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Comment by MrVitaliy | Hacker News Reader

I'm hopeful for hydrogen use in aviation. Either that or some ridiculous breakthrough in battery tech, and the latter seems less likely than former.

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Existing aircraft will be wholly unable to compete on any route where LH2 aircraft operate, but it will take a long time to bring up the infrastructure for it. Expect it to appear first on select freight routes.

Oddly, H2 aircraft seem to be promoted with inboard tanks. The natural place for the tanks is in nacelles slung under the wings, for safety. (Hydrogen would not fit in the wings.) Hydrogen tanks in an enclosed cabin is a formula for disaster.

> Existing aircraft will be wholly unable to compete on any route where LH2 aircraft operate

? Are you suggesting this will be a result of regulatory action? Since it would most likely be more expensive for the first decade, even if I gave you a tap on the airfield labelled "free H2"

It is because the very large difference in fuel weight for a flight leaves radically increased capacity for carrying paying freight. As LH2 produced on-site at airports gets cheaper than Jet-A fuel, the gap widens.

I'd need to see the numbers on this on joule/kg, and then allowing for the increased tankage size.

Volume is cheap, on aircraft. If a normal freighter today takes off with 25t of kerosene, the similar-sized LH2 craft needs only 10t of LH2. That means it can take an additional 15t of paying cargo.

I am fairly sure this is due to the necessity of having pressurized cylindrical storage for the hydrogen. Fitting that kind of tank into the wings is going to be hard.

Tanks would be wholly impossible to fit into the wings. Thus, the under-wing nacelles. But pressure cylinders are a non-starter, because they are heavy. Expect to see, instead, insulated, unpressurized LH2 tanks.

The common criticism of LH2 economics is the energy required for liquification. Are you assuming that this is taken care of by overbuilding solar/wind and creating LH2 opportunistically?

Much the way aluminum producers guarantee load for hydro power producers, airports will consume overproduction by nearby solar farms.

Given the power requirements of flight — they're fuel efficient per passenger-kilometre, but they have a lot of passengers and go a long distance — planes are the only place[0] where I think "overproduction by nearby solar farms" just isn't going to be a thing.

OPEC paving their deserts with PV and synthesising fuel (whatever that is: hydrogen, Sabatier methane, aluminium for burning) or just exporting that electricity along a 2m^2 cross section solid aluminium rod to the other side of the planet? Sure, plausible.

[0] I was going to say "and rockets", but then I realised we don't launch anything like as many rockets as we fly planes, so even then rockets might still be running on green hydrogen or methane derived from it.

Notably, SpaceX is making no visible effort to synthesize methane for their cans. The only gesture in that direction is an announced plan to buy a pre-fab methane refinery that could possibly be adapted to run Sabatier; but no hint at a solar farm to power it, or a place to put one. I guess they could build one across the border in Mexico? CH4 for Cape Canaveral is a separate problem. More likely the refinery will just purify mined LNG.

Solar and wind farms supplying international airports would probably need to send power via HVDC transmission lines. But, yes, the airports will need much more than just overage from the farms, and probably booster shipments of LH2 from farms in the tropics, besides. Imagine how big must be the project of refining, transporting, storing, and distributing kerosene to gates, today. Yet it is made almost invisible.

For SpaceX in particular: if they don't do it, they're not going to Mars.

(That doesn't mean they will do it, it's just the consequence of failure).

Not coming back, anyway.

Why do you believe it will be freight first? Historically, the new aircraft go to pax service, then get converted after 20 or 30 years.

That tendency will delay the transition. But the big economic benefit arises from being able to carry 40% more cargo because the fuel is lighter. Passenger aircraft capacity is maxed out when the seats are full, now that carrying freight in the same aircraft is not allowed. Also, it will take a long time for safety worries to be satisfied.

I think hydrogen for aviation is probably a dead end. I suspect the long term solution for aviation, at least long haul, is biofuels. Battery tech is edging into the realm where it could be used for short haul flights, but we have quite a ways to go before you'll be able to fly across the Pacific, or even Atlantic, on a single charge with reasonable passenger comfort.

Biofuels are fundamentally not green. You might as well suggest we use fossil fuels forever.

What do you mean? The fuel itself should be carbon neutral, even if it requires extra energy to create. I don't see any reason you couldn't make carbon neutral bio fuel, although I'd net on syngas instead.

Growing food and converting it to fuel is not carbon neutral. Farming is very resource intensive at the scales needed. In fact, it is literally the same idea as carbon capture, but via biology instead of synthetic. As a result, it is entirely a bad idea.

Biofuels today have issues, but there's no fundamental problem preventing them from being green. It's just a side effect of shoehorning them into our existing agriculture systems.

In the future when we have more solar power than we know what to do with during the day it may become economical to run the Sabatier reaction with hydrolyzed seawater and atmospheric CO2 to make methane, which can be burned in a lightly modified aircraft turbine.

That isn't biofuel. That's synthetic fuel.

flying machines and hydrogen has an spotless history without any tragedies whatsoever

A catch is that hydrogen does become much safer in high-pressure tanks, because, when such tanks are breached, hydrogen escapes so fast that it can't react with oxygen fast enough to burst into flame. To prove this, Toyota literally shot their hydrogen tank with .50 cal[1].

[1]: https://www.youtube.com/watch?v=jVeagFmmwA0

Practically no one died in the Hindenberg disaster. There were more people injured than died. Most people involved just walked away from it. It was televised spectacle much more than a factual tragedy.

(It's a bit like saying we shouldn't use AC power because look what Edison did with those poor elephants. It's interesting anti-technological propaganda that made sense socio-politically at the time but isn't that useful to today's discussions.)

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