Hydrogen Is a Volume Problem, Not an Energy One
Hydrogen looks like the obvious answer to aviation: it carries nearly three times the energy of kerosene per kilogram and produces no carbon dioxide at all when burned. The catch is not the energy. It is the volume.
Excellent by mass, poor by volume
Liquid hydrogen holds about 120 MJ per kilogram against Jet A's 43 — a factor of 2.79, and for an aircraft, where mass is everything, that is an enormous advantage.
But it is extraordinarily light: about 0.071 kg per litre, against 0.80 for kerosene. Multiply through and hydrogen delivers about 8.5 MJ per litre against Jet A's 34.4 — roughly a quarter of the energy per litre. The fuel energy comparison shows both figures side by side, because quoting either alone misleads.
Four times the tank
The practical consequence: a hydrogen aircraft needs roughly four times the fuel volume for the same energy. Kerosene lives in the wings, in space the wing needs anyway. Four times that volume does not fit in a wing.
This is why hydrogen aircraft concepts look different — tanks in the fuselage behind or above the cabin, blended-wing bodies, unconventional layouts. It is not styling; it is the only place the volume can go, and it costs cabin space and changes the structure.
Cryogenic, not just compressed
Liquid hydrogen has to be held at around minus 253 degrees Celsius. That means insulated cryogenic tanks, which are heavy, and boil-off — hydrogen warms and vents over time, so a fuelled aircraft cannot simply sit for days.
Tanks must also be roughly spherical or cylindrical to handle the pressure, which wastes volume compared to the conformal tanks that make wings such efficient fuel storage. Some of hydrogen's mass advantage is spent on the tank that contains it.
Zero carbon at the engine only
Burning hydrogen produces water, not CO2, which is genuinely significant. Two qualifications.
First, the hydrogen has to come from somewhere. Most hydrogen produced today is made from natural gas, and using that in aircraft would move emissions rather than remove them. The climate case depends entirely on hydrogen made with clean electricity.
Second, water vapour at altitude is not climatically neutral — contrail and cirrus effects are part of aviation's impact, and a hydrogen aircraft emits more water than a kerosene one. This is an active research area and one of the genuine open questions.
Everything on the ground changes too
Airports would need cryogenic storage and refuelling systems, new safety procedures, and trained crews. That is a large, slow, capital-heavy transformation, and it has to happen at both ends of every route an aircraft flies.
It is the strongest practical argument for SAF as the near-term option: SAF needs no new infrastructure at all, while hydrogen needs a parallel system built out before the first commercial route works.
A serious contender, on a long timescale
The physics is favourable and the engineering is hard — which is roughly the opposite of the battery case, where the engineering is tractable and the physics is not. Regional and short-haul hydrogen aircraft are where the credible programmes are aimed, and the timelines are measured in decades.