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Comment by gusgordon | Hacker News Reader

For those curious about the physics of these aircraft, here's an analysis I did of the same concept. The goal is to determine the smallest aircraft configuration that can indefinitely sustain flight: https://github.com/gusgordon/atmospheric_satellite#readme

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Is there a reason you optimized the amount of starting energy in the battery? I know basically nothing about how solar power works, but surely you'd just fill the battery up to 100% with an extension cord on the ground before launching it?

Good question. One requirement for the aircraft in this optimization is that they must have more energy in the battery than they did 24 hours prior. If the aircraft started at full energy, they wouldn't ever be able to satisfy this requirement, so that's why it's an independent variable.

For example, an aircraft could "start" at 50% battery state of charge, then charge to 95% over the course of the day, then come back 24 hours later at 51%, and that would be valid. There are other ways around this, but this is what I came up with at the time.

This is similar to why the starting altitude is allowed to float. The gravitational potential energy of the aircraft can be used as another "battery", but the aircraft is only a valid solution if it's not losing altitude over the course of 24 hours.

Maybe it helps surface how much charge is required to climb to altitude vs how much charge is required to sustain it? This may also help surface how much of a buffer you may have in takeoff time in order to survive night. E.g. if you require 100% charge then it's likely you have to take off at a fairly specific time of day.

The battery is too heavy if it's fully charged.

There are actually electrochemical cells which do change mass as they charge and discharge, metal-air batteries. In these, oxygen from the air is the cathode used to reduce the metal anode. But mass gain goes the opposite direction: the batteries are actually heaviest when discharged.

Various metals can be used. Iron is cheap, whilst aluminium-air cells actually offer some of the best performance in kWh/kg of available materials, exceeding that of LiON cells.

https://en.wikipedia.org/wiki/Metal%E2%80%93air_electrochemi...

Batteries are heavier when fully charged. Takeoff weight matters. It's not much heavier, but it might be enough to affect things at this scale.

edit: remember the context. This is about an abstract optimization to find the minimum viable aircraft.

I can't tell if you're being serious. You are technically correct, but also incredibly wrong in suggesting that the increased weight would be substantial enough to impact anything measurable. Depending on the size and capacity of the cell, you might see a difference due to general relativity on the order of nanograms.

see: https://physics.stackexchange.com/questions/34421/does-the-m...

A nanogram saved is a nanogram earned.

I can't tell if this is satire or not, but taken in good faith, how?

A fully charged battery would necessarily have more mass than a fully depleted battery, but the difference should be so tiny as to be immeasurable. Or am I wrong? We're essentially talking about the sum weight of a bunch of electrons, which are extremely light. There's no other exchange of matter going on when charging/discharging a battery, just the creation/destruction of chemical bonds, and associated movement of electrons.

A battery doesn't have a static charge. When it discharges, electrons move from one side to the other, then back through the batter, and you modify which atoms have which elections. But it continues to have no static charge.

However it has less potential energy. And therefore you change mass by the mass associated with that potential energy.

what kind of batteries are meaningfully heavier when charged? are you just talking about like E/c^2?

A water/gravity battery comes to mind. Definitely not the answer you are looking for, but an answer none the less.

Depends on your definitions. Is the water pumped considered part of the battery before it’s elevated?

The pumping of the water would be "charging" the barrel of water, I guess.

I'm sure this will work great for powering an aircraft, too. You could just let it all hang from an anchor at the highest point in the battery :-P

Gas tanks :D

nice analysis

Is it feasible to operate these at lower altitudes?

e.g. can we have solar/air drones posted every 100 miles of interstate highway?!

Yes it's a lot easier at lower altitudes since you can get more lift, but you might run into some extra regulatory issues with that :)

> e.g. can we have solar/air drones posted every 100 miles of interstate highway?!

What's the usefulness of this or was it just a random example?

As someone who lives somewhere with photo radar, I assume photo radar.

Nobody likes photo radar.

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