Vacuum Balloons

[gamowin]

Balloons float by being less dense than the air that surrounds them. So what if you were to build some sort of rigid balloon that could have all the air sucked out of it to become a vacuum chamber, would it float? If not, would it do anything at all? Part of me feels like this shouldn't work, but I can't figure out why it shouldn't.

[maltesh]

They would work, and would be a bit better than Hydrogen balloons. The problem is building something large enough to contain enough vacuum to lift something useful, with a shell both light enough to lift, airtight enough to keep air out, and strong enough to keep air pressure from crumpling it in an instant.

[Bill Phil]

Well, the vacuum would have to counteract the shell's high density...

[Newt]

Hydrogen and helium are useful because they put outward pressure on the generally fragile skin of blimps and balloons, which permits light structures to be made at large scales, as well as that can expand to maintain the same pressure as the surrounding so as to maintain lift. Vacuums exert no meaningful pressure, so the structures have to be made strong enough to resist the difference between in and outsides. Most materials that can do this well are really heavy--heavier than the benefit of making the pressure that low would be, and so we do not really use them.

[Louella]

that's a railway tank car imploding due to vacuum, as part of a safety demonstration

now they're really built to contain internal pressure, so a vacuum balloon wouldn't be built the same way, but that's still a fairly substantial, and heavy, steel structure, and it crumples that easily.

[kenbobo]

dear god

that tanker

[Tommygun]

Wow that video of that railway tank car is impressive.

I think at sea level air pressure is about 1 kilogram per square centimeter?

So that's about 10,000kg per square meter. I can't imagine any material that can take that and still float.

[cantab]

The chief problem is that buoyancy comes from the difference in density between the lifting gas and the air. So a vacuum only has about 10% more lifting power than helium.

Set against that is that lifting gases can be contained in a lightweight flexible structure. A vacuum requires a rigid shell to hold it. It's almost impossible to make that shell light enough, and no vacuum balloon has ever been demonstrated.

[K^2]

It's fairly trivial to prove that no uniform material is strong enough to build a vacuum balloon. There are some designs that involve composite honeycomb and Mylar that are just barely strong enough to allow for positive net buoyancy. That makes for an extremely expensive and extremely inefficient construction. You are better off building a hot air balloon than a vacuum one.

[GreeningGalaxy]

I love the idea of vacuum balloons, but I don't think they're really something we'll see in our lifetime. I remember reading some excerpt from a contemporary sci-fi story featured in Popular Science that had vacuum airships, with the envelope contained by some sort of handwavium super-strong force field (premise was that we ran out of helium), but that's obviously quite a ways off.

Meanwhile, you can probably get very close to that level of buoyancy by combining hot-air balloons with light-gas ones - a hot-helium balloon, or if you feel like living on the edge and doing the most dangerous things you can possibly think of, a hot-hydrogen one. It's a bit of an engineering challenge to figure out how to heat the gas through a sealed envelope without setting something on fire, but substantially less of an issue than making a lightweight vacuum chamber that size.

[PakledHostage]

I think at sea level air pressure is about 1 kilogram per square centimeter?

I am not trying to pick nits here... Only trying to clarify something about the metric system for our imperial system using friends:

You are basically correct, but your use of units is a bit cringe inducing... Pressure in the metric system is measured in Pascals. Standard air pressure at sea level and 15°C is 101325 Pascals. Pascals have units of Newtons per square metre. A Newton is a unit of force. It has units of kg * m/s². When you write pressure in terms of kg per cm², as you did, you are effectively using mass and weight interchangeably. I'm sure you'll agree that that's not cool on a science forum!

[GreeningGalaxy]

Kilogram weight is a valid unit of force, but I'd agree that SI units are better.

We keep encountering psi in my physics textbook, and it makes me want to yell.

[Tommygun]

Well, I was trying to from a visual image and I wanted to avoid pascals and newtons as they are more abstract.

I guess I could have said 120 Sir Isaac Newtons standing on a metal plate. He was a bit chubby and very Imperial.

[K^2]

Kilogram weight is a valid unit of force

It's icky for the same reason pounds of mass are icky. And pounds of mass are the reason we have a factor of g in rocket equation. Do you want a factor of 1/g in your rocket equation? Because that's what you'll get with kilograms of force.

[Frozen_Heart]

a hot-hydrogen one

Yeah that's not going to end well for anyone...

[Ralathon]

Yeah that's not going to end well for anyone...

Perfectly valid strategy! Just keep away from oxygen! Don't let doomsayers stop the progress of SCIENCE just because of minor mishaps like the Hindenburg!

Seriously though, it would be cool to have a hot hydrogen balloon in the jovian atmosphere to study its composition. I wonder if an RTG has a high enough heat output to keep itself afloat...

[K^2]

You honestly don't need much, so yeah, I think that could be arranged.

[cantab]

It's icky for the same reason pounds of mass are icky.

The pound, like the kilogram, is primarily a unit of mass. Though both pound-force and kilogram-force are widely recognised and convenient sometimes.

[battlefieldaces]

A hot hydrogen balloon sounds feasible as long as you have no oxygen in there. Remember WW1 british pilots: They were struggling to shoot down airships because the holes normal bullets made were too small to drain hydrogen quickly. Even when incendiary bullets appeared they were still ineffective agains their targets due to lack of oxygen in the envelope. The solution was to use belts of alternating explosive and incendiary bullets. The explosive rounds made big holes for oxygen to enter and when the incendiary bullet followed it set off quite a show.

[benzman]

This site:- http://www.rusring.net/~levin/levin3d/dz.htm shows the work of a Russian computer artist. He has drawn what looks to be a Steampunk inspired vacuum dirigible. Utterly impractical, but cool looking.

[Iskierka]

The pound, like the kilogram, is primarily a unit of mass. Though both pound-force and kilogram-force are widely recognised and convenient sometimes.

False, the pound is a force unit. Pound-mass is the abstract, imperial unit's standard mass is the slug, defined the same way as the kilogram, as g * force, making it around 32 pound-masses, or ~14 kilograms.

However, pound-mass, Newton-mass, slug-force and kilogram-force are all units that should be recognised, as while yes, they lead to stray g factors in rocket equations, conversely, they eliminate stray g factors in aircraft and ground vehicle range equations, and can come in useful in other places. All should be used, but should be used when appropriate and clarified that they are not the conventional unit, but the converted equivalent.