NavList:

Call me old fashioned but the National Space Transportation System, definitely forget the economics..., had a far more sublime, approach to recovering both stages intact (sans the external tank).  It worked the first time and all but twice over its long history. Heck STS-51F even demonstrated a remarkable press to orbit abort capability back in July 1985.  That following an automagic center engine shutdown during second stage and a wise call for "limits to inhibit" therby avoiding a likely Trans-oceanic Abort Landing owing to a sensor issue on one of the two operating SSME's.  Try that one in your single engine second stage Falcon 9.