[ravenstine]

How could hydrogen be a feasible approach to lifting any amount of cargo if aerogel of all things is too heavy? I just find it hard to believe that even the volume of aerogel required to support enough hydrogen to lift a useful amount of cargo would approach even a tiny fraction of the weight of the cargo itself, but perhaps I'm not imagining things correctly.

[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?

[dTal]

Usually when you lift cargo, the volume of the cargo is tiny compared to the volume of the lifting gas. Picture a huge blimp with a tiny gondola underneath.

With an aerogel infusion, the volume of the aerogel is the same as the volume of the lifting gas. Picture a huge blimp lifting a huge-blimp-sized lump of something underneath. Even aerogel, light as it is, adds up.

In other words - it's barely useful, and the only reason it works is because of the square-cube law: the weight of the envelope goes up as the square of the dimension, while the lifting power goes up as the cube. If you want to fill the volume with something, you lose the scaling advantage.

The fact that an aerogel-filled blimp might still conceivably be neutrally buoyant is actually testament to how incredibly light aerogel is. If you filled the Hindenburg with water, it would be more than half the weight of the Empire State Building.

[ravenstine]

> The fact that an aerogel-filled blimp might still conceivably be neutrally buoyant is actually testament to how incredibly light aerogel is.

True!

I've always had a soft spot for aerogel. I discovered it as a kid back around 2000 when Disneyland had this NASA/JPL Mars rover exhibit and they had a piece of aerogel on display. When I got home I researched it and was really impressed by all its properties. My mom got me some broken pieces of it off eBay (at the time this was way cheaper than buying an intact slab). My parents were pretty cool in nurturing my nerdiness. hah

The more I think about it, the more it seems like aerogel in a hydrogen balloon isn't the best idea for other reasons. If any moisture makes its way into the balloon, I bet it wouldn't take long for the density to suddenly drop catastrophically.

[lenzm]

The basic principle at work is the square-cube law. The mass of an airship enclosure grows 2-dimensionally while the volume and lifting power grow 3-dimensionally. If you use any 3-d structure to support the airship you will have problems.

https://en.wikipedia.org/wiki/Square%E2%80%93cube_law