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Elon Musk's Next Great Idea? Electric Air Travel (bgr.com)
An anonymous reader writes with this excerpt from BGR: Elon Musk is changing the world one idea at a time. First, with Tesla, the man so many people call the real life Tony Stark has done an incredible job of bringing electric vehicles to the mainstream. Second, Musk has been doing an impressive job over at SpaceX in the realm of space travel. And third, Musk's effective rough draft of a high-speed transportation system known as the Hyperloop is being contemplated and conceptualized in a very real way by some extremely smart people. So where does Musk go from here? Why, Mars of course. Recently, Musk said that he plans to unveil SpaceX's Mars roadmap next September. But on another front, Musk has also been thinking about developing an electric airplane capable of taking off and landing vertically. While answering a few questions during a Q&A session at the SpaceX Hyperloop Pod Award Ceremony last week, Musk was asked what his 'next great idea' was. The answer? Electric-powered air travel.
Batteries do not have the energy density of jet fuel. The primary thing that matters here is energy density, which has two forms, energy per mass and energy per volume. https://en.wikipedia.org/wiki/Energy_density Both need to be much better than they are today for electric airplanes to have any chance (lifespan and and number of cycle uses also need to improve but that's in some ways less of a barrier.) Energy density of batteries by both metrics batteries has increased by 5%-10% a year depending on the exact metric and choice of examples https://www.quora.com/Is-it-true-that-battery-energy-density-improves-5-8-per-year which is exponential growth ( but with a much slower doubling time than something like Moore's Law. One has a doubling about once every 8 or 10 years.) Jet fuel has an energy density of around 45 MJ/kg, The most efficient batteries have a little under 1 MJ/kg. So one needs at least about 5 doublings before batteries can reasonably compete which will start to occur if they have an energy density of around 32/ MJ/kg. Similar remarks apply to energy density measured by joules per volume. However, there are technical reasons to think that batteries will stop doubling before that (see theabove quora link for details which argues that we can't make batteries much than four times as efficient before we start running into serious theoretical limits). At around 20 MJ/kg, one maybe could run planes practically but they would be much less convenient and practical than today's jets and that would be at the very upper end of the plausible limits just from a straight energy density estimate.
However, the situation is even worse than that. When you use jet fuel, you use it up. Depending on the type of airplane, at take off fuel is generally 25% to 50% of the mass of the plane. So one gets serious savings that one doesn't have to move all the used fuel the entire way. That doesn't work with batteries: they are the same mass and volume whether or not they are charged, and dumping them would defeat most of the point. It might be possible to do some sort of staging approach where one uses some set of batteries to nearly empty and then have them break off in a modular plane that returns to the ground site. But that itself would lead to all sorts of additional problems.
So it is likely that we will still see fossil fuels used for jets for the next 40 or 50 years. Indeed, it is likely that they will be the very last use of fossil fuels.
If it takes off vertically, you could possibly provide electrical power from the ground for a certain distance, overcoming the initial lift off energy use at least. But still, better hope battery tech evolves dramatically for any real prospect of fast long distance E flying.
Li-Air have roughly the same specific energy as gasoline. I'm guessing aviation fuel is similar. So if electric engines can be that are more efficient than jet engines...
Who ordered that?
A very expensive Li battery can hit 1 MJ/L. Diesel (jet fuel) is about 36 MJ/L. There needs to be over an order of magnitude improvement before this can work.
When flying, isn't it weight rather than volume that matters?
Confucius say, "Find worm in apple - bad. Find half a worm - worse."
Yes. Also, you can't ignore comparative efficiencies of engines. Or engine mass to weight ratios. Or the length of time to market, and the expected level of battery change during that time period. Or side benefits (for example, the ability to have small, very light engines was made use of in one NASA experiment that placed numerous small engines along a wing, causing an effect that created drastically more lift at low speeds and allowing for a much shorter takeoff distance).
And beyond that, you can't ignore economics. Having reduced range but getting your fuel at a fraction of a cost may ultimately prove to be more desirable. It's a very complex issue that one can't just make all-encompassing statements based on a single figure like "energy density of batteries vs. energy density of fuel".
Anyway, this is hardly Elon's first time to mention it. Years ago he mentioned that he wants to be the first person to have an electric plane break the sound barrier. If there's anything one can say about Elon, it's that he sure doesn't set the bar low...
It's times like this I wish I had a friend named 'The Professor'.
The only practical application of an electric airplane within the near future would be lug around batteries. Which may come handy to Elon with his gigantic battery factory. Stuff the "plane" with fully charged batteries and fly it to the nearest sea port or distribution center.
Well, you see the problem: it doesn't fit in the subject line.
Next time you reference Him, I suggest you use the proper enneagrammaton for The Man Known As The Real Life Tony Stark: TMKATRLTS.
I have discovered a truly marvelous proof of killer sig, which this margin is too narrow to contain.
Electric air planes with lithium-air batteries would weigh the same at landing as they do at takeoff whereas a 747 loses around a quarter of it's weight en-route.
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On the other hand, as the jet fuel is consumed the weight decreases. Batteries stay the same weight for the entire flight.
An interesting point . . . when a jet needs to make an emergency landing with full tanks, it will ditch the fuel before attempting a landing, because of the fire danger. Will this be necessary with Li-Air? Could there be a danger of a fire if the plane needs to land under "extraordinary circumstances? Like, no landing gear?
So will a Li-Air plane need to have a mechanism to ditch the batteries?
And if the batteries land in my backyard, can I keep them . . . ?
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As I posted below, it seems pretty obvious you would use fuel cells instead of batteries for an electric aircraft... from your energy density link compressed hydrogen has an even better energy density (142 MJ/kg) than jet fuel (46 MJ/kg)!
The cost of hydrogen production is estimated to become close to gasoline production over the next decade or so, but there is a huge pollution benefit to using fuel cells which could drive adoption quicker.
The currently very low cost of oil is probably the main thing that would keep airplanes from going electric soon.
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I've only heard it from tech press who are constantly cheerleading Tesla and Musk. I've questioned the number of Tesla stories on Slashdot before and been downvoted by fanboys, their marketing, sales methods, spats with journalists etc just aren't technology stories.
Electric air planes with lithium-air batteries would weigh the same at landing as they do at takeoff whereas a 747 loses around a quarter of it's weight en-route.
It is even worse then that. Li-Air batteries absorb oxygen as they release electricity. They get heavier the lower the electric charge. The only possible advantage is that they are lightest when they require the most power - take-off.
No idea how Elon Musk feels about it, but I think it's not quite appropriate.
The fictional Tony Stark made his money with dubious weapons business.
Frankly Elon Musk is the better man.
NASA has been researching electric aircraft for quite a while. They do have some advantages, although they're not ready to commercialize yet.
http://www.nasa.gov/centers/ar...
http://www.nasa.gov/aero/testi...
http://aero.larc.nasa.gov/file...
http://ntrs.nasa.gov/archive/n...
http://ntrs.nasa.gov/archive/n...
http://www.geoffreylandis.com
Yes, they still do that, though in developed areas it's often flying for longer to burn the fuel rather than just dumping it.
I don't read AC A human right
An interesting point . . . when a jet needs to make an emergency landing with full tanks, it will ditch the fuel before attempting a landing, because of the fire danger.
No, no they don't. The dump fuel because the maximum landing weight on commercial aircraft is much lower than the maximum takeoff weight. Fuel is too damn expensive to dump just because you don't want to explode. They'll dump fuel until they are under the max landing weight, then land.
You can land at heavier than the max landing weight - but you'd better get it right or you'll never fly again - even if the landing is a good one.
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-1 Stupid.
You're forgetting that 60-75% of the energy in hydrocarbon fuel is wasted in the form of heat when you burn it in a combustion engine. Conversion losses for electricity are a tiny fraction of this.