Saturn IB

The Saturn IB was an American launch vehicle commissioned by the National Aeronautics and Space Administration for the Apollo program. It uprated the Saturn I by replacing the S-IV second stage, with the S-IVB. The S-IB first stage also increased the S-I baseline's thrust from 1,500,000 pounds-force to 1,600,000 pounds-force and propellant load by 3.1%. This increased the Saturn I's low Earth orbit payload capability from 20,000 pounds to 46,000 pounds, enough for early flight tests of a half-fueled Apollo command and service module or a fully fueled Apollo Lunar Module, before the larger Saturn V needed for lunar flight was ready.

By sharing the S-IVB upper stage, the Saturn IB and Saturn V provided a common interface to the Apollo spacecraft. The only major difference was that the S-IVB on the Saturn V burned only part of its propellant to achieve Earth orbit, so it could be restarted for trans-lunar injection.