Aerogel

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Arminius
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Aerogel

Postby Arminius » Mon Aug 11, 2014 12:15 pm UTC

I was wondering if Aerogel and its derivates (like Pyrogel) is naturally airtight (I guess not) and what one can do to make it airtight without too much increasing its weight.

Tub
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Re: Aerogel

Postby Tub » Mon Aug 11, 2014 12:52 pm UTC

It's not like something is either airtight or not; it's a gradient. Making something 100% airtight is difficult, if not impossible to do. Just ask anyone trying to maintain a vacuum in a lab.

Go to your kitchen, grab a sponge. It's mostly air, right? Now put it to your mouth and try to breathe through it. Hope you used a fresh one. Aerogels are similar, just with a much finer structure. I suppose there are differences between the different aerogels, but air flow through them is usually minimal. The fact that they're used for thermal insulation should tell you as much.

If you need it to be tighter, you could try spraying on a thin coat of something airtight, clear coat or whatever. Reinforcing the surface seems more efficient than picking a stronger, heavier material for the whole block.

Arminius
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Re: Aerogel

Postby Arminius » Mon Aug 11, 2014 1:44 pm UTC

What you say is certainly true. You could even add that it depends on width of material.

Let's reformulate my question to get the answer I am looking for. For a sheet of Pyrogel be airtight enough to be used/replace the skin of a hot air ship (let's not got into details about weight yet), how thick would that sheet need to be? How would you proceed to calculate that?

Tyndmyr
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Re: Aerogel

Postby Tyndmyr » Mon Aug 11, 2014 6:39 pm UTC

Well...probably a lot less than you'd think. Hot air balloons, etc generally don't have a ton of pressure. Even the old zepplins didn't have a big issue with being shot up until incendiary rounds became a thing, because the gas simply escapes very slowly.

To get a hard number, you need to set specs. Figure out the permeability of your aerogel. Figure out what your acceptable escape rate is, and then you can calculate out how thick it'll need to be to meet that. After that, probably move on to a small demo system to see how it works out. Aerogel is sufficiently light that it *may* be possible.

You may have additional practical difficulties, though. For one thing, building an airship out of flat sheets may pose a few problems, and I don't know of a good source to get aerogel in large custom shapes.

Also, keep an eye on tensile strengths, and consider if you'll need additional reinforcement if you're working with very large areas of very thin sheeting.

Out of curiosity, why are you considering this question in particular?

Arminius
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Re: Aerogel

Postby Arminius » Mon Aug 11, 2014 7:20 pm UTC

Thanks for the answer.

I actually got to that question whilst reading from one article to another. I noticed that there a some problems with helium airships, such as the price and rarity of helium but it is the prefered choice, dispite the disadvantages. I then had a look at current operating temperatures for hot airships and read about enormous losses through the nylon skin. From there, the simple question: lightweight, low loss material -> aerogel and its flexible alternatives.

Despite what one might think, aerogel has many form, some of which can be bent (almost) like cloth.

Any idea how to calculate or find the permeability of aerogel? How would you go about testing it a lab?

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Re: Aerogel

Postby Tyndmyr » Mon Aug 11, 2014 9:34 pm UTC

If you have fairly precise scales, you should be able to measure a container of it filled with air, and the same container again after flushing it with helium, and see how quickly the second approaches the first. Should be able to pull a decent practical estimate based on that and the dimensions of your test vessel.

It's likely that different aerogels will have different properties, and it's a somewhat esoteric field(merely getting a good supply can be a challenge), so you may have to run multiple tests with different materials. You may even see variation between different batches of the same material....

Of course, this is the kind of thing that should get better as it scales up, as the volume/surface area ratio improves. In raw lift/escape rate, anyways. No doubt a giant aerogel airship will have all manner of interesting constraints...but it's a fun exercise all the same. I did see some years back a DIY guide to making aerogel floating about, but I imagine it'd be fiddly stuff, and requiring a goodly sized pressure chamber for your needs. The helium would be much easier, as party supply shops have fairly inexpensive balloon filling kits that are fine for screw-around levels of the stuff.

Arminius
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Re: Aerogel

Postby Arminius » Mon Aug 11, 2014 9:45 pm UTC

I have my doubt about your method to messure permeability to air and here is why:

Air and helium have different permeabilities, wouldn't you be testing the latter?

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Re: Aerogel

Postby Tyndmyr » Mon Aug 11, 2014 10:03 pm UTC

Arminius wrote:I have my doubt about your method to messure permeability to air and here is why:

Air and helium have different permeabilities, wouldn't you be testing the latter?


Sort of...and thus it is imperfect, but pressure should equalize, so you'd expect air to be leaking in about as fast as helium as leaking out, presuming you have no overpressure. A hot air test would be more accurate for your envisioned scenario, but is somewhat harder to pull off. You could compare to a box made of a material of known permeability, I suppose.

However, at that point, you're really tracking retention of heat, not retention of air per se. A pedantic point, probably, given your goal, but aerogel is a pretty good insulator, so you'll want to consider your control box's insulation qualities as well as air permeability. Obviously, this insulation is a good thing for your goal, as you probably care more about heat loss regardless of cause than you do about permeability itself.

Arminius
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Re: Aerogel

Postby Arminius » Tue Aug 12, 2014 8:09 am UTC

I found this but I think you're right.

Image


What you actually care about is if the heat inside the envelope stays constant for my case.

Tyndmyr
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Re: Aerogel

Postby Tyndmyr » Tue Aug 12, 2014 4:42 pm UTC

Aright, now I'm getting curious...don't have any around to test on at the moment, unfortunately. Tensile strength seems like the other potential hangup. The only numbers I could find were here, which seems...as a ballpark estimate, lower than you'd want(materials such as nylon or kevlar are FAR higher). Especially combined with a potential for fracturing. You can somewhat mitigate this by adding more, but...if you're putting significant load on this, it could be sketchy.

There's ways around that, though. I *believe* it has decent compressive strength(though I can't find hard numbers), so, with the right frame, you could probably still make it work.

Now, where to find aerogel in reasonable quantities for a price that isn't impractical...

Arminius
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Re: Aerogel

Postby Arminius » Tue Aug 12, 2014 9:41 pm UTC

No no, not Silicia aerogel. Other types of aerogel are better suited. Pyrogel is what I talked about. Have a look at this: http://www.insulationfabricators.com/downloads/data-sheets/aspen-aerogel-pyrogel-xt-e-ds.pdf

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cjameshuff
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Re: Aerogel

Postby cjameshuff » Tue Aug 12, 2014 11:10 pm UTC

You're overcomplicating things. The reason you want aerogel is as an insulator, you don't need it to have any tensile strength or to be airtight...just line a relatively conventional hot air balloon with insulating material.


Arminius wrote:No no, not Silicia aerogel. Other types of aerogel are better suited. Pyrogel is what I talked about. Have a look at this: http://www.insulationfabricators.com/downloads/data-sheets/aspen-aerogel-pyrogel-xt-e-ds.pdf


http://www.apexballoons.com/balloons/

"Most standard factory-built balloons are constructed from a coated ripstop nylon fabric that weighs approximately 1.9 ounces per square yard"

That's 64 g/m^2. From the numbers on that datasheet you linked, Pyrogel insulation weighs around 1000 g/m^2.

Aerogels can be extremely light, but not everything that uses them is also as light. Thinness and ease of handling seem to have been bigger priorities for the people designing Pyrogel.

Arminius
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Re: Aerogel

Postby Arminius » Wed Aug 13, 2014 9:41 am UTC

Yes indeed. However if I line the conventional hot air balloon, it will just add weight. If I use the insulator as the envelope itself, it will save weight.

Furthermore, the use of pyrogel allows you to go to higher temperature and hence, produce more lift. (nowhere near helium, if I remember right I calculated around 30000°C to get to the lift of helium).

One last thing... I think your comparison between nylon and pyrogel is not fair. The pyrogel fabric they sell is between 5mm and 10mm wide. Way more than the width of nylon envelopes (can't find the width but intuitively).

Edit: Well well, somebody got the same idea. http://www.festo.com/rep/en_corp/assets/pdf/Iso_Ballon_en.pdf

It looks like parachute is the same material as parachutes: http://www.parachutefactory.com/03_mat/mat.php?cnum=41

Fabric/Cloth; Nylon Ripstop, 36"/48"
Thickness: less than 0.101 mm
Weight: less than 54.25 g/m2 (1.60 ounces)

We can approximate slightly more width but lets say for simplicity sake 0.1mm.
5->0,1 divide by 50.
1000/50= 20 g/m^2

Quite competitive no?

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cjameshuff
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Re: Aerogel

Postby cjameshuff » Wed Aug 13, 2014 10:52 pm UTC

Arminius wrote:Yes indeed. However if I line the conventional hot air balloon, it will just add weight. If I use the insulator as the envelope itself, it will save weight.


Not if it's not structurally suitable for an envelope. This pyrogel is just an unwoven fiberglass mat to give an aerogel based insulating layer enough integrity to make it easier to handle. It's not fundamentally any different from lining a balloon, it just starts with materials and construction that aren't very well suited to balloons.


Arminius wrote:Furthermore, the use of pyrogel allows you to go to higher temperature and hence, produce more lift. (nowhere near helium, if I remember right I calculated around 30000°C to get to the lift of helium).


No, the use of an insulator allows that. Pyrogel wouldn't even let you build a balloon that got off the ground.


Arminius wrote:One last thing... I think your comparison between nylon and pyrogel is not fair. The pyrogel fabric they sell is between 5mm and 10mm wide. Way more than the width of nylon envelopes (can't find the width but intuitively).

Edit: Well well, somebody got the same idea. http://www.festo.com/rep/en_corp/assets/pdf/Iso_Ballon_en.pdf

It looks like parachute is the same material as parachutes: http://www.parachutefactory.com/03_mat/mat.php?cnum=41

Fabric/Cloth; Nylon Ripstop, 36"/48"
Thickness: less than 0.101 mm
Weight: less than 54.25 g/m2 (1.60 ounces)

We can approximate slightly more width but lets say for simplicity sake 0.1mm.
5->0,1 divide by 50.
1000/50= 20 g/m^2

Quite competitive no?


No. Pyrogel is not a high tensile strength, low weight rip-proof material suitable for use in hot air balloons, it's a high performance insulation blanket material. It's unlikely that even the fiberglass batting that serves as structural support could be made 0.1 mm thick. Even if it could be scaled down to such a thickness, you would lose the majority of your insulation qualities in the process, not to mention the structural strength needed to contain the hot air and support its own weight and that of the payload.

An insulated balloon is not a bad idea, Pyrogel is just not the material to use for it. That other paper you linked was about experiments using a multi-layer membrane material called Aerofabríx, with an aluminized outer layer and inner flocking:
http://www.ballontec.de/seiten/ecomagic/ecomagic.pdf

There are other alternatives for handling higher temperatures, such as Nomex (which actually is used in parts of some current hot air balloons). Some high-strength, high-temperature resistant polymer fiber and insulating/reflecting layers seem like the way to go. Something closer to aerogel than Pyrogel might be useful, but handling would be much more difficult.

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drachefly
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Re: Aerogel

Postby drachefly » Thu Aug 14, 2014 2:10 am UTC

Arminius wrote:Furthermore, the use of pyrogel allows you to go to higher temperature and hence, produce more lift. (nowhere near helium, if I remember right I calculated around 30000°C to get to the lift of helium).


Off the top of my head, it would be (14*2/4)*300 = 2100°C for high temperature N2 to be as light as ambient He.

Arminius
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Re: Aerogel

Postby Arminius » Thu Aug 14, 2014 6:12 am UTC

My bad. One 0 too much. It's 3000°C.
2100°C gets you 96% percent lift.

But I suppose 40°C outside temperature. To account for worst case scenario. At 20°C you are right indeed.

billy joule
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Re: Aerogel

Postby billy joule » Thu Aug 14, 2014 7:14 am UTC

Isn't hot air ballooning basically a hobby with little practical purpose?

Sure, a better insulated balloon will have greater range & lower fuel costs but the order/s of magnitude increase in volume will make transport and storage difficult to say the least..

Arminius
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Re: Aerogel

Postby Arminius » Thu Aug 14, 2014 10:39 am UTC

cjameshuff I get your argument about high-tensile strength needed and I think you are right. Indeed, other materials might be more suited.

billy joule, it is true that it has nowhere near the lift at regular temperature, but if you get to higher temperatures, it would work better. Around 600°C would help a lot already.

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Re: Aerogel

Postby billy joule » Sat Aug 16, 2014 1:47 am UTC

Arminius wrote:billy joule, it is true that it has nowhere near the lift at regular temperature, but if you get to higher temperatures, it would work better. Around 600°C would help a lot already.



I mean the deflated, storage volume of the balloon. e.g. Most balloons are transported in the basket in a horse float sized trailer which is towed by a pickup. No one will want a balloon that needs a semi truck for transport, no matter how much they'll save on balloon fuel.

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cjameshuff
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Re: Aerogel

Postby cjameshuff » Sat Aug 16, 2014 1:30 pm UTC

billy joule wrote:I mean the deflated, storage volume of the balloon. e.g. Most balloons are transported in the basket in a horse float sized trailer which is towed by a pickup. No one will want a balloon that needs a semi truck for transport, no matter how much they'll save on balloon fuel.


If it can carry a decent propulsion system without being a fuel hog, there isn't much need to transport it by truck. Think of an advanced thermal airship, not just an insulated hot air balloon.

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Re: Aerogel

Postby Tyndmyr » Mon Aug 25, 2014 7:16 pm UTC

billy joule wrote:
Arminius wrote:billy joule, it is true that it has nowhere near the lift at regular temperature, but if you get to higher temperatures, it would work better. Around 600°C would help a lot already.



I mean the deflated, storage volume of the balloon. e.g. Most balloons are transported in the basket in a horse float sized trailer which is towed by a pickup. No one will want a balloon that needs a semi truck for transport, no matter how much they'll save on balloon fuel.


Sure, rigid and semi-rigid airships have seen commercial use before...if the performance improvements are sufficiently great, it's worth the hassle in at least some circumstances.

However, yes, the additional weight/insulation is likely not worth it, and the lack of tensile strength means you can't displace the existing envelope with it, so no real offset there. It's a fun mental exercise, but yeah, you probably need to widen the materials under consideration. It's likely that professionals in the field have already done this, of course, but hey...still interesting to consider.

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Re: Aerogel

Postby Sstpd » Wed Apr 19, 2017 11:23 am UTC

Sat in a bar in Vang Vieng, Laos, and saw a couple of hot air balloons go by. I remembered that aerogels were a thing, and pictured them being useful as a replacement for nylon on a balloon, given the light weight and thermal insulation. A quick google on the air permeability of aerogels led me to this interesting discussion on the same idea, had years ago by strangers.

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Re: Aerogel

Postby Soupspoon » Wed Apr 19, 2017 1:45 pm UTC

It lets me jump in and say (in response to early in the thread) that it depends if the aerogel 'foam' is closed-cell (loads of complete bubbles, forming discrete enclosures) or open-cell (the bubbles have mostly been opened to each other, face-to-face, just leaving the 'corner structures' to form the structure of the end product.

My suspicion (without checking) is that aerogel is purposefully manufactured to retain just the structural 'corners' and 'edges', without the mass of the 'faces' in the matrix, like an aluminium strut with strategic holes along its face to save weight, thus is 'open'.

The navigation of gases through the gel would be impeded, but not really stopped (unless you lined it/left it lined, at least on the high-pressure side, with an impermeable membrane that was supported against the pressure by the honeycomb part), whereas a closed-cell version would impede gas flow as a multiple of the basic permeability of each bubble-on-bubble, as leakage through each interface sets up a pressure gradient that the 'next' bubble-on-bubble out will leak even less, etc. Even if you can't stop helium, for a given mass, the greater thickness might (I'm guessing) do better at being impermeable than the same 'blocking matrix' squished thinner.

It'll also be better thermally, of course, as the temperature gradient is lessened, and relies upon even rarer movements of fast gases around.

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Re: Aerogel

Postby p1t1o » Wed Apr 19, 2017 2:46 pm UTC

Aerogels are generally manufactured in a "subtractive" (correct word?) manner. In that some intermediate product/substance is formed and on exposure/treatment with a solvent, the aerogel "skeleton" is left whilst the substrate is removed.

Im not sure if this generally results in a closed or open cell structure, but it strikes me as relevant.

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Re: Aerogel

Postby Xanthir » Thu Apr 20, 2017 12:34 am UTC

By definition, a subtractive method must leave an open-cell skeleton.
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Re: Aerogel

Postby p1t1o » Thu Apr 20, 2017 8:34 am UTC

Xanthir wrote:By definition, a subtractive method must leave an open-cell skeleton.


Now that I think about it that does seem obvious.

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Re: Aerogel

Postby Sockmonkey » Wed Apr 26, 2017 9:23 am UTC

Assuming that an aerogel can withstand 15 psi without crushing, you could actually do the "vacuum blimp" concept by making a thin shell of aerogel covered with mylar.

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Re: Aerogel

Postby Quizatzhaderac » Thu Apr 27, 2017 7:34 pm UTC

So apparently someone has had this idea before. I'm not really seeing much on actual aerogel on the site. I think they produce a coating for resealing old balloons that is maybe kinda-sorta an aerogel?

@Arminius: For your purposes I'd say the permeability of the aerogel doesn't matter once one knows it's thermal conductivity. I mean, yes fluid conductivity can be a source of thermal conductivity, but you just need to worry about how quickly the hot air in the center becomes cold.

Regarding tensile strength: So from looking up hot air balloons on wikipedia, the envelope seems to consists of a frame (or load tapes, or webbing) and the sections within those frames (or gores).

Some parts of the webbing will obviously need to be able to hold the whole weight of the basket/gondola. But I don't think the gores need be able to hold any more weight than the balloon's dead weight * gore's area/ envelope's area. If the aerogel is too weak for standard size gores, I'd expect smaller gores could be created without adding too much extra material.

The reason the gores are as solid as they are, seems to be a matter of preventing rips, rather than regular use. If they issue of rips could be ignored, it would seem an envelope with a fiberglass webbing and aerogel gores would be lightweight and able to operate at much higher temperatures.
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Re: Aerogel

Postby p1t1o » Fri Apr 28, 2017 7:54 am UTC

Quizatzhaderac wrote:So apparently someone has had this idea before. I'm not really seeing much on actual aerogel on the site. I think they produce a coating for resealing old balloons that is maybe kinda-sorta an aerogel?


Irritatingly enough, I think their name is a smashing-together of "angelic" with an "aero" prefix. Don't think they do anything with aerogel.


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