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Comment by joezydeco | Hacker News Reader

So if we wrap this material in mylar and remove the air, we'd have solid balloons without needing helium? Sounds good to me.

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Sadly, it is highly compressible, so air pressure would just crush the mylar ballon around it. If you could make this material rigid, so the balloon would be able to displace the proper volume of air, then you could make party balloons and airships without wasting helium.

Wouldn't it be just as easy to make a lightweight shell around a vacuum?

Helium and hydrogen at STP are, for the most part, insufficiently different from a vacuum to make one desirable for lifting things.

So basically, what you're saying is hoverboards are possible.

No, because hoverboards are highly positively buoyant when below a fixed distance from the ground, and negatively buoyant when above it. But lighter-than-air vehicles are equally buoyant at all distances above the ground (unless you run out of atmosphere).

That's actually true of LTA vehicles, too, since the atmosphere gets progressively less dense the further from Earth you are.

It's just that the distance above ground at which the buoyancy changes sign is much, much higher :)

fit it with an altimeter so that once it is a meter above the ground it takes in air?

But they should work over water too. ;-)

What would be the economic implications of "float stone", that is a solid material which is lighter than air or at least neutrally buoyant? Say a molecular structure that creates pockets of vacuum at the micro scale.

I wonder if you would create a hazard for rockets, once enough float stones had been dropped and were drifting around whatever layer of atmosphere they were neutrally buoyant with respect to.

I guess it might not be a hazard at all. Firstly, they wouldn't be traveling at orbital velocities, and secondly, since the premise is that they are the same density as the air, hitting a float stone at Mach N might not be any worse than hitting the air at Mach N.

> since the premise is that they are the same density as the air, hitting a float stone at Mach N might not be any worse than hitting the air at Mach N.

Dive into a swimming pool filled with water. Safe, harmless fun!

Dive into a pool filled with solid ice. Ice is 10% less dense, so ten percent more safe and harmless and fun, right?

Density is not the most important physical quality here.

Strap one on your back and you can swim in the atmosphere like a diver in water.

Although it would have to be over 66 cubic meters for an 80kg person in 20°C air. If it is spherical, it would be c.a. 5 m in diameter.

I would make mine in the shape of a giant hand. Then I'd take a picture and send it to Peter Molyneux.

Thanks for the napkin calculation. How does that compare to the volume required for the amount of helium or hydrogen that would be required to achieve the same buoyancy?

This new material is 0.16 kg/m3, presumably if it's filled with vacuum.. Helium is 0.17 kg/m3 at 20°C and hydrogen is 0.09 kg/m3. At the same time, air is 1.2 kg/m3.

So, the volume of each needed to lift 80kg (which would have to include the person and whatever structure to contain the material), is 80/(1.2 - density of material) m3. So:

Helium: 78 m3, or sphere of diameter 5.3 m

Graphene aerogel: 77 m3, or sphere of diameter 5.3 m

Hydrogen: 72 m3, or sphere of diameter 5.1 m

Floating buildings? Floating roads?

Crazy things ...

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Great idea! How to control buoyancy? I guess you need some ballast to keep the whole system close to air density, and a pump to put / remove air, for fine tuning depending on conditions (temperature, ...) and what you want to do (go up / down)

Helium balloons don't need all that. They get by with significant positive buoyancy and a simple string. Seems reasonable for any balloon replacement to do the same.

I was assuming the material is rigid (which is wrong). So my idea (continuing the suggestion of parent) was to put / remove air from the surrounding volume to the mylar balloon (which means, in the aerogel) to modify the density of the system and thus the buoyancy. This would require energy to run the pump, so it is not clear this would be more efficient than normal air balloons (but would be less dangerous, since no fire is needed).

Of course, nothing mentioned here makes any sense since the material is not rigid.

And sorry, I was referring to air balloons (as "flying machines"), not to party balloons. Those are indeed very simple, as you mention.

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