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.

Comments

The technical problems with this are immense.

[JoshuaZ]

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.

Re:The technical problems with this are immense.

[brambus]

So batteries don't necessarily need to directly compete with combustion engines because electric engines could (and I stress "could") have higher efficiencies than either piston or turbine engines (both of which actually lose a fair amount of the energy in the fuel to their engine cycle as exhaust heat). At 20MJ/kg a battery would compare very well to jet fuel. A good high-bypass turbofan can get maybe 40-45% efficiency. An electrically driven fan, might go as high as 80-90%. But AFAIK Musk wanted to make these aircraft supersonic. While a fan-driven engine *theoretically* can go supersonic, in practice it's so horribly inefficient that it's unlikely to be practical. That's where the Brayton cycle comes in and we're back to the 40% efficiency range (regardless if the reaction mass heating is provided by hydrocarbon fuels or an electrically-sourced heating mechanism) and batteries in that case are dead in the water.
The kicker though, as you correctly identified, is mass loss during flight. Aircraft get a lot of efficiency from this mechanism and also significant mission flexibility (for shorter missions you can take less fuel and more cargo). An electric aircraft would pretty much have to be factory-built for max range from the factor. I highly doubt it's ever going to be practical to reconfigure an electric aircraft on the flight line for shorter haul by taking some batteries out - keep in mind how tricky even comparatively tiny electrical systems are (see Boeing 787 Li-Ion battery fires). Plus the red tape on this is would boggle the mind.
Lastly, we needn't rely on fossil fuels. The public at large always thinks "smelly fuel = dirty". Not necessarily. We can synthesize a wide range of synthetic jet fuels already. Provided that the carbon source for that fuel is "renewable" (e.g. dissolved carbonic acid in ocean water), we could keep the venerable jet engine in place and simply source the fuel in a renewable manner. Then the fuel simply becomes a liquid chemical battery with fantastic power density and deployment flexibility.

Re:The technical problems with this are immense.

[Anonymous Coward]

The model airplane community is where a lot of delusions about the possibility of electric air travel come from. I'm sure you've seen the "man-carrying" many-copters that even university teams are working on. Do I need to point out why that is a bizarre and stupid waste of time and resources? Quad copters are a great way of building small vehicles, because small propellers accelerate quickly, so steering by modulating the propeller speed is easy and works well. This doesn't scale up. Large multicopters are hilariously inefficient and difficult to control compared to more conventional helicopter designs. Small electric models beat ICE models hands down because internal combustion engines don't scale down well to that size. Just because something works well when you're flying one or two pounds of foam doesn't mean it's a good idea for an actual plane.

Re: Batteries just don't store enough energy...

[Rei]

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...