I'm not claiming it would be cost competitive with current jet fuel prices. However, if fossil fuel prices skyrocket (either because of a substantial carbon ta

I'm not claiming it would be cost competitive with current jet fuel prices. However, if fossil fuel prices skyrocket (either because of a substantial carbon tax or simple resource exhaustion), then the economics change in hydrogen's favor. I'm also not claiming that existing airframes could be easily retrofitted to fly on hydrogen. Not only do you need a much larger fuel volume, you can't practically store it in wing tanks. On the other hand if you took an existing design and stretched the fuselage to accommodate larger tanks, or attached large external tanks to the wings, you could get something that's economical. That's because you have an aircraft that's larger (more expensive, all other things being equal) but also lighter (cheaper, all other things being equal).