Comment by jmward01 | Hacker News Reader

Comment by jmward01 | Hacker News Reader

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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 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.

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.

You are ignoring the second variable on the consumption of energy dense materials. Weight.

It correlates to the energy density of course, but, weight directly goes into the power consumption calculations for vehicles. Efficiency is just a multiplier afterwards.

You can only ignore weight in non-mobile battery applications, i.e. grid applications.

It is a multi-variate problem and petrol currently wins out by a wide margin.

> Because if the vehicle can do the miles, that's all that matters.

Unfortunately that's probably going to stay fossil for a while. What might matter is things like local ordinances prohibiting it on AQI grounds (especially things like leaded fuel in Cessnas!), as well as more dramatic questions like shortages.

(we're probably never going to get a carbon tax on jet fuel, too much coordination required)

> Because if the vehicle can do the miles, that's all that matters

You conveniently forgot about climate change.

It also very much matters that my grandchildren will be able to breath, will have food to eat, won’t have to live underwater and won’t depend on wars in faraway lands.

If we can synthesize jet fuel cheaply with renewable electricity it's carbon neutral.

You honestly think “If we can just invent a magic solution cheaply we can keep burning stuff”

Is the answer?

The problem with "burning stuff" is twofold:

1. Carbon emissions - not a problem if you synthesize the jet fuel out of carbon dioxide in the air. https://www.climatechangenews.com/2025/06/18/e-saf-jet-fuel-... it's expensive today but with cheap-enough energy, such as with abundant solar and wind power, it won't always be.

2. Air quality - If energy is so cheap that you can literally make jet fuel, it follows that we'd stop "burning stuff" for any other reason. No more coal or natural gas plants, gasoline or diesel engines, heating with natural gas or propane. Jet planes' air pollution is a tiny fraction of all those sources. I could live with that.

Yes, that's the answer. If we want to have long range air transport then we're going to keep burning stuff no matter what.

>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

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...

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.

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.

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.

There's a hydrogen fuel cell version too that has been demonstrated.

This is one potential pathway towards cleaner aviation.

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.

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”.

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.)

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.

If we want clearer aviation then manufacturing synthetic liquid jet fuel (kerosene) is more likely to be viable than hydrogen.

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.

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.

Right, so when you factor in the legally required reserve flight time the battery powered Joby is only capable of very short hops. And that's fine, it's still potentially useful on a few routes and newer models will improve over time.

Sounds about right. A plane of comparable max take-off weight, a Piper Malibu, has a range of ~1500 miles (with reserve remaining).

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.

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.

Just talking about what they've talked about as goals in interviews. And I'm surprised you're willing to make definitive statements about my lawn, we have a large enough area with none of those obstructions you're talking about, we live on a few acres. And if it's running 20 miles here and there, it can do a few trips before it needs to go somewhere to charge or battery swap. That would cover our trip to our nearest international airport.

If these e-VTOL aircraft get used for air taxi service at all it's going to be for short hops in denser urban areas. Not in rural areas where people have acres of open land.

Go watch one of JoeBen's interviews. His original inspiration for the company was making something that could make where he grew up in the redwoods of the Santa Cruz mountains more accessible. His stated long term vision is vertiports embedded in communities. In the short term, I agree that they're going to start in denser areas.

I don't understand how that would be possible with the lack of anywhere to land in denser urban areas. This is a toy to hop over to the golf course.

Does your lawn come with an air traffic control tower?

Heh it doesn't have to be literally on one's own lawn, it could just be a little helipad per community. And my understanding is that the vast majority of private helipads don't have air traffic control - your hospital's roof doesn't have its own air traffic control. Pilots operate under "see and avoid" rules.

Use your imagination a little. So much status quo bias here.

Who would want to privately own a community helipad? Sounds like an insurance and liability nightmare.

This is an impressive enough achievement, but let's not kid ourselves this is going to revolutionaise suburban or semi-rural transport. Its maximum payload weight (450kg) barely covers 5 passengers with no baggage. It's for hopping from the country club to the golf course.

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