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A possible future power source for passenger airline aircraft is liquid hydrogen, and assuming we solve the safety and infrastructure issues, it's doable and much less sci-fi, as for example, a Martin B-57B flew on hydrogen rather than the normal jet fuel, and so did a converted Tu-154 airliner.

However, a brief youtube and web browsing session(nothing as deep as scientific papers) shows me that while the mass to energy ratio is superior to current jet fuel, the volume rather is still terrible, even when cryogenic, meaning that the tanks would need to much more larger, in terms of volume.

Airbus proposals all use a flying wing design, which has the capability to have a much larger internal volume. However, I'm not sure if it would be possible to make a flying wing as maneuverable and as tolerant of high g force as the standard fighter jet layout, due to the longer wings and greater frontal surface area. Since my scenario is around 2050, what would be possible near-future designs future Liquid Hydrogen fighter jets? My starting point is "fifth-generation" fighter jets, which have anti-radar coatings for stealth and slightly different geometry to scramble radar signals, but most importantly, can not have fuel tanks and weapons hanging off the side during flight. That means everything, even all fuel tanks must be inside the aircraft.

For the base of the design, I'm looking at current 5th gen aircraft, like the F-22, F-35, Su-47, and J-20. The J-20 looks the most "chonky", and maybe with a smaller jet engine, I could squeeze multiple large volume fuel tanks, enough to match the flight time of current 5th aircraft? But could you all be be willing to provide input or maybe even just rough sketch for viable designs that use Liquid Hydrogen?

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    $\begingroup$ You will need to clarify, as LOX is to do with liquid oxygen, not hydrogen as stated in the title. Also you'll need to narrow down somewhat, as we deal with one question per post, as per the help center. (I should add, one clear and specific question). $\endgroup$
    – Escaped dental patient.
    Jul 8, 2021 at 0:31
  • $\begingroup$ This is a cool premise but I agree w Ant; be speciifc. Edit it down to one question. If you dig it, you can come back later with a second question and link this one as background. $\endgroup$
    – Willk
    Jul 8, 2021 at 0:41
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    $\begingroup$ I would think that the difficulties and hazards of handling cryogenic fuels for military aircraft and storing them on-board (no self-sealing fuel tanks meaning that any damage is fatal to the craft, bulky insulation, etc.) to be well nigh insurmountable, not to mention the difficulty of a cryogenic-safe coupling for mid-air refueling. It's just not very plausible. $\endgroup$
    – GrumpyYoungMan
    Jul 8, 2021 at 1:58
  • $\begingroup$ @Willk Thanks, just got rid of the entire back half, and just narrowed it down to a possible design. $\endgroup$
    – itisyeetimetoday
    Jul 8, 2021 at 4:44
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    $\begingroup$ @itisyeetimetoday Cryogenic tanks in rockets are very specific to their role. They're topped off on fuel until just before launch because the fuel is constantly boiling out of the tanks. A rocket's main stages are generally burned off within a half-hour or so; thrusters (such as RCX thrusters) that need to last longer than that don't use cryogenic fuels as a rule. It would be possible to use similar designs for some specific military roles, but very tricky. $\endgroup$
    – Cadence
    Jul 8, 2021 at 5:23

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Consider that not everything will switch to alternative energy at the same time. Some things, that are better suited to it, will switch before others.

Given that, it seems to me that fighter jets are just about the worst possible case for alternative energy. It hurts them more than any other technological artifact whatsoever. As you observe, they will be hurt aerodynamically and in terms of stealth profile by bulky fuel tanks. They already suffer for lack of range; volume-constrained fuel tanks would hurt there. They need to deploy from forward bases with limited infrastructure; hydrogen would hurt the ability to do that.

Really, if you have any budget for non-cryogenic liquid fuel – petroleum, palm oil, even ethanol – you should give your fighter jets first claim on it, and make other things subsist on hydrogen etc.

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    $\begingroup$ Biofuels are FAR easier and cheaper to produce than LH2. They are to some extent used in both military & civil aviation now: en.wikipedia.org/wiki/Aviation_biofuel Even if you had some way of producing large amounts of hydrogen at reasonable cost, you'd do far better to react it with some carbon to produce hydrocarbons that are liquid at ambient temperatures. $\endgroup$
    – jamesqf
    Jul 8, 2021 at 4:47
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    $\begingroup$ Thank you for your response. My goal was to have a relatively advanced west coast post-USA country that was focused on environmental issues, and based on how California is planning on banning the sale of new gas cars, I thought that it would only make sense for lawmakers, with free reign, to ban jet fuel too, but I'll either have to make a design that looks Saab Draken with huge fuel tanks in the but huge gas tanks where the intakes go, and maybe add a F-16 style air intakes, but I'll try to find alternatives. $\endgroup$
    – itisyeetimetoday
    Jul 8, 2021 at 5:17

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