My "invention"

[billydkid]

Ok, I was thinking it would be cool if you could create an anti-balloon. By which I mean the equivalent of a balloon, but containing a vacuum and thereby being bouyant in the atmosphere. I wonder if this is possible with today's high tech materials. My first thoughts were something along the line of a lightweight graphite frame with a mylar skin. Another option might be a sphere made out of ultralight weight shuttle skin type foam with a skin around it and draw a vacuum on that. Is this simply ridiculous and implausible? Also, I was thinking of something equivalent to an eggshell made out of lightweight polymer of some sort. Of course, it would have to be big enough that the volume of the vacuum gave sufficient lift to overcome the weight of the structure.

[Genesius]

Buzz Aldrin beat you to it - read his novel "Encounter with Tiber".

Still, even if the idea's been done before it still leaves you in pretty good company.

[Mojo]

This guy (http://www.faculty.fairfield.edu/jmac/sj/scientists/lana.htm) beat Buzz by a few hundred years. Unfortunately I posted it into the other thread...

[fishbob]

Edgar Rice Burroughs also beat you to it. See Tarzan at the Earth's Core for details.

[Ziggurat]

Ok, I was thinking it would be cool if you could create an anti-balloon. By which I mean the equivalent of a balloon, but containing a vacuum and thereby being bouyant in the atmosphere.

The big question is, why? What does a vacuum balloon get you that you can't achieve with a hydrogen or helium balloon? If the shell were hard enough, you might get a bit of a size advantage (less than a factor of two in volume at best on earth, which means much less than that in linear dimensions), but that's not really why you go with a balloon (as opposed to, say, a glider) in the first place. A vacuum balloon would be expensive and fragile (it has to be hard, unlike a gas balloon which can be stretchy and bouncy, and if you do dent the thing at all, the pressure to collapse on a now-weakened structure increases), failure modes are likely to be catastrophic (even a small crack can propagate under the external pressure, with resulting implosion and the whole thing plummeting - whereas a gas balloon can have close to equal pressure on both sides, so tears don't propagate by themselves, most leaks stay small and you only slow descend). So I'm not seeing any reason to try doing something that difficult unless there's some compelling advantage which I don't see yet.

[billydkid]

The big question is, why?

Because it is interesting to think about and would be an achievement if you could do it.

[rwguinn]

Because it is interesting to think about and would be an achievement if you could do it.

It would, indeed. Even spherical objects collapse quite readily at the application of external pressure. You will find that the volume of any object will decrease to essentially zero for any thickness-to-diameter ratio that will give you any boyancy.
The average density of your volume (including the shell material) must be less than that of air for this to work. The lack of stability against external pressure in any manufacturable shape means the thing collapses, and the internal volume goes away...
Heck-you can collapse an aluminum beer can by mouth--and it will contain an internal pressure of several atmospheres...
So come up with a stable shape, first...

[Ziggurat]

Because it is interesting to think about and would be an achievement if you could do it.

Interesting to think about, sure. But even if you could do it, it would be extremely expensive, which means nobody is going to try it just to see.

But let's think about this in a little more detail, since that seems to be the point. The easiest shape to do is a sphere, both because it's fairly straightforward to make (and calculate) and because it's going to be the most stable against hydrostatic pressure. So how much pressure are we talking about here? Well, let's play around a little with some numbers.

Let's see what it takes to make a 1 meter vacuum balloon. A 1 meter diameter sphere has a cross sectional area of 0.785 m^2. Atmospheric pressure is 101 kPa, or about 1x10^5 Newtons/m^2. If we divide our balloon into a top half and a bottom half, we can calculate the force that the two sides apply to each other as cross-section x pressure = 7.9x10^4 Newtons, which is a little under 9 tons of force. This will be applied along an area of 3.14 meters x thickness (assuming a thin shell).

Now, how thin would this shell need to be? For a 1 meter diameter sphere, we're displacing about 0.52 m^3 of volume, which is around 1.2 kg/m^3 near sealevel (good enough for the moment). So we're displacing about 0.63 kg of air, which is the upper limit on the weight of our sphere. The volume of shell is going to be
area x thickness = 3.14 m^2 x thickness, or
thickness = volume/3.14 m^2
Steel is about 7.8 g/cm^3 = 7.8x10^3 kg/m^3. If we made our sphere out of steel, the volume would need to be
mass/density = 0.63 kg / 7.8x10^3 kg/m^3 = 8x10^-5 m^3,
so the thickness would be
8x10^-5 m^3/3.14 m^2 = 2.5x10^-5 meters
or 0.025 mm thick (clearly not enough to support that much pressure)

Of course, we could build it out of something stronger, and also possibly less dense. As one example, we'll consider diamond, with a density of about 3.5x10^3 kg/m^3. Run through the same calculations and you'll get a thickness of about 5.6x10^-5 meters.

So let's suppose we went with diamond. The cross-sectional surface area of our shell is now 3.14 m x 5.6x10^-5 m = 1.76x10^-4 m^2. The force on the two halves of the shell is 7.9x10^4 Newtons, so the pressure is 7.9x10^4 / 1.76x10^-4 m^2 = 4.5x10^8 Pascal.

The Young's modulus for diamond (a measure of how it responds to stress/strain, NOT the breaking point which can be much lower) is about 1x10^12 Pascal. That pressure in tensile mode, if the stress/strain remained linear, would stretch a diamond to twice its length. Diamond won't stretch that much, of course, and will break under tensile strain MUCH sooner than that. It's much stronger under compressive strain, but you still can't expect to approach that kind of pressure and have it hold. Since our pressure is only three orders of magnitude smaller than the Young's modulus, that suggests to me that while a diamond shell might hold, it wouldn't actually hold by a lot. Include the problems of any defects plus the added local pressure if you accidentally bumped the thing, and you're really pushing it.

So: diamond might do it, if you can manufacture a thin spherical shell of the stuff. Steel won't come close. Exotic stuff like carbon nanotubes might be easier to manufacture, but isn't likely to have much better strength/weight ration than diamond for compression. So while it might be possible, even with future materials and fantastic manufacturing processes it's probably cutting it close.

[Jekyll]

It would, indeed. Even spherical objects collapse quite readily at the application of external pressure. You will find that the volume of any object will decrease to essentially zero for any thickness-to-diameter ratio that will give you any boyancy.
The average density of your volume (including the shell material) must be less than that of air for this to work. The lack of stability against external pressure in any manufacturable shape means the thing collapses, and the internal volume goes away...
Heck-you can collapse an aluminum beer can by mouth--and it will contain an internal pressure of several atmospheres...
So come up with a stable shape, first...
It strikes me as fairly obvious, that using conventional materials, the shape would need be internally braced and infact as unlike a sphere as possible.
I would sugest a tetrahedal structure and then allow the sides to bow in under the pressure.

That's unless there's some wonderful polymer which is highly flexible with an increadably high stress when being bent that can be rolled into a large ball.

[rwguinn]

It strikes me as fairly obvious, that using conventional materials, the shape would need be internally braced and infact as unlike a sphere as possible.
I would sugest a tetrahedal structure and then allow the sides to bow in under the pressure.

That's unless there's some wonderful polymer which is highly flexible with an increadably high stress when being bent that can be rolled into a large ball.
What we need here is a supply of unobtanium. The G'vmt keeps it in a warehouse right next to the rigidium...

[Jekyll]

What we need here is a supply of unobtanium. The G'vmt keeps it in a warehouse right next to the rigidium...
That reminds me of the how to instructions on making a wormhole. Take two black holes and a large amount of matter with negitive mass....

[Ziggurat]

It strikes me as fairly obvious, that using conventional materials, the shape would need be internally braced and infact as unlike a sphere as possible.
I would sugest a tetrahedal structure and then allow the sides to bow in under the pressure.

That's an interesting idea (though something with more sides is probably better, since you lose less volume from inward stretching). That gives you potentially much greater resistance to perturbation, but in terms of actual load limits, you've only turned compressive strains into tensile strains for the actual surface. Most of the super-strong materials that we know of have higher compressive load strengths than tensile strengths (eg, diamond). And it's made more inefficient by the fact that now you've got the weight of the struts (which must still withstand enormous pressures if they are to be light enough) PLUS the surface material. So it may come down to what your limiting factor is: ideal strength, or stability against external forces (such as getting bumped). If it's the former, I don't think this gets you anywhere, and may even make things worse. If it's the later, it might work.

[Jekyll]

That's an interesting idea (though something with more sides is probably better, since you lose less volume from inward stretching).
Oh yeh, whoops. Something like a bucky ball would be more appropriate.

That gives you potentially much greater resistance to perturbation, but in terms of actual load limits, you've only turned compressive strains into tensile strains for the actual surface. Most of the super-strong materials that we know of have higher compressive load strengths than tensile strengths (eg, diamond). And it's made more inefficient by the fact that now you've got the weight of the struts (which must still withstand enormous pressures if they are to be light enough) PLUS the surface material. So it may come down to what your limiting factor is: ideal strength, or stability against external forces (such as getting bumped). If it's the former, I don't think this gets you anywhere, and may even make things worse. If it's the later, it might work.
I'll bow out to this, my material sciences is extremely limited, however you can equalise the pressure inside the struts with the inside of the 'ballon' if this would make things easier.

[Lynx2174]

yeah, I had the same idea; AFAIK you can't do it wit current materials science, you might be able to if you had lotsa fullerene/diamond based products to work with.

an advantage of it is that although a hydrogen or helium balloon has the gas diffuse out of the balloon, with a vaccuum balloon, you can just pump out any air that gets in with a simple mechanical pump.

disadvantage is that unlike a zepplin, if it is ruptured, it will just fall down and crash.

it would have really pitiful lifting properties compared to even gas balloons, because of the weight of the container. also, it would have a lower maximum ceiling probably.

[Ziggurat]

it would have really pitiful lifting properties compared to even gas balloons, because of the weight of the container. also, it would have a lower maximum ceiling probably.

That would definitely be the case. High-altitude weather balloons use a balloon bag with potentially much larger volume than the boyant gas inside them at sea level, so that most of the balloon is actually slack when first released. That's not a problem, since the material doesn't need to support any pressure difference, it only needs to contain the gas and support the weight of the payload. As the balloon goes up, surrounding air pressure decreases, but the boyant gas inside simply expands as well, and you don't loose any boyancy until you hit the volume limit of the balloon. So you can get to altitudes where the pressure is many times less than at sea level. But for a vacuum balloon, your altitude limit is basically set by how much stronger your material is than the minimum strength needed where you start: if you're barely strong enough, you won't have much room to go up at all.

[casebro]

That would definitely be the case. High-altitude weather balloons use a balloon bag with potentially much larger volume than the boyant gas inside them at sea level, so that most of the balloon is actually slack when first released. That's not a problem, since the material doesn't need to support any pressure difference, it only needs to contain the gas and support the weight of the payload. As the balloon goes up, surrounding air pressure decreases, but the boyant gas inside simply expands as well, and you don't loose any boyancy until you hit the volume limit of the balloon. So you can get to altitudes where the pressure is many times less than at sea level. But for a vacuum balloon, your altitude limit is basically set by how much stronger your material is than the minimum strength needed where you start: if you're barely strong enough, you won't have much room to go up at all.


Wrong, Ziggy. The less pressure on the outside, the less force trying to colapse your 'egg'. However, as the egg rises, the air is less dense, less weight is displaced by the egg, and that WILL limit flotation.

[Ziggurat]

Wrong, Ziggy. The less pressure on the outside, the less force trying to colapse your 'egg'. However, as the egg rises, the air is less dense, less weight is displaced by the egg, and that WILL limit flotation.

That's what I mean: they're boyancy-limited. If the best you can do is barely withstand pressure at sea level, then you're barely lighter than air, and you can't go up much before you reach equal boyancy. If, however, the strongest "egg" you could make that was equal boyancy at sea level was significantly stronger than it needed to be, this would also mean that if you only made it as strong as it needed to be at sea level, it could be significantly lighter than air and so could reach a reasonable altitude. The requirement for "extra strength" that I meant refered more to the fact that to be able to reach a good altitude, you need to be able to build a lot lighter than neutral boyancy, which requires a lot more strength than the neutral boyancy situation I did my calculations for. I know I didn't make this as clear as I wanted to, but my argument wasn't that the egg would actually be under any more stress at higher altitude, and I hope what I meant is a little clearer now.

[jeffq]

As far as catastrophic failure goes, just include a parachute, protected from the potential collapse of the "vacuum balloon" (as the payload would have to be as well).

Even if it's not obvious what super-lightweight but high-strength material it would be made from, or if it could be used in an obvious practical situation (like passenger or instrument flight), it remains an interesting concept that could have uses we might not think of until such a device is actually built. That's the beauty of abstract research, which seems to be losing traction is this world of quarterly profits and day-trading investments.

[Pope130]

That's what I mean: they're boyancy-limited. If the best you can do is barely withstand pressure at sea level, then you're barely lighter than air, and you can't go up much before you reach equal boyancy. If, however, the strongest "egg" you could make that was equal boyancy at sea level was significantly stronger than it needed to be, this would also mean that if you only made it as strong as it needed to be at sea level, it could be significantly lighter than air and so could reach a reasonable altitude. The requirement for "extra strength" that I meant refered more to the fact that to be able to reach a good altitude, you need to be able to build a lot lighter than neutral boyancy, which requires a lot more strength than the neutral boyancy situation I did my calculations for. I know I didn't make this as clear as I wanted to, but my argument wasn't that the egg would actually be under any more stress at higher altitude, and I hope what I meant is a little clearer now.

I think there's a solution to the altitude limitation inherent in the design. A balloon that will maintain only a slight differential pressure (and lift) at sea level will only rise a short distance. However, a small pump could be used further reduce internal pressure, restoring the differential pressure it had at sea level. This would reduce the weight of the balloon so it could continue to rise. On reaching it's next equilibrium point the pump will be run again. There would still be a limit of course, when the internal pressure was reduced to zero, and the balloon rose to it's final equilibrium altitude.

Robert

[LordoftheLeftHand]

Maybe it would be possible to construct in such a way that it doesn't have to endure quite so much pressure. At lower altitudes (high air pressure) maybe lift could be achieved with just a lower pressure chamber instead of a near vacuum, and as you ascended to higher altitude a pump could continue to evacuate the chamber (maintaining the difference in pressure for as long as possible). Of course even if this worked you would have the additional weight of the pump and its power source...

LLH

*edited to add:
I see Robert has beat me to the punch!

[billydkid]

Ok, how about an open cell silicon foam or carbon foam or metal foam encapsulated in at thin skin - draw a vacuum on that?

[Ririon]

One benefit would be that it could work on a planet with a hydrogen atmosphere. Hey, if the concept is hypothetical, why not the surroundings...

[Just thinking]

Even if it could be made, and behaved just like a weather balloon filled with hydrogen -- its lifting power wouldn't be much greater than the hydrogen balloon; a few percent, that's all.

[DrRyanScarsella]

I remember a couple of years back about a foam that had been invented that would expand upon compression. Something about a collapsed structured that expanded when the material was compressed. This wouldnt be the proper material, but a similar structure in a harder, lighter compound might be a place to start. It wouldnt be anymore efficient then a gas balloon, but it might make a neat science experiment. It might yield some new technology of materials to toy with but would cost a fortune to develope. It would be neat if you could make it the size small enough to float around indoors. I think i read about the foam in pop sci. It expanded in width when pulled axially, and when compressed axially, it got more narrow. I think they described it as a cube with the sides concaved in a four sided manner, like the top of an inverted tetrahedron, like a five sided pyramid with the base being one side of the cube. Im no physicist, but I thinks its a cool idea.

[Ziggurat]

One benefit would be that it could work on a planet with a hydrogen atmosphere. Hey, if the concept is hypothetical, why not the surroundings...

That's a possibility, although the problem there becomes harder since at the same pressure, hydrogen gas is lighter than air by about a factor of 14, meaning that the weight of your vacuum balloon must be about 14 times smaller too, and I don't know if even a diamond sphere could pull it off under such conditions.

But there's still the problem of why use an expensive balloon when you could just use a glider? If the planet is near a sun, then getting a glider which could run perpetually on solar power might be a lot easier to do than a vacuum balloon.

Ok, how about an open cell silicon foam or carbon foam or metal foam encapsulated in at thin skin - draw a vacuum on that?

That might be easier to manufacture than our spherical shell example, and has the advantage that you could make essentially arbitrary shapes, but I suspect it wouldn't give you any advantage in terms of strength/weight ratio limits. So I don't think any metal or silicon foams would cut it, but maybe some hypothetical carbon nanotube foam might be strong enough.

[malaka]

If this "vacuum ballon" were large enough in a sealed room and it "popped" (collapsed), what would happen (as the rest of the air in the room rushed in to fill the space)?

[Jon.]

If this "vacuum ballon" were large enough in a sealed room and it "popped" (collapsed), what would happen (as the rest of the air in the room rushed in to fill the space)?

Presumably, the air pressure in the room would go down proportionally to the degree to which the balloon "filled" the room.

[Ziggurat]

If this "vacuum ballon" were large enough in a sealed room and it "popped" (collapsed), what would happen (as the rest of the air in the room rushed in to fill the space)?

Basically a small explosion: the air surrounding your void would come rushing in, and when the air coming in from all directions collides in the middle, it will have to stop very quickly, which means that it will reach very high pressures at the center, and so you'd have a VERY loud bang. Might spray out bits of your shattered balloon, too.

[Just thinking]

Basically a small explosion: the air surrounding your void would come rushing in, and when the air coming in from all directions collides in the middle, it will have to stop very quickly, which means that it will reach very high pressures at the center, and so you'd have a VERY loud bang. Might spray out bits of your shattered balloon, too.

Sort of like breaking a TV vacuum tube -- do not try this!