Vacuum Balloon!

[coreview2]

Vacuum balloon

 

Yes, my partner and I tried to build one.

 

It was a competition grade balsa and polyethylene construction measuring 4’ x 8’ containing 128 cubic feet of air. At sea level, that’s about 10 pounds of air and the construct weighed 1.25 pounds. We needed to pump 1.3 pounds of air out of the device for it to float.

 

It nearly floated after vacuuming air from it to lower the internal pressure and thus weight, prior to the frame collapsing.

 

What I really needed was a CAD and physics simulation program to design the thing and test it without spending days gluing balsa. It was fun anyway. It nearly flew for a historical first!

 

Cheers,

[Mr Skeptic]

Vacuum balloon

[...]

It nearly flew for a historical first!

 

Has no one ever made one? Several years ago, I thought of a vacuum blimp. I wanted to keep it rigid using air so I could use only tensile material, but calculated that the air pressure to keep it from collapsing would make it weigh too much. Anyhow, I would have expected someone to have made a vacuum balloon before. Now we even have aerogels which weigh less than air.

[coreview2]

Has no one ever made one? Several years ago, I thought of a vacuum blimp. I wanted to keep it rigid using air so I could use only tensile material, but calculated that the air pressure to keep it from collapsing would make it weigh too much. Anyhow, I would have expected someone to have made a vacuum balloon before. Now we even have aerogels which weigh less than air.

 

Aerogels? Do tell.

 

It was a lot of fun building it and it did get very, very light, but alas we needed to build it as a spherical shape -- though that would have taken more free time that we had.

 

NASA has a very hard time keeping big balloons up for a long time and the design of a vacuum balloon would potentially solve most of the problems of Helium balloons if it were worked out.

 

Helium is a very small atom and it is hard to keep in especially with a very thing and light weight membrane. Also as a gas it expands and contracts quite a lot especially at very high altitude due to very intense sun and very cold nights. Also, once ballast or helium are released, there is no control over altitude.

 

A vacuum (negative pressure as buoyancy), unlike helium is is not trying to use a very thin membrane to keep a very small atom in, but keep air's larger molecules out. A vacuum is also not affected by temperature change so there would be no significant expansion and contraction. A vacuum is also a controllable thing given an energy source and pump that can be increased or decreased.

 

Having balloons that stayed in the upper atmosphere for months or years would be of great value.

[YT2095]

there is a little more info in here: http://www.scienceforums.net/forum/showthread.php?t=31317

[swansont]

Has no one ever made one? Several years ago, I thought of a vacuum blimp. I wanted to keep it rigid using air so I could use only tensile material, but calculated that the air pressure to keep it from collapsing would make it weigh too much. Anyhow, I would have expected someone to have made a vacuum balloon before. Now we even have aerogels which weigh less than air.

 

Pedantic man says, "If it was rigid it wasn't a blimp."

[coreview2]

there is a little more info in here: http://www.scienceforums.net/forum/showthread.php?t=31317

 

My thinking exactly. Even with our crude balsa and poly structure, it very nearly did work.

 

Here is a picture of our experiment. Again, we wanted to make it a big sphere with i-beam like balsa for the ribs, but found it beyond the three weeks of free time we had to play with this idea.

 

It makes a lot of sense, but again the way to do it is to have a simulator where designs can be built and tested in a computer quickly before spending weeks gluing balsa. It would make a great classroom experiment or competition.

[Mr Skeptic]

If you built it big enough, you could bend the balsa into a sphere.

[coreview2]

If you built it big enough, you could bend the balsa into a sphere.

 

The tricky part is that it is hard to get balsa in long lengths that do not have serious weak spots (by experience on this). We did attempt to bend it to a sphere, but it was very time consuming as the wood needed to be thin strips that were laminated together to make it curve and then too we wanted to make an I-beam kind of balsa struts.

 

We built the skin from very large dry cleaning bags after much searching for something very thin and very strong. We cut the bags apart and used Scotch double face tape for the seams. That part was more than adequate and the weight was very small. Again the whole construct weighed in around 1.25 pounds (on a reasonably accurate postal scale).

 

 

In any event, it was a lot of fun to try for a world first!

 

http://s113.photobucket.com/albums/n219/coreview2/?action=view&current=VB03.jpg

[swansont]

Where did the structure fail? My gut tells me you'd do better with triangles in a polyhedron shape, like a d20, or hexagons like a soccer ball.

[coreview2]

Where did the structure fail? My gut tells me you'd do better with triangles in a polyhedron shape, like a d20, or hexagons like a soccer ball.

 

The breakage happened mostly near the most reinforced joints.

 

A soccer ball concept with I-beam balsa might have been a good compromise over the classic Roman arch and a lot easier to construct. Even as it was, it felt like it was strong enough, but the thing that surprised me was the amount of stress removing a pound or so of air actually caused!

 

Again, ideally, it would be best to test this concept in a simulator where many variations in design and materials could be subjected to stresses.

 

Ultimately for NASA or such, perhaps a negative pressure combined with helium would be the best solution. Helium under negative pressure would weigh even less.

[swansont]

The advantage of the quasi-sphere is that internal support is all compression if it goes along the diameter, which is going to be a lot more robust. With a cube, there will be lateral forces as well, which may be why a joint failed.

[coreview2]

The advantage of the quasi-sphere is that internal support is all compression if it goes along the diameter, which is going to be a lot more robust. With a cube, there will be lateral forces as well, which may be why a joint failed.

 

Roger that. We knew it would be weaker, but it felt pretty strong for something so very light. Just didn't realize how strong the pressure would be.

 

It would be a great experiment for an engineering CAD/sim program!

[coreview2]

I'd love to find someone who has access to an appropriate engineering cad simulator who was willing to test the concept of a negative pressure blimp.

 

cheers,

[John Cuthber]

I take it this thing was 4 by 4 by 8 feet.

That's pretty big. You want to take about 13%v of the air out of it. That will give a differential pressure of roughly 13% of an atmosphere.

1 atmosphere is about 15 pounds to the square inch so you are after 1.9 PSI difference in pressure. That's 275 ponds to the square foot so for the 4 by 8 foot face you are talking about a load of roughly 4 tons.

 

Challenging.

OTOH, if you can heat it to 60C...

[swansont]

I take it this thing was 4 by 4 by 8 feet.

That's pretty big. You want to take about 13%v of the air out of it. That will give a differential pressure of roughly 13% of an atmosphere.

1 atmosphere is about 15 pounds to the square inch so you are after 1.9 PSI difference in pressure. That's 275 ponds to the square foot so for the 4 by 8 foot face you are talking about a load of roughly 4 tons.

 

Challenging.

OTOH, if you can heat it to 60C...

 

But there are 8 surfaces at 4 square feet each, not one at 32 square feet..

 

What one should probably do is take this calculation and invert it, so to speak. Using the strength of the materials, find the size of surface that will hold up. A similar volume in a spherical shape will have a radius of about a meter, and a surface area of around 12.5 m^2, vs. 14.6 m^2 for the rectangular prism.

[coreview2]

I take it this thing was 4 by 4 by 8 feet.

That's pretty big. You want to take about 13%v of the air out of it. That will give a differential pressure of roughly 13% of an atmosphere.

1 atmosphere is about 15 pounds to the square inch so you are after 1.9 PSI difference in pressure. That's 275 ponds to the square foot so for the 4 by 8 foot face you are talking about a load of roughly 4 tons.

 

Challenging.

OTOH, if you can heat it to 60C...

 

Building cad models I can do as I have Maya. However, Maya is an animation program and not an engineering or physics simulator. It does have a physical dynamics package, but it is gear to special effects not realistic simulations.

 

There must be a program out there that could allow for the testing via simulation of many design concepts.

 

But there are 8 surfaces at 4 square feet each, not one at 32 square feet..

 

What one should probably do is take this calculation and invert it, so to speak. Using the strength of the materials, find the size of surface that will hold up. A similar volume in a spherical shape will have a radius of about a meter, and a surface area of around 12.5 m^2, vs. 14.6 m^2 for the rectangular prism.

 

In designing the structure, our original idea was a sphere, but again we didn't have the time to build it and getting the balsa to bend would have taken a different kind of cut than we had ordered (thin layers laminated most likely). Skinning it was possible, but would have taken more time too.

 

A good cad simulator would allow designs to be tested and if it was reasonably accurate would help provide clues as to where various designs would likely fail.

 

Also, a simulator would allow for bigger models to be tested.

 

The link below is to the "Physics Balsa Bridge Building Contest", but a negative pressure balloon would be more fun )

 

http://www.balsabridge.com/

[swansont]

As I said before, you approximate the sphere with something like a geodesic design. The calculations tell you how small the elements would have to be, and you get 32 surfaces (some pentagons, some hexagons)

[John Cuthber]

"But there are 8 surfaces at 4 square feet each, not one at 32 square feet.."

OK so you have lots of 4 by 4 squares carrying half a ton, each supported by 4 thin balsa struts 4 feet long.

 

Here's the formula I think you need to look at before bothering with CAD/CAM

http://en.wikipedia.org/wiki/Buckling

I haven't done the maths (because I don't have the exact dimensions of the struts and the equation raises that data to the 4th power so there's not a lot of point me guessing it from the picture).

Or you could take the empirical aproach. Stand one of the struts vertically and put a 275 pound weight on top then see what happens.

[coreview2]

OK so you have lots of 4 by 4 squares carrying half a ton, each supported by 4 thin balsa struts 4 feet long.

 

 

Something doesn't feel right here. Removing 1.3 pounds of air from 128 cubic feet (10+ pounds) results in a half a ton of pressure? The pressure was indeed far more than expected, but it was only a vacuum cleaner that sucked the air out (albeit it could pick up a bowling ball).

 

This is only an intuitive "feeling" kind of thing, but that construct got to the point just before collapse that it could be lifted with a delicate touch of two fingers whereas before that would have not been possible. One's sense of lift gets very good after handling a thing for a couple of weeks.

[swansont]

Play with some Magdeburg hemispheres and you'll get a feel for how much atmospheric pressure there actually is.

[John Cuthber]

"Something doesn't feel right here. Removing 1.3 pounds of air from 128 cubic feet (10+ pounds) results in a half a ton of pressure? "

Whatever it "feels like", you saw me do the maths and the answer is 4 tons not half a ton.

Air pressure is a lot bigger than most people realise. The force it exerts on a typical door is about 15 tons

Here's a slightly naff demonstration.

http://youtube.com/watch?v=rX52TsJCuKA

[Mr Skeptic]

This is only an intuitive "feeling" kind of thing,

 

Perhaps it would be more intuitive if you considered that 1 atmosphere of pressure is the same as the (extra) pressure of being about 30 feet under water.

[John Cuthber]

Another way to look at is that it's not removing 1.3 pounds of air that's creating the pressure- it's a whole several mile thick layer of air pressing down that does it.

[coreview2]

"Something doesn't feel right here. Removing 1.3 pounds of air from 128 cubic feet (10+ pounds) results in a half a ton of pressure? "

Whatever it "feels like", you saw me do the maths and the answer is 4 tons not half a ton.

Air pressure is a lot bigger than most people realise. The force it exerts on a typical door is about 15 tons

Here's a slightly naff demonstration.

http://youtube.com/watch?v=rX52TsJCuKA

 

Neat little experiment.

 

Perhaps it would be more intuitive if you considered that 1 atmosphere of pressure is the same as the (extra) pressure of being about 30 feet under water.

 

One of the reasons I like shallow dives in scuba!

 

Still is not a 13% reduction in air equivalent to being under 3.9 feet of water in our case?

 

I after the structure collapsed, I did one more thing. Using a hair drier in the vacuum port, filled it with enough hot air to make it float. Of course, it went right up then.

[swansont]

Still is not a 13% reduction in air equivalent to being under 3.9 feet of water in our case?

 

 

 

Yes. And 128 cubic feet of water weighs just under 4 tons.