Dave: No atmosphere behind means no resistance to the expulsion of stuff. This means they come out moving faster. Total change in momentum of the stuff is greater so the produced thrust is greater.
Irlandes: I didn't actually ask a question. I was a physicist prior to flying helicopters. I have read those books and regularly recommend "Six Easy Pieces" to non-physics types. Also out is "Six Not-so-Easy Pieces" haven't read it yet. The explanation I gave for light is accurate but lacks a lot of explanation. If you want more detail, pm or email me. Not really stuff for this forum.