Hydrogen storage is an open question, with a lot of development underway.

Pressurized or cryogenic storage of neat hydrogen is not the only means of storage, particularly for fuel-cells.

Fueling rockets with liquid hydrogen is one thing, but they generally will consume the liquid fuel quite rapidly in a combustion process. I think you'll always see hydrogen fuel rockets use cryogenic hydrogen for that reason.

A fuel cell application on the other hand, could use a different means of storage.

Methanol as a hydrogen storage media is reasonable. There's 99g of hydrogen in a liter of methanol, vs 77g of hydrogen as a cryogenic liquid. Obviously, methanol doesn't require cryogenic temperatures and pressures, as it is a liquid at ordinary temperature and pressure.

There are fuel cells that directly use methanol, or the methanol could be reformed into hydrogen for a hydrogen fuel cell.

The other area of research is nanomaterials and other solid-state materials which exhibit much higher storage factors than compressed gas or liquid hydrogen.

I'd mark it as a, "not yet, but maybe in the future" thing.

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.

Space isn't cold or hot - no air. The temperature of a surface depends on whether or not its in the sun and its absorption and emissivity.

The moon's surface goes from 250F to -208F day to night.

(quotes all messed up so deleted)

While that is true, we are talking about Joby, and they operate in the terrestrial environment.

So back on topic, there is no practical way to use cryogenic fuels onboard aircraft due to the weight of the containers, and the low density of the fuel stored within.

If you have a half-ton pickup, I can convert it to run on LNG. But you'll give up a 30" x 70" space in the bed and add 650 pounds to the truck. For hydrogen, the space claim will double, and the weight will increase by 30 percent.

My point was that since hydrogen tanks work for very weight sensitive spacecraft (eg centaur upper stage) they can work in air. My comment was to someone who said space was "cold" and that is why hydrogen can be stored there.




While that is true, we are talking about Joby, and they operate in the terrestrial environment.

So back on topic, there is no practical way to use cryogenic fuels onboard aircraft due to the weight of the containers, and the low density of the fuel stored within.

If you have a half-ton pickup, I can convert it to run on LNG. But you'll give up a 30" x 70" space in the bed and add 650 pounds to the truck. For hydrogen, the space claim will double, and the weight will increase by 30 percent.

I don't think you are fully understanding. In space, if you keep the sun off the tank which is relatively trivial, it is "cold" or more importantly, there is little heat transfer. For a launch rocket, the storage time is also brief.

For flight in the atmosphere (or a car) where fuel has to be stored for hours, the tank needs to be much more substantial. If the fuel also has to be storable for days or weeks, as opposed to all your reserves evaporating after every flight, it gets worse.

If you could get past the weight penalties associated with cryogenic fuels, you then have to surmount the refueling infrastructure costs. Which are multiples of the costs of conventional fuel storage and dispensing.

The typical stationary 10,000 to 15,000 gallon avgas tank and pump costs about $45k

To replace that with cryogenic storage and dispensing, make that $300k. A mobile 2500 gallon fueling truck will run $350k. And the larger tanks needed for busy GA airports will run $1-2 million. For airline operations....add a zero.

Self-serve fuel? That goes away with cryogenic fuels. There's no safe way for an airport to allow self-serve fueling due to the cryogenic burn risk. So now add a full-time employee to the cost of fueling.

H2 only makes sense for large aircraft, not hope for GA.

For airliner use (I'm not suggesting GA use) you need only a few days storage at most.

Cryo tanks are generally insulated with super insulation - multiple layers of aluminized mylar in a pretty good vacuum. Helium and I think hydrogen tanks also use the cold boil-off to cool 1 or more intermediate thermal shields, (both He and H2 gas absorb a lot of heat relative to the energy it takes to boil them from liquid).

The limit (at least for liquid He which I've used quite a bit) is the heat leak from the liquid to the thermal shield, that boil off rate is enough provide enough gas cooling that the outside temperature doesn't make much difference.

We have a He tank in the lab that is only 25 gallons, and lasts a week or more and helium is more difficult to deal with than H2.

I haven't worked with H2 (other than some frozen deuterium cooled by liquid He) but I've worked a lot with liquid He, and much colder (milli kelvin systems).

Liquid hydrogen does have some challenges - you need a stylistic to align the molecular spins or it will self boil pretty quickly from the ortho-para transformation. Using it in aircraft will have some challenges but those are understood.


Unfortunately I can't find the ground hold time for a centaur stage.






I don't think you are fully understanding. In space, if you keep the sun off the tank which is relatively trivial, it is "cold" or more importantly, there is little heat transfer. For a launch rocket, the storage time is also brief.

For flight in the atmosphere (or a car) where fuel has to be stored for hours, the tank needs to be much more substantial. If the fuel also has to be storable for days or weeks, as opposed to all your reserves evaporating after every flight, it gets worse.

Where do you get that weight for the tank?
One example of an aerospace H2 tank is at http://www.nasa-klass.com/Curriculum/Ge ... RDG_ET.pdf Page 2 shows a H2 tank dry weight of 29000#, and a full weight of 227,000# so empty weight is only about 12% of full weight,. (this is just the H2 tank).

Non aerospace tanks are much heavier and you are correct that for roadable systems, https://www.osti.gov/servlets/purl/1985749 the H2 weight is <1/4 the total weight. Of course the technology used is very different.


The best source though is this study (which I just found and have only skimmed)
There is an old Lockheed / NASA study here https://ntrs.nasa.gov/api/citations/197 ... 025036.pdf
(The study is old, from before there was politics around "green" fuels and back when NASA was NASA)

Page 41 has their comparison of H2 vs JetA fueled large transport aircraft

Lots of good stuff in the study that I haven't had time to read yet

Where are you going to put that much hydrogen on a 747? Certainly not in the wings.