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Comment by simonebrunozzi | Hacker News Reader

I'm super ignorant on physics and whatnot; but wouldn't it make more sense to use airships covered with solar panels at 5k-6k meter altitude? Thin solar is now a thing, and I'm quite sure that there would be a viable way to engineer an airship in such a way. One could even use graphene aerogel instead of helium or hydrogen as the lighter-than-air filler.

Is this a really dumb idea? Any of you with a more relevant background could tell me where this could be so wrong?

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There have been proposals to use things at that kind of altitude for power, but the ones I've seen have been for wind power rather than solar power.

The idea is to have something that is tethered to the ground and held aloft by the winds at high altitude and that has wind turbines to generate power, which is sent back to the ground by cable.

There are pretty much always strong winds over much of the US at 5k-6k meters, so such a wind power generator would provide 24 hour a day power most of the time.

Here's a site showing the wind over the at around the height [1]. Click where it says "earth" in the lower left to change the height. The height is given in pressure. My link is for 500 hPa pressure which is somewhere around 5500 m. The 700 hPa option would be around 3000 m, and the 250 hPa option would be around 9000 m. Here's a site with a graph of pressure vs height if you want to know the heights for the other pressure options [2]. That graph uses kPa. 0.1 kPa = 1 hPa.

The proposals I've seen I've proposed a few different things to fly. One is basically just a kite carrying turbines. Another is a parasail. Another is some sort of rotor aircraft similar to a quad drone. I believe I've also seen some that use fixed wing aircraft with propellors.

The ones with the rotors or propellers would have electric motors powered from the ground that can be used to get the thing up to operating altitude, and then they switch the motors from taking electricity to drive the rotors/props to letting the wind drive the rotors/props to generate electricity.

[1] https://earth.nullschool.net/#current/wind/isobaric/500hPa/o...

[2] https://www.engineeringtoolbox.com/air-altitude-pressure-d_4...

Makani (Acquired by Google and shut down) built large airplane/kites which transmitted wind power to the ground via a tether. The engineering challenges were... intense. This was all documented extensively and this documentary summarizes all the footage they took: https://www.youtube.com/watch?v=qd_hEja6bzE

One of my favorite engineering projects and documentaries, just because of the sheer ambition and scale, and the grit of the team to keep pushing through repeated failures. Sad the project was cancelled but it makes sense given the cost and complexity.

There are also designs which keep the generators on the ground, and turn it by spooling/unspooling the tether e.g. https://en.wikipedia.org/wiki/SkySails

Seems like some of the bad parts of terrestrial solar (fighting atmospheric conditions, wind loading, etc) without the good parts of orbital solar (once you're in orbit, staying there is relatively easy and you have very little in the way of mechanical stresses for your solar array, so you can make it really huge). I might not be thinking about it hard enough.

> 5k-6k meter altitude

That's not high enough to put you above bad weather (airliners fly higher, and they divert around storms.) Bad weather is bad news for airships and historically destroyed about as many as hydrogen fires. You'd need to bring your airships down into hangers when the weather got bad.

Also, wouldn't the graphene aerogel be filled with air and be heavier than air? Aerogel doesn't just float away. Unless you mean for these to be vacuum airships, but those seem very far fetched.

I don't know if a lighter than air brick of aerogel has ever been manufactured. Would helium in the crevices generate enough lift to make a neutrally or negatively buoyant solid?

If so, I want one.

You can get one today, it’s called a balloon

Helium and hydrogen would immediately leak out of an aerogel.

> Vacuum airship

Wow what a fascinating idea! Just thinking out loud here, but the compressive force is proprtional to area, while the boyancy is proportional to volume. So I wonder if making it big enough would allow using some fairly standard materials. What about a cube made of 6 big (huge) sheets of some strong yet light material supported by a sort of skeleton inside?

Or maybe we can use electro-static repulsion to keep it from imploding? Like how a normal balloon compresses the air inside because the rubber tries to shrink but if you had a balloon that tried to expand instead, because of say electric repulsion, or maybe something else, then it would put the air inside at lower than atmospheric pressure.

Last time I read about that, the problem seemed to be buckling [1]. E.g. if you look at Euler's critical load [2] then the pressure that a slender column can withstand just depends on the aspect ratio of the column (imagine a vacuum airship that uses internal columns to withstand the air pressure). So scaling things up means, that the column's weight has to scale proportionally with total airship volume (and the (air) pressure is a constant anyways). See also these discussions here [3]. Disclaimer: I don't claim to understand any of this.

[1] https://en.wikipedia.org/wiki/Buckling

[2] https://en.wikipedia.org/wiki/Euler%27s_critical_load

[3] https://en.wikipedia.org/wiki/Vacuum_airship

I did check the wiki article on Vacuum airships before writing my initial comment, and they are talking about the buckling of a hollow sphere, not of a column - hence why Akhmeteli and Gavrilin suggest considering alternative structures.

But regarding your point that the columns would have to scale with volume you might be right about that. I was thinking that it should scale with the total force, but the bigger your structure the longer distance the force has to be transmitted. So yeah I think I got that one wrong.

For some better context using numbers everyone can understand: 5 km to 6 km is ~16,400 feet to ~19,700 feet. Airliners normally fly around 35,000 feet give or take a couple thousand. Military aircraft can fly upwards of at least 50,000 feet.

Some of us are in countries that use the metric system. So "everyone" is quite a sweeping statement.

The aviation industry uses feet for measuring altitude. Doesn't matter if you're a metric or imperial fanboi, the standard is feet for measuring altitude.

Only a handful of countries use meters for measuring altitude, and as far as I'm aware they are feeling the pressure to reform to using feet for sake of consistency (read: safety).

Besides, denoting both meters and feet like I did will guarantee everyone will understand one way or another.

Fair enough, except that we aren't talking about the aviation industry here.

Except we are, airships are part of the aviation industry unless I'm missing something.

Biggest advantage of solar in geostationary is that you get power 24/7, so you don't need a lot of storage. You're not in shadow at all except for a little bit around the equinoxes. Total uptime is 99.5%.

Is graphene aerogel lighter than air? I thought it _was_ mostly air, which is why its so light.

About 7 times lighter than air, AFAIK.

https://en.wikipedia.org//wiki/Aerographene It is approximately 7.5 times less dense than air. Note that the cited density does not include the weight of the air incorporated in the structure: it does not float in air.

You made me smirk with the combination of "I'm super ignorant on physics" and "I'm quite sure that there would be a viable way to engineer an airship in such a way" (:

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