Originally Posted by
tdracer
Denti - hydrogen will always be problematic for long distance travel (at least for subsonic travel). The problem is density - it would take a massive volume of H2 to equal the energy of a thousand liters of Jet-A and either cryogenic or high pressure means spherical or cylindrical fuel tanks to be even remotely viable - you're not going to be storing it in the wing. So you're looking at devoting much of the fuselage to fuel - which of course makes it unavailable for payload (not to mention the issues with safely segregating the fuel from passengers). The equation changes rather dramatically when you start talking something like hypersonic - but that's going to require a major leap in technology before it becomes even remotely viable.
Don't be quick to dismiss biofuel - you're correct if biofuel was limited to things like turning corn or soybeans into jet fuel - but there are other options that look far better. One of the most promising is using algae - it takes a small fraction of the area per unit of fuel that land crops do, the downside being ti currently takes large amounts of fresh water. If they can come up with a variety of algae that produces a similar amount of fuel in salt water it could quickly become a viable option.
Yes, i don't see hydrogen in itself as feasible either, sorry if that didn't show through in my post. As you said, volumetric energy density is simply not nearly enough, and weight, while still important, does not matter in this case where density is so low.
I take your point about algae based fuels. Those could be a possibility, if and when they discover or engineer the correct kind of those to grow without too much of a downside. And i know, that P2F is currently in its infancy and very power hungry, basically it generates fuel out of power via hydrogen, using CO2 from the atmosphere to generate liquid fuels that can be easily engineered to be simply Jet-A1. However, it is extremely energy inefficient as of yet. Still could be a possibility.
In the end it could be a combination to generate the required amount of fuel.