Approximating air as nitrogen... nitrogen cannot exist as liquid phase at room temperature - it can exist as a supercritical fluid though at 5MPa (so 50 atm). A

Approximating air as nitrogen... nitrogen cannot exist as liquid phase at room temperature - it can exist as a supercritical fluid though at 5MPa (so 50 atm). At that point it has a density of roughly 56 kg/m^3. At ~1 atm, nitrogen becomes liquid at -194C and has a density of roughly 800 kg/m^3. So your mass density of the system is like 14 times better with liquid nitrogen, and the pressure loading requirements are stupidly lower.