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Hi there, I read your IAmA where you had a reply to a message, now deleted, in which you must have been asked about aerial circumnavigation. You mentioned that

Hi there, I read your IAmA where you had a reply to a message, now deleted, in which you must have been asked about aerial circumnavigation. You mentioned that you'd use a solar balloon, and that you merely would have to have a strategy for darkness. I've spent a fair amount of time thinking about this, and it may interest you to know that: Water vapor has both a lower molar density than air The phase transition of water vapor gives more than enough energy to heat up more than the equivalent volume of air to displace the reduced volume of water vapor turning to liquid. So, a possible strategy for darkness is to have a two bladdered balloon, one with water vapor + air, one with just air. Then, when the water vapor condenses, transfer the heat to air sourced from outside the balloon via a counterflow heat exchanger. Put that warmed air in the air bladder. You won't stay at precisely the same height, but you will retain volume and you can stay buoyant. Many things work well when your thermal energy storage is positively buoyant. Regards, Come visit the next time you're free around Berkeley or Oakland.

lutusp 14y ago
This is an interesting altitude-control scheme. When I think about this problem, I consider a method that involves a flexible helium container (in the simplest form, a helium balloon) and a control system that involves pumping helium out of the balloon into a high-pressure tank to descend, and releasing it from the tank to the balloon to rise. It's a variation on the submarine buoyancy method, which also requires power to run pumps. The advantage is that the method doesn't just throw away the helium, it recycles it, but the recycling activity require a lot of power. And it's way complicated compared to your method.
- DaniFong 14y ago
  Yup, and things stay around atmospheric pressure throughout. I intend to build it at some point. (There's a long list, obviously, but this is a keeper) so if you're ever interested in it, let's chat. :-)

lutusp 14y ago

This is an interesting altitude-control scheme. When I think about this problem, I consider a method that involves a flexible helium container (in the simplest form, a helium balloon) and a control system that involves pumping helium out of the balloon into a high-pressure tank to descend, and releasing it from the tank to the balloon to rise. It's a variation on the submarine buoyancy method, which also requires power to run pumps. The advantage is that the method doesn't just throw away the helium, it recycles it, but the recycling activity require a lot of power. And it's way complicated compared to your method.

DaniFong 14y ago
Yup, and things stay around atmospheric pressure throughout. I intend to build it at some point. (There's a long list, obviously, but this is a keeper) so if you're ever interested in it, let's chat. :-)