NASA is heading back to the moon, and it’s planning to use the long-delayed Space Launch System (SLS) to get there. The agency is working to assemble the first SLS rocket, which will be the most powerful in the world upon completion. Some of that power will come from four RS-25 engines on the core stage. If they look familiar, that’s because the RS-25 has a storied history in NASA’s Space Shuttle program, having first debuted in the 1970s. Now, NASA has just finished installing them on the SLS.