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I am etching out a spec evo project involving small carnivorous sapient beings. I intend these creatures to never discover the secret of heavier-than-air flight, so instead they construct highly efficient blimps capable of reaching speeds from 80 to 90 mph, such as passenger air-buses to sky-battlecruisers.

However, I don’t want them to repeat the mistakes of man by using fossil fuels, and the main material used to create envelopes for hot air balloons is polyester, which is made from these. So, what alternatives to polyester materials could be used to construct an airship envelope?

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    $\begingroup$ Airships have never been built as hot-air balloons. They use hydrogen or helium, at ambient temperature. Another problem is that developing efficient engines and propellers for airships will cause them to discover how to build wings, and this heavier-than-air aircraft. $\endgroup$
    – John Dallman
    Nov 27, 2022 at 10:37
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    $\begingroup$ The envelopes of actual historical airships were made of fabric, not any kind of plastic. And actual historical airships were definitely not blimps -- they had a rigid structure. (And in in actual historical airships, the envelope was just a fairing, not a gas container. The lifting gas was contained in cells inside the envelope.) $\endgroup$
    – AlexP
    Nov 27, 2022 at 12:23
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    $\begingroup$ "the main material used to create envelopes for hot air balloons is polyester" - Historic airships used cow intestines, not polyester: German Zeppelins Were Made with Cow Intestines And It Led To Sausage Restrictions During WWI $\endgroup$
    – TessellatingHeckler
    Nov 27, 2022 at 19:47
  • $\begingroup$ Why are you worried about fossil fuels in the envelope materials? How is your very draggy balloon going to go 80-90 mph without fossil fuels? Also, depending on how small the creatures are, heavier-than-air flight would be significantly easier than it is for humans. Squirrels can glide long distances, and birds and bats fly easily. It's when you get to human size that flight gets difficult. This is a function of air density and gravity. $\endgroup$
    – Dan Hanson
    Nov 27, 2022 at 21:12
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    $\begingroup$ 80-90 mph suggests a huge amount of drag to overcome. If you can squeeze that much power into an airship, heavier-than-air power is trivial. $\endgroup$
    – user535733
    Nov 27, 2022 at 23:02

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The Hindenburg used gelatine coated layers of cotton to produce gas tight lift cells. It also used cotton cloth varnished with cellulose acetate butyrate and iron oxide and impregnated with aluminium powder (to reflect UV light) on the outer hull. So no need for man made fibres.

Given the fictional situation you describe, there would be plenty of scope to suggest a wide range of naturally occurring materials for use in an airship. Everything from fictional spider silk to bamboo fibres and from fibre-glass reinforcement to naturally occurring epoxy resin glues. The world is your oyster.

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This answer is the same Idea as Willk's but......

BLADDER PLANTS FULL OF HYDROGEN

Think about it, a large plant that fills the same niche as trees do (tall, forest-building plants) but instead of using reinforced Cellulose trunks to get above other plants, they use hydrogen-filled sacs to fight the tug of gravity.

So then your species comes along and uses them to travel in the sky, eventually figuring out how to keep the plants alive and floating, selectively breeding them for larger and larger hydrogen bladders, until they get to Hindenburg-sized ballons.

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Whole animals.

goat balloon

https://www.ripleys.com/weird-news/inflated-animals/

Here as an example is a whole pygmy goat balloon and your people have many such - usually used in "herds" but one is enough to lift one of your small carnivorous people. Your people also make balloons out of larger animals including camels, hippopotami, elephants, whales and giant toads. They make many more animal balloons than they actually need for the construction of airships.

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    $\begingroup$ Yeah, that's not at all disturbing lmao. All those balloon animals I've made at b-day parties were just cheap imitations of the ʀᴇᴀʟ ᴅᴇᴀʟ. $\endgroup$
    – BMF
    Nov 27, 2022 at 20:51
  • $\begingroup$ It's remarkable, but how practical is this? I had been thinking of bladders or stomach tissue as balloons, but this seems like a lot of unnecessary load, and what exactly is containing the gas here? $\endgroup$
    – Joachim
    Nov 27, 2022 at 23:23
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    $\begingroup$ Listen @Joachim - these creatures are not going to repeat the mistakes of man! Enough said. $\endgroup$
    – Willk
    Nov 28, 2022 at 0:29
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TL;DR If you want polyester, make it from plants

From this article:

The Japanese chemistry giant and functional fabrics specialist Toray has developed the world’s first fully plant-based polyester yarn made exclusively from renewable raw materials.

The new ecodear™ plant based polyester can be used for fabrics and membranes for garments as well as for bottles and packaging. The polymer is chemically identical to existing polyester from crude oil, and it can still be used for all functions and applications.

Admittedly, making polyester from plants was a goal in our world, starting with polyester from oil as a template. In your case, there will have to be a method to come up with independently, but that is certainly possible. Synthetic rubber is mostly made from petroleum, but it is patterned (initially at least) after natural rubber from plants.

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