Can a Vacuum Balloon Really be Built Using Aramid Fabric and Carbon-Epoxy Truss?

[SkepticJ]

I've been enamored with the idea of vacuum balloons for the better part of a decade. I know they're an extremely difficult engineering challenge, and may not actually be possible even with the strongest materials.

I've come across a novel approach to the problem, and I'm curious what actual engineers think?

Suppose the balloons are made of aramid fabric sandwiched to a gas-tight membrane. Could this actually work?

 

[boneh3ad]

It's a pretty terrible idea. You are just transferring the same problem you would always have of the exterior pressure tending to crush the balloon from the outer skin onto the inner skin.

Consider the (presumably) original case, a plain, single-walled, evacuated sphere surrounded by 1 atm pressure. There would be 1 atm of pressure across the entire surface exterior and 0 pressure on the inside, so a net 1 atm pressure acting inward.

Now consider the proposal in the link. You have that same 1 atm acting on the outer skin exterior, but now you have 2 atm acting on the outer skin from the plenum (space between the two skins). You would have a net outward pressure of 1 atm acting on that area then, meaning the outer skin is not stressed any less, just in a different, less strong direction. On the inner skin, you would have 2 atm acting on the plenum side, and 0 atm acting on the interior for a net of 2 atm acting inward, doubling the problems faced by the original case.

Now since both skins are connected, that will help out some, but it would still end up with an overall net pressure of 1 atm acting inward with with a lot more stress in the structure.

Basically, this model just makes things worse.

 

[SkepticJ]

That's very true.

What made me think it might have potential is because the solid matter in the design is only under tensile stress. The strongest, lightest materials are strong in tension, not compression. The gas turns the outside compressive load into a tensile.

An aramid thread about a mm in diameter can support an average person's weight*, but has virtually no compressive strength--of course, it's a piece of string. Carbon nanotubes are the same way; their compressive strength is unremarkable.

I might be wrong about this, but I don't think a column of diamond a mm in diameter could support an average person's weight under compression, and diamond is one of the strongest materials in compression that can exist.

I'm having difficulty finding the company again, but there's one out there that makes inflatable arches that can support tonnes of weight.

*Assuming it was properly connected to them, otherwise it would cut through them like a wire through cheese.

 

[Vadar2012]

Ah Bonehead, always fun to read your posts. So harsh yet interesting.

 

[boneh3ad]

Ah Bonehead, always fun to read your posts. So harsh yet interesting.

Ha! Harsh? I don't try to be harsh. You can blame my advisor for his bluntness rubbing off on me, haha.

 

[AlephZero]

From the OP's link:

If a flexible balloon was constructed in collapsed form, the interior sealed while collapsed, and the double skin inflated, no evacuation pump would be needed as the interior would be largely free of air.

I suggest they stop thinking about balloons and start marketing ioflatable vacuum chambers

 

[AlephZero]

To be fair to the link, it is halfway to doiing the right thing, but for the wrong reasons. A thin curved structure like the baloon skin will not fail through compressive stress in the material, but by buckling. It is more efficient to make a double skinned structure separated by ribs, because this will be much stiffer in local bending (i.e. resisting buckling) than a solid plate with the same mass.

But internal pressure is irrelevant to this, and the arrangement of the ribs doesn't suggest that is what the designer was thinking about.

This is a better way (sorry it's a poor quality copy of a rather ancient document): http://femci.gsfc.nasa.gov/Isogrid/NASA-CR-124075_Isogrid_Design.pdf

Note the "second skin" is isogrid is not actually continuous, but the tops of the T-section ribs do the same job as a continuous skin, and it can be machined or chemically etched from a solid plate, without the fabrication or welding required to make a hollow double-skinned structure.

 

[SkepticJ]

Would an isogrid better resist buckling if its members weren't I-beams, but were more like bamboo?

I've long been impressed by how strong bamboo is. The wood is different than tree-wood, but it looks like it's mostly down to its shape.

Assuming cost was no object, could a vacuum balloon fly that was built using the isogrid principle? Maybe it's five times the size of Cape Canaveral's Vehicle Assembly Building, and it's made of hundreds of billions of dollars worth of composite materials, but it flies.

 

[AlephZero]

Would an isogrid better resist buckling if its members weren't I-beams, but were more like bamboo?

The problem is you have to fabricate the structure somehow, unless you can just grow it.

But there are ways to do that ... http://core.materials.ac.uk/repository/eaa/talat/3805.pdf

(Superplastic forming works rather like blowing balloons out of chewing gum, so this isn't completely off topic)

 

[Wiggin]

Clearly a major error in the first post's link is that the material is flexible, but the inner skin is not drawn as bulging inwards under pressure.

Have a look at this link instead:

http://launchloop.com/KristianAndresen?action=AttachFile&do=get&target=VacuumTunnel.pdf

 

[Enthalpy]

Description there
http://saposjoint.net/Forum/viewtopic.php?f=66&t=2520
with a truss of carbon-epoxy tubes (available in kite shops) and tension-working film it doesn't look so difficult.