Reading this question and answers, I wondered what would happen if there was no atmosphere, everything else being the same on Earth.
What would be the payload capacity to LEO of Falcon 9 in reusable mode, with boost back to launch site?
It seems obvious that in fully expandable mode, payload to LEO would be greater since there would be no drag to fight against during ascent.
On the other hand, in reusable mode, there would be no drag either to slow the booster down after boost back burn. And this booster will accelerate until landing burn. How much of a payload weight difference would this represent?
(grid fins would be useless, and their weight replaced by bigger RCS tanks.)