This is an interesting article from a couple of days ago about tracking diy balloons long distance: https://spectrum.ieee.org/explore-stratosphere-diy-pico-ball.... Given the tracker can be built for $14 it might be worth it to test a version with just the custom hydrogen enclosure and tracker and see how far it gets.
A new sailing record was set recently, which didn't _quite_ beat 40 days - it took them 40 days and almost 11 hours.
If you've ever seen a SailGP boat flying up out of the water on their hydrofoils and doing 100kmh - imagine a 32m long 26m wide ocean going trimaran doing the same thing in a non stop circumnavigation of the globe. I think they _averaged_ 27knots or 50kmh!
And when the world was still flooded with money and the music was awesome (so 2004), Jeremy Clarkson did a series about inventions, one of them being the jet plane, and he flew commercial around the world in 120 hours: https://www.youtube.com/watch?v=o8D_UzMhjH8
Hydrogen containment: you might want to look at topping up hydrogen along the way. Have a small compartment inside the gas bag with lithium shavings and a small servo that can release water into this compartment.
You'll get about 1/15-th of the lithium+water mass as hydrogen. So if you want to replenish 1 m^3 of hydrogen at STP (or around 1/8-th of the gas bag volume), you'll need around 600 grams of reagents.
> Finally, can it be flown legally? Most of the trajectory can pass over the oceans, but skipping land completely would take too much of a detour, and likely be incompatible with prevailing winds. Although the political climate may be hostile, it is still legal to fly civil craft over other countries.
Fair points. I am aware that it is a UAV, and that there are some harsh regulations around the world (e.g. drones are completely banned in Morocco). I am also willing to accept my vast ignorance. I don't know exactly what regulations apply to an autonomous airship: balloon, UAV, recreational amateur aircraft? Actual laws for the handful of countries involved in the actual trajectory (Nicaragua, Thailand, Egypt perhaps) are not easy to find online. So I changed the first paragraph to:
"Finally, what are the legal implications of flying an airship drone over foreign air spaces? This very complex area clearly requires in-depth study and careful consideration, and it lies completely out of my depth. The primary objective is just to avoid being shot down since I'm not planning to visit any foreign countries in person, so allow me to just touch over the main issues."
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