However, I don't of any society in the world that is actually ready to ban airplanes.Transport without fossil fuels is the hardest problem (renewables and nuclear can handle the needs of power generation).Although the much-hyped hydrogen economy has proven to be a dud (and will remain so -- there are simply too many problems with H2 for it to be a profitable energy storage medium), commercial aviation is one niche for which H2 might actually work. To really get the energy density needed for hydrogen to be used as a transport fuel requires liquifying the hydrogen, and this is expensive since it requires high pressure or very cool temperatures (20K at atmospheric pressure). This is completely uneconomical for passenger cars, but potentially becomes feasible for larger vehicles. The relative cost of containing liquid H2 decreases with the size of the fuel tank because the cooling costs go as the surface area of the containment vessel, which increases only as the 2/3 power of the fuel mass. So keeping large fuel tanks cool costs less per gallon of H2, or Joule of energy produced by combustion. And the weight of liquid H2 is low for the amount of energy available. This is precisely why liquid H2 is used as a rocket fuel (along with the liquid O2 oxidizer).I don't believe there's any fundamental reason why commercial jets couldn't fly using liquid H2 instead of avgas. Overall, even with the pressurized containment tank, the weight of a full tank would probably be less than for avgas.Liquid H2 might also be feasible for the trucking industry and diesel locomotives as a replacement for diesel fuel.These uses are probably the only situations that H2 will ever make economic sense as a fossil-fuel substitute.Phil
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