The Saturn IB (pronounced "one B", also known as the Uprated Saturn I) was an American launch vehicle commissioned by the National Aeronautics and Space Administration (NASA) for the Apollo program. It replaced the S-IV second stage of the Saturn I with the much more powerful S-IVB, able to launch a partially fueled Apollo Command/Service Module (CSM) or a fully fueled Lunar Module (LM) into low Earth orbit for early flight tests before the larger Saturn V needed for lunar flight was ready.