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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.)

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    $\begingroup$ +1 This is a really interesting question! It might be too hard for people here though. I wonder if it would be better if you just asked about the atmospheric component to a F9 1st stage core reentry. That's already a hard question. Once that's answered it might be easier to address the whole FH scenario. $\endgroup$ – uhoh Apr 15 '19 at 13:29
  • $\begingroup$ @uhoh, except for the gravity, the same equations that determine landing and take-off between the moon and orbit would apply here too. So I don't think it would be that difficult a question to answer (not that I'm going to attempt it). $\endgroup$ – Ray Butterworth Apr 15 '19 at 14:29
  • $\begingroup$ @RayButterworth "not that I'm going to attempt it" self-calibrates your comment. $\endgroup$ – uhoh Apr 15 '19 at 15:40
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    $\begingroup$ if there were no atmosphere on earth falcon 9 would be designed very differently, and fly a different trajectory $\endgroup$ – JCRM Apr 15 '19 at 17:08
  • $\begingroup$ @JCRM great comment, oberth ascent profile would be the norm. $\endgroup$ – qq jkztd Apr 15 '19 at 18:53

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