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dr_jiang t1_iy5ar4z wrote

Reply to comment by Robot9P in 'Landmark achievement': Rolls-Royce and easyJet hail successful hydrogen jet engine test by Wagamaga

Commercial aviation burned 95 billion gallons of jet fuel in 2019. Hydrogen has one fourth the energy density of jet fuel, and takes 2.4 gallons of water per kilogram of hydrogen if we're making it through electrolysis.

There's plenty of water in the Great Lakes (for now: we're currently turning it saline). There's not plenty of spare electricity to power the electrolysis plants, or plenty of spare money to build them, or plenty of hydrogen transport and storage infrastructure to get it from the Great Lakes to everywhere planes need to take off from.

Hydrogen has plenty of use cases where it makes sense, and its weaknesses as a fuel can be mitigated. Aviation is not one of them.

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Nyrin t1_iy5gq4n wrote

>Hydrogen has one fourth the energy density of jet fuel

Just to call out: as density decreases, fuel requirement goes up a lot worse than linearly. You have to burn fuel at the beginning of the flight to transport the fuel you'll use for the rest of the flight, and the more fuel you need, the more dire that picture looks.

The exact numbers would depend on a lot of variables (weight and distance chief among them) but most flights, even the smaller and shorter ones that are feasible with hydrogen's energy density, will need way more than four times the fossil fuel amount in hydrogen.

I do think that hydrogen (and even BEV in narrow situations) might have a place in limited aviation scenarios (very short/light flights) but completely agreed that the math just doesn't work for either of these electric modalities to replace fossil fuels in-place for long, heavy flights — and that's not a "point in time, technology will keep getting better" thing.

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dr_jiang t1_iy5ip33 wrote

I'd forgotten about the "need gas to haul gas" math. Honestly, I stopped taking hydrogen as aviation fuel seriously when the white papers came in describing the plane passenger economics.

The reference escapes me, but the bottom line was that converting existing airframes to hydrogen meant ripping out 14-20% of seats and paying 60% higher fuel costs. Barring science-fiction level advancements in the underlying technologies, commercial air travel as we know it can't exist in a hydrogen-fueled world.

No industry, no government, and no passenger is going to tolerate that.

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