And because we are using those engines, which lack sufficient thrust at liftoff, we have to use the Solid Rocket Boosters. Those were supposed to be recoverable but the SLS just drops them into the ocean now too.
>it would cost $23 million to refurbish a used SRB and $12~70 million to refuel it.
A unconfirmed sources, that worked at NASA claim that Thikol employee explained to him. That reuse cost 3 time more, than a expendable SRB https://forum.nasaspaceflight.com/index.php?topic=51959.0
https://space.stackexchange.com/a/45894
Basically the SRB had multiple modules and some were more reusable than others, so some got recovered and refurbished a lot more.
RS-25s were the three main engines. They were very expensive, designed for reuse and were recovered with the rest of the orbiter they were bolted on to. Not in the ocean. Then refurbished with a much greater amount of effort and money than initially expected, and eventually reused on a future mission..
But the SLS first stage doesn't fly itself back to Cape Canaveral after 2 weeks like the Shuttle orbiter did. So those now FOUR very expensive "reusable" engines are now chucked into the ocean never to be seen again.
The shuttle's SRB's were fished out, refurbed, refueled, and reflown.
The SLS's SRB's are left to sink to the bottom.