4 ms·

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 pl

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

burkaman 5y ago
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.
- Robotbeat 5y ago
  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.
- Dylan16807 5y ago
  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.
sudosysgen 5y ago
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.
- rsj_hn 5y ago
  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.
  - sudosysgen 5y ago
    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.
    - rsj_hn 5y ago
      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

burkaman 5y ago

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.

Robotbeat 5y ago
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.
Dylan16807 5y ago
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.

sudosysgen 5y ago

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.

rsj_hn 5y ago
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.
- sudosysgen 5y ago
  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.
  - rsj_hn 5y ago
    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