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Comment by bagels | Hacker News Reader

To fly through the night, you need energy. I'm wondering if it's more efficient to store it in the batteries, or as stored kinetic energy by flying to a high altitude and descending through the night.

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It's a good idea, in principle. But according to the sources I could find, to get enough gravitational potential energy to equal the electrical capacity of Zephyr's batteries, you'd have to lift the entire craft by about 70km.

https://www.aviationpros.com/engines-components/aircraft-air...

https://en.wikipedia.org/wiki/Airbus_Zephyr

I can imagine you'd want to take advantage of gravity to a certain extent, but it seems a bit tricky to estimate how much. Presumably there's a particular altitude at which the aircraft's overall efficiency is at a maximum. Deviating from this altitude allows you to store a bit of extra energy (which means you can get away with a smaller battery) but you don't want to deviate too far, or you'll lose more energy to various inefficiencies than you're saving. And you probably need to keep some electrical reserve power anyway, in order to be able to actively navigate away from unfavorable winds.

There's also the fact that this is optimizing for power usage in the air, when really what we'll want to optimize for most likely is power usage while doing some specific task in the air, and that task may necessitate specifics of what altitudes it can function at.

That's the real challenge. Being able to, for example, loiter over a suspected clanlab 24/7 for a month to gather evidence is far more taxing than flying an optimized route.

Assuming you didn't care about the path, could you also take some optimal path where you go east to west to prolong the days, then go west to east to shorten the nights. Could probably play with south/north too depending on the time of the year to get longer days. Or maybe you can go far enough to the pole where it's always day.

If you fly fast enough you can outrun the night, but that opens up a whole new set of problems.

Also, if you raise altitude high enough, you also can get rid of air resistance, so you won’t need to spend any energy to maintain altitude.

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