composite vacuum chamber

Discussion in 'Fiberglass and Composite Boat Building' started by jim lee, Sep 20, 2009.

1. Joined: Feb 2007
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jim leeSenior Member

I'm thinking 3' radius to start. If I can get the composite to weigh in less than 1 lb/sq foot, my calculations show, it'll float.

In air.

-jim lee

2. Guest625101138Previous Member

The 1lb/sq.ft is OK for a layup having sufficient robustness to take the local loading and allow a triaxial arrangement of weave.

My calculations show you need to go much bigger with a skin that heavy. Post your calcs.

Rick W

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jim leeSenior Member

Weight of skin < weight of air

4 pi r^2 x (skin lbs/sq ft) < 4/3 pi r^3 x (air lbs/cbe ft)

I don't remember weight of air off the top of me head. Give me a few.. To look it up again..

-jim lee

4. Guest625101138Previous Member

If you google vacuum balloons you will find some interesting stuff.

From a practical point of view I think you would be much better off using a cylinder. You will find forming a sphere to be very difficult.

You should be able to do a single layup of glass cloth into a cylinder that is not too delicate to handle that has a skin weight around 300g/sq.m. It only has to handle the hoop stress from 100kPa. The ends could be domed, strutted or thick panels with light core.

The attached pictures shows a light structure that almost floated before it collapsed. It is not the best shape from a stress point of view but shows an alternate of using struts rather than relying on the skin stress.

In any case it is possible to make a vacuum balloon that has some lifting power.

Rick W

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5. Joined: Feb 2007
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jim leeSenior Member

Well, here I am surrounded by fiberglass infusion equipment. So I figured all I need to do is make a big hemisphere plug -> Mold. Pop out some big 'ol hemispheres, glue 'em together and see if it worked.

So, I looked at a bunch of the "will it work" stuff and pretty much everyone says it won't. Odd, I have to go back over my math, but I think it would.

I'm surprised that cube thing worked at all.

-jim lee

6. Guest625101138Previous Member

How will you get a spherical layup that is not creased? Any creasing will create a local buckling failure.

Rick W

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jim leeSenior Member

Creases? I don't see that as a problem. I figure it'll be just a big 'ol infused hull shape that happens to be real close to a sphere. I plan on doing it as a cored composite. That should give some poking resistance.

From what I've been reading, I'm wondering if I have a decimal point off somewhere. Seems no one has been able to do this. I'm going to go through the numbers again before trying it.

-jim lee

8. Guest625101138Previous Member

Typically hulls do not have continuous compound curves like a sphere. That is why I suggested a cylinder. I imagine you could layup about a dozed pieces with overlap.

For a 3ft radius you will need a layup less than 1.3oz per sq.ft for it to be buoyant at room temperature. Maybe possible.

Rick W

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jim leeSenior Member

Really? How big does the radius get when the layup is 1 lb / sq ft?

10. Guest625101138Previous Member

13m radius or in your British units say 80ft in diameter. It will weigh 11 tonne before the vacuum is pulled.

It is not a trivial task. If it was you would expect to see a lot of vacuum lifters around rather than cranes.

Rick W

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jim leeSenior Member

Yikes! looks like I did move a decimal place over one somewhere.

Rats!

-jim lee

12. Guest625101138Previous Member

You should check my calculation.

I also think a much lighter layup is possible so the 6' diameter might be practical but I do not think it will be done with infusion. It will require something like 100gsm cloth (more like film than cloth) either side of a very low density core.

A 6' diameter ball would be around 10lb to get buoyant.

Rick W

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rxcompositeSenior Member

Jim,

Actually, it is a little bit of "rocket science". I programmed the calculations for Rocket Motors and Pressure Vessels for you.

I reversed the pressure and used compression instead of tension for the stress.

I am attaching the excell program so you can play around with it. Seems the 14.7 psi vacuum will require only 0.001" of skin so the problem would be how to support the skin since it will collapse unto itself. Finding a fiber that is less than 0.001" is also impossible.

The netting analysis is an entry level method for predicting the sizing of materials. No interlaminar shear, no stiffness analysis.

You gave me an idea. I can use this to design submarines.

Rx

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14. Joined: Feb 2007
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jim leeSenior Member

Well, to solve the collapsing problem I was going to core the thing. This would also go a long way for solving the "It implodes if it touches anything" problem as well. I have a bunch of 1/4" core left over from the Dart Project.

From what you are saying, it seems one would need very little glass to hold the pressure as a sphere. So mostly the deal is the weight of the core.

-jim lee

15. Joined: Jan 2005
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rxcompositeSenior Member

yes Jim,

Stiffness would be the problem and cored composites is the best approach.

In cored laminates, the skin receives the highest stress, the core receives compression and shear.

Try also increasing the safety factor. Aerospace generally use 2, Marine uses 3.

Rx

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