The main drawback is the low density which would require larger fuselages, but that should be offset by the significantly lower weight.
- It'll be a long time before fuel cells are cost competitive with gas or electric cars, but for planes the cost of fuel and operation is much more significant than with a car, so the upfront cost for building the plane seems less significant in comparison
- Refueling infrastructure is much easier to implement at a (relatively) small number of airports than it is to build many small consumer refueling stations everywhere
- Space is more important than weight in a car, but the opposite may be true for a plane
- Crash safety is a much bigger problem for cars than planes - plane crashes are rare, and if a plane crashes there's a good chance everyone dies anyway, but if you're designing a car you're going to have to be really careful to make sure every other fender bender doesn't result in a hydrogen explosion
Of course there's the issue that most hydrogen today is created from fossil fuels, but perhaps there are ways to generate it sustainably and still remain competitive with the cost of jet fuel?
Electric cars can be a very competitive alternative to gasoline cars, but electric batteries are not yet energy-dense enough to make long-distance air travel viable, so it feels like hydrogen planes might be the least bad solution if we want to get off of fossil fuels.
That's actually not the case. Overall your odds of surviving an accident in a plane are something like 95%. Still above 50% for "serious" incidents. Sure, if the plane drills the ground everyone is going to die, but planes crash somewhat regularly with most/all surviving.
Otherwise you'll have a really giant bag of gaseous hydrogen which does help your bouyancy, but then you've made an airship instead of a plane and a spark accidentally turns it into the Hindenburg.
Compressed at 690 bar (about 680 atmospheres) it goes all the way up to 5 MJ/L, only 7x as much volume of fuel required compared to jet fuel. But the tanks to pressurize something that much are presumably very heavy and not a reasonable solution in an airplane.
Liquifying it gets that up to 10 MJ/liter and I don't think you need as much pressure to store that (as long as you keep it very very cold).
Per weight instead of volume you beat jet fuel (by 3-4x), so that aspect is a win. Having enough volume to store it and keeping the storage equipment light are the challenges.
It can be burned in a gas turbine, but for now the NOx emissions are pretty horrible.
Pressure vessels though, you're better off making them round.
Either way, the Soviets tried gaseous fuels in the 80s - See Tu-155. They ran on liquid H2, CNG.
If I recall, the weight increase didn't make it practical. On the other hand it was a turbine powered plane. It might work out with fuel cells (although you'd need a big and heavy compressor to run it)
Until recently, making hydrogen by electrolysis was not a benefit, environmentally, as a lot of the electricity would have come from fossil fuels. I think decomposition of methane is an option that would allow the carbon to be sequestered.
This charge seems to indicate liquid hydrogen would require something like three times the volume as jet fuel / kerosene/ diesel.
https://upload.wikimedia.org/wikipedia/commons/thumb/c/c6/En...
Most rockets use hydrocarbons for this very reason.
Shape will be more interesting - storing the fuel within the wings is not a very bright idea in this case.
This one is a non-problem. I don't think there is any civilian airport with enough fuel storage for two weeks.
(But the tank weight, well that is a problem.)
The ones that aren't supplied by pipelines have trains or fleets of trucks supplying them. Those would see a gain on using water, electricity and some mildly expensive machinery.
Of all the very relevant problems of using hydrogen on planes, long term storage is not one of them.
What problem are we solving by moving to hydrogen?
Safety is critical on aircraft. Hydrogen is difficult to contain, and has very low viscosity due to its small molecules. It burns with an almost invisible yet very hot ultraviolet flame. It easily mixes with air to form an explosive mixture and has a very low minimum ignition energy (about a tenth of gasoline).
Combustion of hydrogen in a heat engine for propulsion would be very inefficient compared to a battery electric. Fuel cells are in between, at about 40-60% efficiency, but have a substantial weight cost.
But don't forget that the stated purpose of funded research is often a lie[0] and this is not necessarily viable outside the lab.
[0] You get other benefits from the research including subsidizing your national industry, training your workforce, doing research you're interested in with a more politically palpable purpose.
~methanol Renewable CO2 recycling and synthetic fuel production in a marine environment https://www.pnas.org/content/early/2019/05/28/1902335116