The Saturn IB (alternatively known as the Uprated Saturn I) was an uprated
version of the Saturn I rocket, which replaced its S-IV second stage with the much more powerful S-IVB. Unlike the earlier Saturn I, the IB had enough throw weight to launch either the Apollo Command/Service Module (partially fueled), or the Lunar Module into Earth orbit. This made it invaluable for testing the Apollo spacecraft while the larger Saturn V needed to send it to the Moon was still being developed.