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Comment by dogma1138 | Hacker News Reader

Jet enegines have a combustion efficiency of 100% at low altitude and 98% at cruising altitudes. High bypass jet engines will operate 90%+ propulsive efficiency when you reach cruising speed and altitude.

This can include torbo props.

https://en.m.wikipedia.org/wiki/Propulsive_efficiency

Fuel has also a very big advantage over batteries and that is that you shed off weight when you use it while an empty battery will weight as much as a full one so you’ll end up carrying much more dead weight for the entire flight.

Fuel cells might be a possibility but I don’t think airplanes with hydrogen tanks are a good idea.

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You are confusing combustion efficiency with thermal efficiency. Having a combustion efficiency of 100% does not mean that 100% of the energy stored in the propellant is transformed into kinetic energy (the thing the plane is interested in).

The ICE engine with highest thermal efficiency reaches just 50%: The Wärtsilä-Sulzer RTA96-C turbocharged 2-stroke Diesel [1]

[1]https://en.wikipedia.org/wiki/Thermal_efficiency

That's quite an engine - I just looked up a picture of it https://pbs.twimg.com/media/DQv24mYUMAAmvQs.jpg

The latest F1 tech has started to exceed this using their jet ignition system.

https://www.autosport.com/f1/news/131772/mercedes-engine-hit...

https://www.enginelabs.com/news/f1-tech-what-is-turbulent-je...

Combined cycle gas turbines are actually higher efficiency than this. The current state-of-the-art is somewhere around 63% thermal efficiency.

True, but those are usually not on a moving vehicle and have the luxury of being as heavy and big as desired. Engines on ships instead are limited by weight and size, engines on cars/trucks a little bit more, engines on planes much more and at the edge of technology we have the engines on rockets which have to be as light as humanly possible whilst still providing high thrust-specific fuel consumption

The propulsive efficiency as defined in the Wikipedia article is the product of the cycle efficiency (the efficiency turning heat into mechanical power) and the mechanical efficiency (the efficiency turning the mechanical power generated by the engine into movement of the airplane). The mechanical efficiency can be close to 100%, but the cycle efficiency is only 40% or so.

Out of my depth here. Why are tanks of Kerosene safer than tanks of hydrogen? Pressure?

Hydrogen is gaseous at normal temperatures. Both Kerosene and hydrogen are flammable, but being gaseous means that the resulting exothermic reaction will happen much faster and thus the energy will be released much more violently (think:explosion).

Additionally, as you said, storing hydrogen densely enough means pressured containers, or extreme cooling (like for rockets).

Hydrogen leaks out of literally any container it's stored in, if you fill up a pure lead bottle(a 25kg bottle only holds 1 litre of hydrogen) it will be empty after 2-3 weeks of doing absolutely nothing - hydrogen molecules are so small they just permeate through any safe to handle material. Obviously planes couldn't use tanks made out of lead because of weight, so whatever material they used would leak even faster - and as hydrogen forms an explosive mixture with air, that's not a good thing.

How do consumer-level hydrogen fuel-cell vehicles solve this issue? Does this mean you can't store them in a garage, because the leaking hydrogen could cause an explosion?

They do not - you're not supposed to keep them in enclosed spaces because of this exact issue.

Can't you just vent to outside of the plane? It's not like they are usually in enclosed spaces.

Hangars?

How does the metal cylinder of anaerobic gas mixture in my lab that has a percent of hydrogen in it keep it in indefinitely then?

Don't know - I only ever handled industrial hydrogen containers and I know they couldn't keep hydrogen in for more than couple weeks. I suspect that for it to be useful it needs to be stored as liquid, which means huge pressures = much higher leakage rate. If it's kept at gas it's probably still leaking,just not at a rate that would matter for a while.

Other posters already mentioned the storage factor, so I'll mention the safety factor:

Hydrogen is flammable, but kerosene is not; kerosene is instead combustible. Here's the technical difference between the two[0], but in practical terms it means that you can drop a lit match or cigarette into a tank of kerosene and nothing will happen. You can even heat a tank of kerosene directly with a propane torch without igniting it[1].

[0] https://en.wikipedia.org/wiki/Combustibility_and_flammabilit...

[1] https://youtu.be/7nL10C7FSbE?t=105

Liquid hydrogen is really, really, bad as a fuel.

The energy density per mass is very high, but it's also the least dense gas, so the volumetric energy density is absolute crap, even at high pressure. A bucket of kerosene contains a lot more hydrogen atoms than a bucket of liquid hydrogen, and unlike the hydrogen, it's safe to keep at room temperature, at normal pressure, and it's neither corrosive, flammable, or explosive.

I would say kerosene is flammable...

Flammable has a very specific definition - it must be easily ignited at room temperature. Kerosene is not easily ignited at room temperature (e.g. you couldn't light a puddle of it with a match), so it is not considered "flammable". It does burn at room temperature though, so it is considered "combustible"

Would Li-air batteries be advantageous in this regard if they get them working?

You could use fuel cells that work with more convenient fuels.

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