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Skylon

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1 hour ago, sevenperforce said:

Superheavy still has to have the heaviest loads during boost when the tanks are under compression, so it has to be strong enough to support itself from the base for that reason.

Yeah but you get all the extra ring stresses from the tank pressurization, which generates tension, which negates the compression. Pressurization and compression-tension is a different thing since we're talking about a tank skin that's used to store pressures inside larger than outside.

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18 hours ago, JoeSchmuckatelli said:

Why do we pressurize with helium and not nitrogen? 

[edit] I didn't see that somebody else already provided the answer I was going to provide, about helium being a gas at much lower temperatures than nitrogen.

Edited by mikegarrison
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In addition to the things other about helium people have mentioned, (low density, resistance to being a liquid, inert), probably the most important thing is that helium provides exceptional pressure per kg.

The ideal gas law is PV=nRT.

R is the ideal gas constant, so at similar T and V, P depends solely on "n", the number of atoms/molecules in the volume. Because helium gas has very low atomic weight, it has a very high "n" per kg, better than any other substance except hydrogen (which due to having diatomic molecules is not as ideal a gas). This means you need less mass of helium to pressurise a tank compared with pressurising it with another gas.

High pressure per kg is such a key feature of rocket propellant pressurant. 

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