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Comment by echelon | Hacker News Reader

Helium is extremely limited and cannot be manufactured at any appreciable scale (you're not dealing with typical chemistry, but physics. Helium is an element.)

What we have naturally comes from radioactive decay deep in the earth. This will continue for some time, but it's a fixed, slow rate.

Helium evaporates into space. Once it's gone, it's gone for good.

The other options we have are hydrogen (extremely reactive), heated oxygen (also reactive), methane (reactive), ammonia...

It's a hard problem.

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Don’t forget about “null” gas.

Better the hydrogen, a few small technical problems aside.

https://en.m.wikipedia.org/wiki/Vacuum_airship

That won't work on earth due to weight of materials and material strength requirements. It might work on a dense atmosphere, such as Venus.

> It might work on a dense atmosphere, such as Venus.

Gravity also matters here.

Implosions would be much worse imo.

One more problem that will be solved by fusion energy!

> Helium is an element

Nitpick: So is hydrogen.

Yeah, but only one of them is a noble gas that is rarely bound into molecules that would be found in solid form. As a consequence, it tends to just leak out of the atmosphere without a lot of sources for replenishment.

Yes.

More precisely, helium is an element that doesn't readily form compounds. Hydrogen does, and as such can be extracted from, say, water via electrolysis.

The secondary issue with helium is that it's much lighter than air -- this is why helium and neon are both much rarer than, say, argon (which is actually the third-most common element in the air after nitrogen and oxygen).

> The secondary issue with helium is that it's much lighter than air

Actually, hydrogen is lighter, and so a better lifting gas, than helium.

https://www.wikiwand.com/en/Lifting_gas#/Hydrogen

Yes, but while hydrogen (H) has half the mass of helium (He), H is only 8% better than He in providing lift in the air, if I computed correctly (as that is generated from displacing much heavier air), so it's not a huge deal:

He: 0.1786 kg/m^3

H: 0.08988 kg/m^3

Air: 1.225 kg/m^3

Air vs He: 1.0464 kg/m^3

Air vs H: 1.13512 kg/m^3

To put it differently, for helium, the lift is about 86% of the weight of the displaced air, while for hydrogen it's 93%.

Both very good, and both are much better than hot air!

(which gives a lift of only about 25% of the weight of the displaced air, at typical temperatures of about 120 deg C)

According to the wikipedia page I linked to:

> In a practical dirigible design, the difference is significant, making a 50% difference in the fuel-carrying capacity of the dirigible and hence increasing its range significantly

But I wouldn't know :)

Oh, very interesting. The linked TIME article from 1924 (!) gives a bit more detail:

> While helium is exceedingly light as compared with air, it is somewhat heavier than hydrogen. The total lift of a helium-filled dirigible is accordingly some 10% less than that of the hydrogen-filled airship. The difference does not appear important at first sight, but the total lift of the gas carries the structure, the motors and the crew. It is only the last 20% or so that is available for carrying fuel, and hence a difference of 10% in the gross lift may spell a difference of 50% in the fuel-carrying capacity. On long-distance flights this difference is vital.

> Nor is the danger of fire totally eliminated with the use of helium; the gas-tanks and the fuel system generally are still vulnerable. But when a ship is properly designed and carefully handled, the danger of fire is comparatively small, even with hydrogen.

https://web.archive.org/web/20110209024932/http://www.time.c...

Note that a contributing problem to the Hindenburg disaster was the exterior canvas paint was chemically about the same as rocket fuel.

That's what I remembered, so I was surprised it wasn't mentioned in TFA.

> Both very good, and both are much better than hot air!

No.

Hydrogen is highly explosive/flammable — that is why Helium is preferred.

Yes, both helium and hydrogen are much lighter than air.

But that doesn't significantly impact rarity of hydrogen, as it reacts very readily and is typically found in compounds.

You don't find much hydrogen gas in nature (~1ppm in the atmosphere, vs 5ppm for helium).

We can theoretically do vacuum zeppelins, which is of course lighter than helium and unlimited tho it has its own problems too.

I was curious about this and it looks like it's currently impossible to have a material that is both strong enough and light enough to support a vacuum balloon:

https://physics.stackexchange.com/questions/71027/is-it-poss...

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