What is "s-ib"

The S-IB stage was the first stage of the Saturn IB launch vehicle, which was used for Earth orbital missions. It was composed of nine propellant containers, eight fins, a thrust structure assembly, eight H-1 rocket engines, and many other components. The propellant containers consisted of eight Redstone tanks (four holding LOX and four holding RP-1) clustered around a Jupiter rocket tank containing LOX. The four outboard engines gimballed to steer the rocket in flight, which required a few more engine components. It burned for nearly 2.5 minutes before separating at an altitude of .