Shuttle was canceled because its bay wasn't needed, it was overly expensive, and its service record was pretty accurately tracking the projected 1 shuttle loss for every ~50 flights. There wasn't a strong commitment to the missions at the time, so making something completely new was out of the question. But there was a lot politically and strategically riding on Shuttle, so they had to find some way of keeping some lights on. Those points can be elaborated, but the decision really wasn't much more complicated than that.

The F1 and SSME are apples and oranges. They have separate uses and achievements for their times. I haven't heard of either being widely considered objectively superior to the other.

At what stage was a shuttle loss per 50 flights projected? Can you expand on this, do you have a link? Was that due to the boosters or some other design constraints? Would a smaller shuttle have fared better in terms of reliability?

They started PRA of Shuttle after Challenger [1]. I got that number from the SLS PRA and Reliability people, and might have read it in a report too, but I don't remember many more details about it. There's a lot that goes into PRA [2].

Shuttle size was dictated by intended cargo and maneuvers, so it's not really possible to say how changing size would affect it since you'd have completely different design constraints and objectives.

[1] https://ntrs.nasa.gov/citations/20150002964

[2] https://www.nasa.gov/pdf/293261main_62868main_1_pmchallenge_...

thanks for the links!

The F1 isn't superior to the SSME. It doesn't really excel at anything (other than being very big maybe.)

From another article on that website: "The RS-25 is still considered to be about the best engine ever made with a fairly high thrust to weight ratio and unmatched efficiency." [1]

[1] https://everydayastronaut.com/raptor-engine/

IIRC the F1 still holds a few thrust records (thrust per one engine maybe ?) but is certainly much less efficient than modern engines from Spacex/Ble Origin & Russian construction bureaus (much worse ISP & TVR).