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I've thought of a super-earth with 30% higher gravity, a denser atmosphere, which facilitate the willing suspension of belief for my giant fireflies that terrorize the local humanoids. Due to the higher oxygen levels gunpowder firearms are a no-no, as the wrong shot can kick start a flaming inferno.

In any case, I'm a tad off track here anyway, the question is:

How would pneumatics in this world change due to the denser atmosphere?

For example, inhabitants of this pseudo-steampunk Anasazi society use a tube system to deliver letters, food, and other such material. Airguns don't necessarily outclass firearms but they don't set fire on everything so there's that.

Would the denser atmosphere hamper airguns by a considerable amount that people would default to armor bows and swords instead?

Lastly, would there be metals available in say, 15th century Europe that could withstand this increased pressure? or should I use some handwavium on that instead.

(forgive my bad grammar, and any horrifically inaccurate terms)

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In the timeframe you mention, the only available pumps for gases were volumetric pumps: something alike to the bellow you see in some old fireplaces, or the pump you see for bicycles.

They work by taking a volume of gas and reducing it by mechanical means, i.e. halving the volume of the gas one can roughly double its pressure. As such, they are not affected, in first approximation, by a denser atmosphere. Such devices struggle when the gas is too rarefied, but this is not the case.

The main problem with pressurized fluids is not the pressure itself, but rather the pressure differential that induces stress on the containment structure.

The main effect would be that the achievable pressure differential would be lower: in the example I gave above, imagine you have a pump where you can half the volume and double the pressure. Starting from 1 bar you could go to 2 bar, with a pressure differential of 1 bar (and I think this is already far fetched). If you start from 10 bar you could go to 20 bar, with a pressure differential of 10 bar. But all the sealings you would have in place (mainly leather rings) would not work with that large differential, thus you would have bigger leaks reducing the practically achievable pressure.

All the above would happen way before you reach the condition of challenging the metal resistance.

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1) More pressure means more eneregy per volume. First "atmospheric" steam engines and Stirling engines would produce more force and power, hot air and light gas balloons would produce more lift (even methane could be used as a lift gas). But since more pressure means more airfriction - overall effectivness of pneumatics would drop down. It will loose more energy on pumping (and thus producing more heat - but dense air also have higher termal capacity and conductivity), would requier more excess pressure for same flow speed.

2) For airguns not the absolute pressure matters, but excess pressure (P1-P2). One and the same excess pressure would produce the same force on bullet and thus the same speed at the end of the barrell (if it is short enough). But then comes airdrag - since airgun bullets are relativly slow and light, they would travel much less distance in dansere air. And since even in 1atm airguns were never a weapon of choise for a battle, in more then 1atm world they would not be such for sure.

3) You do not need superstrong metals or handwavium. Common materials (skins, wood, copper, bronze) would be enough to operate even inside 100atm pressuer. The only thing that you should keep in mind - it is the formula PV/T = const, wich meens that say changing the temperature (due to friction or direct sunlight) in an tank from 0C to 30C (273->303 K abot 10%) would rise excess pressure of 0.1 atm on Earth and 10 atm on 100atm planet. It means that random fails would be more frequent and more destructive (shockwave strikes harder in densere atmosphere).

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