[jodrellblank]

Airship Hindenburg had 200,000 cubic meters of lift gas[1].

Air masses 1.2 Kg per cubic meter, so 240,000Kg of air in that volume normally.

Helium masses 0.18Kg per cubic meter[2], so replacing that volume with Helium gets it down to 36,000Kg.

Hydrogen masses 0.08Kg per cubic meter[2], so replacing that volume with Hydrogen gets it down to 16,000Kg.

Huge balloons containing almost-nothing, as soon as you replace the inside with something it gets heavier. Aerogel is 1Kg per cubic meter without the air in it, says Wikipedia. So adding AeroGel to Hydrogen it would be 216,000Kg in that volume displacing 240,000Kg of air. Hardly buoyant at all.

Wikipedia has something called AeroGraphene mentioned[4] which is down to 160g per cubic meter. If that could be scaled up to the same volume with vacuum in it, it would be 32,000Kg and filled with the mass of Hydrogen, 48,000Kg, but that's still less buyoant overall than using Helium lift gas. Hydrogen isn't really "a feasible approach to lifting any amount of cargo", if it was then airships would be everywhere. The rest of the structure of the ship was heavy in the Zeppelin days, leaving little extra lift for people or things. Out of a Whitehouse sized vehicle it could lift low hundreds of tons. Maybe better today with carbon fibre and lightweight engines and such.

[1] https://www.airships.net/hindenburg/size-speed/

[2] https://www.aqua-calc.com/calculate/volume-to-weight

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

[4] https://en.wikipedia.org/wiki/Aerographene

[nousermane]

This. Also, even if aerogel tech improves drastically, there is another, more insidious problem.

Small-molecule gases diffuse through solid matter (search for "helium leak iphone" if you haven't read that fascinating story), so hydrogen will be slowly leaking out of aerogel, much like it does leak from a normal balloon.

Air diffuse back inside at a much, much slower rate - with net effect that pressure inside drops. This will crush/crumple aerogel, with no easy "top it off" option available for normal balloons.

[goohle]

But we just need a thick but lightweight shell, which is hard to puncture, not a whole volume made with aerogel.

[freemint]

You are aware that aerogel is porpous?

[dTal]

Indeed. Popular materials for this "envelope" are nylon fabric or mylar =)

[goohle]

Never heard about 10cm thick mylar or nylon fabric envelope.

[ravenstine]

Wow, great explanation! Kind of disappointing since it sounds like an awesome solution, but now this makes sense to me.

[m4rtink]

It would still have some compressible strength, right ? While not really being crushed under its own weight by default.

So couldn't you build, say a giant pyramid reaching out into the stratosphere using hydrogwn infused aerogel ? :)

[jodrellblank]

You could build a giant space pyramid out of anything in principle, according to Isaac Arthur on YouTube[1]. The limiting factor quickly becomes the size of "giant" needed for most materials and the limited size of Earth to have a wide enough base. Maybe being Hydrogen-infused would mean less pressure on the lower levels and able to build it higher.

Really though, doesn't AeroGel work by being mostly nothing? The more gas and less substance it has, at some point you may as well use a cloth balloon and only have the surface area size, no?

[1] In this video on Space Elevators - https://www.youtube.com/watch?v=dc8_AuzeYKE

[m4rtink]

Yeah, in principle you could make the whole thing inflatable as well, with some compartmentalization here and there to avoid catastrophic single point failure.

[Hypx_]

You have to be careful with the calculation here: The airship must also include the weight of the structure, and aerographene could potentially need much less structure than giant gas balloons.

[jodrellblank]

I don't know how strong AeroGraphene might be, but is it a reasonable approximation to say that these are like a sponge and the way they get less dense is to have more larger voids and less solid matter; that is, the lighter they are, the weaker they are?

AeroGel.org says: "Aerogels can usually hold a gently applied load of up to 2,000 times their weight and sometimes more. But since aerogels are so low in density, it doesn’t take much force to achieve a pressure concentration equivalent to 2,000 times the material’s weight at a given point. The amount of pressure required to crush most aerogels with your fingers is about what it would take to crush a piece of Cap’n Crunch® cereal." - https://www.aerogel.org/?p=3 (it goes on to say Aerogels differ in strength, but most can be made stronger by making them denser and heavier).

I have seen a suggestion, probably in a HN submission, that remaking classic Zeppelin design but replacing the duralumin structure, metal tensioning wires, coated cloth outer shell, and animal guts lift bags with modern carbon fibre, kevlar, and foils could knock 80-90% off their mass. What is all the structure in a Zeppelin doing, why can't they be shaped like hot-air balloons?