Liquid hydrogen is the key. It's frickin' cold, as in liquid nitrogen is positively tropical in comparison, but the density is high enough that you just need sl

Liquid hydrogen is the key. It's frickin' cold, as in liquid nitrogen is positively tropical in comparison, but the density is high enough that you just need slightly larger wing volume for tanks. Vacuum based insulation, which is what they use, is pretty light. As I said, Airbus did a large R&D project on this and they concluded it's technically and economically feasible for passenger jets. I'm taking their word for it, seeing as they build airplanes for a living.