It's going to run out of old Space Shuttle boosters eventually.
By Ryan Whitwam October 23, 2023
https://www.extremetech.com/aerospace/n ... nch-system
NASA's return to the moon, known as the Artemis Program, is powered by its shiny new Space Launch System (SLS). However, not every part of that rocket is new. It relies on RS-25 engines repurposed from the Space Shuttle program, but NASA's supply of old engines is finite. The agency has begun a series of "hot fire" tests to help engineers build a new family of RS-25 boosters that will catapult SLS into space beginning in the late 2020s.
The Space Launch System is currently the world's most powerful operational rocket. That will change as soon as SpaceX gets the kinks in Starship worked out, but the SLS is still an obscenely powerful launch vehicle. When it clears the tower, the SLS is propelled by two enormous solid rocket boosters and four RS-25 engines. The solid boosters account for most of the power, but those RS-25s are essential to keep the rocket flying after the solid boosters are exhausted.