# Would vacuum balloon fly

1. Sep 23, 2011

### calis

would a baloon or a barrel 'filled' with vacuum act as a hellium balloon and float in air?

of curse that would require hard shell balloon...

density of air at sea level is 1.22521 kg/m3.

so if we could make a vacuum chamber one m3 big that would weigh 1.22 kg it would float right?

p.s.if the container would be a sphere than it should weigh 252 grams/m2

2. Sep 23, 2011

If you had a rigid balloon then yes. If the rigid balloon itself was too heavy and would make the whole system more dense than the equivalent volume of air, then no.

3. Sep 23, 2011

### Curl

everyone should realize that the helium in helium balloons is just to push the rubber into the balloon form, there's nothing magical about helium that makes it fly.

anything floats in a fluid as long as the density of the object is less than that of the fluid.

4. Sep 23, 2011

Staff Emeritus
The answer is yes, it would if there were a balloon material strong and light enough to contain the vacuum. But there isn't.

5. Sep 23, 2011

### gsal

CurL: Of course there is something magical about helium....it is lighter than air!

if you just went to the back porch and fill up a balloon with propane from the grill...would it fly? I don't think so.

...anyway...just to be proper

6. Sep 24, 2011

### bbbeard

I gave this problem a cursory look about ten years ago and didn't find an acceptable solution, either. But is this really a "law" of physics? Surely not. What precisely is the limitation? What if we allowed the balloon to have a rigid frame covered by a membrane? Are there still no materials that could make a vacuum balloon? (Is this a job for Mythbusters?)

7. Sep 24, 2011

### bbbeard

What if we filled the balloon with pure nitrogen?

BBB

8. Sep 24, 2011

### gsal

BBB: you don't have to "what if" one gas at a time, just look up gas densities...if the gas you are wondering about has a density lower than air, than the balloon has a chance of floating. Just remember, there needs to be enough difference in weight between the volume of air and the volume of the gas to match or over come the weight of the material the balloon would be made out of.

9. Sep 24, 2011

Staff Emeritus
It's actually a size problem, not materials problem per se.

The requirement is that the pressure vessel needs to be lighter than air, i.e.

$$\frac{3}{2}PV\frac{\rho}{\sigma} < V \rho_{air}$$

or

$$\frac{\sigma}{\rho} > \frac{3}{2} \frac{P}{\rho_{air}}$$

Which means I need a stress-to-density ratio of just over 105 Nm/kg, and remembering a little bit about the speed of sounds in solids, recognize that anything with a speed of sound > 320 m/s or so will be strong enough. That's pretty much any metal.

What's the problem then?

Imagine a vacuum balloon with radius 0.15m. The most the aluminum (let's pick that, because it's got a very high speed of sound) can weigh is 18g. That means it can only be 24 microns thick. A foil that thin won't hold vacuum. I don't know whether it's imperfections, or chemistry, or what - but it won't. Adding a rigid framework only makes this thinner.

10. Oct 12, 2011

### akhmeteli

I agree that a vacuum balloon with a homogeneous shell made of existing materials cannot be both strong enough and light enough (the main failure mode is buckling though). However, finite-element computations demonstrate that a vacuum balloon with a sandwich shell made of existing materials can be both strong enough and light enough (https://www.physicsforums.com/showpost.php?p=1754588&postcount=11 ). The relevant designs are scalable (so the problem of foil not holding vacuum can be solved by multiplying all dimensions by a common factor), but they are difficult and expensive to manufacture. By the way, even your equations suggest scalability.

11. Oct 12, 2011

### DaveC426913

Beryllium eh? A rigid beryllium balloon, filled with vacuum, that floats...

And if you let it go outside, it would float up into the sky till it hovered. But unlike a H or He balloon it would not burst at high altitude. In fact, it would get stronger.

12. Oct 12, 2011

### bbbeard

I think the strategy would have to be to cover a rigid framework with a thin strong foil such that the foil is in tension, not compression. Imagine a cubical frame with concave walls. I haven't figured out the best way to deal with vertices, but maybe the optimum geometry simply has curved frame elements, like two circles in parallel planes (e.g. the circles that form the edges of a circular cylinder) supported by some internal brace. Obviously, the heavier the frame, the thinner the foil has to be, but I haven't run the numbers on any particular frame concept.

BBB

13. Oct 12, 2011

### akhmeteli

As I said, it's not easy, and it's not cheap. But it is possible.

Well, the altitude will be limited compared to that for lighter-than-air-gas balloons. As you said, a vacuum balloon would rise until the air density is the same as the average density of the balloon - the volume would not change.

14. Oct 13, 2011

Staff Emeritus
But that's the problem - metal foils this thin, especially large ones, don't hold vacuum.

15. Oct 13, 2011

### akhmeteli

I have little to say about the structure discussed by bbbeard, but, as I said, the sandwich structures mentioned in post 10 in this thread are scalable, so if the face sheets are too thin to hold vacuum, one can scale up all dimensions and get a structure that is at least equally viable. For example, the radius can be increased from 0.15 m to 0.3 m, and then the thickness of the face sheets can be doubled, and so on.

16. Oct 13, 2011

Staff Emeritus
I'd like to see someone do an actual calculation, as opposed to "surely this other design will work". Tell me how much the framework would weigh, how thick the foil needs to be, and show that someone makes a window the size and thickness of the face sheets that holds vacuum. Remember, it has to be pinhole free over the entire surface.

One can always come up with materials that have theoretically good enough properties. My point is that if you need to have something that you can actually make, you need to scale it up, and if you scale it up (perhaps to blimp size), the odds of a fatal manufacturing error (like a pinhole) also go up. Making this fly is so far past present technology, it's fair to call it "impossible" - just like a car that weighs 10 pounds.

17. Oct 13, 2011

### akhmeteli

For example, we performed computations for a radius of 1 m (however, the computations are intrinsically scalable, so nothing changed when we tried 2 m or 0.5 m). For a zero-buoyancy vacuum balloon, one of the viable structures had two beryllium face sheets each approximately 0.1 mm thick and aluminum honeycomb core approximately 2.8 mm. No, nobody "makes a window the size and thickness of the face sheets that holds vacuum", and cannot make in principle: the face sheets of this size and thickness without core just cannot withstand the differential pressure because of buckling. As for pinholes, see below.

I don't think pinholes are a showstopper. If, however, they are a problem, a thin (<4 micron - see, e.g., http://www.spaceref.com/news/viewsr.html?pid=6248 ) pinhole-free coating can be applied for a small weight penalty.

18. Oct 13, 2011

### chrisbaird

I find this thread highly engaging. I see nothing that physically rules out the construction of a vacuum balloon. It just seems to be difficult engineering work. Engineering challenges are usually overcome in time.

19. Oct 13, 2011

### DaveC426913

It would not be apparent until you examined the ability for a material of a given weight to withstand a pressure of one atmo over a given surface area.

It may be possible, it may not. Most solutions won't be.

It may be possible that there's one particularly creative construction technique that comes in under the wire of tensile strength over material weight.

But it's also entirely possible that there simply is no such solution with any known material.

20. Oct 14, 2011

### bbbeard

Well, the closest I've been able to find "off the shelf" is a Kevlar window implemented on the NA48 particle physics experiment at CERN.

This window is a dome with diameter 2.4 meters with vacuum on the concave side. The wall thickness is 0.9 mm. The specific gravity of the Kevlar is 1.35. If this were a full dome, the mass would be about 22 kg. The mass of the displaced air is 8.87 kg.

So this doesn't seem as far off as a 10 pound car.

I'm not sure what calculations went into the design of the CERN window. I'm pretty sure that the Kevlar was chosen because of its extraordinary strength/weight ratio. If we could keep the 0.9 mm thickness (which we know will hold a vacuum), I estimate the "break-even" sphere diameter is 6 meters.

BBB