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4 tons of water is smaller than a cube 6ft on the side. Water flows where you want it, it’s disposable without special procedures, and is literally available anywhere out of the tap.

This doesn’t seem like the hard part.

Edit: sibling comment points out that helium can also be compressed. I don’t think “too much lift” is an intractable issue.

Do they not compress the helium they have internally? Seems like an obvious solution so I must be missing something.

You need a large compressor and carry the energy around.

When loading/unloading you just tether it to the ground. If you must fly it without a specific cargo you probably just fill it with water.

There are many routes where cargo needs to be moved both ways