Replacement for shuttle must serve as bridge craft, Steve Lindsey, Houston Chronicle
"NASA spent several years studying architectures and researching every available commercial rocket to find the design that could best accomplish those dual mission objectives in the safest, most reliable way. The conclusion we reached was that a space shuttle and Apollo derived vehicle was the best choice. Many have challenged this decision and still are advocating the use of existing commercial launch vehicles. Those have been studied extensively and fall short of our requirements for several reasons. First, existing commercial launch vehicles can lift only a fraction of the mass required for station and lunar missions in a single launch. Second, existing commercial launch vehicles have been designed and built to carry unmanned payloads; they would need to be heavily modified to meet our human rating requirements."