That wholly misses the point.

We need hydrogen production in unlimited amounts for myriad processes. Want synthetic ammonia? Need hydrogen. Methane? Hydrogen. Kerosene? Hydrogen. Green steel production? Hydrogen.

Furthermore, liquified hydrogen is the future of aviation.

So a tech that produces hydrogen efficiently anywhere there is air is intrinsically important.

> Furthermore, liquified hydrogen is the future of aviation.

What makes you think so? Why not eg methane? Or other synthetic carbohydrates?

Performance-wise there's nothing close. You fly twice as high because hydrogen burns so lean, and you only need roughly 1/5th the equivalent mass of kerosene in fuel to get you the same distance. There are obvious barriers in the way (cost, certification, handling), but the potential benefits have been known for a long time.

The main problem with LH2 is that it won't fit in the wing tanks like kerosene. The tanks may be very light, because the pressure is low and the LH2 weighs nothing, but they will be big!

Some bet LH2 aircraft will have a wholly new shape, more like a lifting body, with a lot more inboard room for the bigger tankage. But another possibility is long underwing nacelles, alongside the engines. Those might even be retrofitted to existing airframes.

Some people dislike the idea of sharing the cabin with inboard hydrogen tankage, as a safety concern. Fuel tanks shearing off with the wings in a crash has saved lives. Probably the nacelles would be rigged to be dropped in an emergency, to leave behind detonation risk.

Yeah that's right, you need roughly twice the fuel volume, though that's easier to accommodate than needing to deal with a heavier fuel like ammonia, for example.

Blended wing bodies and flying wings might make more sense with LH2. They haven't happened so far because they're harder to certify (essentially from scratch vs a standard design which uses evidence from previous designs to expedite certification), and they don't work too well with existing airport infrastructure. If you're going through the trouble of certifying LH2 anyway, that could provide justification for a new design, and working with airports to get the infrastructure going.

I am betting compatibility with past practice wins over incremental efficiency.

But I have no way to evaluate whether retrofitting existing airframes with underslung tankage will be practical.

I tend to agree. Performance-wise, kerosene is "good enough". The only area where I see LH2 being worth the effort is military aircraft, especially unmanned.

Of course, that's not a knock against green hydrogen itself (which I'm sure some will interpret it as) since kerosene is really just hydrogen with extra steps. But there are more factors that go into an effective propulsion system than just raw performance and efficiency.

> Performance-wise, kerosene is "good enough". The only area where I see LH2 being worth the effort is military aircraft, especially unmanned.

I'm not so sure. The military tends to put a premium on not exploding, even when fired up on. Even at the expense of efficiency.

Case in point: TNT vs dynamite.

> TNT has never been popular or widespread in civilian earthmoving, as it is considerably more expensive and less powerful by weight than dynamite,[12] as well as being slower to mix and pack into cylindrical boreholes; for its part, dynamite has never been popular in warfare because it degenerates quickly under severe conditions and can be detonated by either fire or a wayward bullet. TNT's primary asset is its remarkable insensitivity and stability: it is waterproof and incapable of detonating without the extreme shock and heat provided by a blasting cap (or a sympathetic detonation); this conveniently also allows it to be melted at 81 °C (178 °F), poured into high explosive shells and allowed to re-solidify with no extra danger or change in the TNT's characteristics.[13] Accordingly, more than 90% of the TNT produced in America was always for the military market, with most filling shells, hand grenades and aerial bombs and the remainder being packaged in brown "bricks" (not red cylinders) for use as demolition charges by combat engineers.

From https://en.wikipedia.org/wiki/Dynamite

See also https://en.wikipedia.org/wiki/Multifuel#Underperformance_iss... and https://en.wikipedia.org/wiki/JP-8 (they add special stuff to make military fuel auto-ignite at a higher temperature than civilian fuel).

However, you might be right that in unmanned situation the military might be ok with more dangerous LH2. Though I doubt it, because the LH2 storage tanks on the ground also need to be relatively close to the front lines. And unless you make it on-site, you need to have a logistics train to get the LH2 there.

LH2's extreme mass-energy density will make other aircraft unable to compete, because the weight savings may be sold as additional paying cargo capacity.

That presumes a joule of LH2 is the same price as of kerosene. But LH2 may end up much cheaper, increasing the gap.

One would suppose mass energy density is even more important for rockets.

In practice, only some rockets use LH2. Many rockets use souped up kerosene (RP1) or, more recently, methane.

Compare https://en.wikipedia.org/wiki/Liquid_rocket_propellant#Hydro... and https://en.wikipedia.org/wiki/Hydrogen-powered_aircraft

Also:

> With materials available in the 2020s, the mass of tanks strong enough to withstand this kind of high pressure will greatly outweigh the hydrogen fuel itself, largely negating the weight to energy advantage of hydrogen fuel over hydrocarbon fuels. Hydrogen has a severe volumetric disadvantage relative to hydrocarbon fuels, but future blended wing body aircraft designs might be able to accommodate this extra volume without greatly expanding the wetted area.

From https://en.wikipedia.org/wiki/Aviation_fuel

The quote concerning mass of tanks is talking about compressed hydrogen storage, which people don't really consider feasible for large aircraft. Cryogenic liquid hydrogen storage is the way.

The need for higher mass density exhaust to provide enough absolute thrust to get off the ground overwhelms H2's energetic efficiency. Thus, SLS, like STS before it, depends on solid-fuel rockets until well clear of the pad, and then jettisons them.

Aircraft operation involves nothing analogous.

Hydrogen is pretty neat as a storage medium. It's easier to store a few terawatthours in gas than in batteries.

Most of all it's an intermediate for even neater chemical storage mediums. The entire range of non-biological regenerative fuels (or non-biological, period, if we include fossils in the biological group), including reduced Fe dust cycles, ammonia or plain old "e-fuel" hydrocarbons, they all start with hydrogen as the first stage. It's the natural short term storage medium that will be used as an input buffer for downstream processes so that they don't have to follow supply peaks as closely as they would have to without that buffer.

Ammonia and synthetic hydrocarbons will be built from green hydrogen though.