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Old Mar 21, 2015, 06:38 PM
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Joined Mar 2015
6 Posts
Idea
Avoid Helium Loss/Degredation

Hi all,

Firstly let me state I have no experience with actual construction or design of blimps or with forum posts but have started researching with the hopes of developing a long range blimp for a science project. From what I've heard, the biggest issue with Blimps is the loss or degradation of Helium. Whilst I lack the skills to implement this I was curious if anyone has tried to avoid this with a second envelope. By having two airtight (easier than helium tight?) envelopes one over the other you could pump in and contain (indefinitely?) a layer of higher pressure air that would stop helium leaking from the inner envelope due to the pressure difference.

Thoughts?
Thanks,
ManTheGoo
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Old Mar 21, 2015, 06:51 PM
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Joined Feb 2009
467 Posts
I think you would still have migration of the helium from the inner envelope into the outer air envelope regardless of the pressure difference. The problem is that helium molecules are so small that they will weasel their way between the molecules of any thin film used to try and contain it. I think it is also not possible to have more pressure in the outer envelope than in the inner envelope, since the inner envelope would then get compressed until the pressure in both envelopes were the same. If the walls were made out of a rigid material like metal then you could do that, but with flexible films you would just crush the inner envelope until the point where the pressure inside it was the same as in the outer envelope. If you think of the outside atmosphere as an outer envelope that is already at one atmosphere of pressure and then just a regular single envelope blimp as the inner envelope, you can see how it is not possible to have less pressure inside the blimp than outside without basically creating a vacuum bag. As with any vaccum chamber (defined as having less pressure on the inside than on the outside), the container wall must have the strength to resist the crushing forces. A flexible blimp envelope has almost zero resistance in that regard.

The better approach is to just use a material that has good helium retention, such as the metalized films used in foil balloons. If you've ever had one of those foil balloons in your house, you'll notice that they stay buoyant for about a month.
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Last edited by pyronaught; Mar 21, 2015 at 06:59 PM.
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Old Mar 21, 2015, 08:37 PM
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Joined Mar 2015
6 Posts
Thanks for the response pyronaught. I can't believe I didn't consider such a simple flaw! So you think even with a rigid structure there would be no point? I guess if a rigid structure was used then you could just us lower pressure helium in a single envelope couldn't you? That would even give a buoyancy advantage due to the lower density. It'd be interesting to see if anyone had done any tests on low pressure balloons. Also, does anyone know what material Google's project loon uses for their balloon envelopes? They have a normal lifetime of 100 days (in that air) with the record at 187 days. Even then it was brought down in a controlled manner, no one knows exactly how long they could theoretically keep enough helium to remain at that altitude.

EDIT:
btw saw your thermal blimp project - very cool (or hot?)
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Old Mar 22, 2015, 09:21 AM
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Joined Feb 2009
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Low pressure balloons contained inside a rigid outer structure are called Zeppelins and were invented in the early 1900s. They have several advantages over a blimp, but are vastly more expensive and complex to construct and fly. They tend to be much larger too due to having to lift the weight of the outer framework. There hasn't been a real zeppelin constructed since the 1930s, with the Hindenburg's twin sister ship being the last one ever flown right up to the beginning of WWII. Unless you count that Dragon Dream project being created for the military, which I have a feeling is going to be just another expensive project that gets mothballed.

I haven't researched the google balloon project, but knowing google the details are probably secret anyway. I think metalized nylon or PET is going to be the most practical solution in your situation. I'm sitting on 11,300 feet of 50" wide .26 mil aluminized PET I can sell you for 50 cents a linear foot if you want.
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