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I am really excited to se electric aviation start to enter the market. A lot of people point out the battery density / jet-A difference and it is valid, but it

I am really excited to se electric aviation start to enter the market. A lot of people point out the battery density / jet-A difference and it is valid, but it isn't the whole story. Jet-A has a much lower conversion to useful work than a battery, an electric power train (minus the batteries) has a lot of opportunity to shed weight (no bleed-air, fuel plumbing, less need to safety systems). There are a lot more opportunities to explore interesting airframes because electric can be placed in unique and more efficient ways (hence the eVTOL in this story). The basic physics change a lot too. We will see how high altitude flight shakes out but there is a big potential to go higher so that more efficiency can be gained, again needing less energy. The big point here is you can't simply compare electric to gas turbine and only swap the fuel for batteries. It is a totally different set of design parameters and it has so many amazing opportunities to be better.

jillesvangurp 6mo ago
Comparing battery energy density to fuel energy density usually ignores the fact that combustion engines aren't that efficient. A great counterpoint is that if you apply the logic to a 60kwh EV, it should have the range of a 2 gallon petrol car. Which of course is not the case. Most medium sized petrol cars would have at least 8-10x that for a similar range to that 60kwh EV. A more useful metric is $ per mile of range. Because if the vehicle can do the miles, that's all that matters. With the first generation of passenger drones their range isn't amazing. But their cost per mile is. And these Joby things have a useful enough range to do JFK to down town Manhattan. I've been following the market a bit. There are a few interesting vehicles moving through certifications. Beta Aviation was touring all the airshows last summer with their ALIA CX300. It's a simplified ctol model of their vtol where they kept the pusher prop but removed the other props to speed up certification. So it's more like a conventional plane. It has a range of around 300 nautical miles depending on the battery configuration (modular). They flew it coast to coast in the US and all around Europe. It should get through certification by 2027 or so. Their vtol version has been flying for a while as well but will take longer to certify. It has less range because landing and taking off vertically just eats a chunk of battery. But once it is up in the air it flies pretty much the same as the ctol. Of course the arrival of solid state batteries is going to shake things up. Everything that is close to being certified is flying without those. A potential doubling of energy densities is going to be a big deal. But certifying the batteries is going to take years.
- mahsa32 6mo ago
  >Comparing battery energy density to fuel energy density usually ignores the fact that combustion engines aren't that efficient. >>Jet-A has a much lower conversion to useful work than a battery
  - coredog64 6mo ago
    Jet-A that has been combusted doesn't require any lift. Edit: Since I have an aerospace engineering degree, I'll post the 100 level concept. https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node98.html https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes...
    - jmward01 6mo ago
      But at the same time you had to design an aircraft to be able to shed that much weight and handle a load that sloshes around. That has cost. Again though, the core point is you can't compare directly. It is more about the final design and final purpose. eVTOL to NYC shows this. We may see quieter, less polluting cheap taxi service, basically a superior ability to function, because of being electric. If you only look for reasons it is worse, you will find them and not progress.
      - nDRDY 6mo ago
        Given the current state of battery technology, this is probably the only possible commercial use case. Even then, you have serious limitations like low passenger capacity, takeoff/landing still requires a helipad, recharge time, and the questionable safety in the event of motor or hinge malfunction.
        jmward01 6mo ago
        And once this gets in use improvements will lead to bigger, longer range, etc etc etc. Batteries too are rapidly improving and this will push more emphasis on that. The first Write flyer couldn't do much either.
bb123 6mo ago
I'm much less optimistic. Even when factoring in the poor thermal efficiency of gas turbines (~30-40%) compared to electric (>90%), the usable specific energy gap remains immense. Jet-A still delivers roughly 14 times more useful work per kilogram than modern batteries. Removing fuel plumbing and tweaking airframes won't overcome that fundamental physics. Also the issue with the high-altitude efficiency argument is that batteries, unlike liquid fuel, don't lose mass during flight meaning the aircraft to haul its maximum takeoff weight from departure to arrival. It's a double whammy.
- ericd 6mo ago
  Well, in this case, we don’t need to argue about theory. The Joby has a tested range of 150 miles. They also tested it with hydrogen fuel cells and got >500.
  - nDRDY 6mo ago
    Sounds about right. A plane of comparable max take-off weight, a Piper Malibu, has a range of ~1500 miles (with reserve remaining).
    - ericd 6mo ago
      This isn’t meant to slot into the role of other planes, though, it’s meant for rideshare. It can take off and land on my suburban lawn. There’s a lot to figure out before we can get to that point, so they’re just displace helicopters for the moment, but it can be a lot more. It’s basically the long awaited flying car, in nascent form.
      - nradov 6mo ago
        No, it can't take off and land on your suburban lawn. The wires and trees overhead would make that ridiculously dangerous, a last resort only for emergencies. Plus they need to recharge for the next flight. These e-VTOL aircraft will operate from dedicated pads.
Gravityloss 6mo ago
There's a hydrogen fuel cell version too that has been demonstrated. This is one potential pathway towards cleaner aviation.
- sehansen 6mo ago
  Hydrogen has a volume problem, though. A 1st generation Toyota Mirai contains 5 kg of H2, equivalent to 197 kWh. That would take up 55 m3 at atmospheric pressure which is why the Mirai stores it at ~700 atmospheres. That's still a 78 liter tank. AFAICT 200 kWh of petrol takes up 25 liters, i.e. a third. On top of that the high-pressure tank in the Mirai weighs 87 kg.
  - JumpCrisscross 6mo ago
    Hydrogen also sucks in that it puts you in your own scaling lane. Relying on batteries means EVs, grid storage, et cetera drive down your costs for “free”.
    - card_zero 6mo ago
      Bertha Benz faced a similar problem in 1888, and had to refuel the Patent-Motorwagen by seeking out pharmacies. Drivers of the steam cars that were popular in France could just pick up a bag of coal from anywhere. (Wait, that doesn't sound right. A bottle of kerosene, then.)
    - deleted 6mo ago
      [deleted]
  - Intralexical 6mo ago
    I like the idea of fuel cells, but hydrogen's going to have an image problem as soon as people see the failure mode, if it's just being stored as H2 in compressed tanks. Liquid fossil fuels and electric batteries burn with a gradual flame. Hydrogen suddenly detonates, with a supersonic, shattering shockwave, if it's mishandled. Even with Cold War money, Lockheed's famed Kelly Johnson couldn't make the logistics work for the CL-400.
- 6mo ago
deleted 6mo ago
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jillesvangurp 6mo ago

Comparing battery energy density to fuel energy density usually ignores the fact that combustion engines aren't that efficient. A great counterpoint is that if you apply the logic to a 60kwh EV, it should have the range of a 2 gallon petrol car. Which of course is not the case. Most medium sized petrol cars would have at least 8-10x that for a similar range to that 60kwh EV. A more useful metric is $ per mile of range. Because if the vehicle can do the miles, that's all that matters. With the first generation of passenger drones their range isn't amazing. But their cost per mile is. And these Joby things have a useful enough range to do JFK to down town Manhattan. I've been following the market a bit. There are a few interesting vehicles moving through certifications. Beta Aviation was touring all the airshows last summer with their ALIA CX300. It's a simplified ctol model of their vtol where they kept the pusher prop but removed the other props to speed up certification. So it's more like a conventional plane. It has a range of around 300 nautical miles depending on the battery configuration (modular). They flew it coast to coast in the US and all around Europe. It should get through certification by 2027 or so. Their vtol version has been flying for a while as well but will take longer to certify. It has less range because landing and taking off vertically just eats a chunk of battery. But once it is up in the air it flies pretty much the same as the ctol. Of course the arrival of solid state batteries is going to shake things up. Everything that is close to being certified is flying without those. A potential doubling of energy densities is going to be a big deal. But certifying the batteries is going to take years.

mahsa32 6mo ago
>Comparing battery energy density to fuel energy density usually ignores the fact that combustion engines aren't that efficient. >>Jet-A has a much lower conversion to useful work than a battery
- coredog64 6mo ago
  Jet-A that has been combusted doesn't require any lift. Edit: Since I have an aerospace engineering degree, I'll post the 100 level concept. https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node98.html https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes...
  - jmward01 6mo ago
    But at the same time you had to design an aircraft to be able to shed that much weight and handle a load that sloshes around. That has cost. Again though, the core point is you can't compare directly. It is more about the final design and final purpose. eVTOL to NYC shows this. We may see quieter, less polluting cheap taxi service, basically a superior ability to function, because of being electric. If you only look for reasons it is worse, you will find them and not progress.
    - nDRDY 6mo ago
      Given the current state of battery technology, this is probably the only possible commercial use case. Even then, you have serious limitations like low passenger capacity, takeoff/landing still requires a helipad, recharge time, and the questionable safety in the event of motor or hinge malfunction.
      - jmward01 6mo ago
        And once this gets in use improvements will lead to bigger, longer range, etc etc etc. Batteries too are rapidly improving and this will push more emphasis on that. The first Write flyer couldn't do much either.

bb123 6mo ago

I'm much less optimistic. Even when factoring in the poor thermal efficiency of gas turbines (~30-40%) compared to electric (>90%), the usable specific energy gap remains immense. Jet-A still delivers roughly 14 times more useful work per kilogram than modern batteries. Removing fuel plumbing and tweaking airframes won't overcome that fundamental physics. Also the issue with the high-altitude efficiency argument is that batteries, unlike liquid fuel, don't lose mass during flight meaning the aircraft to haul its maximum takeoff weight from departure to arrival. It's a double whammy.

ericd 6mo ago
Well, in this case, we don’t need to argue about theory. The Joby has a tested range of 150 miles. They also tested it with hydrogen fuel cells and got >500.
- nDRDY 6mo ago
  Sounds about right. A plane of comparable max take-off weight, a Piper Malibu, has a range of ~1500 miles (with reserve remaining).
  - ericd 6mo ago
    This isn’t meant to slot into the role of other planes, though, it’s meant for rideshare. It can take off and land on my suburban lawn. There’s a lot to figure out before we can get to that point, so they’re just displace helicopters for the moment, but it can be a lot more. It’s basically the long awaited flying car, in nascent form.
    - nradov 6mo ago
      No, it can't take off and land on your suburban lawn. The wires and trees overhead would make that ridiculously dangerous, a last resort only for emergencies. Plus they need to recharge for the next flight. These e-VTOL aircraft will operate from dedicated pads.

Gravityloss 6mo ago

There's a hydrogen fuel cell version too that has been demonstrated. This is one potential pathway towards cleaner aviation.

sehansen 6mo ago
Hydrogen has a volume problem, though. A 1st generation Toyota Mirai contains 5 kg of H2, equivalent to 197 kWh. That would take up 55 m3 at atmospheric pressure which is why the Mirai stores it at ~700 atmospheres. That's still a 78 liter tank. AFAICT 200 kWh of petrol takes up 25 liters, i.e. a third. On top of that the high-pressure tank in the Mirai weighs 87 kg.
- JumpCrisscross 6mo ago
  Hydrogen also sucks in that it puts you in your own scaling lane. Relying on batteries means EVs, grid storage, et cetera drive down your costs for “free”.
  - card_zero 6mo ago
    Bertha Benz faced a similar problem in 1888, and had to refuel the Patent-Motorwagen by seeking out pharmacies. Drivers of the steam cars that were popular in France could just pick up a bag of coal from anywhere. (Wait, that doesn't sound right. A bottle of kerosene, then.)
  - deleted 6mo ago
    [deleted]
- Intralexical 6mo ago
  I like the idea of fuel cells, but hydrogen's going to have an image problem as soon as people see the failure mode, if it's just being stored as H2 in compressed tanks. Liquid fossil fuels and electric batteries burn with a gradual flame. Hydrogen suddenly detonates, with a supersonic, shattering shockwave, if it's mishandled. Even with Cold War money, Lockheed's famed Kelly Johnson couldn't make the logistics work for the CL-400.
6mo ago

deleted 6mo ago

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