The Saturn V rocket was the most powerful rocket ever built. The first stage had 5 F1 engines. The second stage had 5 J2, while this third stage has one J2. It required only one, because by this time, it was well away from the strong pull of the earth gravity, plus it had far less weight to push, since it had lost two previous stages and all of the fuel they first contained.
The phrase "rocket science" has multiple meanings; the science it takes to plan and navigate the path into space, the physics of gravity vs. lift, and the mechanics of creating and building something so complex as to push out the amount of thrust these engines created.
And to think this was state of the art for the 1960s. I can't even fathom the complexity of what it takes to build a station in space or create a reusable vehicle like the Space Shuttle. I guess that's why I just take the photos and leave the science to the real heroes, those scientists that create it.