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For the story I'm coming up with, there's two planets in a binary orbit, close enough to be travelled with early spaceflight. Over time, both planets are inhabited, and due to evolution, plants on one planet emit roughly half as much oxygen as the other, processing it to be more energy-efficient. This leads to two roughly earth-sized and shaped planets, except planet 1 has an oxygen level of 25%, and planet 2 has an oxygen level of 16%.

Now, with two different planets, there is a rebellion on the lower-oxygen planet as a sort of "revolution" from Earth. What I'm wondering is if gunpowder weapons will be altered by the lower oxygen concentration. If the explosion size is different, then by how much?

I know from a quick explosion search that high explosives are unaffected, but gunpowder is a "low-explosive" and thus burns.

The firearms used vary from muskets and cannons to modern firearms.

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  • $\begingroup$ One detail I forgot is that the firearms used vary from muskets and cannons to modern firearms. $\endgroup$ Oct 23 '20 at 13:23
  • $\begingroup$ The amount of oxygen released by plants depends basically on the amount of plants (= how many kilograms or pounds of plant mass are there), not on their "efficiency". For every kilogram of plant mass (= mostly cellulose, lignin, starch, sugars) the plant has had to take in about 1.5 kg of carbon dioxide and 0.6 kg of water, and combine them releasing about 1.1 kg of oxygen. This is fixed, cannot be changed. "Efficiency" in the case of plants means how much sunlight and water the consume to do this. $\endgroup$
    – AlexP
    Oct 23 '20 at 16:40
  • $\begingroup$ Unrelated but if you're dealing with combustion in lower O2 environments then this might be of some use: en.wikipedia.org/wiki/Limiting_oxygen_concentration - at 16% wood would not readily sustain fire and would self-extinguish. $\endgroup$
    – Zac Walton
    Oct 23 '20 at 20:29
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All explosives -- even gunpowder -- contain their own oxygen, so will function even in a vacuum, never mind ambient oxygen levels lower or higher than ours.

The only issue with black powder weapons might be that the early ones depended on burning flakes of metal shaved by a flint to ignite the powder charge -- and if the oxygen level is too low, the metal might not burn and hence not stay hot enough to ignite the powder in the pan. Mainly this means some other method of ignition will be required, and it's possible it might not matter. Maybe we could ask our favorite YouTube experimenter to try a flintlock in a vacuum chamber (with just the pan charge, for safety)...

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    $\begingroup$ Much as I enjoy checkmarks, you might want to wait a while (24 hours?) to see if someone else comes up with a better answer. $\endgroup$
    – Zeiss Ikon
    Oct 23 '20 at 13:55
  • $\begingroup$ Okay. I'm still pretty new to this website, sorry. $\endgroup$ Oct 23 '20 at 13:59
  • $\begingroup$ Everyone was, at some time or another. And welcome! $\endgroup$
    – Zeiss Ikon
    Oct 23 '20 at 14:06
  • $\begingroup$ Practically correct, but technically not ALL explosives. There's a class of fuel-air explosives that depend on atmospheric oxygen: en.wikipedia.org/wiki/Thermobaric_weapon Sometimes this occurs "naturally", as when dust in grain elevators or flour mills explodes: en.wikipedia.org/wiki/Dust_explosion So the OP might make a plot point out of such explosions being more likely on the high O2 planet. $\endgroup$
    – jamesqf
    Oct 23 '20 at 16:58
  • $\begingroup$ @jamesqf There are fuel-air explosions but the "explosive" is just fuel. But yes, worth noting that the high-oxygen planet will be more prone to wildfires. FAE won't be any more likely, just require a richer mixture (and therefore give higher pressures). There's also a class of oxygen-free explosives (azides) and others that explode by decomposition rather than combustion (fulminates, styphnates, nitrogen halides), but historically, neither class is really important compared to combustion-driven materials. $\endgroup$
    – Zeiss Ikon
    Oct 23 '20 at 17:37

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