The Saturn IB (pronounced "one B", alternatively known as the Uprated Saturn I) was an American launch vehicle commissioned by the National Aeronautics and Space Administration (NASA) for use in the Apollo program. It was an uprated version of the Saturn I rocket, which replaced its S-IV second stage with the much more powerful S-IVB, which gave it enough payload capability to allow it be used for early testing of the Apollo spacecraft while the larger Saturn V needed to send Apollo to the Moon was still being developed. Unlike the earlier Saturn I, it could launch either the partially fueled Apollo Command/Service Module (CSM), or the fully fueled Lunar Module (LM) into low Earth orbit.
The Saturn IB was used for two unmanned CSM suborbital flights, one unmanned LM orbital flight, and for the first manned CSM orbital mission (first planned for Apollo 1, later Apollo 7). It was also used for one orbital mission, AS-203, in which it carried neither a CSM nor a LM, so that the S-IVB would have unburned liquid hydrogen fuel remaining in its tank in orbit. The purpose of this mission was to support design of the restartable version of the S-IVB used in the Saturn V, by observing the behavior of the liquid hydrogen in weightlessness.
After completion of the Apollo Moon landing program, the Saturn IB was used to launch the Apollo CSM on three crew missions to the Skylab space station, and a joint US-USSR space mission, the Apollo Soyuz Test Project.