On airships of old, this was actually a potential problem as well. They tried to avoid venting lifting gas whenever possible and investigated several technique

On airships of old, this was actually a potential problem as well. They tried to avoid venting lifting gas whenever possible and investigated several techniques to avoid it as much as possible. Extracting water from the air or weather to use as ballast as your airship becomes lighter and lighter (by consuming fuel for instance) is a particularly clever idea I think. http://en.wikipedia.org/wiki/Buoyancy_compensator_(aviation) http://en.wikipedia.org/wiki/Buoyancy_compensator_(aviation)