[jfengel]

That's a clever approach, using tensioned pulleys (a bit like a bicycle wheel, which also uses tension to resist a crushing force).

The specs call for a 20 meter "balloon", which can lift 41,000 kg. The structure itself (made with existing materials) would weigh 37,000 kg, yielding a 4 tonne payload.

I'm not competent to evaluate whether the calculations are correct. It would be really cool to see such a thing constructed. I imagine it would cost hundreds of thousands, at a minimum.