"In the 1950s and 1960s, the term "man rating" was coined to describe the process of converting the military Redstone, Atlas, and Titan II vehicles to the requirements of manned spaceflight. This involved a number of factors such as pogo suppression, structural stiffening, and other details not particularly germane to today's expendable vehicles. The concept of "man rating" in this sense is, I believe, no longer very relevant."
"... Logically, therefore, launch system reliability is treated by all parties as a priority of the highest order, irrespective of the nature of the payload, manned or unmanned. While there is no EELV flight experience as yet, these modern versions of the Atlas and Delta should be as inherently reliable as their predecessors. Their specified design reliability is 98%, a value typical of that demonstrated by the best expendable vehicles. If this is achieved, and I believe that it will be, and given a separate escape system with an assumed reliability of even 90%, the fatal accident rate would be 1 in 500 launches, substantially better than for the Shuttle. Thus, I believe that launching OSP on an expendable vehicle would pose no greater risk - and quite likely somewhat less risk - for human spaceflight than is already accepted for the Shuttle."
"The Space Taxi could initially be launched on a heavy-lift Evolved Expendable Launch Vehicle (EELV), currently under development by U.S. industry and the U.S. Air Force. Together with a small cargo carrier located behind the Space Taxi, this system would be used to meet future ISS servicing requirements. Later, a two-stage, commercially developed RLV, under study by Orbital, would replace the EELV in launching the Space Taxi system at a significantly lower cost."
"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."