NationStates Jolt Archive

Now hot air balloons work because hot air is less dense than cooler air. Helium balloons because helium is lighter than air. So surely a vacuum as an absence of anything has to be lighter than all of these. Would it be possible to have a large balloon (made out of something like mylar tethered all round to a lightweight frame so it keeps its shape) to hold the vacuum. Are there any physicists here who can actually work out for a given volume of vacuum how much weight it could lift? Or as normal am I going slightly mad walking to work and thinking up impossible ideas?

Uh...I'm reasonably certain this would either not work entirely, or be too problematic and/or expensive.

from wiki:

First proposed by Italian monk Franceso de Lana in 1670, the vacuum balloon would be the ultimate expression of displacement lift power. A frequent topic of blue sky thinking, the basic principle has remained the same: A container strong enough to preserve a vacuum that displaces sufficient air to lift the container and an additional load. However to avoid crushing by atmospheric pressure would require materials far stronger than any currently available (see unobtainium)

from wiki:

Thank you.

A Casimir vacuum? Or just plain old fashioned gas vacuum? With the latter, simple experiment.

Vacuum seal a plastic bag. Take an open one. Throw both in the air. Watch them both hit the ground.

So much for that idea. :p

A Casimir vacuum? Or just plain old fashioned gas vacuum? With the latter, simple experiment.

Vacuum seal a plastic bag. Take an open one. Throw both in the air. Watch them both hit the ground.

So much for that idea. :p

A vacuum sealed plastic bag though is not displacing much in the way of air. I am suggesting having it in a cubic frame with tethers from the surface of the envelope attached to the frame, so the sides are held in a shape as close to a sphere as possible so that it displaces the maximum amount of air.

A vacuum sealed plastic bag though is not displacing much in the way of air. I am suggesting having it in a cubic frame with tethers from the surface of the envelope attached to the frame, so the sides are held in a shape as close to a sphere as possible so that it displaces the maximum amount of air.

An air ship works by having a zone that is lighter than air. With a vaccuum bag that area gets reduced to zero.

So basically I'm backing you up Rambhutan

Good luck mining your unobtainium.

Good luck mining your unobtainium.

How I mine for fish?

An air ship works by having a zone that is lighter than air. With a vaccuum bag that area gets reduced to zero.

So basically I'm backing you up RambhutanAren't you contradicting yourself?

Now hot air balloons work because hot air is less dense than cooler air. Helium balloons because helium is lighter than air. So surely a vacuum as an absence of anything has to be lighter than all of these. Would it be possible to have a large balloon (made out of something like mylar tethered all round to a lightweight frame so it keeps its shape) to hold the vacuum. Are there any physicists here who can actually work out for a given volume of vacuum how much weight it could lift? Or as normal am I going slightly mad walking to work and thinking up impossible ideas?

hot air balloons are huge compared to the size of basket they lift.

this leads me to assume that a vaccuum baloon would also be too big to be practical.

from wiki:

That was my first thought.

This might interest you
http://www.newton.dep.anl.gov/askasci/phy00/phy00844.htm

This might interest you
http://www.newton.dep.anl.gov/askasci/phy00/phy00844.htm

Thank you that is exactly what I was looking for.

Thank you that is exactly what I was looking for.

Doesn't tell you how to build your contraption but it's a start.

Impossible, vacuums are not lighter than air, like hot air or hydrogen; they are no air. Therefore, a vacuum balloon would be just the weight of the basket and piloting structure, therefore it wouldn't fly.

Impossible, vacuums are not lighter than air, like hot air or hydrogen; they are no air. Therefore, a vacuum balloon would be just the weight of the basket and piloting structure, therefore it wouldn't fly.Depends on just how heavy the non-vacuum bits are.

If something is lighter than the volume of air it displaces, it will be pushed by the air, away from the strongest source of gravity. If you could preserve the integrity of a vacuum, it would be pushed into space. If you could attach just enough weight to keep the vacuum & whatever's stuck to it, lighter than a similar volume of air, it would also get pushed by the air, away from the strongest source of gravity. The principle is exactly the same as a conventional hot air balloon, or a bit of wood floating on water.

But as some helpful poster already pointed out, there's no way to preserve a vacuum, without the help of structures that weighs more than the volume of displaced air. So.. It works in theory, but can't be done in practice.

Vacuums are actually quite difficult to support on their own. Building something that's incredibly light, rigid, and air tight would be a nightmare. Building something incredibly light, air tight, rigid, and weighing less than an equal volume of air is pretty well impossible. Not saying future materials won't do it, but right now we can't.

Plastics for instance aren't air-tight, metals and glass are, but not plastics. Metal and glass both weight a lot more than plastic.

Uh...I'm reasonably certain this would either not work entirely, or be too problematic and/or expensive.

That's why it won't work.

The idea of filling a balloon with hot air or a gas lighter than nitrogen (air is 75% nitrogen) is that you have the same pressure on the inside and on the outside of the balloon: this way, the balloon has very little structural stress, and you can use a light fabric to build it. 1 atmoshpere on the inside, 1 atmosphere on the outside = 0 pressure difference.

Remember: perfect gases (hot air, helium etc are almost perfect gases) follow the state equation pV=nRT.

If you were to use a vacuum balloon, you'd have a 1 atm of difference of pressure between inside and outside. This means, at sea level, about 100 kPa , that is 100000 N of force every square metre of balloon surface, or, if you prefer, about 10000 kgf (kilogramme-force) every square metre.

You'd need a very robust material to build the balloon, and such materials:
1.cost a lot
2.weigh a lot, hence you'd have to build an enormous balloon (I don't want to do the calculations right now, however it would have to be ridiculously huge) , because the need for fabric or whatever (hence, mass!) increases as the surface (the second power of the balloon radius), while the lift of the balloon increases as the volume (the third power of the balloon radius).

So, a vacuum balloon it's not impossible: it isn't just practical.

That's why it won't work.

The idea of filling a balloon with hot air or a gas lighter than nitrogen (air is 75% nitrogen) is that you have the same pressure on the inside and on the outside of the balloon: this way, the balloon has very little structural stress, and you can use a light fabric to build it. 1 atmoshpere on the inside, 1 atmosphere on the outside = 0 pressure difference.

Remember: perfect gases (hot air, helium etc are almost perfect gases) follow the state equation pV=nRT.

If you were to use a vacuum balloon, you'd have a 1 atm of difference of pressure between inside and outside. This means, at sea level, about 100 kPa , that is 100000 N of force every square metre of balloon surface, or, if you prefer, about 10000 kgf (kilogramme-force) every square metre.

You'd need a very robust material to build the balloon, and such materials:
1.cost a lot
2.weigh a lot, hence you'd have to build an enormous balloon (I don't want to do the calculations right now, however it would have to be ridiculously huge) , because the need for fabric or whatever (hence, mass!) increases as the surface (the second power of the balloon radius), while the lift of the balloon increases as the volume (the third power of the balloon radius).

So, a vacuum balloon it's not impossible: it isn't just practical.
I knew I was on the right track. At least I haven't forgotten everything I learned in school.

Impossible, vacuums are not lighter than air, like hot air or hydrogen; they are no air. Therefore, a vacuum balloon would be just the weight of the basket and piloting structure, therefore it wouldn't fly.

I hope, for your physics grades, that you're joking.

In case you aren't, or someone else has doubts:

Vacuum is lighter than air (in this case, "less dense" would be more accurate). In the very same way that 0 is less than 1.

By claiming that a vacuum balloon would be just the weight of its structure THEREFORE it wouldn't fly, you show poor understanding of Archimedes' law.

A hot air balloon's mass is its structure PLUS the mass of the air in the balloon. The difference is the displacement of the outside atmosphere. Archimedes' law (you can obtain it combining the laws of gravity and fluidodynamics) states that the lift of a body submerged in a fluid (here, the atmoshpere) is equal to the weight of a volume of this fluid equivalent to the body's volume. This is why balloons fly and ships (even METAL ships) float!

Not because they are LIGHTER (or less massive) than air; because they're LESS DENSE than air.

Vacuums are actually quite difficult to support on their own. Building something that's incredibly light, rigid, and air tight would be a nightmare. Building something incredibly light, air tight, rigid, and weighing less than an equal volume of air is pretty well impossible. Not saying future materials won't do it, but right now we can't.

Plastics for instance aren't air-tight, metals and glass are, but not plastics. Metal and glass both weight a lot more than plastic.

I was thinking of metal coated boPET
http://en.wikipedia.org/wiki/Mylar
for the spherical envelope with a light but rigid exoskeleton of beams forming a dodecahedron holding the envelope in shape from the outside.

But it clearly has little gains over helium or hydrogen...

I was thinking of metal coated boPET
http://en.wikipedia.org/wiki/Mylar
for the spherical envelope with a light but rigid exoskeleton of beams forming a dodecahedron holding the envelope in shape from the outside.

But it clearly has little gains over helium or hydrogen...

I was thinking a honeycomb metallic film inflated with air encasing the vacuum. Very strong structurally and light.