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The Ozoa are a peculiar race. They are native to a large gas giant similar to Saturn and physically look similar to what we would call "jellyfish". Within their "head" the Ozoa contain a lifting gas their body naturally produces and regulates to keep the Ozoa afloat. The tentacles are used for propulsion and grasping. "Swimming" similar to how a squid moves through the water.

But these traits are similar to many examples of gas giant species across the galaxy. The Ozoa are truly remarkable for being "naturally" spacefarring with elderly Ozoa able to escspe the atmosphere of their homeworld.

Ozoa are born rather miniscule but throughout their lifespan the Ozoa grow to rather monstrous size with some reaching the sizes of zeplins. When Ozoa reach peak "ship size" their outer layer of skin begins to harden into a tough exoskeleton that's also airtight and to keep themselves afloat the Ozoa begin to process the hydrogen atmosphere into fuel. Which they use to eventually escape their planet.

What I'm wondering is how they would make hydrogen into fuel. The two things I'm interested in is: how do they "process" hydrogen into a fuel they can store and second how do they effectively "burn" or "light" the hydrogen without, you know, exploding?

Notes:

Evolution of the species isn't overly important for the question they just did.

They have a similar life cycle to a jellyfish, but past physical appearance are very different internally.

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    $\begingroup$ Off the top of my head, I don't know if this would work at all. Hydrogen is a great rocket fuel, but you have to mix it with oxygen in order for it to burn, and oxygen is not a common element in gas giant atmospheres. $\endgroup$ – Morris The Cat Sep 26 '19 at 15:47
  • $\begingroup$ @Morris The Cat yeah. I knew they'd need to make an oxidizer somehow. But that solves the whole burning down their entire ecology dilemma. $\endgroup$ – Celestial Dragon Emperor Sep 26 '19 at 15:49
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    $\begingroup$ Hydrogen is a fuel. And I don't fully understand what a "lifting gas" would be in a hydrogen atmosphere -- there is no gas lighter than hydrogen. $\endgroup$ – AlexP Sep 26 '19 at 15:52
  • $\begingroup$ @AlexP ...warm hydrogen :-) (the difference is not so great though. You'd need a very large volume for the volume to surface ratio to allow for lifting the surface material itself, let alone the "gondola" creature). $\endgroup$ – LSerni Sep 26 '19 at 16:40
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    $\begingroup$ @AlexP: Hydrogen is not a fuel in a hydrogen atmosphere, any more than oxygen is a fuel in Earth's atmosphere. Unless you're filtering out deuterium/tritium and doing fusion, of course :-) $\endgroup$ – jamesqf Sep 26 '19 at 17:16
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As comments said, Hydrogen is a fuel, but it needs oxygen or other oxidizer to burn. And there is no gas lighter than hydrogen. So here are couple ideas to resolve that:

A. Change atmosphere to methane (like neptune). Your creature breaks it down into Hydrogen and Carbon; Hydrogen is used to float, Carbon is building blocks for the body.

B. keep your hydrogen atmosphere, but let your creature heat up the hydrogen inside of it, turning it into a hot gas baloon. it will take a bit of energy to do that. Maybe its skin is black to absorb sunlight, maybe it has a radioactive isotopes to provide heat.

Now to fuel. Hydrogen is readily available in either scenario, the critical resource will be oxidizer. I know Jupiter has some water, and maybe other gas giants have it too, so your creatures could find a way to collect water and break it down into oxygen and nitrogen. Oxygen will be highly corrosive and explosive in their environment, so it will take a lot of technology to use it safely.

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    $\begingroup$ Oh, the Methane thing is clever. Oxygen is still the big problem though. It's only present in trace amounts in Gas Giant atmospheres, and critically, not anywhere close to the same altitudes. Getting to where any oxygen might be in Neptune's atmosphere would be like going to the bottom of the Marianas Trench for us. $\endgroup$ – Morris The Cat Sep 26 '19 at 16:32
  • $\begingroup$ @Bald Bear Methane it is $\endgroup$ – Celestial Dragon Emperor Sep 26 '19 at 17:09
  • $\begingroup$ @Morris The Cat maybe that's why they evolved the exoskeleton bit $\endgroup$ – Celestial Dragon Emperor Sep 26 '19 at 17:10
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    $\begingroup$ @CelestialDragonEmperor Well, probably not... most earth creatures that have to manage extreme pressure changes deal with it by being elastic, not rigid. Squid, for example. $\endgroup$ – Morris The Cat Sep 26 '19 at 17:22
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    $\begingroup$ If oxygen is deep down, they will have to drill or dredge for it. They have the technology, don't they? Or they can dive for it like our sperm whales dive for giant squid. $\endgroup$ – Bald Bear Sep 26 '19 at 18:42
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Given that the atmosphere must contain something more than hydrogen, or lifting would become a significant problem, why not go all the way? (Even if, from an evolutionary standpoint, it's unlikely in the extreme).

The Ozoa, unique among their brethren, have the capacity to differentiate between different hydrogen isotopes. In their old age, through careful membrane sifting, they begin to accumulate deuterium in a specialized lift cell. Also, they accumulate reaction mass and sufficient quantities of handwavium catalyzer.

Finally, they engage a biological catalyzed cold fusion drive - it needn't be very efficient nor very powerful, as it simply generates a large amount of heat, enough to power a jet engine. For a long time, the Ozoa rises ever higher in the atmosphere, toughening and acclimatizing to lower and lower pressures, before falling back down in the rich troposphere and filling up its tanks.

With time, the combustion chambers become capable of withstanding very high temperatures and pressures, and the Ozoa relies less and less on its natural buoyancy in favour of powered flight. This allows it to adopt a more streamlined shape, and reach higher and higher up the gravity well (which is quite deep, this being a gas giant).

This also serves to train its reflexes and computation abilities to cope with concepts such as microgravity, heat of reentry, delta-vee and the laws of Kepler. Many Ozoa fail to learn these lessons in time, but this one does not.

Until one day it finds itself capable of fully powered flight in near-vacuum, and it is free to roam the oceans of space.

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