As a gas at 1 atmosphere, wikipedia says 0.011 MJ/L, compared to jet fuel at 35 MJ/L. You'd save weight (it weighs less than air so this would make you float) b

As a gas at 1 atmosphere, wikipedia says 0.011 MJ/L, compared to jet fuel at 35 MJ/L. You'd save weight (it weighs less than air so this would make you float) but need 3000x as much space if it's not compressed. Compressed at 690 bar (about 680 atmospheres) it goes all the way up to 5 MJ/L, only 7x as much volume of fuel required compared to jet fuel. But the tanks to pressurize something that much are presumably very heavy and not a reasonable solution in an airplane. Liquifying it gets that up to 10 MJ/liter and I don't think you need as much pressure to store that (as long as you keep it very very cold). Per weight instead of volume you beat jet fuel (by 3-4x), so that aspect is a win. Having enough volume to store it and keeping the storage equipment light are the challenges.