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.