Actually if you bring back almost any liquid rocket engine, and it has not been damaged by its use, it could be reused. But you are correct in that it was weight and performance that governed cost in the case of the SSME. I don't at this time see it being re-designed in any way, so when the shuttles are retired so will the wonderful (I worked on these engines for some 20 years of my own career) but too costly SSME's.<br /><br />When the RS68 was being designed for Boeing's entry into the EELV designs it was (realiability and therefore quality assurance are ALWAYS number one, if not then you don't have any business in the rocket engine business!) designed with lower performance and higher weight, but this was a natural exchange for far lower cost! I think the entire RS68 engine costs just about the same as just the turbopumps for the SSME!<br /><br />Cost has rightfully become the chief aim of getting into space, so I see where the RS68 engines are now going to be NASA's choice for the CALV. These engines will neither need to be reuseable nor man-rated. <br /><br />They also have the advantage over the SSME of being far more powerful engines. At 665 K thrust you could use only three such engines instead of five (400 k) thrust SSME's. However, I suspect that NASA is going to use four such angines, thus giving the CALV at least 500 k thrust more than a vehicle powered by five re-designed SSME's!<br /><br />I also heard a rumor from some friends (I am retired) at Rocketdyne that an upgrade to 1,000 k was being considered for the RS68. We shall see....