The cryogenic storage, handling equipment, fuel cell are all weight to be dealt with, though.

Handling liquid hydrogen is hazardous. It is below 20 K (-423 F).

If it spills, it quickly boils into gas and mixes with oxygen. That can explode, like the Hindenburg.

Mike C.[/quote]

Exactly. Aside from the storage issues on the aircraft itself, the ground infrastructure required to support hydrogen fueled aircraft is a huge factor, vacuum insulated dewars, transporation to/from production facilities, etc. The current domestic production capability of Lh2 is barely enough to support existing needs.

and volume.....

remember the density is quite different which requires large liquefied bottles that have to go somewhere.

It takes care, but its doable - we have liquid and solid hydrogen in my lab. IF (big if) you design the infrastructure correctly, leaking hydrogen will just float upwards, and is safer than jet fuel. (if you do it wrong and leave spaces where it can accumulate, then you can have a big problem).

Hydrogen planes don't make any sense now, but as we get more non-constant renewable energy (wind, solar) we will have more situations where power is being thrown away because there are no customers. That will bring down the production cost of hydrogen a lot.




Exactly. Aside from the storage issues on the aircraft itself, the ground infrastructure required to support hydrogen fueled aircraft is a huge factor, vacuum insulated dewars, transporation to/from production facilities, etc. The current domestic production capability of Lh2 is barely enough to support existing needs.[/quote]

Agree, volume is a really big deal. It would be interesting to see a tradeoff study between jet fuel and hydrogen counting weight and volume.

It would require new airliner designs which is of course an enormous expense.

Also a new design of turbines - but in a lot of ways hydrogen fueled turbines are easier, less corrosion and injecting cold gas or possibly liquid along with the lower combustion temperature keeps the turbine temperatures lower for the same efficiency.

I can't imagine fuel cells having enough power to weigh to be useful for large aircraft

The energy per cubic foot of hydrogen is comparatively quite low. The volume needed for a transatlantic airliner would be problematic. Even for shorter flights it won’t make a lot of sense.

If you are using excess electricity to make synthetic hydrogen, you are probably better off making a synthetic kerosene or other fuels with better volumetric energy density.

If the carbon for your synthetic hydrocarbon comes from the air, it will still be carbon neutral when burned.

Energy per volume:

Liquid hydrogen: 10.1 MJ/L
Liquid methane: 23.3 MJ/L
Jet fuel: 34.4 MJ/L

Mass per volume:

Liquid hydrogen: 0.071 Kg/L
Liquid methane: 0.42 Kg/L
Jet fuel: 0.8 Kg/L

Energy per mass:

Liquid hydrogen: 142 MJ/Kg
Liquid methane: 55.5 MJ/Kg
Jet fuel: 43 MJ/Kg

Boiling point (1 ATM):

Liquid hydrogen: -253 C (20 K)
Liquid methane: -162 C (91 K)
Jet fuel: ~ +200 C

Some rockets use liquid hydrogen as a fuel, but they are characterized by having huge tanks and weight is more of an issue than volume for them. An example is the Delta4 Heavy which famously sets itself on fire during launch (due to hydrogen floating upwards from the engines during start).



The SpaceX Starship uses liquid methane which fits between liquid hydrogen and jet fuel numbers. The higher boiling point is very similar to liquid oxygen (-183 C) and thus easier to manage. Additionally, it is thought to be easier to make liquid methane on Mars than liquid hydrogen.

Mike C.

Note that the H2 energy density per weight is 3X jet fuel which corresponds to a similar reduction in fuel weight. but you do need 3.5X the tank volume.

For rockets, the higher isp of H2/O2 is useful for upper stages for operation beyond LEO, not clear whether or not its a win for LEO (but probably not). The ISP mostly matters when you need a lot of delta-V and are far up the exponential

For aircraft, the weight advantage is larger because you aren't carrying the oxidizer.

For a 787-9, MTOW is 561,000 lbs, fuel weight (for Jet-A) is 224,000 llbs. With hydrogen that fuel weight would drop to bout 70,000 lbs. leaving about 140,000 pounds extra load capacity. (depending on how much more the LH2 tanks weigh). Of course it would really need to be a clean sheet design to use the larger tanks.

Cryogenic tanks for liquid hydrogen and methane fuels are extremely heavy double wall stainless steel. The LNG tank I use for off road trucks contains 285 gallons of fuel, and weighs 2200 pounds. You can't use space-conforming tanks, you have to use round or cylindrical tanks due to the temperatures (-260 to -180 dF) and pressures (0 to 160 PSI) involved. Methane is the closest in btu's to jet fuel, and is still takes 1.6 gallons of methane to equal one gallon of jet-a. Hydrogen is not even worth talking about. The tanks alone would weigh more than the useful load of the plane.

Not necessarily. The LH2 tanks for spacecraft have to be light enough that its a net win over Kerosine / LOX. For aircraft the weigh benefit of H2 over kerosine is even higher than in spacecraft where the oxidizer is already most of the fuel weight.

as an example: https://en.wikipedia.org/wiki/Centaur_(rocket_stage)
has an empty weight of 4500 pounds and carries 45,000 pounds of fuel. That is a mix of H2 and lox. I don't have the numbers but that is probably >5000# of LH2 since rockets run H2 rich compared to stochiometrid

Even if the tank to H2 weight is 1:1, the total weight is considerably less than for kerosine for the same energy.

I agree the tanks need to be round, presumably would be built into a larger fuselage.





Cryogenic tanks for liquid hydrogen and methane fuels are extremely heavy double wall stainless steel. The LNG tank I use for off road trucks contains 285 gallons of fuel, and weighs 2200 pounds. You can't use space-conforming tanks, you have to use round or cylindrical tanks due to the temperatures (-260 to -180 dF) and pressures (0 to 160 PSI) involved. Methane is the closest in btu's to jet fuel, and is still takes 1.6 gallons of methane to equal one gallon of jet-a. Hydrogen is not even worth talking about. The tanks alone would weigh more than the useful load of the plane.

This only works in rockets because the dwell time in the tanks is so short. The fuel only needs containment for minutes, not hours or days. SpaceX uses liquid methane, and the only way it works is they literally have to fuel the rocket and launch immediately. Otherwise the methane boils off and they lose it quickly.

Liquified gaseous fuels are maintained in the liquid state by super cooling, and the tanks are thermos bottles. Only stainless steel can handle the extremely low temperatures. Which makes them very heavy.

I've been dealing in cryogenic fuels for over a decade, and I can tell you there is no way to use them in conventional aircraft. None. The energy density is far too low, and the containment methods are far too heavy. The same space that accommodates 1100 gallons of diesel fuel can only accommodate 300 gallons of liquid natural gas. We even struggle with this on 240 to 360 ton capacity mine-haul trucks, where the operators need to give up three tons of payload per trip in order to use LNG in place of diesel fuel. That's a loss of 14,000 to 26,000 tons of payload per year, which is counted against the total fuel cost savings in the payback calculations.

The instant that government-subsidized research on liquid hydrogen aircraft applications ceases, the business will also cease to exist.

https://www.ulalaunch.com/docs/default- ... 6-7270.pdf

suggests 8-10 hours useful time for centaur, and multi-day with a proposed upgrade

[quote="Josef Frisch"]https://www.ulalaunch.com/docs/default-source/extended-duration/centaur-extensibility-for-long-duration-2006-7270.pdf

suggests 8-10 hours useful time for centaur, and multi-day with a proposed upgrade


[quote="Glenn Juber"][quote]

The proposal is to reduce boil off of the fuel while traveling in space, where the temperature is nominally -255 dF. They really only have to shield the tanks from the sun to remain that cold.

Back here on earth, we don't have that cold to assist us, and we're stuck with tanks that would add 1800 pounds to a Baron, while only containing enough fuel for four hours of flight.

Works out that that 40 kg of liquid H2 is over 152 gallons.

Some of that research (e.g. from GTL [ PDF link ]) shows notable improvements in boil-off with an all-composite structure that notably reduces your 1800-lb value from cryo tanks with liners.

Using current TRL9 tech, I absolutely agree with you that hydrogen doesn't have a role in airplanes. But there's a lot of TRL3-6 tech around cryo-compatible composite resins that I expect will change that in the coming decade.