What specific challenge does the super-cooling required for $ ext{LH}_2$ introduce over time, especially on long missions?

Answer

Boil-off

Because no insulation system is perfect, heat slowly leaks into the cryogenic tanks, continuously turning liquid hydrogen back into gas, which results in critical propellant mass loss during long on-orbit residency.

What specific challenge does the super-cooling required for $	ext{LH}_2$ introduce over time, especially on long missions?
hydrogenfuelrocketpropellant