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Old 05-20-2017, 08:25 PM
Shalmanese Shalmanese is offline
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Have we been able to construct vacuum buoyant structures yet?

If you build a structure that is simultaneously light enough and rigid enough to stand up to atmospheric pressure, you could suck out all the air and make it float. Have we developed the engineering prowess to make something like this yet?

If not, how close are we and what are major engineering constraints that still limit us?
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Old 05-20-2017, 09:35 PM
LSLGuy LSLGuy is offline
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Long and sometimes contentious thread from 2016 on point: http://boards.straightdope.com/sdmb/...d.php?t=802451

The short answer is it's essentially impossible with real world materials. Including ones we haven't invented yet. You need almost pure unobtanium to get something that rigid, strong, and light.

The key thing about an equal pressure balloon is the gas pressure inside is the same as outside. The difference in gas density is what makes the lift.

Going from that equilibrium to a vacuum-filled balloon you only gain lift equal to the weight of the gas you remove. But you lose the gas inside pushing out to hold the vessels shape. So now your structure needs to withstand all that pressure instead of zero pressure.

IOW, remove 10 lbs of helium, gain 10 lbs of lift, and install 100 tons of steel to hold it together. That way does not lie a successful balloon.

Last edited by LSLGuy; 05-20-2017 at 09:40 PM.
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Old 05-20-2017, 09:43 PM
JWT Kottekoe JWT Kottekoe is offline
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Nor is there much point. Helium is one seventh the weight of Nitrogen gas, so you've reduced the weight of the volume by 86% of the displaced air using Helium, so there is only 14% left to gain and only 7% if you use Hydrogen, not counting the weight of the enclosure and a balloon is going to be much lighter than a pressure vessel.

Last edited by JWT Kottekoe; 05-20-2017 at 09:45 PM.
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Old 05-20-2017, 10:23 PM
Shalmanese Shalmanese is offline
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Originally Posted by JWT Kottekoe View Post
Nor is there much point. Helium is one seventh the weight of Nitrogen gas, so you've reduced the weight of the volume by 86% of the displaced air using Helium, so there is only 14% left to gain and only 7% if you use Hydrogen, not counting the weight of the enclosure and a balloon is going to be much lighter than a pressure vessel.
Except we're running out of Helium but we have literally infinite supplies of vacuum.
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Old 05-20-2017, 10:30 PM
Shalmanese Shalmanese is offline
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Originally Posted by LSLGuy View Post
Long and sometimes contentious thread from 2016 on point: http://boards.straightdope.com/sdmb/...d.php?t=802451

The short answer is it's essentially impossible with real world materials. Including ones we haven't invented yet. You need almost pure unobtanium to get something that rigid, strong, and light.

The key thing about an equal pressure balloon is the gas pressure inside is the same as outside. The difference in gas density is what makes the lift.

Going from that equilibrium to a vacuum-filled balloon you only gain lift equal to the weight of the gas you remove. But you lose the gas inside pushing out to hold the vessels shape. So now your structure needs to withstand all that pressure instead of zero pressure.

IOW, remove 10 lbs of helium, gain 10 lbs of lift, and install 100 tons of steel to hold it together. That way does not lie a successful balloon.
The answers in that thread all seem to be talking about a completely hollow internal space which is a structurally difficult item to build. But there's no requirement for the item to be hollow, adding internal reinforcement significantly decreases the weight.

I'm imagining something like aerogel wrapped in a thin, impermeable film. If aerogel can resist the crushing force of a person standing on it, it should also be able to resist atmospheric pressure.
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Old 05-20-2017, 10:33 PM
Shalmanese Shalmanese is offline
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Originally Posted by JWT Kottekoe View Post
Nor is there much point. Helium is one seventh the weight of Nitrogen gas, so you've reduced the weight of the volume by 86% of the displaced air using Helium, so there is only 14% left to gain and only 7% if you use Hydrogen, not counting the weight of the enclosure and a balloon is going to be much lighter than a pressure vessel.
Also, I'm not talking about economic practicality. I'm merely wondering if material science has advanced to the point where such a thing could be constructed as a proof of concept.
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Old 05-21-2017, 03:49 AM
Isilder Isilder is offline
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Except we're running out of Helium but we have literally infinite supplies of vacuum.
Has alpha radiation emmission by the rocks stopped ? I think it keeps on coming.
It might be more expensive in future when natural gas reserves are depleted. Then they have to go back to the old natural gas sites and extract the gas just to get helium.
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Old 05-21-2017, 04:38 AM
Shalmanese Shalmanese is offline
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Has alpha radiation emmission by the rocks stopped ? I think it keeps on coming.
It might be more expensive in future when natural gas reserves are depleted. Then they have to go back to the old natural gas sites and extract the gas just to get helium.
By that logic, we're not running out of coal, oil or natural gas either.
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Old 05-21-2017, 08:06 AM
TriPolar TriPolar is online now
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Originally Posted by Shalmanese View Post
Except we're running out of Helium but we have literally infinite supplies of vacuum.
The supply of hydrogen is not infinite but may be second only to vacuum. I know there's a safety issue there, but vacuum vessels have a very high risk of implosion and a tiny leak in one may be a greater risk than igniting hydrogen.
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Old 05-21-2017, 08:18 AM
LSLGuy LSLGuy is offline
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Originally Posted by Shalmanese View Post
The answers in that thread all seem to be talking about a completely hollow internal space which is a structurally difficult item to build. But there's no requirement for the item to be hollow, adding internal reinforcement significantly decreases the weight.

I'm imagining something like aerogel wrapped in a thin, impermeable film. If aerogel can resist the crushing force of a person standing on it, it should also be able to resist atmospheric pressure.
The key tradeoff is whether your aerogel+ vacuum mixture is more or less dense than helium. If more, you're losing lifting performance not gaining it.

The cool thing about a gas is that if you're OK with a non-rigid structure, AKA a balloon, the envelope plus the gas inside have near infinite pressure resilience for almost zero weight. Your balloon may dynamically smoosh in a wind gust or whatever, but it also dynamically smooshes back out. Try duplicating that trick with a semi-rigid material. It's darn near exactly the formal definition of the difference between gas phase and solid phase.


The critical engineering point remains that compared to Earth atmosphere, helium already weighs very close to zero. There's so little left to be gained by going to true zero that you can't afford to spend more than a tiny fraction of that gain on getting there.
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Old 05-21-2017, 01:09 PM
Stranger On A Train Stranger On A Train is offline
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Originally Posted by Shalmanese View Post
Also, I'm not talking about economic practicality. I'm merely wondering if material science has advanced to the point where such a thing could be constructed as a proof of concept.
No, not even remotely. The problem isn't one of material strength as it is geometry, i.e. such a rigid wall vessel would have to have extremely thin walls, and all thin-walled structures are sensitive to local buckling regardless of the tensile and shear strength. Filling it with a reinforcing material such as aerogel helps resist buckling but at the expense of additional weight, and even the lightest aerojels weight more than a low density gas.

Meanwhile, hot gas and inert low density atmosphere inflatables work just fine. If we were truly running out of helium (we aren't as helium continues to be produced by decay processes, and will be a waste pruduct of deuterium-tritium fusion when we finally make that practical, but the United States foolishly decided to sell off the National Helium Reserve, so our standing reserves are depleated) we could simply design high temperature adibatic membranes and use very hot gas to produce an amount of lift per unit volume approaching that of vacuum. As LSLGuy notes the theoretical advantage of a rigid vacuum hull is so minimal there is little reason to expend effort on it even if it were physically practicable.

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Old 05-21-2017, 01:15 PM
watchwolf49 watchwolf49 is offline
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We could cleverly build our rigid structure in the shape of a boat ... we wouldn't need to evacuate the air for it to float ... I think we'll find that if we put this matter to a vote ... the odds of anything coming along will be quite remote ...

Last edited by watchwolf49; 05-21-2017 at 01:16 PM.
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Old 05-21-2017, 01:36 PM
GusNSpot GusNSpot is offline
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Economical, useful, etc. is not the question. Can we make a structure that will float on Earth out of a material that will displace more enough water to float that if for spaces, would not. Correct??

The glass vacuum inserts on old style thermos bottles. Fill just that with water and will it still float?

Battle ships float and all it's parts individually do not. The trick is to displace displace enough water I am thinking.

Now, to float in the air. What density of air? On a cold dry day of high pressure in a polar area at sea level with a double walled glass flask of some dimension that also can stand some internal partial vacuum is impossible? Artificially cold compressed air but not to a liquid state?

Steel drums with only a tiny amount of air are found floating below the surface of the water.

A glass thermos bottle will not be able to do that in some state of air which is compressible but is still air?

Proof of concept???
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Old 05-21-2017, 02:14 PM
Chimera Chimera is offline
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Nope, sorry. As above, any material you care to use is significantly heavier than any air that might be displaced by the construct, no matter what you build it from.
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Old 05-21-2017, 02:23 PM
Chronos Chronos is online now
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And the answer has already been given: We cannot. We already know the most efficient shape for something like this, and we can calculate the needed material strength for an enclosure of that shape, and the needed strength is beyond anything we can imagine actually making.
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Old 05-21-2017, 08:15 PM
Stranger On A Train Stranger On A Train is offline
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And as for the notion that "we're running out Helium: Wired: "The Dire Helium Shortage? Vastly Inflated."

We actually need helium more as a coolant and working fluid than for inflatables, and we have sufficient known and predicted reserves which exceed the expected needs for the foreseeable future, even if we start building helium-cooled fast fission reactors. (The helium cycle is closed so the need for each reactor is finite, and there are arguably better concepts for breeding fertile material into fissile fuel and burnup of actinides.) We don't need to build rigid vacuum structures for a few percent improvement over low mass elemental gases.

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Last edited by Stranger On A Train; 05-21-2017 at 08:15 PM.
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Old 05-21-2017, 08:30 PM
Shalmanese Shalmanese is offline
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I'm perfectly happy to concede that vacuum buoyant structures are not and will never be economically practical as anything but a novelty.

Still, exactly how close are we to building them now? Are we any closer than we were 5, 10, or 50 years ago? Are there any promising areas of research that might yield one on the future if pursued far enough? Or does there exist some physical law that would make materials approaching the limit of what's needed to build them theoretically impossible?
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Old 05-21-2017, 08:40 PM
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Quote:
Originally Posted by Shalmanese View Post
I'm perfectly happy to concede that vacuum buoyant structures are not and will never be economically practical as anything but a novelty.

Still, exactly how close are we to building them now? Are we any closer than we were 5, 10, or 50 years ago? Are there any promising areas of research that might yield one on the future if pursued far enough? Or does there exist some physical law that would make materials approaching the limit of what's needed to build them theoretically impossible?
Well, post not in order but ...
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Originally Posted by Chronos View Post
And the answer has already been given: We cannot. We already know the most efficient shape for something like this, and we can calculate the needed material strength for an enclosure of that shape, and the needed strength is beyond anything we can imagine actually making.
Now if you choose not to believe Chronos and others who have said pretty similar, fine, but why keep asking?
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Old 05-21-2017, 09:05 PM
LSLGuy LSLGuy is offline
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Seriously OP: read the thread I cited in post #2. For awhile it gets bogged down in the practical utility of blimps/dirigibles. But then on page 2 it gets fully back on track about vacuum vessels for flotation. There's further refs to earlier threads. The thread spins in and out focus on this issue several times before it's finally done.

But along the way real material & structural engineers provide calculations. A vacuum balloon needs gossamer weight with the compressive & bending strength of high end specialty steel.

Maybe someday when we can grow flawless single molecules of polymeric carbon crystal (graphene) the size of houses in whatever shape we want we'll be able to pull it off. Maybe.

But right now it's like asking how much closer we are to the first hyper-lightspeed starship than we were in the Space Shuttle era. To a first approximation the answer is: we're not making progress at all. It's utterly beyond our reach. So nobody's really trying for that as such.

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Old 05-21-2017, 09:09 PM
Stranger On A Train Stranger On A Train is offline
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Originally Posted by Shalmanese View Post
I'm perfectly happy to concede that vacuum buoyant structures are not and will never be economically practical as anything but a novelty.

Still, exactly how close are we to building them now? Are we any closer than we were 5, 10, or 50 years ago? Are there any promising areas of research that might yield one on the future if pursued far enough? Or does there exist some physical law that would make materials approaching the limit of what's needed to build them theoretically impossible?
We cannot build rigid-wall structures that are light enough to be buoyant in sea-level atmosphere. I don't know how many more ways to say this. It is not a matter of progressive development of tensile or shear strength of the materials; it is a result of the geometry of sufficiently thin-walled structures beng sensitive to local buckling regardless of material properties, and there is no material sufficiently light to resist compressive loads that would be lighter than air.

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  #21  
Old 05-22-2017, 01:22 PM
Leo Bloom Leo Bloom is offline
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Originally Posted by LSLGuy View Post
Long and sometimes contentious thread from 2016 on point: http://boards.straightdope.com/sdmb/...d.php?t=802451...
I remember that one. An excellent thread on the SD and science coupled with one of the infrequent occasions when GQ posters grind another's hopes and dreams mercilessly to dust (to mix metaphors), which always gives enjoyment by mixing in the human element for the popcorn crowd.

Last edited by Leo Bloom; 05-22-2017 at 01:24 PM.
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Old 05-22-2017, 02:26 PM
Scylla Scylla is offline
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And the answer has already been given: We cannot. We already know the most efficient shape for something like this, and we can calculate the needed material strength for an enclosure of that shape, and the needed strength is beyond anything we can imagine actually making.
We could never imagine Hannah Montana swingingnaked on a wrecking ball, but that happened, so you know, I'm still hopeful for my vacuum blimp.
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Old 05-22-2017, 02:35 PM
Machine Elf Machine Elf is offline
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Maybe someday when we can grow flawless single molecules of polymeric carbon crystal (graphene) the size of houses in whatever shape we want we'll be able to pull it off. Maybe.
Still gonna go with no on that.

Imagine a bicycle spoke, long and thin. If you stretch it in a materials testing machine you can probably develop about 250 pounds of force before it breaks. Now try compressing one instead: I'll wager you'll peak at a few pounds of resistance before it buckles and bends. The problem is that as soon is the material deforms and moves away from the load axis, you create a large bending moment that assures larger stresses and larger deformations, unto complete structural failure.

For very short very fat columns (think "hockey puck"), buckling isn't an issue, and so it just comes down to compressive strength. But at some sufficiently high aspect ratio (somewhere between "hockey puck" and "bicycle spoke"), buckling becomes a concern, and the only solution is rigidity, which comes from both cross-section geometry and the material's modulus of elasticity. Example, Imagine a fat tubular aluminum column, and a skinny solid steel column. The aluminum column, despite having a lower modulus of elasticity and lower total compressive strength (compared to the skinny solid steel column), may have greater resistance to buckling failure because of the greater width of the column.

If we're trying to make an atmospherically buoyant vacuum vessel, then the skin will need to be so thin as to be a 2-dimensional analog of "bicycle spoke" in terms of the thickness compared to span. In fact, even thinner.

According to that Wikipedia link, graphene has a stiffness about 5X that of steel - which means the section modulus of your vacuum vessel's skin can be decreased by a factor of five, which in turn means you can only lower your skin thickness by a factor of 51/3 = 1.7 compared to the thickness of steel that you would need to avoid buckling. Bottom line, anything larger than a microballoon will still have to be built far too heavy to be buoyant, even if it's made of graphene.
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Old 05-22-2017, 02:55 PM
Darren Garrison Darren Garrison is offline
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We could never imagine Hannah Montana swingingnaked on a wrecking ball
Maybe you couldn't...
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Old 05-22-2017, 03:06 PM
Scylla Scylla is offline
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I've actually done a lot more thinking about the concept of a vacuum dirigible since my last thread and have a brans new idea for a hot air/vacuum hybrid airship for everybody to shit on and crush my dreams.

Picture something similar to aerogel or a honeycomb. Because it is hard to make something big that holds any amount of negative pressure, we simply stack lots of smaller ones together into a honeycomb type of structure. These micro capsules are not airtight. They are somewhat permeable to air, so that they slowly acquire whatever the current surrounding pressure is. Your pressure envelope is made out of a 10 foot thick layer of this honeycomb structure with a hollow center in which you have your heater.

When you turn the heat on it heats the the air inside the envelope which expands, is outgassed and provides lift. It also heats your microbead/honeycomb structure, the air inside the microbeads heats up, expands, and is also outgassed throught the permeable membrane of the beads themselves.

Such a setup retains heat better than a standard hot air balloon as that honeycomb will be a great insulator. When you turn the heat off, the air will take some time to flow into the microbeads while that's happening, you are generating some (it might be tiny) amount of lift from the low pressure inside the microbeads and the pressure envelope itself.

Picture a giant styrofoam shell that leaks really slowly.


Ta-da!
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Old 05-22-2017, 03:25 PM
Chronos Chronos is online now
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So what you're positing is a well-insulated hot air balloon. The "slow leak" thing doesn't help you: If it's slower than the cooling, then the pressure difference will crush your balloon, and if it's faster than the cooling, then it's just hot air as a lifting gas, which is nothing new.
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Old 05-22-2017, 03:58 PM
Stranger On A Train Stranger On A Train is offline
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We don't insulate hot air balloons because it isn't necessary; the loss of heat is low because the air inside the balloon and the surrounding atmosphere don't directly interact, so there is essentially no convection, only conduction, and because the density of the gas is very low conduction is very slow. (There is also radiation but because the temperature difference between in the inside and outside of the balloon is so small radiation is nearly insignificant. However, if we filled a vessel with very hot gas we would want a reflective layer on the inside to prevent radiation losses.) Hot air balloons, once aloft, can float for many hours with minimal use of the burner provided that there aren't shifts in air density.

Having a bunch of smaller nearly evacuated vessels is not an improvement on the concept; you now have the parasitic mass of each of these vessels each of which are orders of magnitude more dense than air. This is really a basic scaling problem and there is no size or shape of a rigid vessel made of a real material which can be evacuated to the extent that it will be buoyant in air. This is why we use pressure supported flexible skins (sometimes with an internal structure to help control inflation or provide aerodynamic properties) for lighter-than-air-flight.

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Old 05-22-2017, 04:01 PM
Scylla Scylla is offline
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So what you're positing is a well-insulated hot air balloon. The "slow leak" thing doesn't help you: If it's slower than the cooling, then the pressure difference will crush your balloon, and if it's faster than the cooling, then it's just hot air as a lifting gas, which is nothing new.
Saying that it's "just" a well-insulated hot air balloon reveals a knee-jerk depecratory stance. A well-insulated hot air balloon is a very neat thing as it saves dramatically on fuel necessary to maintain lift.

The second part is also untrue. Sure, there would be a buckle rate for the microspheres but the greater the pressure differential, the faster they would equalize, and while they are equalizing we are gaining something.

It's ok, you can admit it. It's a cool concept
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Old 05-22-2017, 04:07 PM
Scylla Scylla is offline
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We don't insulate hot air balloons because it isn't necessary; the loss of heat is low because the air inside the balloon and the surrounding atmosphere don't directly interact, so there is essentially no convection, only conduction, and because the density of the gas is very low conduction is very slow. (There is also radiation but because the temperature difference between in the inside and outside of the balloon is so small radiation is nearly insignificant. However, if we filled a vessel with very hot gas we would want a reflective layer on the inside to prevent radiation losses.) Hot air balloons, once aloft, can float for many hours with minimal use of the burner provided that there aren't shifts in air density.

Having a bunch of smaller nearly evacuated vessels is not an improvement on the concept; you now have the parasitic mass of each of these vessels each of which are orders of magnitude more dense than air. This is really a basic scaling problem and there is no size or shape of a rigid vessel made of a real material which can be evacuated to the extent that it will be buoyant in air. This is why we use pressure supported flexible skins (sometimes with an internal structure to help control inflation or provide aerodynamic properties) for lighter-than-air-flight.

Stranger
Ok. These are good objections, but if the temperature loss is so low why does my yeti cooler need such thick walls? Why isn't it just made of balloon skin? Why does my oven have thick insulation? Why isn't that made of balloon skin?

We are talking a lot of surface area to radiate heat from in a hot air balloon. I find it difficult to believe that loss of heat over that huge are is not an issue when it is such a big issue in coolers and ovens.

If you were well insulated you could easily maintain a greater temperature differential. The hotter the air inside your envelope the less the volume that envelope needs to be.
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Old 05-22-2017, 04:09 PM
Dr. Strangelove Dr. Strangelove is offline
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I think I have a solution that's on the edge of working, though some might consider it a cheat.

The problem isn't raw material strength. Extant materials have the compressive strength requirements; the problem is buckling resistance, which is a problem with all compressive structures. The way to improve buckling resistance is to make the walls thicker, lowering the average density, but the walls themselves are made from stuff and have internal structures which may themselves buckle.

So it would be nice to convert a compressive structure into a tensile one. Luckily, there's a way--using inflatables! A column built from a high tensile strength fabric and filled with compressed gas can be made as fat as necessary and thus has arbitrary buckling resistance.

I propose making an icosahedron out of fat columns filled with compressed hydrogen. An envelope surrounds the whole thing and the interior is pumped to a vacuum.

The solid material (fabric) is always in tension and so there are no buckling problems in that regard. Can we ensure there's enough pressure that the columns as a whole don't buckle?

I posit that the columns can always be made large enough that it works--if you imagine the limiting case with really fat columns, the structure becomes not all that different from a superpressure balloon with a tiny bit of vacuum at the center.

Of course that indicates why it's pretty much a cheat, since obviously the dominant component of the lift is coming from the air displaced by the gas-filled structure and not the vacuum. So take that as you will, though my intuition is that you can get at least half the lift from the vacuum if you designed it right.
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Old 05-22-2017, 04:20 PM
Chronos Chronos is online now
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To have enough strength, your hydrogen tubes would have to be at high pressure, high enough that the mass of the extra hydrogen inside would more than offset the vacuum.
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Old 05-22-2017, 04:24 PM
Leo Bloom Leo Bloom is offline
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We could never imagine Hannah Montana swingingnaked on a wrecking ball, but that happened, so you know, I'm still hopeful for my vacuum blimp.
But she didn't do it from a skyhook.
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Old 05-22-2017, 04:25 PM
Dr. Strangelove Dr. Strangelove is offline
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Hydrogen is so much less dense than air that you can get away with a lot of compression. Suppose a cross-section of your structure is 50% gas/50% vacuum. The gas needs to be at least 2 atmospheres to resist the overall forces along that plane. But hydrogen can be at 14 atmospheres before it reaches the same density as air, so there's plenty of headroom.

Of course the whole thing is in every way worse than a simple hydrogen balloon, but no one asked about that...
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Old 05-22-2017, 05:08 PM
Stranger On A Train Stranger On A Train is offline
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Ok. These are good objections, but if the temperature loss is so low why does my yeti cooler need such thick walls? Why isn't it just made of balloon skin? Why does my oven have thick insulation? Why isn't that made of balloon skin?

We are talking a lot of surface area to radiate heat from in a hot air balloon. I find it difficult to believe that loss of heat over that huge are is not an issue when it is such a big issue in coolers and ovens.

If you were well insulated you could easily maintain a greater temperature differential. The hotter the air inside your envelope the less the volume that envelope needs to be.
First of all, heat loss from a hot air balloon is primarily by conduction, not radiation, as addressed previously. Because the air, both inside the balloon and in the atmosphere is of such low density, the rate of conduction is low under normal conditions. The air inside the balloon does flow (cold air from the outside falls and forces hot air to rise through the center) but because of the large mass of air and small temperature differentials the flow is slow, and the difference in average temperature is not large; hot air balloons have an average internal temperature of around 200 F, whereas your stove probably doesn't even have a measurement below 300 F, and the primary reason for insulating your stove is to prevent your kitchen from getting hot (unless you have a convection stove) as it cooks largely by radiation. Your Yeti cooler, on the other hand, has to protect a small volume from heat coming from the ambient air into a cold temperature reservoir with essentially infinite heat capacity, and unless you are using dry ice you have a working temperature of not much less than 32 F which is maintained as long as the total energy flux doesn't exceed the latent heat of fusion of your ice or other coolant.

It isn't that a hot air balloon doens't lose heat energy, but it does so slowly enough that it is not worth adding the extra mass of insulation to offset any savings of fuel for the propane burner. Complex schemes that add more mass in order to achieve a marginal increase in theoretical buoyancy are highly unlikely to be a net improvement.

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Originally Posted by Dr. Strangelove View Post
So it would be nice to convert a compressive structure into a tensile one. Luckily, there's a way--using inflatables! A column built from a high tensile strength fabric and filled with compressed gas can be made as fat as necessary and thus has arbitrary buckling resistance.

I propose making an icosahedron out of fat columns filled with compressed hydrogen. An envelope surrounds the whole thing and the interior is pumped to a vacuum.

The solid material (fabric) is always in tension and so there are no buckling problems in that regard. Can we ensure there's enough pressure that the columns as a whole don't buckle?

I posit that the columns can always be made large enough that it works--if you imagine the limiting case with really fat columns, the structure becomes not all that different from a superpressure balloon with a tiny bit of vacuum at the center.

Of course that indicates why it's pretty much a cheat, since obviously the dominant component of the lift is coming from the air displaced by the gas-filled structure and not the vacuum. So take that as you will, though my intuition is that you can get at least half the lift from the vacuum if you designed it right.
This isn't a cheap per se, but realize that once you start increasing the pressure of the hydrogen inside the columns you are increasing density and thus reducing buoyancy. Compared to the external loads (14.7 psia AMSL) which add up quickly you're going to need a considerable amount of internal structure to support that. You've proposed an icosohedral structure (presumably because it is the simplest "sphere-like" classic solid with triagonal faces) but it would be instructive to look at an even more simple structure for scaling without dealing with only axial loading, i.e. take an annular structure, like a backpacker's alcohol stove, seal and fill the annulus with hydrogen or helium, and cap the open ends (we'll assume for the moment that the end caps are perfectly rigid) to form an evacuated chamber in the center. In this case buckling is not a concern as long as you keep the pressure in the annulus sufficient to assure positive tension. Now calculate the buoyancy of the structure (both the evacuated center and the high pressure annulus) versus the mass of some real world textile material for the skin. You'll find that the necessary ratio of pressure in the annulus to the ambient to support the end load is larger than the inner diameter of the annulus squared over the difference of the squares of the outer and inner walls. In short, you'll end up with a very large annulus to support an evacuated center, even assuming rigid caps. The scaling on a icosohedran will be different but it is actually worse because of the lower mechanical advantage of that configuration. Getting more than a small fraction of lift from the evacuated volume just isn't plausible.

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Old 05-22-2017, 05:18 PM
dtilque dtilque is offline
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To have enough strength, your hydrogen tubes would have to be at high pressure, high enough that the mass of the extra hydrogen inside would more than offset the vacuum.
Also hydrogen is going to leak and the higher the pressure, the faster the leak. And where will it leak to? All directions to some extent, but if there's a vacuum on one side and air on the other, it'll leak more to the vacuum side, since there's less resistance that way. Anyway, what you end up with is a hydrogen balloon.


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Of course the whole thing is in every way worse than a simple hydrogen balloon, but no one asked about that...
Indeed, since you'll have all that internal tubing, which will be useless mass.
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Old 05-22-2017, 06:01 PM
LSLGuy LSLGuy is offline
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It's interesting to me how this idea of vacuum balloons seems to get a hold on people. I wonder what's special about it? Probably conceptual simplicity when looked at through the lens of Warner Brothers physics.
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Old 05-22-2017, 06:05 PM
Peter Morris Peter Morris is offline
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I asked some similar questions to the OP in a recent thread.

http://boards.straightdope.com/sdmb/...d.php?t=822718
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Old 05-22-2017, 06:08 PM
Peter Morris Peter Morris is offline
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It's interesting to me how this idea of vacuum balloons seems to get a hold on people. I wonder what's special about it? .
Because hydrogen is dangerous and helium is rare and running out (until we are able to mine the Sun)
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Old 05-22-2017, 06:15 PM
Dr. Strangelove Dr. Strangelove is offline
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For me at least, it's because it falls in the domain of extreme engineering problems. It doesn't violate any laws of physics, and existing materials seem almost good enough if you could arrange them right, but it's never been done. So it's like nanomachines and nuclear rockets and electromagnetic launchers and maglev vacuum tunnels and so on.
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Old 05-22-2017, 06:32 PM
Dr. Strangelove Dr. Strangelove is offline
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You've proposed an icosohedral structure (presumably because it is the simplest "sphere-like" classic solid with triagonal faces)
Pretty much. Can you describe your proposed structure a bit better? I know what an alcohol stove looks like but if you tried to inflate one with a gas, the inner part would collapse as it's under compression, not tension. So I must have an incorrect picture of the proposal.

To get a slightly better feel for my intuition, consider just the material required by an infinite cylinder at 2 atm (1 atm above ambient), 1 m radius. Zylon fiber has a tensile strength of 5800 MPa and density of 1540 kg/m^3. Hoop stress is Pr/t, which if you solve for t (wall thickness) gives 1.75e-5 m. Since Zylon is a fiber with anisotropic strength, you have to handle the axial stress separately. That's half of hoop stress so the wall thickness ends up as 2.62e-5 m. The final density of the cylinder is thus 0.253 kg/m. The density of the hydrogen is 0.564 kg/m, so the fabric is actually the smaller part. And if you work the numbers a bit further, you find that you can get a neutrally buoyant cylinder compressed at 7.65 atm. That's a heck of a structural member that weighs the same as air. Well, I'm ignoring the end caps but they're relatively small if the cylinder has a decent aspect ratio (10:1, say).

Anyway, this isn't proof that it's possible to get a decent ratio of vacuum but hints that there is reasonable margin available with this approach.
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Old 05-22-2017, 06:36 PM
LSLGuy LSLGuy is offline
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...
Still, exactly how close are we to building them now? Are we any closer than we were 5, 10, or 50 years ago? Are there any promising areas of research that might yield one on the future if pursued far enough? Or does there exist some physical law that would make materials approaching the limit of what's needed to build them theoretically impossible?
Not trying to pile on. Peter Morris's post led me on a voyage of discovery back through the even earlier history on this perennially popular question.

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For me at least, it's because it falls in the domain of extreme engineering problems. It doesn't violate any laws of physics, and existing materials seem almost good enough if you could arrange them right, but it's never been done. So it's like nanomachines and nuclear rockets and electromagnetic launchers and maglev vacuum tunnels and so on.
It's the "seems" part I wonder about. Though I readily admit your relevant engineering chops are far beyond my decades old academics.


Ref this post from 2008 by an ME/SE: http://boards.straightdope.com/sdmb/...53#post9581053

The strength/weight shortfall between the best known material (diamond) and that required for an engineeringly perfect sphere of the same material is 10 million to 1.

We've got aways to go to engineer an improvement by 10 million X over the current best known natural or artificial material.

This isn't one of those "How much do I have to work out to be able to lift 250 lbs?" questions. It's more like "How much do I have to work out to be able to high jump over the Moon?"

Last edited by LSLGuy; 05-22-2017 at 06:38 PM.
  #42  
Old 05-22-2017, 06:47 PM
Dr. Strangelove Dr. Strangelove is offline
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The strength/weight shortfall between the best known material (diamond) and that required for an engineeringly perfect sphere of the same material is 10 million to 1.
That post certainly proves that any thin-wall solution is doomed to failure; that is, anything with a dense, homogeneous wall material. But that just indicates that you need thick, low-density walls instead. It's like claiming that large cranes are impossible because a solid bar of steel has too high a bending moment. Well no, it just means you need to structure the material better, into a truss or otherwise.
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Old 05-22-2017, 07:01 PM
Stranger On A Train Stranger On A Train is offline
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Because hydrogen is dangerous and helium is rare and running out (until we are able to mine the Sun)
Hydrogen is fairly dangerous (high detonability and diffusion) but helium is not running out. We have (foolishly) elected to liquidate the National Helium Reserve because of the costs of storage (even though those are largely administrative) but nearly every natural gas field has at least a small fraction of helium that ends up being a large volume, and some produce 10% or more by mass. In many cases the gas is vented into the atmosphere once separated from natural gas not because it is in any way difficult to collect or refine, but simply because it isn't worth enough for natural gas producers to bother to store or distribute it. Just as there are vast known and presumed untapped resources of natural gas, there are also volumes of helium which could be extracted if the value were sufficient. We can also produce helium by neutron bombardment of 6Li, and it is a byproduct of tritium synthesis by this mechanism. Once we make nuclear fusion practicable helium (not going to lay money on when that happens but it will almost certainly occur at some point), helium (in the form of ionized alpha particles) is a product of the most probable fusion reactions up to p-11B and 3He-6Li. And of course, the Sun throws off thousands of tons of helium every second. While it will likely never make economic sense to collect interplanetary helium for delivery to and use on Earth, it could certainly be a nearly limitless supply of helium for space use, from supercoolant to working fluid to fuel. It is literally the second most abundant material in the Universe, dwarfing everything except hydrogen, and we are nowhere near its disappearance.

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For me at least, it's because it falls in the domain of extreme engineering problems. It doesn't violate any laws of physics, and existing materials seem almost good enough if you could arrange them right, but it's never been done. So it's like nanomachines and nuclear rockets and electromagnetic launchers and maglev vacuum tunnels and so on.
Existing materials are no nearly good enough to produce a buoyant vacuum vessel. You can say that it "doesn't violate any laws of physics," but material science and structural mechanics are just as much a part of physics as electrochemistry and gravitation, and unless you can postulate a material which is capable of exhibiting virtual rigidity through just a few tens of layers of molecules or lattice planes, vastly beyond any known or hypothesized material, this is just as much science fiction as warp drives and teleportation. Nanomachines are practicable--nature has been producing them on our planet for over four billion years--and nuclear thermal and electric rockets are largely more a matter of engineering (albeit yet-to-be-proven concepts) than basic physics as they operate on known principles. The kind of hyperrigid materials to be buoyant in air while continuing a vacuum, on the other hand, defies basic material science as we know it.

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Old 05-22-2017, 07:24 PM
Dr. Strangelove Dr. Strangelove is offline
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and unless you can postulate a material which is capable of exhibiting virtual rigidity through just a few tens of layers of molecules or lattice planes, vastly beyond any known or hypothesized material, this is just as much science fiction as warp drives and teleportation.
Again, this is only true if you maintain the thin-wall criterion. Dense materials are doomed to fail. But nanostructured materials don't have to be dense. There are all kinds of possible carbon structures that achieve whatever density you wish to have without significantly affecting the specific strength.

If you really want to reach physical limits, you can go way further than that, with electromagnetic or kinetic structures. Want to achieve really impressive compressive strength with no buckling and unlimited length? Get two pairs of electromagnetic decelerators/accelerators and have them shoot bearing balls at each other. Maintaining stability is a bitch and we're nowhere close to building useful systems like this, but they don't exceed any basic material limits or break any laws.
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Old 05-22-2017, 08:04 PM
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No, not even remotely. The problem isn't one of material strength as it is geometry, i.e. such a rigid wall vessel would have to have extremely thin walls, and all thin-walled structures are sensitive to local buckling regardless of the tensile and shear strength. Filling it with a reinforcing material such as aerogel helps resist buckling but at the expense of additional weight, and even the lightest aerojels weight more than a low density gas.

No lighter-than-air aerogel is commercially available (now): http://www.aerogeltechnologies.com/faqs

But not "all thin-walled structures are sensitive to local buckling". Remember Eulers buckling theorem depends on length and edge conditions: F=k/(x**x). When x is very small and the ends are constrained, stuctures don't buckle.

The design of a rigid gas is an interesting problem: if it's rigid, it's not a gas. If it's rigid then it's not equally incompressible in all directions at the molecular level. And if it's rigid, and not isotropic, then it's molecular, so it's denser.

Still, if you think of a theoretical aerogel that is a constrained gas, rather than starting with the idea of a crystal, it is an area that may still see more development.

Last edited by Melbourne; 05-22-2017 at 08:05 PM.
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Old 05-22-2017, 08:11 PM
LSLGuy LSLGuy is offline
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ETA: @Dr Strangelove just above:

Agree that I ignored the structural design issue. Which is an oversimplification too far. OTOH needing to make up a materials deficit of 10 million to 1 via smarter structures implies a hefty amount of smarter.

You gave me an idea. Could you do something with electrostatic repulsion? I'm imagining a rigid conductive sphere that could not withstand much (any?) negative atmospheric pressure. Build the sphere, but with normal atmospheric pressure inside and out.

At the center, place a relatively small but powerful electron source or sink. Crank up the power so the central anode/cathode is repelling the enclosing spherical cathode/anode. Then evacuate the air out of the empty space.

Lotta challenges. And making the entire power supply chain weigh less than the sphere-equivalent volume of H or He is simply not gonna happen. But it might be a way to improve buckling resistance with less weight within the sphere proper.

After all, it's essentially molecular-scale electrostatic repulsion that's creating the gas phase phenomenon we call "pressure."

Last edited by LSLGuy; 05-22-2017 at 08:13 PM.
  #47  
Old 05-22-2017, 08:16 PM
TriPolar TriPolar is online now
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Originally Posted by Dr. Strangelove View Post

I propose making an icosahedron out of fat columns filled with compressed hydrogen. An envelope surrounds the whole thing and the interior is pumped to a vacuum.

A tensegrity sphere makes sense for this, unless that's what you were thinking off. Buckminster fuller proposed such designs for hot air floating cities using floating members. For anything of size you need far more faces than an icosahedron.
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Old 05-22-2017, 08:43 PM
Chronos Chronos is online now
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LSLGuy, I think you overlooked the biggest advantage of that design: You can literally and honestly tell people that it's held together by a forcefield.
  #49  
Old 05-22-2017, 08:45 PM
Dr. Strangelove Dr. Strangelove is offline
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You gave me an idea. Could you do something with electrostatic repulsion?
I believe so (and vaguely hinted as much above). There are limits to what you can achieve before the high voltage discharges to ground through the air, but the material can be arbitrarily thin--like gold leaf. It should be self-smoothing due to the 1/r^2 falloff; any parts that get too close will get pushed away by the electric force; too far and air pressure pushes it back.

In principle, you get also get away with an arbitrarily small power supply, but I guess free ions in the air will eventually discharge it. Not sure offhand what that rate would be.
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Old 05-22-2017, 08:54 PM
Dr. Strangelove Dr. Strangelove is offline
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A tensegrity sphere makes sense for this, unless that's what you were thinking off.
It's close to a Fuller-like design in that all elements are purely compressive or tensile. Actually, simpler to think about is a tetrahedrons made of of 6 of the cylinders mentioned above. They're neutrally buoyant, so they don't contribute anything to the weight, but clearly have plenty of structural margin. Inside the tetrahedron is a tetrahedral bag pulled apart by tethers attached to the corners of the big tet. You'll get some amount of vacuum inside the bag no matter what tension you can achieve.

I would say that whether this ends up as a cheat or not depends on exactly how much vacuum is achieved. If you need 1000 m^3 of structure to get 1 m^3 of vacuum, then it's silly to call it a vacuum balloon. But if the numbers are in the same ballpark then I think it's fair. I haven't run the numbers carefully enough yet to know if this is possible.
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