[koolba]

I'm not at the forefront of this field so maybe I missed some great advances in battery technology or airplane efficiency, but how could this possible work when the energy density of a battery (.3-.9 MJ/kg) is 50x less than that of jet fuel (46 MJ/kg)[1]? Or is that simply enough for (very) short haul flights?

[1]: https://en.wikipedia.org/wiki/Energy_density

[fdej]

That's correct, but note that an electric motor is about twice as efficient as a combustion engine, reducing the factor to about 25x (at least ideally). Typical medium range aircraft burn about 3 kg fuel per km [1], so to fly 1000 km you would need about 3 * 25 * 1000 kg = 75 tonnes of batteries. This should be in the ballpark of what we can pack into an aircraft already, but I guess at least another 2x improvement is needed to make it economical (we want to haul some useful payload, not just the batteries).

[1] https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft

[mikro2nd]

There are other considerations, though... The thought of a Li fire on board an aircraft is something that would keep me awake nights.

I'd guess that anybody wanting to put Li technology batteries aboard aircraft is likely to spend the next 10 years jumping through regulatory hoops, so unless rechargeable battery technology is "ready to deploy" right now on aircraft, I don't see any way this is happening within a decade.

[fab13n]

> The thought of a Li fire on board an aircraft is something that would keep me awake nights.

Whereas you wouldn't think twice about jet fuel fires, because you're used to it rather than because fuel isn't a fire hazard :-)

[ams6110]

Jet fuel isn't very flammable unless it's atomized or vaporized, something which only happens in the engine. And it's pretty straightforward to extinguish an engine fire, and all aircraft engines have systems to do this.

[walshemj]

And extinguishing a metal fire is not trivial

[JumpCrisscross]

Doubly surprising given the fear is expressed in a thread specifically about the 50x energy-density differential between jet fuel and Li-on batteries.

[fdej]

Trivia: there was about as much chemical energy in the fuel of either Boeing 767 that struck the twin towers as the gravitational potential energy released by the towers collapsing.

[JustSomeNobody]

What's the turnaround for the aircraft though? If they can't turn around an aircraft in 15-30 min, we'll need more aircraft or prices will skyrocket (no pun).

[notahacker]

Any sensible proposal for electric systems for commercial aircraft is going to aim for batteries to be safely swappable in 10-15 minutes. (Though they can still make delays from unexpected diversions to airports without a stock of batteries worse)

[jpm_sd]

The structural penalties for making "most of the weight of the aircraft" removable will probably lead to a plane that doesn't fly.

[greglindahl]

... for example, putting the batteries in the existing form-factor of removable cargo containers used in jumbo jets.

[JustSomeNobody]

Hmmm... I wonder. Let's say they design the aircraft around an existing battery (size/weight). If later technology allows for a lighter battery, would they they have to add the weight back in order to keep the cg correct?

[dx034]

And then charge overnight and be used as energy storage for the time they're not in use. As flight times are very predictable, they should be a good use case for battery storage.

[fab13n]

They say they plan to package batteries are packaged as standard freight containers.

Since current turnarounds include freight container swaps, I don't see why a battery swap wouldn't fit in it.

[rwmj]

Supercapacitors could solve that, but unfortunately they currently have much less density even than batteries. Apparently graphene supercaps may solve that, but not at the moment.

[wlll]

Tesla style battery swap perhaps?

[polshaw]

Yes the idea for this will be v. short haul for now which is why they talk about London to Paris; ~215 miles. 25 times further (50x lower energy density, 2x better engine efficiency) would be under 5,400 miles.

Looking at the empty weight, the weight of max fuel capacity and range[1], it seems realistic to fly 200-300mi on batteries without any kind of jettisoning.

So it seems feasible, the next question is where could it be in terms of commercial viability.

1. https://en.wikipedia.org/wiki/Boeing_777

[aphexbr]

FTA: "The company is relying heavily on innovation in battery technology continuing to improve at its current rate. If not, the firm will not be able to build in enough power to give the plane the range it needs."

So, it presumably can't work now but they're putting their money on the idea that it will in the near future.

[jacquesm]

At which time everybody and their brother will be doing the same. The hard part is the batteries.

[6stringmerc]

See also: Moeller Air Car

[tankenmate]

The equation isn't just energy density; it's also the efficiency of harnessing that energy. Good electric motors are way more efficient than a turbofan.

[blutack]

Electric motors would need to be orders of magnitude more efficient than turbofans (which they aren't). You also need to compare the overall efficiency developed as thrust, not just thermal efficiency.

There's also the issue of the mass of batteries not going away during the flight, unlike fuel, which will require maximum landing weight to equal maximum takeoff weight - this will impose a structural penalty versus IC aircraft.

[cjrp]

> There's also the issue of the mass of batteries not going away during the flight

Excellent point; currently as fuel is burned off, planes can climb to higher, more efficient altitudes (known as a step climb). What's the performance penalty for not being able to do this as your batteries aren't getting lighter as the flight progresses.

[gsnedders]

Such short flights also spend a large proportion of time on the ground taxiing, etc., where turbofans are very inefficient. They spend incredibly little time at their most efficient high-altitude cruise.

One can imagine an electric aircraft relying on powered landing gear (as previously trialled by Airbus/Lufthansa Technik) which would further improve efficiency during taxiing.

[neolefty]

Maximum efficiency is ... 100%. So if a turbofan is 30% efficient, then the best you can do is about 3x.

[ericd]

Good point, but something to keep in mind is that the efficiency of the motors in turning stored energy into motion is only one part of the efficiency of the aircraft. If a plane is able to fly much higher due to the engines no longer being air breathing, that would also change the equation, as drag would be much lower with the thinner atmosphere.

[Gravityloss]

Also you can tune the location and amount of propulsors, and have a high efficiency rotation rate with direct drive electric motors.

The current turbofan engines are close to the size limits in relative fan size and bypass ratio without a gearbox, which adds cost and mass. (Only one manufacturer is in the process of introducing a geared turbofan.)

Because of no high speed jet exhaust and large area thrusters, these aircraft have the potential to be able to take off really steeply and to be really quiet.

[dredmorbius]

That buys you roughly a factor of three.

Subtract again for the fact that you're not shedding half your takeoff weight in fuel over the course of a flight. You land with as much battery as you started with.

More if you're using metal-air batteries.

[koolba]

Not necessarily!

Imagine a system where the plan actually jettisons the spent batteries along the way. They could glide/parachute down to a collection depot to be recharged and sent back to the airport for installation.

[nradov]

You've got to be kidding. The civil aviation authorities would never allow airlines to jettison solid objects over populated areas for safety reasons.

[J-dawg]

What if the jettisoned objects were autonomous aircraft that could land accurately at a specified location?

[srssays]

A better idea would be to dig oil out of the ground, process it into kerosene, and use that to power jet turbines on the aircraft.

[dredmorbius]

I'll give you credit for out-of-the-box^Wairframe thinking there....

[J-dawg]

Speaking as someone with no aviation knowledge at all this seems like an awesome idea.

The plane could take off with battery packs slung under the wings which are jettisoned and become drones, gliding down to a depot.

I'm now waiting for someone who actually knows what they're talking about to point out the flaws. One that springs to mind is the logistics of collecting the drone-batteries and transporting them to be reunited with the parent aircraft.

[dredmorbius]

Keep in mind that all protrusions are going to generate significant drag. So if you can find ways to incorporate the batteries within the airframe, you'd be better off.

There's also the problem of mass-transfer. In general, aircraft should keep the center of mass behind the center of lift, and ... bad things happen when this isn't maintained. With liquid fuel, tanks are actively pumped to retain both forward-aft and left-right balance, something difficult to achieve with solid battery packs.

There's the problem of both energy consumption profiles and battery delivery/drain cycles. An aircraft generally needs maximum power to get off the ground (hence: all batteries delivering at or near their maximum output), but only partial power once airborne. What you'd like to do is to drain a few batteries completely in the take-off and ascent stages, then jettison them, but this doesn't match the batteries' own power delivery capabilities. You might be able to switch to jettisonable packs after TOaA, to completely drain those.

If fit between wing spars, you might end up with a roughly rectangular package which could be ejected aft of the aircraft from the wing, with a door sealing off the cavity. The battery itself would require some sort of deployable wing itself, as well as guidance and control systems and surfaces, possibly a small propulsion unit. A guided descent stage might actually be one of the more viable options.

It's also possible that jettisoning additional batteries on final approach would make landing dyanamics for the aircraft itself simpler.

On whole, though, I'm questioning the usefulness of this, particularly given coplexities, a likely low airspeed, and competition with ground-based alternatives (high-speed rail, Chunnel) which would bypass the power storage requirements entirely, and would likely operate at equivalent or greater speeds, direct to city centres.

[J-dawg]

Thanks for the comprehensive reply. I find it interesting to consider ideas like this, even if they ultimately turn out to be unworkable.

[kgabis]

I would also add safety and the fact that 70% of our planet is covered in water/not easy to retrieve batteries from.

[dredmorbius]

The aircraft in question is being offered for a relatively short-range flight (London-Paris), 345 km (215 mi). Odds are good this flight configuration wouldn't be used on long-distance trans-oceanic flights.

Though it might be best-suited to short hops between islands, islands and mainlands, or across deep peninsulas. That's a somewhat limited set of markets.

For travel between heavily-populated continental points, ground-based rail would almost certainly be more efficient and practicable, and even in the case of the suggested London-Paris route, there is a ground-based alternative.

[jsjohnst]

The risk of puncture would likely cause the safety precautions required to make this cost prohibitive. That's just one battery specific reason, there's hundreds more why jettisoning isn't done except with military aircraft and even then only rarely.

[Gravityloss]

More likely to be a tow line (like are used already for gliders) or a tow aircraft?

[JustSomeNobody]

That sounds like something that could go very wrong and either cause the plane to crash or hurt/kill people on the ground.

[jpm_sd]

That's called a "bomber".

[jakozaur]

Maybe we should think outside of the box. What if, aircraft doesn't need to store all of the required energy on-board?

E.g. transmit energy wirelessly through microwave and just have enough fuel for emergencies.

[wlll]

Planes I've been on regularly fly above the clouds, I wonder how much energy could be generated by covering the entire top of the aircraft with solar panels? I don't work in this field so as far as I know it could be a drop in the ocean.

Clearly only possible during the day though.

[gambiting]

It is a drop in the ocean. 1 square meter of surface receives at most 1kWh of energy from the sun, and that's only at the equator, on clear day, and without taking any conversion losses into equation.

But for the sake of fun - For a Bombardier Q400(that was the only reliable data I could find)[0], the total surface area is ~213 square meters. That's obviously total surface area, so it includes both the top and bottom of the airplane. For simplicity sake, let's say only half is in full view of the sun - so 107 square meters.

Even if we assume magical solar panels that can actually convert 100% of the energy into electricity, and assume that the plane is flying in full daylight, that's only ~100kW.

The aircraft uses two PW100 engines, each producing....3700kW, so the total output of the aircraft is around 7400kW.

So even if the conditions were ideal, we had magical solar panels, and covered every inch of the aircraft, they would produce....less than 1% of power necessary to fly it. To be fair, a normal 2.0L petrol engine produces more power than what those solar panels would produce.

In reality, those panels could maybe power the onboard lights and computers? Just about?

[0]http://www.supersonicinstitute.org/docs/Q400SurfaceAreaRepor...

[jsjohnst]

> In reality, those panels could maybe power the onboard lights and computers? Just about?

Doubtful. I don't have the exact numbers handy, but the power draw there is much higher than you think. That said, if you're unicorn farting skittles scenario was actually possibly (100kW solar per plane), which we know it isn't, then sure, it's enough power for that.

[brians]

Durability is a problem here too: these things fly through hailstorms and thunderstorms, and it'd be nice to have power coming out the other side.