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Comment by schwarrrtz | Hacker News Reader

The specific energy of kerosene is ~40 MJ/kg [0] while the specific energy of lithium ion batteries is ~1 MJ/kg [1].

Unsure how much energy you'd save from the difference in drag, but I'd guess that significant improvements in electric energy storage would be required to make an electric plane doable.

[0] https://en.wikipedia.org/wiki/Kerosene#Properties [1] https://en.wikipedia.org/wiki/Lithium-ion_battery

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To be fair to our electric friends, electric motors are >90% efficient while turbofans top out at somewhere near 35% efficiency (as far as I can find).. So what appears to be a 40:1 advantage to kerosene is more like a 15:1 advantage -- substantial but much more accessible with ongoing battery improvements. Though as another commenter mentioned, fuel planes do definitely benefit from shedding weight the entire time they're moving.

A jet engine (e.g. turbofan) relies on combustion in the engine in order to function. The ignition of the fuel from within the engine is key… it is not just there to just create energy to drive rotational force. You would you use an electric motor on a plane without going back to what is essentially a propeller design?

An engine is just a mechanism for converting stored energy into forward motion - the details are, well, details. There's nothing wrong with propellers (probably impellers will be more efficient), and those efficiency numbers are (presumably) including that.

No, the details are completely critical to making it work!

I'm kind of surprised that Elon Musk, who clearly knows his stuff in rocketry, is handwaving the difficulties of an electric jet. Personally I see it as far more likely that we'll continue to use conventional aircraft but start synthesising fuel from renewable energy. The US Navy is already looking at doing this to make jet fuel from spare carrier nuclear power:

http://www.nrl.navy.mil/media/news-releases/2014/nrl-fuel-fr...

You can get supersonic by having air intake, expansion chamber, blade, high pressure chamber, exhaust port.

Think something like this: <=|=>

Importantly the air flows through the compression chamber at Subsonic speeds.

PS: While simplified this is the general approach for low mach speeds as burning stuff in a supersonic combustion chamber is much more difficult.

Actually, a good 80% of a modern airliner's thrust comes from the huge fan in front of the engine.

A turbine after the combustion chamber steals some of the energy to move the compressor and the fan. Most of the air (>90%) just goes through the fan and bypasses the engine. You could argue that these engines are _already_ cleverly camouflaged propellers :)

https://en.wikipedia.org/wiki/Turbofan https://www.quora.com/How-much-of-a-jet-engines-thrust-is-de...

To be fair, you can manufacture kerosene from renewable electricity. If you use carbon from the atmosphere, it's even completely carbon neutral. Think of it as a less cost efficient, but more energy dense battery. I kind of doubt that batteries are the way to go when energy density matters.

Plus a conventional airplane sheds weight as it travels.

Nothing that recoverable drop-tank batteries can't solve. I'm mostly kidding.

Refueling via amazon drone?

And yet Musk claims transcontinental range (NY-LA) would be possible with 400 Wh/kg batteries and mid-70% battery mass fraction.

http://www.aviation.com/general-aviation/elon-musk-toying-de...

A 70% battery mass fraction seems like a huge engineering challenge for an aircraft. I did a quick calculation of fuel mass fraction for the various Boeing 777 models [0] which ranged from ~13% to ~36%.

Not saying it's impossible. Just hard.

[0] https://en.wikipedia.org/wiki/Boeing_777#Specifications

I don't see that the engineering is hard. You could just take an existing design and replace most of the cabin with batteries.

Building an economically viable mass-market aircraft with a 70% mass fraction might be another question, but if we're just talking about the engineering challenge of building it at all I don't think that's hard?

And on top of that, "refueling" wouldn't cost the 250k figure that usually gets thrown in comments. I imagine it being more in the space of a 1-2k$ ; in which case the reduced passenger capacity might still be worth the premium in batteries.

Specific energy of hydrogen is 142 MJ/kg. Why not a hydrogen fuel cell powered electric jet?

Because hydrogen is very hard to store in any reasonable size or shape, and that's even without focusing on the inefficiency of generating it.

Ammonia doesn't have this or several of the other drawbacks of hydrogen, but is otherwise equivalent.

Drag on a bigger airframe.

exactly

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