It seems a shame to throw away SSMEs. It's kind of like throwing away your cadilac at the end of your trip. I'm just guessing but don't think they have been making many of them except for the ones needed for the shuttles, and the upgrades. I guess what I'm trying to say is that it's not like there's an assembly line in which they are rolling these things out all the time. They just made what is needed for the present shuttles. If they are going to be expendable I would think the specs. Would be different since it's not meant to be reused. Perhaps they could choose a cheaper expendable engine already in use today.