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Hydrogen, for rocket fuel, tends to leak even through metal containers. Launchers carry hydrogen cooled to liquid in order to save storage space for aerodynamical reasons, but that is unnecessary in free space. Hydrogen is the most efficient rocket fuel and needs to be stored in space during long travel times until for example a landing burn, or in the future maybe after having been extracted from airless celestial bodies.

Would hydrogen gas in inflated huge balloons leak less than liquified hydrogen in pressure tanks?

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    $\begingroup$ Since flow rate is roughly proportional to delta pressure, there may be some advantage, but there are a lot of factors to consider depending on your imagined system. For example, are your balloons more vulnerable to penetration by micrometorites, and will that make them leak more than if you left the h2 liquid? Will you have to recompress the H2 to use it in your engine? If not, your mass flow rate is likely to be very low and so will your thrust. Bottom line, without some kind of system design in mind, it's pretty impossible to say if this is advantageous. Maneuvering with huge balloons? $\endgroup$ Dec 22, 2015 at 14:15
  • $\begingroup$ Hydrogen has the highest energy per mass, but that doesn't make it the most efficient fuel. Efficiency is measured by total system requirements, not energy/mass. The extra dry mass from much larger tanks, cryogenic cooling systems and lower thrust causes Hydrolox to be less efficient than dense fuels for first stages. Once in space low thrust is less of a problem, but it still has to accelerate enormous, heavy tanks, and long trips have a lot more leakage. A fuel with twice the energy per Kg, that loses more than 50% due to leaks is less efficient without even considering dry mass. $\endgroup$ Oct 26, 2022 at 1:25

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Hydrogen escapes from solid-walled containers by solid diffusion. This is exponentially related to temperature. For instance, hydrogen diffuses through iron 75 times as fast at 100 °C as at 10 °C. (https://en.wikipedia.org/wiki/Mass_diffusivity) By comparison, the diffusion rate is linearly related to both the concentration (partial pressure) and container area.

Bottom line: Least H2 diffusion is in a small cryo container rather than a large container at ambient temperature. I couldn’t find diffusion rates for balloons, but I bet dollars to donuts it is higher than steel.

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