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Comment by MegaButts | Hacker News Reader

> reduced weight in other parts of an aircraft

The bigger problem is that the overall weight increases. Rearranging the COG doesn't really matter when most of your energy is spent literally fighting gravity.

This is the first thing that popped up in google when I wanted to compare gravimetric density between gasoline and lithium ion batteries. Gasoline is still approximately 30x denser. That is at least one revolutionary breakthrough in battery technology away, if not several.

https://research-archive.org/index.php/rars/preprint/downloa...

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Considering the thermal efficiency of a modern jet engine, the usable energy compared to a lithium battery will be ~15 higher per kg, still bad, but not as bad.

Also some napkin math using common examples gives a range of 0.2 - 1.2 horsepower / kg for gasonline motors, and 8 - 21 horsepower / kg for electric. So even though the batteries weigh more, the motors weigh less.

Doesn't it compound since it costs fuel to carry fuel in a flying machine?

I'm not sure about math but isn't it like 1/15th Isp, even with that maximally optimistic value?

This is not correct for electric trucks. Replacing diesel motor and gearbox with battery pack and electric drive train is close to a zero sum game according to https://youtube.com/@electrictrucker?si=RjdWBQQXansebUyJ

Definitely not a huge penalty.

Fair, but the context here is planes (and boats I guess though that seems less difficult than planes)

Container boats are ridiculously carbon efficient because they move unimaginable amounts of weight (amortizing any fixed costs like keeping the engine running) slowly (low drag) over a perfectly level surface (no loss from going up hill).

Almost any carbon reduction scheme that involves doing anything other than using them doesn’t work.

For instance, the embodied carbon of an apple that goes from China to the US, then is driven to a Walmart in a diesel train / semi is probably lower than the carbon footprint of one from the local farmers market (unless the farmer drives the apples to market in an EV and the local power grid is low carbon).

Right and it's precisely because they can have unimaginable amounts of weight that it's a more tractable thing to solve compared to electric planes.

I don't think the article is debating cargo ship vs car carbon footprint here, it's just the feasibility of electric cargo ship vs bunker fuel cargo ship. (And planes, which seems way harder)

Airplanes don't get to do regenerative braking except briefly upon landing, but you're not going to put generators in the wheels just for that, and you need active thrust reversers, so really there is simply no room for regenerating power in electric planes.

There’s no reason why you couldn’t do regen when descending with an electric-powered prop plane. It would give a steeper descent than normal, and may not be more efficient overall than cutting power earlier and descending more gradually, but it could be done.

Electric will start smaller and build up to big just like everything else. The single and twin engine world is pretty dominated by structure so the gains are easy to see there. As new capabilities and designs are prooven out things will grow or, and this is what I really hope for, we will find that we don't need the massive aircraft to be profitable anymore and we will just get more smaller electric aircraft providing the service in a more point to point nature. One of the biggest efficiency gains we could have is not forcing people into a hub and spoke mega airport model, something that isn't practical with the current massive aircraft tech.

> One of the biggest efficiency gains we could have is not forcing people into a hub and spoke mega airport model

The airlines have already switched from that model.

That would be terrifying.

Wheeled vehicles have lots of opportunities for braking, but airplanes and boats not so much, so even if you could do regenerative braking (which you probably can't) it'd not be enough to be worth doing. It's a loss compared to wheeled EVs of 30%-50%.

And then airplanes typically need to be lighter when landing than when taking off, and jet fuel has the nice properties that a) as you use it up what you've left weighs less, b) you can toss enough jet fuel to get to landing weight if need be. Batteries have neither of those properties, which means that electric airplanes would have to be built much sturdier (i.e., heavier, therefore more expensive and less efficient) to handle heavy landings, or would have to carry less cargo / fewer passengers per unit of stored energy (i.e., less efficient).

I'm afraid that no matter how good wheeled EVs get, it's going to require a whole new kind of battery before you can ever get to practical. large electric airplanes.

The point was the other parts of the plane. No lines moving all that gas around and the pumps and the plumbing in and around the engine and the bleed air piping and the and the and the.... There are a lot of potential places to shave weight when you go electric. An aircraft optimized for electric will be massively different if done right.

The way jetliners scale shaving the weight of those things (and note that you're not counting the weight of electric cables that can carry hundreds of amps) is nothing. It might be something for tiny airplanes, but that's it.

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