S-IB

• 1. Introduction

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  The S-IB stage ignited eight Rocketdyne H-1 engines and burned for nearly two and a half minutes before dropping away from the rest of the rocket. It was the first stage of the Saturn IB, built for Earth orbital missions, and it flew ten times across nearly a decade. Its tanks were not purpose-designed for spaceflight. They came from two different Cold War missiles. What does it mean when a rocket built from borrowed parts becomes the vehicle of record for America's first crewed Apollo mission? And what happened to the stages that were built and tested but never left the ground?

• 2. Borrowed Tanks

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  Eight of the S-IB's nine propellant containers came from the Redstone missile program. Four of those held liquid oxygen and four held RP-1, a refined kerosene. The ninth tank, at the center of the cluster, derived from the Jupiter rocket and also held liquid oxygen. The nominal LOX capacity across all tanks was 66,277 US gallons, including a 1.5 percent ullage volume to allow for gas space above the liquid. RP-1 capacity was 41,000 US gallons. That mixed parentage gave the stage a distinctive silhouette: a wide Jupiter-derived cylinder at the core, surrounded by eight narrower Redstone-derived cylinders packed tightly around it. The arrangement let engineers sidestep the cost and risk of certifying a single large tank from scratch.

• 3. Eight Engines, Four That Steered

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  Four of the eight H-1 engines were gimballed, meaning they could swivel to redirect thrust and keep the rocket on course. These were the four outboard engines; the four inboard engines fired straight down and provided only upward push. Gimbaling required additional components on the outboard engines, adding mechanical complexity to that part of the stage. The S-IB also carried an ODOP transponder, which allowed ground stations to track its precise position during ascent. Eight fins ran along the base of the stage, and the whole assembly sat on a thrust structure designed to distribute load from all eight engines evenly into the airframe above.

• 4. Five Apollo-Era Missions

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  S-IB-1 launched on the 26th of February 1966, flying a suborbital path on the mission designated AS-201. S-IB-3 followed on the 5th of July 1966 as the AS-203 orbital test, and S-IB-2 flew the AS-202 suborbital test on the 25th of August 1966. Those three flights validated the stack before any crew rode it. S-IB-4 carried Apollo 5 into orbit on the 22nd of January 1968 for an unmanned test of the lunar module. On the 11th of October 1968, S-IB-5 launched Apollo 7, the first crewed Apollo mission. Apollo 7 put three astronauts into Earth orbit and confirmed the command and service module was ready for missions beyond Earth.

• 5. Skylab and the Soviet Handshake

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  S-IB-6 lifted the first Skylab crew on the 25th of May 1973, opening a new phase for the rocket. S-IB-7 followed on the 23rd of July 1973 carrying Skylab 3, and S-IB-8 launched the Skylab 4 crew on the 16th of November 1973. All three Skylab crew rotations relied on the same stage design that had been flying since 1966. The final Saturn IB flight came on the 15th of July 1975, when S-IB-10 carried the American crew for the Apollo-Soyuz Test Project, a joint docking mission with a Soviet Soyuz spacecraft. That mission ended the Saturn IB's operational record, closing a span that ran from early suborbital tests through the first US-Soviet joint spaceflight.

• 6. Stages Left Behind

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  S-IB-9 was stacked on a Mobile Launcher Platform as a ready rescue vehicle for the Skylab program. It never flew, but it survived. It now stands on display at the Kennedy Space Center Visitor Complex. S-IB-11 had a longer and stranger afterlife: displayed vertically at the Alabama Welcome Center for years, it was dismantled in late 2023 after weathering made it unsafe to keep upright. S-IB-12 is presumed scrapped at Marshall Space Flight Center sometime in the late 1970s. S-IB-13 and S-IB-14 were also scrapped. S-IB-15 and S-IB-16 were cancelled by NASA in 1968 before they were built, the same year S-IB-4 flew Apollo 5.

• 7. Concepts That Stayed on Paper

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  In 1960, engineers studied a larger S-IB-2 variant to power the proposed Saturn C-3. That concept specified a height of 34.50 meters and a diameter of 8.25 meters, driven by two F-1 engines rather than eight H-1s. A companion study, the S-IB-4 variant, was examined for the Saturn C-4 using four F-1 engines. By 1965, the S-IB-A was being studied for the Saturn IB-A and IB-B, substituting eight H-1c engines for the standard H-1s. In 1966, three more concepts appeared: the IB-11 stage proposed for the Saturn INT-11, INT-13, and INT-14 paired eight H-1b engines with UA1207 solid boosters; the IB-15 stage was designed for the Saturn INT-15 using H-1b engines alongside Minuteman first-stage strap-ons; and the S-1B-4 for the Saturn INT-12 combined four H-1b engines with UA1205 solid boosters. None reached hardware. S-IB-9, waiting on its launcher at Kennedy, represents the peak of what the actual design achieved.