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Comment by DennisP | Hacker News Reader

Electric airplanes need significantly better energy density.

"significantly" sells it short. Electric airliners need a couple orders of magnitude of improvement to even come close to viability.

Jet engines are significantly less efficient than electric engines so it’s lithium ion has 1/20th the useful Specific Energy at takeoff, though that gets worse as the flight progresses and fuel is burned or batteries age. On the other hand for ultra long range flights electric jets can cruise as higher altitudes which makes up for some of that. Also having 1/3 the 15,000 km range of some aircraft is perfectly viable.

IMO, Due to cost savings electric aircraft would likely become common at around 4x the current Specific Energy and fully take over at about 10-20x.

PS: Energy density is fine for lithium ion, the issue is energy per weight (Specific Energy) not energy per volume (Energy Density).

An airplane will weigh less, the more jet fuel it uses. A battery powered airplane does not have an obvious way of losing weight in this manner, further limiting range.

Talking about 5.000 km is hard: While it won't get you from London to New York without some post-crash landing swimming, the current largest electric plane can fly ~100 miles at around 114mph, with 9 passagers.

I see that you take the laws of physics into account, but allow me to state this for the sake of the conversation: In order to replace jet fuel, we must match what we could call it's useful energy density (energy/kg * efficiency). While electric engines are probably more efficient, we're still up to 2 orders of magnitude away – that's requires a 50-100x improvement of energy density, while maintaining the needed high discharge characteristics, developing a sufficiently efficient electric jet engine and taking into account the increased weight. All at a price where people won't need to save up for years, in order to visit their families.

While I hope to see a zero emission future in my lifetime (actually a negative-emissions might be needed for at time), I feel like putting batteries into airplanes is a bit too much "Silicon Valley Sophism" – shoving the same solution at each and every problem, rather than figuring out what the core of the problem is.

I do love electric cars, but they are not for everyone('s budget). Perhaps the solution is synthetic fuels, combined with carbon capture and a cheap, clean and reliable energy source, such as nuclear power. I know this isn't exactly cheap or readily available either, but it seems like one would bypass a lot of the issues above, by focusing on the source of the problem: Non-neutral use of carbon fuels.

From what I've read, ONE order of magnitude higher density is all we need for commercially viable eletric airplanes. We don't need an additional 10X, as you are suggesting.

No they don’t. 400Wh/kg is sufficient when combined with structural efficiency and aerodynamic improvements. Over 1000 mile range should easily be feasible there in a new, clean sheet design incorporating best of everything and a 400Wh/kg battery.

Serious question, would nuclear powered planes be viable? As far as energy density goes it seems pretty hard to beat!

https://xkcd.com/1162/

Yes, both USA and Soviet experimented with nuclear powered planes during the 60s. The problem was how to shield the pilots from radioactivity. https://en.wikipedia.org/wiki/Nuclear-powered_aircraft

They also experimented with nuclear powered drones. https://www.popularmechanics.com/military/research/a33565112...

And Russia now claims to have a nuclear engine that can make a cruise missile fly for days. https://en.wikipedia.org/wiki/9M730_Burevestnik

What if there were a reactor type which would require no, or much less shielding because all it emits is mostly hard ultraviolet light?

[0] https://en.wikipedia.org/wiki/Nuclear_lightbulb

I read that as using UV light to transfer energy to the working fluid, not that the actual nuclear reaction itself would be any different to any other nuclear reactor.

Even when it reads "It would be operated at temperatures of up to 22,000°C where the vast majority of the electromagnetic emissions would be in the hard ultraviolet range." ?

edit: Sigh.. disregard, cf. https://web.archive.org/web/20080109011832/http://www.nuclea...

Another problem is crashes or accidents

I don't know. Sure, if you can build batteries with the energy density of gasoline, you can build an electric plane with them. But is this really necessary?

Right now, we have relatively large, relatively fast, jet planes. Do we need that size and that speed to be commercially viable? Could we also run a larger fleet of smaller, more automated planes or helicopters? If there was no kerosene left by tomorrow morning, would there really be no alternative?

I probably have a skewed perception because I play with small fpv drones, but any weight saving like this usually opens up a whole world of new possibilities we never bothered to imagine before. This will certainly help existing delivery drones and drone taxi projects.

> If there was no kerosene left by tomorrow morning

I suspect the medium term solution would be to make some, either from other hydrocarbons or plant cellulose, or even by the US Navy seawater CO2 to liquid fuel process.

> I don't know. Sure, if you can build batteries with the energy density of gasoline, you can build an electric plane with them. But is this really necessary?

Only if you value overseas travel... It currently takes ~16 hours to fly from SF to Singapore depending on weather. Any longer, and you're going to see a lot of that travel disappear.

Only if you value overseas travel... It currently takes ~16 hours to fly from SF to Singapore depending on weather. Any longer, and you're going to see a lot of that travel disappear.

It takes that long because it's more efficient to do it in a single flight. Taking off and landing are a massive waste of time and fuel. If we only had short range planes people would change their destinations in some cases or they'd plan layovers as part of the trip. I don't think you'd see that many people just stop travelling abroad.

In the era when planes had to do that, there was not much trans-oceanic travel. The Pan Am flying boats, the "China Clippers", went from San Francisco to Hawaii to Midway to Wake to Guam to Manila to Macao to Hong Kong. It took a week.[1]

[1] https://tripsintohistory.com/wp-content/uploads/2012/10/Clip...

Indeed. Though like in the olden days the large number of stopovers might cut down demand somewhat:

https://en.wikipedia.org/wiki/Kangaroo_Route#Multiple_stops

it's not really true that ultra long range direct flights (eg. from sfo to sin) are more efficient.

the efficiency gain by not taking off and landing is eaten away by the efficiency loss of having to carry so much fuel (which also comes at the expense of useful load..)

It's not difficult to imagine in-flight recharging from something like a KC-135 running a big fat generator. If you're recharging capacitors, connect time would probably be a minute or two.

I'm not so sure about that being an acceptable solution for civilian transportation… Even assuming automation can eliminate the risks and training overhead associated with in-flight refueling, there are still be some major hurdles that would need to be addressed before this could be considered a viable solution. The 3 big ones that come to mind are:

What happens if the KC-135 misses the rendezvous? What if the charging port fails to work for some reason? What about the carbon footprint of these airborne generators?

Plus, now you have the weight of the capacitors to carry around all the time, further reducing the effective payload.

> What happens if the KC-135 misses the rendezvous? What if the charging port fails to work for some reason?

The same thing that happens today with combustion aircraft: the crew makes an emergency landing if they can, or bails out and waits for a Seahawk to pick them up.

This actually happens with fuel-driven aircraft regularly today.

Bailing out seems a bit problematic for passenger aircraft.

I kinda enjoy the ability to get to the other side of the world in 24 hours.

> if you can build batteries with the energy density of gasoline

Actually significantly worse than that. A plane gets lighter the longer it flies, especially at takeoff when everything is at max thrust. A battery becomes more and more dead weight the longer you fly.

I wonder if this could be solved for with detachable take-off booster. Gets you up in the air, then returns to the airport.

Couldn't you just start with a cable that detaches and is pulled back to the landing strip?

A "third rail" along the runway and a dodgem/Scalextric type conductor springs to mind.

Just make sure it detaches properly, you don't wanta a dozen wire to yank you in mid ascend

I keep seeing numbers around 500 miles range floated for 8-12 passenger electric aircraft.

Fire up google earth and start drawing 500 mile radius circles around metro areas.

You probably don’t need gasoline density equivalents for short haul flights, but we’re still really far off from where we need to be.

the problem with jets is stratospheric water vapor in addition to all the CO2 they emit (quite a lot in itself).

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