“Variable buoyancy propulsion system.”
If that means that they move to the air layer going in the right direction, google loon have done that successfully.
They bob and weave… going up and down by changing buoyancy they move up and down through the atmosphere, this creates flow over the surfaces which can give a propulsion relative to the local wind.
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So you’re not going backwards quite as fast…
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Could be a good analogy for human behaviours in some areas.