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Comment by Robotbeat | Hacker News Reader

The technology is independent of scale. If it works for a nine Seater it also works for a 100 Seater.

EDIT: and YES this is true. Battery specific energy is 200-400Wh/kg. That is PER kilogram. And larger planes are NOT less efficient per passenger than smaller ones. Small and large jets or propeller aircraft have a out the same life to drag ratio. If anything, larger tend to be better due to Reynolds Number effects.

There is maybe not one misconception about electric flight that I’ve seen repeated most often by Internet skeptics than this false idea that “bigger is disproportionately harder for electric planes.” It is simply. Not. True.

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That's not true unfortunately, batteries are too heavy to make bigger planes workable right now. We need denser batteries and more powerful electric motors, and there are companies working on both those problems, but they aren't solved yet.

Look at the specific energy of jet fuel compared to batteries on this page: https://en.wikipedia.org/wiki/Energy_density

Specific energy is independent of scale! It’s right in the units: energy divided by mass. Watt-hours per kg. Where are you getting the claim from that larger planes can’t technically be made electric even if smaller ones can?

And no, don’t switch topic to range. That is a known constraint. Focus on SCALE. It is scale-independent to zeroth and first order at least!

This comes up every single time, and I feel like I’m taking crazy pills. :D

I mean I'm not really qualified to have this debate, but I'll do my best. Intuitively, if you double the weight of the plane, now you need twice as much battery to fly it. But now you've added a bunch of battery weight, so you need even more batteries to lift those batteries. You also will need more or bigger motors, which adds even more weight. At some point, your plane isn't gonna get off the ground.

I am only a lowly software engineer so I don't know that much about planes or batteries. If I'm wrong then I'm wrong, and I hope I am.

No, that IS how it scales for range to some extent (where if you just add fuel/battery, you now need to add more power, etc), but it’s NOT how it scales for size. To first order, everything scales linearly. If you doubled the size of the aircraft, you already increased everything to compensate. This works for everything with two caveats: 1) Reynolds number means you get more efficient as you scale up in size. The effect is slight and there is nuance with respect to laminar vs turbulent flow, but it is true. 2) larger size means at some point you need truss braced wings. But this is only at pretty large sizes and only usually if you’re also shooting for high aspect ratio wings. NASA/Boeing/etc have researched truss braced wings, and it is most certainly doable.

But both of these are second order effects. And they mostly cancel out.

I am a materials scientist with a background in physics and I have also developed electric motors for electric aircraft. I have a good understanding of how the systems scale. You were mixing up the exponential range equation with simple size scaling. if it were true, then large aircraft of any type would be much less efficient than smaller aircraft of the same type. This is most certainly not true. In fact, usually the efficiency increases with scale. A 777x is much more efficient than a Gulfstream or some smaller regional jet per passenger mile.

You're describing something like the rocket equation, but that's for increasing final speed, not increasing mass.

If you double the weight of "the plane", that includes the batteries and motors.

If you start with doubling just the structure, then you might need to add 50% more batteries, not "twice as much". Then to lift those batteries it takes another 30%. Then you add bigger motors which adds even more weight, etc. etc. By the time you finish, the total weight will be about 2x your initial weight.

There is difficulty in scaling planes, but it's not because more mass requires more fuel. A fixed percentage devoted to fuel works fine, if you're assuming the same route for both planes.

Airplanes are sadly not infinitely scalable in this way. If you attempt to naively scale up an airplane you need stiffer materials, eventually you run into the issue of the weight of the structure increasing more rapidly than the area, and this is exacerbated by heavy batteries. We sadly cannot make large battery powered airplanes with a usable range and speed unless we get up the gravimetric density.

I don't think battery airplanes are going to be a big thing, but I think hydrogen powered airplanes could well be a big thing. It's certainly something I'd invest a lot of R&D into.

Also, the airplane manufacturer business is ripe for disruption. You have an extremely bureaucratic and dysfunctional Airbus battling an even more dysfunctional and mismanaged Boeing. Surely there is room for a hydrogen startup to come in and devastate these two monopolies. Often I wonder how much the world would be different if we had 100 Elon Musks instead of just one.

I agree that hydrogen in airplanes has some potential, though there are certainly issues - you need cryogenic cooling to avoid super heavy pressure vessels, and if it fails..., but there are similar fuels that could work.

As far as disruption, the issue is that there is no opportunity for competition. Countries are rabidly protectionist about their airplane industries, and any newcomer even with a completely superior product (See: Bombardier C-Series) will get absolutely destroyed by sanctions, tarrifs, or even secondary export controls (Gripen, Avro Arrow, etc...). You cannot succeed unless you are American, and even in America Boeing has massive undue influence.

All true. I do think there is a window to succeed in the U.S. In Europe it's just much harder to innovate more generally.

Also the SAFs are looking pretty good. Point is, I don't think we should decide that planes are bad of all sudden, that doesn't seem like a very wise thing to do given the massive benefits of air travel. Rail is also great but rail shines for cargo and large volume traffic between urban cores

One of the fields that isn't true is civilian aerospace. Innovation cycles are long, regardless of country, regulations are more or less the same. Nobody only certifies according to EASA or FAA, everyone does both. And with Airbus and Boeing forming a duopoloy on civilian aircraft, both sides have equal interest, and funding, for innovation.

Throw in all the current eVtol start-ups and I don't see that much difference between the US and Europe.

Airbus and Boeing have plenty of competition in the smaller jet range: Embraer, Comac, Sukhoi, until recently Bombardier. If it was easily to scale up, it would have happened by now.

I do agree that an electric/hydrogen jet is the kind of disruptive tech that could upend the market, but due to the long lead times, stringent certification and very political markets it's an arguably even tougher market than cars or rockets.

Bombardier Aviation was bought by Airbus, Embraer almost by Boeing. Funny enough, there was always a lot more competition in the regional market then for bigger single aisle planes (B737 and A320).

No, this is false. The stiffness issue is a second order issue (which is why larger conventional planes actually have LOWER dry mass per passenger than 9 seat passenger aircraft) and IT AFFECTS ALL PLANE TYPES, and can be addressed via truss braced wings as I said. There are more important minimum gauge type issues at small scale that tend to dominate and make small scale worse performance than large scale.

The claim that larger size in particular is a problem is false. Efficiency, including structural efficiency, actually tends to improve with scale beyond 100 seats.

> We sadly cannot make large battery powered airplanes with a usable range and speed unless we get up the gravimetric density.

This, again, is often repeated but false from a technical standpoint. 9 seater and 100 seater, for the same range, scale just fine between them and with clean sheet design we can use existing 200-400Wh/kg gravimetric density. Scale. Isn’t. The. Problem.

Large planes tend to be used on longer routes, in fact. Smaller planes are needed for their granularity on short routes.

The reason we see 9 passenger electric planes first is pretty obvious: cheaper and faster to develop small planes than large. Big aircraft manufacturers are much more risk averse as well. A large new clean sheet aircraft can take literally decades to develop. Not true for small aircraft. And again, generally the granularity of small planes means they’re more useful than large planes for shorter routes. This all reinforces the false notion that there is some technical reason why we “CAN’T” make large electric planes when there really, truly isn’t.

For instance, consider the Learjet 70/75. 8 passenger, dry mass of 6 some tonnes. Over 700kg per passenger. 14 passenger Falcon jets do better at about 500kg per passenger. 737 Max8 with 210 passengers gets about 215kg/passenger. So if anything, overall structural efficiency improves with scale! At least up to ~200 passengers. (Caveat is that these are typically private jets with a different seating arrangement... but so are the example electric aircraft. So they'd similarly benefit from scale up to larger seat capacity.)

I think you seriously underestimate the mass of a 100 seater with 500 mile range.

The 737 Max8 has a massive advantage over the LearJet of massively lower surface area per passenger. It's not comparable at all. In an electric aircraft where the bottleneck is the battery as far as weight goes this is not a relevant comparison.

You have two regimes. One where the dominant factor is structural efficiency is mass. One where the dominant factor is area. You cannot compare the two. It is simply a completely deficient comparison.

Do you want an actually correct comparison? Keep the 737 Max 8. Now compare it to a PAC P-750XL, dry mass of 1.6 tons over 9 passengers. That's 181kg per passenger, less than the Max-8.

Now that's the aluminium version. The Max-8 uses a massive amount of composites - the composite version of the P-750XL is expected to be significantly lighter still.

It's absolutely not a fiction. A 787 Max-8 consumes 18 liters (!!!) of kerosene at >60% thermal efficiency, per passenger, per hour. That's 175kW of useful energy consumption per passenger. To fly one hour at 400Wh/kg assuming 100% energy efficiency you need to add 437 kg of batteries PER PASSENGER.

PAC P-750XL Is unpressurized so is able to significantly lower the dry mass per passenger. I noticed this difference while researching the different dry mass numbers. The low speed unpressurized utility aircraft like the P-750XL ARE able to get lower per passenger mass, but the experience is not comparable as they are unpressurized. Among pressurized jet aircraft, the larger jets do universally better per passenger than the smaller.

The real key to making electric aircraft feasible is to increase the efficiency dramatically. (Also, jets don’t have >60% thermal efficiency if you use the HHV of the fuel.) just a swap out of the fuel for battery gets you to a few hundred miles of range at best. Enough maybe to compete with the California HSR, but not much more.

It’s like how a Tesla Model S is not a battery swapped sedan. It’s a custom clean sheet design. That’s why you can get 400 miles of range in it instead of just 150 miles.

Likewise, you need a clean sheet battery electric aircraft design. That’s why the Eviation Alice is able to get significant range. Incorporate 777x-like folding wingtips and possible truss braced high aspect ratio wings, natural laminar flow like sailplanes, you could increase L/D to about double current aircraft. Things like that allow 1000 mile range. Substantial enough for short haul trips, which are about half of all passenger miles.

If you're actually trying to scale aircraft up and down, you can do the math and figure out that a scaled down aircraft will be slower. It's a perfect comparison.

A Tesla Model S doesn't have any more weight efficiency than a battery swapped sedan. It's just as heavy as a battery swapped sedan, it's just built to handle the weight better.

The main factor in the Alice having a lower L/D is flying at less than half the airspeed and thus drastically reducing parasitic drag. There are no easy gimmes. You're going to have to sacrifice range and speed.

So far electric airplanes are somewhat worse than piston aircraft. They will never rival jets until we drastically increase energy density.

The turbine efficiency of modern turbojets is actually around 60% and there are turbines with more than that. The overall efficiency is actually lower because of propulsive inefficiency, but electric aircraft have that exact same problem.

This doesn't seem right to me as it goes against my intuitive sense of how other factors generally scale with size/weight/volume (e.g. rocket equation), but I admit I'm definitely not an expert. However Wikipedia claims [1] that 250-300 Wh/kg is sufficient for small aircraft but something the size of an Airbus 320 would need 2 kWh/kg. A random recent paper about scaling electric aircraft [2] seems to imply that there are challenges to scaling size as well:

> All-electric designs have been demonstrated for small air vehicles. However, such prototypes have not been scaled up to more than ten passengers due to the specific energy (E*) limitations of current battery technology. [...] A significant proportion of the energy expenditure would be used to transport the mass of the batteries; this mass would not decrease during a flight as would that of conventional fuel.

Are these sources wrong, or too simplistic in their analysis, or am I misinterpreting what they're saying?

[1]: https://en.wikipedia.org/wiki/Electric_aircraft#Batteries

[2]: Structural Power Performance Targets for Future Electric Aircraft - https://www.mdpi.com/1996-1073/14/19/6006

The rocket equation, or its airplane cousin the Breguet range equation talk about ratios of mass, not absolute mass. These larger airplanes (eg an A320) often fly much faster and longer distances

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