Philip wrote:

And I guess if I had thought about more, the SSMEs used on the core
boosters are not recoverable from MECO altitude.


Which makes it strange to use them, as they are not a cheap engine by
any means. I'd have gone with J-2S, despite its lower ISP.
It could be that they are just going to take all the existing SSMEs and
use them up on the stick, figuring that at only one SSME per launch they
will have plenty to work with for a fairly substantial program. Engine
burn time will be a lot shorter in this usage also, so that probably
lowers the level of reliability that an engine is acceptable at,
particularly if they operate at lower than standard thrust.

Pat