I wonder if you could use vacuum instead of helium for the lighter-than-air medium? That way, you just have a vacuum pump as the energy expenditure. Also, that

I wonder if you could use vacuum instead of helium for the lighter-than-air medium? That way, you just have a vacuum pump as the energy expenditure. Also, that solves the problem of the ship being too light after unloading: just let a controlled amount of air back in to make it neutrally buoyant. Edit: "The main problem with the concept of vacuum airships however is that with a near-vacuum inside the airbag, the atmospheric pressure would exert enormous forces on the airbag, causing it to collapse if not supported. Though it is possible to reinforce the airbag with an internal structure, it is theorized that any structure strong enough to withstand the forces would invariably weigh the vacuum airship down and exceed the total lift capacity of the airship, preventing flight." https://en.wikipedia.org/wiki/Vacuum_airship https://en.wikipedia.org/wiki/Vacuum_airship