[jrexilius]

It got me wondering if the increase lift capacity would be added factor, but quick calculation shows that it would only gain an extra ~1% of lift. So it would really just be a cost question: https://en.wikipedia.org/wiki/Lifting_gas

[ryandvm]

Totally unrelated, but a fascinating tidbit from that wikipedia link:

> Venus has a CO2 atmosphere. Because CO2 is about 50% denser than Earth air, ordinary Earth air could be a lifting gas on Venus. This has led to proposals for a human habitat that would float in the atmosphere of Venus at an altitude where both the pressure and the temperature are Earth-like.

[zokier]

Not just on Venus; Buckminister Fuller proposed to build floating cities on earth:

https://en.wikipedia.org/wiki/Cloud_Nine_(sphere)

[JumpCrisscross]

How much would a mile-wide geodesic sphere cost to build?

[jiggawatts]

That’s an interesting What If? question!

One cost-reduction approach would be that the beams could be mass-produced for efficiencies of scale. The top panels could be simply plastic sheeting.

[Gravityloss]

You could make it out of UHMWPE, the best current strength to weight material available. It's a plastic with density slightly under 1 kg/L. And it's UV resistant. Joining the parts is the real problem though. You could make it out of continuous thread in single piece with 3d knitting to avoid glue or fasteners. But all kinds of problems would arise. Also repairability would be bad. So probably you would use some sections and glue.

[bawolff]

This seems like the perfect premise for a scifi novel. I wonder why we don't see it more.

[chris1993]

Charles Stross has used it in a couple of novels: 'Saturn's Children' and 'Accelerando.'

[thechao]

I tried to convince a group that:

1. Hot air balloons get their lift from hot air; and,

2. Dirigibles can get their lift from hydrogen; thus,

3. Hot hydrogen dirigible?

Blah blah blah ... exploding dirigibles ... blah blah blah. The lifting efficiency is outrageous.

[Gravityloss]

Even if you used weightless atoms to fill your balloon, the lift is only the weight of the air it displaces. So you don't really get much better since hydrogen already weighs so little.

Air is about 30 grams per mole and hydrogen is two grams per mole. So going entirely weightless you could go from 28 grams of lift per mole to 30 grams of lift per mole, a 7% increase.

[thechao]

But 100% more firing a flamethrower into your hydrogen filled envelope while looking your customers in the eye.

[scoopertrooper]

Just crazy enough to work. Hydrogen doesn't autoignite till 500 degrees celsius, while your average hot air balloon only heats gas to 100 degrees.

You'd want to be damn sure about the reliability of your thermocontroller though.

[b112]

I think the fuels, and components to heat without flame, might reduce the benefit a lot.

I have often wondered about super strong materials, a sphere with vacuum inside.

[FlyMoreRockets]

I've often wondered if a double envelope would be worthwhile. Use a thin outer envelope containing He, and an inner lifting envelope with H. The outer envelope would contain any H that leaked through the inner membrane.

[chriswarbo]

Unfortunately, Hydrogen doesn't lift as much compared to Helium as we might expect, since it occurs as H2 molecules rather than isolated atoms.