Skybergs

Nov. 20th, 2013 03:36 am
dichro: (Default)
[personal profile] dichro
I haven't given up on the whole floating thing. The notion that's been rolling around my mind of late is aerogel-filled balloons. It isn't a new idea, and even SEAgel is easily floatable at STP.

But the more interesting compound is the recently synthesized aerographene, which not only boasts an absurdly low density, but remarkably high compressive strength.

One of the things mentioned by smarter balloon people than I was the magnitude of the thermal stresses caused by solar heating. You have an envelope floating around, filled with gas, when the sun comes up. In short order, the sun heats the envelope and gas, raising the pressure inside significantly. Your envelope either bursts or vents. If it vents, then when the sun subsequently goes down again, envelope temperature will return to equilibrium, and the envelope will shrink, causing its lifting power to be dramatically reduced, thence likely plummeting to earth, dragged down by the weight of your payload.

If the envelope is filled with aerogel, two things come into play. One, the aerogel resists being compressed, meaning that the envelope will be at a slight negative pressure to the outside air, boosting buoyancy. The aerographene paper is irritatingly vague on compressive modulus, but it does cite two useful facts: compressive modulus is 8 kPa at a density of 5.1 mg cm-3 and varies with the square of the density. Since the paper claims that the lowest synthesizable density is 0.16 mg cm-3, one could handwave, but it's too hard to run up a spreadsheet at 3am to verify my suspicion that that's too small an effect to make much difference. I think the density of aerogel required to make a meaningful difference at any particular pressure significantly outweighs the air it's saving.

The other potentially useful property is insulative. These payloads all have electronics inside them, and electronics generate heat. Nesting the electronics inside an aerogel balloon might result in a high enough temperature to make a meaningful difference. 3am is once again preventing me from contemplating Stefan-Boltzmann's law and the diurnal stratospheric temperature curve with the focus those calculations require, insomnia notwithstanding.

There's a few other interesting points in the aerographene paper. The synthesis doesn't appear enormously complex to my ignorant eyes (although the synthesis of giant graphene oxide sheets, a precursor, doesn't fill me with enthusiasm). The raw materials aren't enormously expensive for experimental quantities. Lastly, the paper claims that production can be easily scaled to volumes of m3.

I dream of people using their swimming pools to grow aerographene skybergs, attaching a lawn chair, and floating away.

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Miki Habryn

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