Once the rationale for this particular dual-launch EOR scenario is understood, the next question is, logically, "why don't we use the existing EELV fleet for the smaller launch?" I'm sure you will understand when I tell you that I get this question all the time. And frankly, it's a logical question. I started with that premise myself, some years back. To cut to the chase, it will work - as long as you are willing to define "Orion" as that vehicle which can fit on top of an EELV. Unfortunately, we can't do that.
The adoption of the shuttle-derived approach of Ares I, with a new lox/hydrogen upper stage on a reusable solid rocket booster (RSRB) first stage, has been one of our more controversial decisions. The Ares V heavy-lift design, with its external-tank-derived core stage augmented by two RSRBs and a new Earth departure stage (EDS), has been less controversial, but still not without its detractors. So let me go into a bit of detail concerning our rationale for the Shuttle- derived approach.
