But the new SRB's and engines are expendable!
They figured out how to expend them. Probably at increased cost...
Strangely, at the SLS's lower flight rate, Thiokol and Rocketdyne might not even have to increase their production rate of building brand new engines.
Early in the development of the Shuttle they considered putting the SSME's under the external tank (the aeronatical engineers were saying it was very advantageous, especially early in the flight, since the thrust vectors would be more aligned with the center of mass). But they still wanted to re-use the expensive engines, so they explained that after main engine shutdown the engines needed to somehow swing over from the ET to the back end of the shuttle, allowing it to jettison only the ET. The mechanical engineers started screaming about how hard that would be, a design nightmare that would probably weigh a lot, never work well, and where any failure (engines stuck halfway) would definitely result in the loss of vehicle and crew.
Now, I've read a lot about the shuttles development over the years and never heard of anything like this. Either A) There was one lone idiot that came up with the idea and he was quickly shouted down, or B) This story is bullshit. Either way, I gotta ask, source?
It seems to me a more sensible option would've been to mount the engines on their own re-entry capsule (heatshield forward like a firewall) and have the engines re-enter shortly after ET seperation, probably after a partial orbit so they could be fished out of the Pacific near Rocketdyne's plant.
Several versions of this have cropped up over the years. You're not the first to have the idea.