An interesting, though probably semi-insane, alternative. What about having a tank on-board, that normally is filled with air, that, when you drop off the payl

An interesting, though probably semi-insane, alternative. What about having a tank on-board, that normally is filled with air, that, when you drop off the payload, gets filled with liquid air instead? It'd be power-hungry, but meh. I mean: liquid air has a density of ~0.87g/cm^3 - it's not so far off that of water. Tank mass would kill you, but as you say, the actual lifting capacity is relatively cheap.