The Saturn IB^{[lower-alpha 1]}(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-pound-force (400,000 N), 43,380,000 lb-sec total impulse), with the S-IVB (200,000-pound-force (890,000 N), 96,000,000 lb-sec total impulse). The S-IB first stage also increased the S-I baseline's thrust from 1,500,000 pounds-force (6,700,000 N) to 1,600,000 pounds-force (7,100,000 N) and propellant load by 3.1%. This increased the Saturn I's low Earth orbit payload capability from 20,000 pounds (9,100 kg) to 46,000 pounds (21,000 kg), 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.

The Saturn IB launched two uncrewed CSM suborbital flights to a height of 162 km, one uncrewed LM orbital flight, and the first crewed CSM orbital mission (first planned as Apollo 1, later flown as Apollo 7). It also launched one orbital mission, AS-203, without a payload so the S-IVB would have residual liquid hydrogen fuel. This mission supported the design of the restartable version of the S-IVB used in the Saturn V, by observing the behavior of the liquid hydrogen in weightlessness.

In 1973, the year after the Apollo lunar program ended, three Apollo CSM/Saturn IBs ferried crews to the Skylab space station. In 1975, one last Apollo/Saturn IB launched the Apollo portion of the joint US-USSR Apollo–Soyuz Test Project (ASTP). A backup Apollo CSM/Saturn IB was assembled and made ready for a Skylab rescue mission, but never flown.

The remaining Saturn IBs in NASA's inventory were scrapped after the ASTP mission, as no use could be found for them and all heavy lift needs of the US space program could be serviced by the cheaper and more versatile Titan III family and also the Space Shuttle.

In 1959, NASA's Silverstein Committee issued recommendations to develop the Saturn class launch vehicles, growing from the C-1. When the Apollo program was started in 1961 with the goal of landing men on the Moon, NASA chose the Saturn I for Earth orbital test missions. However, the Saturn I's payload limit of 20,000 pounds (9,100 kg) to 162 km would allow testing of only the command module with a smaller propulsion module attached, as the command and service module would have a dry weight of at least 26,300 pounds (11,900 kg), in addition to service propulsion and reaction control fuel. In July 1962, NASA announced selection of the C-5 for the lunar landing mission, and decided to develop another launch vehicle by upgrading the Saturn I, replacing its S-IV second stage with the S-IVB, which would also be modified for use as the Saturn V third stage. The S-I first stage would also be upgraded to the S-IB by improving the thrust of its engines and removing some weight. The new Saturn IB, with a payload capability of at least 35,000 pounds (16,000 kg),^[4] would replace the Saturn I for Earth orbit testing, allowing the command and service module to be flown with a partial fuel load. It would also allow launching the 32,000-pound (15,000 kg) lunar excursion module separately for uncrewed and crewed Earth orbital testing, before the Saturn V was ready to be flown. It would also give early development to the third stage.^[2]

On May 12, 1966, NASA announced the vehicle would be called the "uprated Saturn I", at the same time the "lunar excursion module" was renamed the lunar module. However, the "uprated Saturn I" terminology was reverted to Saturn IB on December 2, 1967.^[2]

By the time it was developed, the Saturn IB payload capability had increased to 41,000 pounds (19,000 kg).^[2] By 1973, when it was used to launch three Skylab missions, the first-stage engine had been upgraded further, raising the payload capability to 46,000 pounds (21,000 kg).

IBM built the instrument unit at the Space Systems Center in Huntsville, Alabama. Located at the top of the S-IVB stage, it consisted of a Launch Vehicle Digital Computer (LVDC), an inertial platform, accelerometers, a tracking, telemetry and command system and associated environmental controls. It controlled the entire rocket from just before liftoff until battery depletion. Like other rocket guidance systems, it maintained its state vector (position and velocity estimates) by integrating accelerometer measurements, sent firing and steering commands to the main engines and auxiliary thrusters, and fired the appropriate ordnance and solid rocket motors during staging and payload separation events.

As with other rockets, a completely independent and redundant range safety system could be invoked by ground radio command to terminate thrust and to destroy the vehicle should it malfunction and threaten people or property on the ground. In the Saturn IB and V, the range safety system was permanently disabled by ground command after safely reaching orbit. This was done to ensure that the S-IVB stage would not inadvertently rupture and create a cloud of debris in orbit that could endanger the crew of the Apollo CSM.

Acceleration of the Saturn IB increased from 1.24 G at liftoff to a maximum of 4.35 G at the end of the S-IB stage burn, and increased again from 0 G to 2.85 G from stage separation to the end of the S-IVB burn.^[8]

AS-206, 207, and 208 inserted the Command and Service Module in a 150-by-222-kilometer (81-by-120-nautical-mile) elliptical orbit which was co-planar with the Skylab one. The SPS engine of the Command and Service Module was used at orbit apogee to achieve a Hohmann transfer to the Skylab orbit at 431 kilometers (233 nautical miles).^[8]

The first five Saturn IB launches for the Apollo program were made from LC-34 and LC-37, Cape Kennedy Air Force Station.

The Saturn IB was used between 1973 and 1975 for three crewed Skylab flights, and one Apollo-Soyuz Test Project flight. This final production run did not have alternating black and white S-IB stage tanks, or vertical stripes on the S-IVB aft tank skirt, which were present on the earlier vehicles. Since LC-34 and 37 were inactive by then, these launches utilized Kennedy Space Center's LC-39B.^[9] Mobile Launcher Platform No. 1 was modified, adding an elevated platform known as the "milkstool" to accommodate the height differential between the Saturn IB and the much larger Saturn V.^[9] This enabled alignment of the Launch Umbilical Tower's access arms to accommodate crew access, fueling, and ground electrical connections for the Apollo spacecraft and S-IVB upper stage. The tower's second stage access arms were modified to service the S-IB first stage.^[9]

For earlier launches of vehicles in the Saturn I series, see the list in the Saturn I article.

In 1972, the cost of a Saturn IB including launch was US$55,000,000 (equivalent to $401,000,000 in 2023).^[14]

• Comparison of orbital launchers families
• Comparison of orbital launch systems