NASA's RS-25 rocket engine powered the space shuttles and four of them will be used to power the core stage of NASA's new Space Launch System (SLS), the launch vehicle that will bring humans to deep space missions like landing on asteroids and Mars. This two minute time lapse video shows how NASA assembles it.
You can read more about the RS-25 here. NASA writes:
In this two-minute time-lapse video, see how a powerhouse of a rocket engine, the RS-25, is assembled by team members from Aerojet Rocketdyne at NASA's Stennis Space Center near Bay St. Louis, Mississippi. Four RS-25 engines will power NASA's new rocket, the Space Launch System, on missions to deep space, including to an asteroid and ultimately to Mars.