envelope as a whole is lighter because you have a thin membrane for most of its surface given support by the hoops. You make the hoops as small as possible (to reduce the weight) and the membrane area as large as possible (to increase the volume).
> 
> This is great when using something like Hydrogen or Helium (or even hot air), because the internal gas also gives internal support. It is useless if you are trying to use vaccuum, because air pressure at one atmosphere will crush things very effectively. If you can put up with the music, there's a nice demonstration here. What appears to be the slo-mo version is here.
> 
> By the time you have sufficient internal strength, you also have far too much weight.


Show me a submarine built out of hoops then? It is a surprisingly similar problem.

Hoops are an inefficient use of material because they only make use of the material strength in one direction, while a sphere uses 2. Cylinders still use 2, but because they are not balanced, they are less efficient. The only advantage hoops have is they might be less likely to buckle due to concentrating the forces, but this comes at the cost of symmetry, and this is a major problem. Unless your membrane is perfectly designed it will be unevenly loading your hoops and that will cause them to buckle well before you would expect. Stability can be improved with extra tensile elements, but every time you do that you also further increase compressive loads. Thats before you even consider that your membrane bends inwards, losing you volume that way.

Optimal design is probably a sphere to take most of the loads coupled with a honeycomb lattice to prevent it buckling. This is still extremely hard to do, because your lattice orientation breaks symmetries, and has to have special points if it is regular. You probably actually want a 'glassy' lattice, created from a Voronoi of blue noise. It loses some load bearing capacity, but is macroscopically uniform across a sphere. You also want it to be fractal, addressing all scales of instabilities well. Good luck manufacturing that though.

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## georgie_leech

> Show me a submarine built out of hoops then? It is a surprisingly similar problem.
> 
> Hoops are an inefficient use of material because they only make use of the material strength in one direction, while a sphere uses 2. Cylinders still use 2, but because they are not balanced, they are less efficient. The only advantage hoops have is they might be less likely to buckle due to concentrating the forces, but this comes at the cost of symmetry, and this is a major problem. Unless your membrane is perfectly designed it will be unevenly loading your hoops and that will cause them to buckle well before you would expect. Stability can be improved with extra tensile elements, but every time you do that you also further increase compressive loads. Thats before you even consider that your membrane bends inwards, losing you volume that way.
> 
> Optimal design is probably a sphere to take most of the loads coupled with a honeycomb lattice to prevent it buckling. This is still extremely hard to do, because your lattice orientation breaks symmetries, and has to have special points if it is regular. You probably actually want a 'glassy' lattice, created from a Voronoi of blue noise. It loses some load bearing capacity, but is macroscopically uniform across a sphere. You also want it to be fractal, addressing all scales of instabilities well. Good luck manufacturing that though.


Erm, seeing as how Submarines are fundamentally built out of a stronger skeletal hull using hoop-like support structures with a comparatively much thinner (still metal) skin between them, and are also noticeably not sphere shaped, you might want to reconsider your example if you're trying to prove a point against the general concept...

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## Manga Shoggoth

> Show me a submarine built out of hoops then? It is a surprisingly similar problem.


In addition to georgie_leech's response, there is a huge difference in scale between submarines and what we are discussing.

Submarines get their boyancy by displacing water with air, which means they get a huge payoff (water is much denser than air). Thus, a submarine can afford to have much heavier superstructure than an airship, and as a result can resist much greater pressures.

With an airship the difference between the density of air and the inside of the gasbag are much, much smaller, so every piece of weight matters. While that's workable with a gasbag (the gas itself gives internal support) trying to build a lattice will simply introduce too much weight.

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## Lvl 2 Expert

I'm going to just start with a basic calculation at surface level here, just to illustrate the problem with the concept of a vacuum balloon better. Helium is roughly 14% the density of air (based on nitrogen, the presence of oxygen makes the number a little lower still), and hydrogen is half that so 7% of the density of air. Either of them offers complete neutralization of the air pressure from outside due to how gasses work. An airship filled with vacuum would at best only be that extra 7% more efficient than a hydrogen airship. If the structure you need weights more than 7% of the air displacement of the size of your balloon you've already lost the efficiency advantage, 14% if we're comparing to helium, which, sure, fine.

Air weighs about a kg per cubic meter, give or take. (Americans: just imagine I say "yard" whenever I mention a meter, it works well enough, and like 2 pounds when I say 1 kg.) So to lift a kg you also need roughly a 1 m^3 balloon. So airships are big. If you want to lift ten thousand kg, about 100 people or one very large elephant with a dream of flying, you need a ten thousand m^3 airship, which is a sort of cilinder of 10*10*100 meters. That volume in regular air weighs 10,000 kg. (That may sound like a lot of weight for air, just remember that the same volume of water would weigh ten million kg.) So to compete with helium you would need structural braces for the whole 10*10*100 meters of hard vacuum that weigh 14% of 10,000 kg = 1,400 kg. A vacuum chamber the size of a small skyscraper (1 pretty decent sized apartment or 6 student rooms per floor, 25 floors tall, 30 if the building is from a few sets of building regulations ago) and the box itself is allowed to weigh the same as two small horses or a pickup truck that won't quite get you invited to parties in Texas. That's not doable. I'm not even sure we could make a vacuum box that will float at all, let alone better than helium.

Now as all the others said the altitude will lower the pressure but also your air displacement, and thus the amount of weight you can use. So at the point where you've only got half the pressure to deal with you also have only half the weight available within your engineering limits. So now you need to make a vacuum chamber that could withstand 50% of sea level atmospheric pressure the size of the same small skyscraper for the weight of a single small horse or a car that won't quite get you invited to the small city car lovers club. That's not really any better, unfortunately.

Lucky for people who want to travel between mountains there are flying things that do benefit from lower air pressure: airplanes. Small planes in particular fly more efficiently at high altitude than they do even skimming across water using the ground effect. If people on mountains want to travel efficiently and realistically I recommend fixed wing aircrafts.

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## halfeye

> Lucky for people who want to travel between mountains there are flying things that do benefit from lower air pressure: airplanes. Small planes in particular fly more efficiently at high altitude than they do even skimming across water using the ground effect. If people on mountains want to travel efficiently and realistically I recommend fixed wing aircrafts.


Except that combustion is less effective with less oxygen, and we don't yet have production lines for electric aircraft.

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## Manga Shoggoth

> Except that combustion is less effective with less oxygen, and we don't yet have production lines for electric aircraft.


We do have jet engines, and they operate at much greater altitudes. And runways aren't necessarly a problem if using a design like the Harrier.

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