[srb-]

Like most, I originally assumed that because batteries have poor specific energy compared to fossil fuels, there was no point in considering them for aircraft.

But Musk's assertion has made me reconsider. Has anyone here done the math? What I think he's talking about is NOT a conventional plane with a battery simply replacing the gas tank, but instead essentially a flying battery, where 90% of the plane's mass is battery.

This sounds ridiculous at first, but when you consider how cheap electricity is compared to jet fuel, as well as the other benefits the OP mentioned, there might be something to it (for short-haul flights.)

[mikeyouse]

Just so people have a basis for his figures, Musk's calcs rely on 400wh/kg batteries and ~75% cell-mass fraction which he thinks will be sufficient for transcontinental flights. Another way to improve efficiency is be removing 'unneeded' components on the planes like tails, rudders, and elevators by just gimballing the electric fans for control. An article with a few embedded clips where he discusses his proposed VTOL supersonic electric plane:

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

[masklinn]

> What I think he's talking about is NOT a conventional plane with a battery simply replacing the gas tank, but instead essentially a flying battery, where 90% of the plane's mass is battery.

Weight is the premium on planes, so the 10% of non-battery you're talking about is a full plane minus fuel tanks. An E-170 is 21t of empty plane, up to 9t of fuel and up to 36t total (MTOW). Given batteries have 1% the energy specificity of aviation fuel (so you need 100 times the weight in batteries for the same stored energy), the math doesn't quite work out.

> This sounds ridiculous at first

And at second, and at third.

> when you consider how cheap electricity is compared to jet fuel

Doesn't compensate the problem of needing a plane 10 times as heavy with 1/10th the payload.

> as well as the other benefits the OP mentioned

Which don't really make sense, lower air density means your turboprops have nothing to work with, there's no such thing as turbofans let alone turbojet so the gains in drag are killed by the inability to actually go forward.

[blktiger]

At Airventure last week I listened to a talk on an electric motor glider (http://electricmotorglider.com). One of the takeaways that I had was that with current battery technology, for electric motors to make more sense than a gasoline powered motors you have to have a flight time of less than 23 minutes. Above that, the reduced weight of fuel beats out the weight of batteries. We need additional research in the battery technology space before it's going to make sense for aircraft to be electric.

[sokoloff]

Electricity is not especially cheap compared to jet fuel.

Jet Fuel has about 128kBTU/gal and is available around $2/gal, so 64kBTU/$.

Electricity is about 3400BTU/kWhr and a kWhr is about $0.05, or about 68kBTU/$.

[coryrc]

Except, electricity will be about 85-90% efficient and a turbofan is about 30% efficient [1], roughly tripling the cost advantage.

Fuel costs are about 35%[1] of operating costs, so only a ~20% cost reduction is possible for fuel change, so your point is still mostly valid.

[1] https://en.wikipedia.org/wiki/Jet_engine#Energy_efficiency

[2] http://www.ajc.com/news/business/airlines-keep-adapting-to-h...

[masklinn]

> Except, electricity will be about 85-90% efficient and a turbofan is about 30% efficient [1], roughly tripling the cost advantage.

But then batteries store 1% the energy per mass fuel does so even assuming triple the efficiency, the energy storage is 30 times easier (assuming everything else stays constant which it can't because now your craft is an order of magnitude heavier and your short-haul plane is as heavy as a 747)

[srb-]

Ah, thanks for putting some numbers out there everyone. Very interesting. That's why I love HN.

Musk has said several times that if he wasn't working on space and EVs, he'd be working on hyperloop or electric planes. Wonder what potential he sees in the latter.

[csours]

Except, you will be carrying more weight, due to the weight of the battery.