Hi GB

I think it's a technical issue. Man-rated means the engines can be throttled back so the crew isn't subjected to excessive gees when the mass-ratio has dropped to near burn-out level. Many unmanned boosters can hit 30 gee or so during the last part of their burn because they don't ease off the thrust as the tanks near depletion.

For example the Shuttle and ET massed about 150 tons dry, but the SSMEs could push upwards at about a gee with a full ET. The ET could hold 750 tons, so as the last of the propellant is burnt the peak acceleration would hit 5 gees if not throttled back.

The CEV launcher using an STS style SRB would have put the crew through 10 gees at burn out (SRBs put out 1174 tons-force of thrust, and the payload (liquid upper stage and 18 tons loaded CEV) was only 118 tons.) Not sure how they were planning to get around that.

Adam