What would be a lighter-than-air gas that is also not flammable, and would be fairly easy to produce in a modern-style world?

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    $\begingroup$ What's wrong with helium? $\endgroup$ – AlexP Sep 7 '19 at 22:40
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    $\begingroup$ Hard to get? Helium is a by-product of oil and gas extraction. It costs about 300 US dollars for a million standard cubic feet. (It used to be cheaper because the USA was selling off a mind-bogglingly huge century old strategic reserve, which they had put together at the beginning of the 20th century when they thought that airships would be very important in wars... That useless stash is gone now.) $\endgroup$ – AlexP Sep 7 '19 at 22:47
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    $\begingroup$ Yes, but it is incredibly hard to get more, new helium. $\endgroup$ – ILoveAllGolems Sep 7 '19 at 22:48
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    $\begingroup$ Helium is a by-product of oil and natural gas extraction... Where do you think the US Bureau of Land Management got their gigantic stash? $\endgroup$ – AlexP Sep 7 '19 at 22:50
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    $\begingroup$ All atoms of all elements are present in a finite amount on Earth. Did you want a gas that can be conjured by magic? $\endgroup$ – The Square-Cube Law Sep 7 '19 at 23:02
  1. Hot air. Cheap, readily available, nonflammable. The hotter, the liftier. You can replenish your supply anywhere you can breathe.

  2. Gaseous water. Cheap, readily available, nonflammable. The hotter, the liftier. You can replenish your supply a lot of places. You will need a container that can withstand the steam.

  3. Anhydrous ammonia. Dark horse candidate and never used as a lift gas to my knowledge. But definitely lighter than air and boiling at -33 C, a gas at most earthly temperatures. Caustic. Technically flammable but only at high oxygen levels so no explosion risk. Ammonia can be synthesized from N2.

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    $\begingroup$ Those were so blindingly obvious I didn't even think about them. Thanks! $\endgroup$ – ILoveAllGolems Sep 7 '19 at 22:55
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    $\begingroup$ All I can think about are blindingly obvious things, so I have an advantage there. $\endgroup$ – Willk Sep 7 '19 at 22:56
  • $\begingroup$ Also, I looked up the material safety data sheet for anhydrous ammonia. https://msds.orica.com/pdf/shess-en-cds-010-000031098301.pdf. Nasty. $\endgroup$ – ILoveAllGolems Sep 9 '19 at 5:03
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    $\begingroup$ Ammonia definitely blinding ;-) $\endgroup$ – user45032 Sep 10 '19 at 20:02
  • $\begingroup$ Yes it is indeed. $\endgroup$ – ILoveAllGolems Sep 14 '19 at 0:42

If you can produce a thin rigid shell for your airship out of carbon or silicon, then you use the atmosphere itself as your lift gas by creating a vacuum inside the shell. Neal Stephenson’s Diamond Age used this principle to make light evacuated cells that were used for tiny drones and giant airships.

  • $\begingroup$ That is incredibly cool, thanks! $\endgroup$ – ILoveAllGolems Sep 14 '19 at 0:30

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