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Electric aviation for aircraft larger than 4 passengers is extremely hard. Batteries don't have the energy density needed, and fuel cells are barely at the prototype stage for aviation. ARPA-E has been looking at this for while, see slide 6 in [1] for an overview of payloads by energy storage medium. If you read a little more through the deck you'll see there are also issues with power output of electric motors in this application.

I'm much more optimistic about low-carbon jet fuels. Biofuel options here have already been trialed successfully. And some new synthetic fuel companies [2] that use direct air capture CO2 are looking promising.

[1] https://arpa-e.energy.gov/sites/default/files/Grigorii-Solov... [2] https://en.wikipedia.org/wiki/Carbon_Engineering

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It's also hard because batteries don't get lighter as they deplete, so you're landing a plane with the exact same weight as what you took off with, meaning you need much more rugged (and heavy) landing gear. This is extremely significant when you consider that a fully loaded 747-8 is 970,000 lb, of which up to 326,000 lb is fuel (per Wikipedia's numbers).

It's a hard problem, and most proposed solutions are extremely novel and risky, like having an all-battery tugboat that tows the plane up to cruising speed/altitude before returning to the airport to charge or get a battery swap— obviously bad because risky, also adds 50% to runway capacity to land all the tow planes.

Aren’t planes already designed for landing at full weight for cases where the plane needs to turn around and land where it departed?

Yes, but they are only intended to use it in rare emergencies. It stresses the airframe components and reduces the life of the aircraft. To build an aircraft that lands heavy every time you need a much stronger undercarriage and frame.

and how often are planes fully loaded with fuel? A plane only needs enough fuel to reach the next destination. Saves the weight and money.

Some planes are capable of that and some aren't. Some have fuel dumping systems to get weight down.

https://en.wikipedia.org/wiki/Fuel_dumping

Many are equipped with fuel dumps. Others have mandatory “overweight landing inspections” prior to further revenue flight.

Your car can run past redline, it just shouldn't all the time.

Just drop them before landing... There could be a batteries landing place where they'd be collected.

Is it too crazy?

It turns out that aircraft have a very sensitive centre of gravity that needs to be close to the centre of lift. If you were to design a "battery bomb" you'd have to carefully engineer the pre- and post-bombing centre of mass and ensure that your control surfaces can respond adequately. In addition, you'll need bomb-bay doors and associated mechanics which adds mass and complexity. All of this takes a toll of efficiency, cost and reliability. However, it's an intriguing idea, especially if you use steerable parachutes that can neatly fly themselves back home, maybe all the way into a recharging slot.

Since a big chunk of the power is expended on takeoff, the ideal would be to drop part of your battery then rather then waiting until after landing. But either way, you're stuck with the significant complexity and expense of having to prepare a parachute and landing gear specifically for the battery on each flight, plus having to collect and charge those batteries.

Accepting that we're just throwing out interesting-sounding ideas, I wonder if having a powered runway rail (or something of the sort) to provide takeoff power could work. With that you wouldn't have to worry about dropping anything.

But I do wonder how much power is actually expended at takeoff versus climbing or cruising flight. According to [1], only ~19% is used for both takeoff and climb, so electrifying the runway to save the fraction of the 19% used while in contact with the ground is probably not worth it.

[1] https://aviation.stackexchange.com/questions/47262/how-much-...

Could use an electromagnetic rail to slowly but surely accelerate and launch it up to to cruising altitude https://en.m.wikipedia.org/wiki/Rocket_sled_launch

SupersonicScrub's comment (a sibling to yours) says that the engine can be significantly more efficient if it doesn't have to meet the power demands of take-off. Their link says "at least 30%" fuel savings on a one-hour flight (although "the fuel savings will partly come from the latest gas turbine technology"). So maybe the rail system you propose isn't such a crazy idea. Also, it's not totally different in concept from an aircraft carrier's catapult; hopefully gentler...

>Also, it's not totally different in concept from an aircraft carrier's catapult; hopefully gentler...

Nah. If someone can't handle a few g's in takeoff, maybe they shouldn't be flying, and should should stay home, or maybe in a nursing home. I know it sounds mean, but seriously, think about it: aircraft transport isn't always comfortable. Turbulence happens, and it can be extremely rough at times, even injuring people severely if they're not belted in. If someone is too frail to handle being launched by an aircraft carrier catapult (which, remember, also launch those big AWACS planes, not just small fighter jets), then they're not suited for handling turbulence either.

Also, don't forget, the latest generation of catapult technology is actually pretty gentle. The USS Ford's "EMALS" catapults are electromagnetic, rather than steam, and one of the big stated benefits of this is that it's gentler on the airframes than the old steam catapults. It's still going to subject the pilots/passengers to several g's, but probably lower peak acceleration than the older technology did.

There's a better electrically driven solution that takes advantage of the fact that take-off and climb-out requires the most power.

Maximum power requirements come from take-off and climb-out, so the engine size is designed around the max take-off power. This means that the engine is over-designed and less-efficient for the majority of the flight.

By implementing a hybrid engine, the gas-powered component and the electric powered component work together to provide take-off power requirements. By the time the aircraft reaches cruising altitude, the batteries are dead, and the gas-powered component works alone. This allows the engine to be designed for the cruise power requirements, which results in a much more efficient engine. UTC is currently experimenting with this concept.

https://www.flightglobal.com/news/articles/utc-to-test-hybri...

Yes it's not carbon neutral, but it's a step in the right direction.

Not just for take-off, but for the possibility of finishing the take-off with one engine failure. That's quite a bit of spare capacity.

However I also wonder why, if electric motors are a solution here, there hasn't been a comparable fuel-powered extra-takeoff-engine. Why would this not have the same benefits? Or are extra engines simply too complicated mechanically unless they are motors?

(The liked article is about modifying a Dash 8, which is a turboprop, and perhaps it's easy to have oversize propellers for takeoff, and just connect a motor to the same gearbox.)

Probably because a jet engine costs like $30MM. It may well be that the incremental cost ($, weight, complexity) of adding an electric booster to an existing engine design is much more practical than just adding more engines.

> take-off and climb-out requires the most power

Military aviation has dealt with this in various ways:

1. Carried aloft by a larger plane

2. Towed aloft be multiple other planes

3. Jet assisted takeoff

4. catapults

5. drop tanks

6. mid-air refueling

You still need enough power to make a late go-around at the destination, in case of problems due to weather on final approach, arrival spacing, or a problem on the runway. I've been on planes that had to go around from low altitude three times, so it's not even that rare. Either the engine needs to be large enough to generate full take-off power, or you need a large margin of battery power that will never be used on an uneventful flight.

Wouldn't this also mean that full engine power is unavailable in the event of an emergency? Seems like this solution makes edge cases more likely to be fatal.

Perhaps you could trail a long unwinding power cable behind you which disconnects when fully extended.

Heck, it might be doable to just lay the power cable in a permanent route on the ground and use electricity to propel the craft along that route! You wouldn't even need wings!

Aircraft carrier catapults do that. When launching the aircraft, all the energy is expended from the ground. Looking forward to such gigantic machines on airports, how much they would charge per flight, and problems like terrain slowly moving due to the gigantic forces at play.

Catapults accelerate aircraft to the point of generating enough lift for powered flight. They allow you to take off from a short runway like a carrier but don't get you to altitude. You can get a sense of how much power is being used to reach cruising altitude by listening to the engines on take-off. They don't fully throttle back until you reach a high enough altitude to permit level flight.

>Catapults accelerate aircraft to the point of generating enough lift for powered flight.

Yes, but the reason they're used is because it's infeasible or too expensive to build an aircraft carrier long enough to have jets reliably take off from them without this extra boost. Even so, those catapults are fairly short: notice they're only a fraction of the length of the carrier.

They're not used in regular airports because they're not absolutely necessary, the way they are on aircraft carriers. It's more expense, and planes can do without them. However, there's a cost to this: the planes need bigger engines and more fuel to take off reliably. And bigger engines are less efficient overall (as in cruising). So there would be a significant fuel savings if you could count on having catapults on the ground to help get the plane to takeoff speed: you could downsize the engines, and carry less fuel, saving a lot of money overall per flight (not to mention all the carbon pollution).

Finally, the extra g-forces shouldn't be a big deal. There's high g-forces in a carrier launch because the catapults are very short. And until recently, they were steam powered. This needn't be the case with commercial aviation: they can use modern electromagnetic launchers, and they can make the catapults very long (as long as much of the runway), to have lower peak acceleration. Basically, the planes and passengers would not need to experience higher g-forces than they already do; they'd just be getting them from the catapult instead of solely from the engines.

Good point - if you watch carrier launches, the planes actually fall a little as they go off the end of runway because they aren’t lifting yet. But presumably with a longer runway like you have at an airport, you could get to higher speed and truly liftoff.

But having also launched from a carrier myself, I wouldnt want that g force everytime I fly commercially.

> the planes actually fall a little as they go off the end of runway

If I was a carrier pilot, I doubt I'd ever get used to that!

I would love to launch from a carrier. Was in the Air Force but not flight crew. Saw a lot of great aircraft there...from the ground. :)

> Looking forward to such gigantic machines on airports, how much they would charge per flight

It's actually an interesting supposition, because whether it's gasoline or electric, it should be cheaper and more reliable to run a ground operation than to pack fuel that is expelled before anything meaningful has happened.

Or build a several kilometers high tower, using either Space fountain or floating spheres.

https://en.m.wikipedia.org/wiki/Space_fountain

https://en.m.wikipedia.org/wiki/Cloud_Nine_(tensegrity_spher...

What happens when it mechanically fails and doesn't disconnect? Have a 2 mile long cable being drug behind a 200mph flying machine?

That seems quite dangerous - to both the aircraft and everyone onboard, as well as to anyone on the ground in the path of this cable.

What happens when the plane mechanically fails? Having 600 tons of metal raining down on a population sounds quite dangerous.

> What happens when the plane mechanically fails? Having 600 tons of metal raining down on a population sounds quite dangerous

Simply put, that's just not how aviation works.

Mechanical failures aren't uncommon in aviation, but for an aircraft to not be able to divert and return to the ground, a lot has to go wrong all at once. It's infinitesimally rare for all systems to fail at once on an airliner. There simply isn't a problem of airliners dropping out of the sky, ever. They're incredibly robust machines.

However, with your idea, we'd be relying on a single system to prevent catastrophic failure. That's a massive step backwards for aviation.

That doesn't even begin covering the problems with delivering enough electricity over this cable to power an airliner at takeoff... That cable would need to be a very thick gauge of wire and very heavy - causing all sorts of it's own problems.

You can have your doubts but I'm off to the patent office tomorrow.

It wouldn't be viable for flights over oceans but I wonder if having batteries that eject at 95% exhaustion and then fly/glide themselves to a collection point with the last 5% of energy would work. There are too many variables for me to even do cocktail napkin math on it but with ever increasing drone tech, perhaps it would be possible.

Hard to say if giving the battery its own avionics is better or worse than equipping it with a parachute or steerable glider. Either way, it's the kind of thing that SpaceX decided was worth trying for the sake of fairings worth a few million apiece. Asking them how it went is probably a good place to start.

Regardless of the specifics, it's obvious that any of these schemes is a logistical and technical nightmare compared to "taxi up to the terminal, plug in the refueling hose while people and cargo are loaded onto the plane." It's unlikely that the private sector will be too enthusiastic about investing in anything like this without heavy incentives.

I wonder if their are a pair of liquids that could be used to make a flow battery and that would be safe to dump they are expended?

The liquid is gasoline, and you can make it carbon neutral. This whole effort toward battery powered aircraft is misguided: technically, hydrocarbon fuels work better than anything else. What you want to do is make carbon neutral hydrocarbon fuels.

The real advantage of electric airplanes is simplicity of electric motors, better aerodynamical properties of a plane using many small propellers, and possibility of vertical takeoff.

Ideally when we have kerosene based fuel cells we could have small electic aeroplanes that will be much easier to control, and would allow more frequent direct flights, instead of having to collect 100s of people together make them wait several hours, and drop them at an inconvenient location.

Sure. I have no objection to electric power trains for aircraft. But at that point, we're not talking about eliminating carbon emissions anymore, so the climate angle in this article ceases to be relevant.

In a word, yes. Dropping the batteries out of an airliner in a way that left the batteries recoverable and reusable would add a tremendous amount of weight. It would also add a tremendous amount of danger. Neither of those things are attractive to a manufacturer.

However, reducing the cost of fuel by 1/10 would be very attractive to basically all airlines, so the manufacturers may very well step up for it. Bombs away!

The reason Boeing developed the 737MAX was a 15% fuel savings. Pretty darned attractive.

Here is what happens when you've got a tank rolling around downstairs:

https://www.youtube.com/watch?v=7QYlueabGDg

Jettisoning 200,000 lbs of batteries would be even worse.

I'm not sure that's relevant. Said tank was an uncontrolled and unexpected movement. Military air transports are certainly capable of doing controlled air drops of tens of thousands of pounds.

Batteries tend to have chemicals that are both fire and corrosion hazards. Subjecting their cases to the stresses of repeatedly landing (even on a padded catcher) could lead to containment problems. You don't want that on an airplane.

Apparently they do actually get lighter, but not usefully so.

Why is that? I’m not a battery expert but it would seem all that charging a battery does is move electrons in the cells around, no?

e=mc2 I think - there’s a Tesla forum thread about it, and Quora articles and things if you look. I’m not a physicist.

The mass depletion is less than a milligram...

Yeah I read the same threads - that’s what I was referring to.

Chemical bonds have (often negative) mass. A chemical reaction will break and/or form bonds. Thus it slightly changes the mass. In a battery there will be chemical reactions happening at the anode and cathode. One reaction produces electrons and another absorbs them.

https://physics.stackexchange.com/questions/34421/does-the-m...

Why wouldn't you just ride on the tugboat?

It doesn't have the range for useful travel. It's designed to get the vehicle up to speed, using almost all its energy, and then drop away. Think of the first stage of a rocket.

> Electric aviation for aircraft larger than 4 passengers is extremely hard.

There is at least one airline transitioning to only electric aircraft already [1]. It's one that focuses on short range flights. And as energy density continues to increase, the range and applications of all-electric aircraft will too.

[1] https://electrek.co/2019/03/26/harbour-seaplanes-electric-ai...

Ignore the PR fodder and go run the actual numbers. The actual range/endurance/payload capability of these aircraft is a tiny tiny fraction of their Jet-A/100LL counterparts. It will eventually get better, and a lot of organizations are smart to keep that in mind, but we're not there yet and won't be for a decade at least.

I specifically mentioned that the range and applications of this initial version is very limited. I was mostly just pointing out a company (with a very unique service) that is at the point where it's possible. It won't make its way to major carriers for a long time (i think a decade would be quite optimistic, given lead times in aviation)

Solid Oxide Fuel Cells are reaching 2.5kw/kg power densities, Electric motors are reaching 15kw/kg power densities. Combined, that is already better than current jet engines. With cogeneration, SOFCs are reaching 80-85% efficiencies, double that of jet engines. Liquid hydrogen has an energy density of 39kwh/kg, compared to JP8 at 13kwh/kg, but SOFCs could run on pretty much any hydrocarbon. They don't even need cryogenic storage, because fuel consumption rates would be higher than evaporation rates.

All the pieces exist for fuel cell powered jets. They've already got the performance levels needed. All they need is to put them together into a cohesive package, and put a shit ton of effort into reliability engineering.

> Combined, that is already better than current jet engines.

Wikipedia lists the GE90-115B at 10kW/kg, four times better than your SOFC. Did you just combine motor and fuel cell to get 17.5kW/kg? That would be hilarious.

> With cogeneration, SOFCs are reaching 80-85% efficiencies

That's creative accounting. You're counting waste heat as if it was useful.

Cogeneration isn't much use on a plane...

Granted a little heat for the cabin is handy, but the rest is largely useless.

I meant in the context of energy conversion. Solid oxide fuel cells produce high grade heat (~800C) that is perfect for conversion via organic rankine cycles (which will convert ~30% of that heat to electricity) or supercritical co2 brayton cycles (~50% heat to electricity with extremely high power densities[0]).

Even without heat capture, SOFCs run at 60-65% efficiency, which is probably good enough.

[0] https://www.ge.com/reports/call-ecomagination-ge-building-co...

Energy density of jet fuel is 45MJ/kg. Fuel is a significant portion of the weight on the plane. Requiring a 10x+ increase in fuel weight is a non-starter no matter how light the engines.

If you're measuring in MJ/kg, hydrogen is 141MJ/kg. Hydrogen, in terms of weight, is absolutely better than jet fuel, not worse. It's not very dense (kwh/l) though, and that could be a problem.

What doesn't make sense is to use number of passengers as the figure of merit, here. Technically, that's no limit at all. Aircraft scale up just fine from 2 people to 20 to 200 people.[0] "Power" is even less of a concern: fundamentally, electric motors are just as high performance as turbine engines.

The appropriate figure of merit is range and specific energy (i.e. useful energy stored per unit mass), and that IS a limitation, but range of up to 500 miles is feasible for pure electric using existing batteries and structure and aerodynamic efficiency tech. Double or quadruple that is possible with cleverness and more exotic approaches. Even long-haul is possible with certain approaches to pure electric flight (either lithium-air, which has just as high theoretical useful energy as something like gasoline, or staging).

The only way "large aircraft" or "high power" is a limitation is just the speed at which things can be certified. Large passenger aircraft cost billions to bring to market, so companies are super conservative about it. But this isn't a fundamental technical limit of electric flight.

[0]Technically, there are structural scaling laws which detract from efficiency as you get bigger, but they don't become strong until you're very large... And they're countered by perhaps even more important positive Reynolds Number scaling effects, i.e. your lift to drag improves as your scale increases.

> staging

I'd like to see a prototype of an electric airliner which can fly forever, but every half hour or so a car-sized battery with little stubby wings flies up and docks into the back of the plane.

Not sure about the utility of an airliner flying forever unless you've got passenger pods attached to the flying batteries :D

...And now we've invented zeppelins again

Don't forget that most long-haul planes can't land at their MTOW - so the batteries need to be scaled down apropriately.

Or more attention must be paid to engineering landing gear and structures to handle high landing weights. One must approach the problem with an eye to identifying solutions or we'll never get anywhere.

There's more to it than just structural integrity - the heavier plane is generally less safe in an emergency, and despite some ideas proposed around you can't "get rid" of takeoff-power engines unless you drop Go-Around capability.

There are other ways to ensure safety during an emergency. Not having flammable fuel on board can help a lot (and requires a non-flammable electrolyte to be developed for lithium batteries, which is already a thing for solid state batteries). Additionally, not all civil aircraft even have go-around capability. Sailplanes don't, for instance, and their safety is approximately the same as other general aviation aircraft.

We'll have to get rid of the fiction that we're going to be able to just plop in electric motors and batteries in existing civil transport vehicles without any change in architecture or operations.

Are you going to get into a transport airplane that can’t go around? I’m not going to. I’ve been in two low level go arounds on commercial flights in VFR conditions (good weather) and one missed approach (bad weather) in the last two decades. Is that three times that I would have crashed? Maybe not; Delta probably could have stuck the downwind landing in Vegas and USAir probably could have stopped before the airplane that was slow in clearing at BWI. Delta into AMS in the snow and fog probably would have been a crash if they couldn’t execute the missed.

Sailplanes are probably not the right model to go flying IFR to minimums, something that airlines do regularly.

If it was engineered to be compatible with no-go-arounds safely, then absolutely I would. I've done it.

Even for aeroplanes with just two people, the most usable, all-electric endurance I've seen is about 1 hour[1]. Just about usable for training (mostly circuits around the airfield), but not much else.

[1] https://www.pipistrel-aircraft.com/aircraft/electric-flight/...

Useable in the loosest possible sense of the word since charging it would then take a few hours. By which point lack of daylight or a change in the weather would probably mean no more flying for the day. And even if that could somehow be made to work (perhaps with swappable batteries), it ignores the fact that anyone trained in flying an electric aircraft would be poorly equipped to fly a conventional light aircraft with issues like carburettor icing, shock cooling, leaning with altitude etc.

I imagine there'd have to be swappable batteries, but you're absolutely right that you'd be missing out on learning all the other aspects of flying. Although by the time electric aircraft become mainstream for training I'd hope we'd seen the end of carbs!

I thought these were in use at flight schools already. With spare batteries being charged on the ground. Perhaps you can't practice quite everything, but as a cheaper way for people to get some of the hours it sounds great.

I had a bit of a look, I found one flight school that had committed to buying some electric aircraft and had apparently trialed them before. No real indication of how it works practically. Bearing in mind that a Tesla battery pack weighs 500kg (1200lb) or so. Changing them over wouldn't be a simple operation.

Looks like this is what I was remembering:

https://www.pipistrel-aircraft.com/locations/#tab-id-2

https://www.dw.com/en/electric-flight-finds-its-wings-at-slo...

They have electric planes, and they have flight schools, but whether the two actually intersect or not isn't immediately clear.

Or you rethink completely the problem, and beam the energy to the plane.

This is more on an engineering challenge, unlike much better batteries which will require a fundamental scientific breaktrough.

https://en.wikipedia.org/wiki/Beam-powered_propulsion

Beam propulsion is impractical for aviation due to weather effects and line of sight. It's more than an engineering challenge, it just won't work (except for maybe a few limited cases of very short range flight in clear conditions).

You can beam from over the clouds (beaming antenna on a tethered balloon or even drone), and have a huge line of sight to boot.

It's an absolutely huge engineering challenge, but at least it doesn't require a discovery worth of the Nobel prize in chemistry (which I guess much improved batteries would warrant).

Having a huge line of sight doesn't solve the problem. The receiver has to be oriented roughly perpendicular to the incoming beam in order to get any reasonable efficiency. And the receiver has to be large in order to account for beam dispersion and aiming inaccuracy. For aerodynamic reasons the receiver can only be on the top or bottom of the aircraft and can't be angled much. So therefore the power transmitter must be close to directly above or below the aircraft.

Battery energy density is steadily improving every year. The 2019 Nobel Prize in Chemistry went to the inventors of the lithium ion battery.

https://www.nobelprize.org/prizes/chemistry/2019/press-relea...

I know some very smart people who are working on this. http://el-sky.com/

I wish them luck but I doubt they'll find a magic solution to beaming power through clouds.

"Electric aviation for aircraft larger than 4 passengers is extremely hard"

To be honest, the worst parts of flying are usually all those other passengers anyways. If you could scale up a fleet on-demand small electric planes. Flying would improve. If you account for all the waiting time, it might be just as fast too.

That would also make point to point flights easier to coordinate. It might not make sense to have a direct commercial flight from Birmingham, Alabama to Fort Collins, Colorado with today's aircraft but if you only needed to fill four seats, such pairings might work. Not sure how much quieter an electric airplane would be but if it is significantly quieter, smaller airports spread out in greater numbers would also make point to point more viable.

Four seater aircraft have been around since the first decade of flight, and I'm not sure batteries are going to solve the cost and logistics issues of using them as taxis, except in edge cases where there's a mountain in the way.

Flying anywhere near as far as from Birmingham, Alabama to Fort Collins, Colorado is also an exceptionally hard problem for electric aircraft.

This is the same for the train and shipping industries. I’ve always thought those vehicles had ample opportunities to capture solar rays and wind since they’re out in the open all the time, but we simply don’t have batteries and engines powerful enough yet.

I mean, basically every train engine you'll see that isn't steam is powered by electric motors. The diesel engine just makes electricity for them, and is not used to directly drive them via transmission like in a car or truck.

Could we use Hydrogen that is generated sing clean electricity or is too dangerous?

It's possible, but even liquid hydrogen is not very dense and requires a lot of insulation.

https://en.wikipedia.org/wiki/Tupolev_Tu-155

Liquid hydrogen has about 4X the specific energy of Jet-A. But yes, the system as a whole isn't great once you consider what it takes to keep the H2 liquid...

Energy density per unit of mass is about 3x in favor of hydrogen. Energy density per unit of volume is about 4x in favor of Jet-A.

Sorry you are absolutely correct. Volume matters a lot, maybe almost as much as weight, for aircraft design.

> Batteries don't have the energy density needed

... today.

There's a lot of money in batteries these days.

But from what I gather, it's not just about the batteries, but also about coming up with a design that takes advantage of the possibilities offered by simple electric motors.

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