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by mikeyouse 3741 days ago
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.
1 comments
SeoxyS 3741 days ago
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?
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gruturo 3740 days ago
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...

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lmm 3740 days ago
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.
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pjc50 3740 days ago
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...

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Retric 3740 days ago
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.

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