Saturn IB

The Saturn IB (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 uprated the Saturn I by replacing the S-IV second stage (90,000 pounds-force [400 kN], 43,380,000 lb-sec total impulse), with the S-IVB (200,000 pounds-force [890 kN], 96,000,000 lb-sec total impulse). The S-IB first stage also increased the S-I baseline's thrust from 1,500,000 to 1,600,000 pounds-force (6,700 to 7,100 kN) and propellant load by 3.1%. This increased the Saturn I's low Earth orbit payload capability from 20,000 to 46,000 pounds (9.1 to 20.9 t), enough for early flight tests of a half-fueled Apollo command and service module (CSM) or a fully fueled Apollo Lunar Module (LM), 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. The S-IVB on the Saturn IB needed all of its propellant to achieve Earth orbit.