Long term coal price., page-158

Will never happen.. They spent $10 million in 1950s on the project and never took it on to this day. Speaks volumes about their findings. We would likely go back to burning diesel for aircraft engines if jet fuel magically disappeared. But all diesel is derived from crude oil through processing, much like jet fuel. So if crude oil disappears we will have much bigger economic problems than air travel.

Ironically, if you track the process far enough you would see that even liquid hydrogen would be impossible without crude oil supporting infrastructure and equipment.