Vacuum Balloon!

[John Cuthber]

It's not the weight of water that matters, it's the weight of the air, and there's a lot of that.

[coreview2]

Actually building the thing and feeling it get very, very light with negative pressure (a couple of minutes of vacuuming air out of the construct), it really didn't feel like it was so far from floating. Then too, the broken, but un-punctured remains did float very quickly when I pumped hot air into it with a hair drier. That's the real world "lab test," primitive though it may be.

 

One of the things that a cad simulator could do would be to define with good accuracy what kind of strength, weight and materials would be needed with various design configurations (even if such materials were yet to be designed) to achieve the best combination of buoyancy and strength.

 

Again the advantages would be substantial for long term and very high altitude balloons which lose helium and suffer the effects of gas expansion and contraction (which is a big part of the reason NASA can't keep them up there that long).

[coreview2]

Is there a difference between the forces of negative pressure (air being kept out) and the forces of hot air held in (air being kept in)? Is this not approximately the the inverse?

 

The thing had no trouble lifting off with hot air and the very thin bag didn't rupture (displacement did of course change so it was not exactly equal in those terms).

[swansont]

Is there a difference between the forces of negative pressure (air being kept out) and the forces of hot air held in (air being kept in)? Is this not approximately the the inverse?

 

The thing had no trouble lifting off with hot air and the very thin bag didn't rupture (displacement did of course change so it was not exactly equal in those terms).

 

Is there a pressure difference when the air is hot?

[coreview2]

Is there a pressure difference when the air is hot?

 

It had to displace about 1.3 pounds of air in order to lift off somewhere around 1/10th of an atmosphere worth given the expanded size, so I'd say yes the hot expanded air was pushing outward and upward at a pressure greater than that of the negative pressure when the sticks broke. I'd guess that the big difference is that the hot air was itself part of the structure.

 

It was a fun project. A real working negative pressure blimp would have great utility in high altitude research. Besides, it would have been a first.

[John Cuthber]

Traditional hot air balloons have a bloody great hole in the bottom.

Does anyone think they contain air at a significantly different pressure from the air outside?

[coreview2]

Traditional hot air balloons have a bloody great hole in the bottom.

Does anyone think they contain air at a significantly different pressure from the air outside?

 

The atmosphere is still pressing in on the expanded and lighter air is it not? Because we can swim to a depth or 30 or 300 feet it does not mean that the pressure is not there only that our bodies are made mostly of water contained in a sack of skin rendering the water as part of the structure (of course it takes about 10 times the amount of air to fill our lungs at 300 feet).

 

I loved building hot air balloons as a kid!

[John Cuthber]

"The atmosphere is still pressing in on the expanded and lighter air is it not?"

Yes, at exactly the same pressure as the air inside, otherwise it would escape through the afforementioned bloody great hole.

[SH3RL0CK]

Swansont,

 

to answer your question if the pressure is different, I'd say probably not significantly due to leakage (i.e. the balloon was broken in the attempt for a vacuum).

 

But consider the basic equation:

 

PV = nRT.

 

If T increases (hot air), and the volume remains the same, and the pressure is the same (evidenced by the holes in the balloon), then n must be less. Since n is smaller, there are fewer atoms in the balloon and therefore less mass contained within. As such, the balloon becomes more bouyant, and floats.

[swansont]

Swansont,

 

to answer your question if the pressure is different, I'd say probably not significantly due to leakage (i.e. the balloon was broken in the attempt for a vacuum).

 

But consider the basic equation:

 

PV = nRT.

 

If T increases (hot air), and the volume remains the same, and the pressure is the same (evidenced by the holes in the balloon), then n must be less. Since n is smaller, there are fewer atoms in the balloon and therefore less mass contained within. As such, the balloon becomes more bouyant, and floats.

 

Precisely. The hot-air balloon does not have to withstand the kind of pressure differential that leads to a significant force on the superstructure, which is the case for a vacuum device.

[coreview2]

Precisely. The hot-air balloon does not have to withstand the kind of pressure differential that leads to a significant force on the superstructure, which is the case for a vacuum device.

 

Actually, in this case the sealed bag was filled with hot air and not an open hot air container. Of course, it did expand quite a bit, and I guess it can be looked at as if the expanded air in its entirety was part of the "structure" that displaced .13 of an atmosphere. Ten pounds of air displacing 11.3 pounds of air and it floats no matter how you capture or contain it.

 

The real question is what would it take to make the vacuum version work? Any practical ideas anyone?

[akhmeteli]

 

The real question is what would it take to make the vacuum version work? Any practical ideas anyone?

 

You may wish to look at our US patent application 20070001053 (11/517915). We propose an evacuated sandwich spherical shell with two thin face sheets and a light core between them. Finite element analysis confirmed that the structure using commercially available materials (e.g., boron carbide face sheets and aluminum honeycomb core) can be light enough to float in air and strong enough to withstand the atmospheric pressure with decent safety factors for strength, buckling, and intracell buckling. Actual manufacturing, while definitely possible, is not easy.

[Think it thru]

Aerogels? Do tell.

 

This site should let you gather knowledge of aerogels.

http://p25ext.lanl.gov/~hubert/aerogel/

This one as well via NASA.

http://stardust.jpl.nasa.gov/tech/aerogel.html and from that site--

It is 99.8% Air

 

Provides 39 times more insulating than the best fiberglass insulation

 

Is 1,000 times less dense than glass

 

Was used on the Mars Pathfinder rover

---------

I can't wait to build a new home using it for windows.

[SteamPunkGeek]

Maybe a construction of balsa like the roof of a house would work better? Who ever said it had to even in any shape or form?

[insane_alien]

Maybe a construction of balsa like the roof of a house would work better? Who ever said it had to even in any shape or form?

 

spheres are best in this scenario as it allows for even distribution of the forces and has the least surface area for a given volume.

[rhubarbpie]

I think a geodesic sphere is well suited and perhaps too much concern is being paid to light weight. Two points:

 

* Does the pressure per square unit differ depending on sphere size? For instance, if a sphere has X radius with a given pressure per square meter, does that pressure per square meter differ with a sphere of say, 10X?

 

* Can a geodesic sphere be made larger without increasing the size of the base triangles? Using the above example, if the base equilateral triangles have strut lengths of 1, the larger sphere could have the same. More triangles, but base triangles of the same size.

 

My point is, if the base triangle can withstand the pressure, good things happen with larger spheres. There'd be a beneficial trade-off between surface area (weight) and volume. However, if the triangles were made larger, the covering and struts would have to be made stronger.

[swansont]

"Pressure per square meter" is redundant; pressure is force per unit area. So no, the size does not matter for the pressure. It does, however, matter for the force, because for a given pressure, the force increases as area increases.

[padren]

Actually, in this case the sealed bag was filled with hot air and not an open hot air container. Of course, it did expand quite a bit, and I guess it can be looked at as if the expanded air in its entirety was part of the "structure" that displaced .13 of an atmosphere. Ten pounds of air displacing 11.3 pounds of air and it floats no matter how you capture or contain it.

 

The real question is what would it take to make the vacuum version work? Any practical ideas anyone?

 

I know this is an old post, but just some thoughts to describe the various things going on in a manner that may make more sense "intuitively" and all:

 

 

When filling a skin with hot air, or a lighter than air gas, there is some some outward pressure, but that has more to do with buoyancy, (and if it has an elastic skin like a normal party balloon, the tension of the stretched skin) but this is very different than the pressures created by a 'vacuum' balloon.

 

The truth is you don't even need a skin - just watch a bubble of air rise in a water tank. To 'catch the ride' you have to at least wrap the top of it. Leaving an opening in the bottom is also handy as it will expand as it ascends into lower pressure levels and that can lead to the 'wrapper' exploding.

 

So while there is outward pressure (even with an open bottom) that is the result of the lighter gas trying to move up through the heavier air, in proportion to buoyancy, not an atmospheric pressure difference.

 

 

A vacuum is pretty different:

 

Consider how well you can stick a toilet plunger onto a flat smooth surface, and the force to break the seal by just pulling "up" on it. In space, the plunger would not stick at all.

 

In Earth's Atmosphere, as you pull up on the plunger, you increase the volume of space inside the plunger's head without air being able to get inside - thus forcing you to 'displace' the air around you to accommodate the increasing volume. The difficulty in lifting the plunger until the seal finally breaks is due to 'pushing all that atmosphere out of the way' and, though it doesn't seem like much is being displaced (the plunger has a pretty small head after all) you can really feel it when you lift the thing.

 

That pressure is all around us all the time - we just really don't feel it because just like deep sea fish, we are at a pressure equilibrium on the Earth's surface.

 

You can feel it in a hurry when you disrupt the pressure equilibrium, such as with the plunger, and balsa frames are likely to feel it even faster.

 

 

Note: swansont or anyone, if I am way off on this (since it's what I suspect is going on) please correct me. I think this is an accurate assessment but could be complete bunk as I am no expert.