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Comment by GuB-42 | Hacker News Reader

There is more than that. Planes become lighter as they burn fuel, batteries don't become lighter as they drain.

Lighter planes are more efficient, and most airliners are not designed to land with their tanks full. They have a option to jettison their fuel in case of emergency.

It means that just matching the energy density of fuel is not enough, batteries must exceed it. Jet engines are also quite efficient: around 40% now, and it may reach up to 60% in the future. It means that the more efficient electric engine is less of an advantage compared to cars for example.

The only battery technology that seems viable in order to replace fuel would be lithium-air. Petrol is just too good at storing energy.

Right now, I'd say the only chance for a "zero-emission" plane would be using biofuels.

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> There is more than that. Planes become lighter as they burn fuel, batteries don't become lighter as they drain.

Flow batteries can and open cycle fuel cells do; electric isn't just the kind of batteries that retain reaction products.

I get the fuel cell concept: fuel (H2) and oxygen combine to make water, which is thrown overboard. But, I don't see how this pertains to flow batteries. Generally, the flow battery material (e.g. vanadium oxide) is highly toxic and/or reactive. Do you propose sending that type of material overboard?

I'm pretty sure the "emergency fuel jettison" is for "I'd rather it not splash about everywhere and catch fire in the case that I put this plane down hard enough to split the tanks open" rather than "It's impossible to land this plane safely at the same weight as it took off".

(Not that I'd want to be hurtling at the ground barely in control with many tons of fully charged LiPo battery on board either...)

It's true. Take a 787 Dreamliner as an example. The maximum take-off weight is over 100,000 pounds more than the maximum landing weight.

If the landing speeds are the same, a heavier aircraft requires a longer landing roll. But a heavier aircraft also requires a higher landing speed. Those two phenomena compound.

Nope, look at the difference between the MTOW and MLW columns here: https://en.m.wikipedia.org/wiki/List_of_airliners_by_maximum...

The issue is the brakes overheating and the impact on the landing gear.

Thanks! Yet another thing to add to "the list of stuff I'm wrong about"... :-)

It's possible to turn atmospheric CO2 and water back into hydrocarbons, which would make for a carbon-neutral, closed-loop fuel system. It takes extraordinary amounts of energy to do that, but improvements in that process, and/or the ability to generate energy more efficiently than we can now, would help.

> batteries don't become lighter as they drain.

Right, but they can be thrown overboard :-)

> Right, but they can be thrown overboard

That might work for disposable rockets launched from the south Pacific, but it does not work for a reusable aircraft flying over populated continents.

Obviously you dispatch localized fleets of drones to catch the discarded batteries.

Then you just need a fleet of smaller planes to carry those drones.

I wonder if you could split the battery pack into thirds - each with a modern steerable parachute type glide-return system?

You climb out to cruise altitude - then drop off the "climb battery" which would autonomously return to your takeoff point (somewhere nearby where if can easily/safely land, be retrieved, and recharged for the next flight). You'd cruise to your destination on the "cruise battery", which you'd jettison near the landing point, so it could fly itself there for charging. You'd land with just the "landing battery" which'd have sufficient capacity for go-arounds and/or diversions, just like your fuel reserve in a conventional dinosaur-burning aircraft.

Dropping batteries off and moving the remaining ones around while maintaining safe centre of gravity might be tricker than just pumping liquid fuel around, but surely not insurmountable (the military seem to be able to jettison stunning weights of munitions while flying...) Perhaps youd got for batteries divided into six spread equally for and aft from the cog - and drop them in matched pairs?

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