> The main differences between Commercial Crew and previous US manned space programs are that a) two different vehicles were built, not one, and b) NASA does not own or operate the vehicles; their builders do. Everything else is the same: NASA provided specifications for what it wanted in a manned spacecraft capable of reliably carrying people to ISS, various companies bid based on the specifications—their designs varying greatly from one another—and NASA chose the winning bids.
I think there is a big difference you haven't mentioned.
With Apollo, with the Space Shuttle, with the US components of the ISS, with SLS/Orion – NASA owned the design and made the big design decisions. They hired contractors to do a lot of the grunt work of the engineering – analysing different options, fleshing out high-level designs into detailed designs, etc – but the big picture design decisions were made by NASA. (Or, in the case of SLS, dictated to NASA by Congress.)
By contrast, with Commercial Cargo/Crew and HLS, NASA is just writing the requirements spec and letting the contractors make the big design decisions. And then the contractors have to convince NASA's engineers that the design actually fulfils those requirements. But NASA isn't making decisions like "what fuel should the launch vehicle use"–so long as they can convince NASA that what they are doing meets the requirements spec, the contractor can do almost anything they like. Whereas, on earlier programs, NASA was making the final call on many of those high-level design decisions.
I think there is a big difference you haven't mentioned.
With Apollo, with the Space Shuttle, with the US components of the ISS, with SLS/Orion – NASA owned the design and made the big design decisions. They hired contractors to do a lot of the grunt work of the engineering – analysing different options, fleshing out high-level designs into detailed designs, etc – but the big picture design decisions were made by NASA. (Or, in the case of SLS, dictated to NASA by Congress.)
By contrast, with Commercial Cargo/Crew and HLS, NASA is just writing the requirements spec and letting the contractors make the big design decisions. And then the contractors have to convince NASA's engineers that the design actually fulfils those requirements. But NASA isn't making decisions like "what fuel should the launch vehicle use"–so long as they can convince NASA that what they are doing meets the requirements spec, the contractor can do almost anything they like. Whereas, on earlier programs, NASA was making the final call on many of those high-level design decisions.