S-IVB

• 1. Introduction

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  The S-IVB, pronounced "S-four-B," is the rocket stage that sent human beings to the Moon. Built by the Douglas Aircraft Company and powered by a single J-2 engine, it was the third stage of the Saturn V and the second stage of the Saturn IB. For every lunar mission it had to fire not once but twice: first to settle the spacecraft into Earth orbit, then to ignite again and hurl the crew toward the Moon in what engineers called translunar injection. That second ignition, performed in the cold silence of orbit after coasting for hours, was one of the most consequential moments in spaceflight. How did a single engine stage come to carry that responsibility? And what happened to these stages after they finished their work?

• 2. From S-IV to S-IVB

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  The S-IVB descended from an earlier stage called the S-IV, which was the upper stage of the Saturn I rocket. The S-IV used a cluster of six RL-10 engines, but it burned the same propellants the S-IVB would later use: liquid hydrogen and liquid oxygen. The name itself carries a trace of that heritage. The stage was originally intended as the fourth stage of a planned rocket called the C-4, so it was designated S-IV. When plans shifted toward the larger C-5 configuration, later named Saturn V, engineers decided to replace the cluster of six engines with a single, more powerful J-2. That uprated design became the S-IVB.

  Eleven companies submitted proposals to build the new stage by a deadline of the 29th of February 1960. NASA administrator T. Keith Glennan made the decision on the 19th of April, awarding the contract to Douglas Aircraft Company. Convair had come in a close second, but Glennan was wary of one company dominating the liquid hydrogen-fueled rocket market. Convair was already building the Centaur stage for the Atlas-Centaur rocket, and that was enough. Douglas got the work.

  The S-IVB also served the Saturn IB, a smaller rocket designed to test Apollo spacecraft in low Earth orbit without the expense of a full Saturn V launch. That dual role shaped how the stage was built, producing two distinct versions with meaningfully different designs.

• 3. Two Versions, One Engine

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  Douglas built twelve 200-series stages for the Saturn IB and sixteen 500-series stages for the Saturn V, alongside three test stages. The two series looked similar but solved different engineering problems.

  The 200 series did not need to restart its engine. Its mission ended once it delivered a spacecraft to low Earth orbit, so it carried less helium pressurization and no mechanism for restarting the J-2. It also had three solid rockets to separate it from the Saturn IB booster during staging. The 500 series had to perform that second ignition in orbit, the one that sent crews to the Moon. To make that restart reliable, engineers added two small Auxiliary Propulsion System modules, known as APS modules. These modules settled the liquid propellants toward the engine before ignition, a process called ullaging, preventing the engine from gulping gas instead of fuel. The 500 series interstage also flared outward to match the larger diameter of the S-II stage beneath it.

  The S-IVB carried 73,280 liters of liquid oxygen, with a mass of 87,200 kilograms, and 252,750 liters of liquid hydrogen, massing 18,000 kilograms. Empty, the stage itself weighed 10,000 kilograms. During coast phases when the J-2 was silent, the two APS modules handled attitude control across all three axes. Each module contained two 150-pound-force thrusters for roll and pitch, a third 150-pound-force thruster for yaw, and one 70-pound-force thruster dedicated to ullage. Each module carried its own propellant supply: 150 pounds of dinitrogen tetroxide and 115 pounds of monomethyl hydrazine, pressurized with compressed helium.

  Funding limits eventually cut production short. An order for four additional 200-series stages was canceled in August 1968 before any hardware was assembled. Two more 500-series stages, intended for Saturn V rockets 516 and 517, were canceled around the same time.

• 4. Testing in Fire and Orbit

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  The first S-IVB to fly launched on the 26th of February 1966, on a suborbital Saturn IB mission designated AS-201. It impacted the Atlantic Ocean at coordinates 9.6621 South, 10.0783 East. A second suborbital test followed on the 25th of August 1966, and during that flight a camera mounted on the first stage captured the J-2 engine igniting for the first time on film.

  The most revealing test came with the mission designated AS-203, launched on the 5th of July 1966. That flight carried no payload at all. Its only purpose was to observe how liquid hydrogen behaved in weightlessness, data that engineers needed before they could trust the J-2 restart on the 500-series stages. The stage was eventually destroyed during a bulkhead test at the end of the mission, and its debris decayed from orbit. The behavior data it gathered went directly into validating the restart procedure that would later send Apollo crews toward the Moon.

  Not every test succeeded. The stage designated S-IVB-503 was destroyed during ground testing and never flew. A rebuilt replacement, S-IVB-503N, flew on the 21st of December 1968 as the 500-series stage for Apollo 8, the first crewed mission to leave Earth orbit. The Apollo 6 mission on the 4th of April 1968 saw a J-2 restart failure after pogo oscillations in the earlier stages damaged the engine. Mission controllers used the Apollo Service Propulsion System to accomplish some of the planned objectives instead.

• 5. Where the Stages Went

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  Starting with Apollo 13, launched on the 11th of April 1970, the 500-series S-IVB stages were deliberately crashed into the Moon after delivering their payloads. Seismometers left on the lunar surface by earlier Apollo missions recorded the impacts, giving scientists a controlled way to probe the Moon's interior structure. The S-IVB for Apollo 13 struck the lunar surface on the 14th of April 1970. Stages from Apollo 14, 15, 16, and 17 followed, each adding data points for characterizing what lies beneath the Moon's crust.

  The earlier 500-series stages, from Apollo 8 through Apollo 12, were sent into heliocentric orbit. One of those, the stage from Apollo 12 designated S-IVB-507, may have returned to the vicinity of Earth decades later. Astronomers believe it was discovered as an asteroid in 2002 and given the designation J002E3, though this identification rests on orbital analysis rather than certainty.

  The 200-series stages that launched crews to Skylab decayed from low Earth orbit after completing their missions. One exception was S-IVB-209, which was kept as a rescue vehicle throughout the Skylab program and later served as a backup for the Apollo-Soyuz Test Project in July 1975. It was never needed for either role. S-IVB-211 became part of a Skylab trainer at the U.S. Space and Rocket Center in Huntsville, Alabama. The stage S-IVB-D, a dynamic test article assembled in 1964 and delivered to Marshall Space Flight Center in January 1965, is also on display at that same center today.

• 6. Skylab and the Repurposed Stage

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  The most unexpected destination for an S-IVB was not the Moon or a heliocentric orbit but low Earth orbit, where one of the stages was transformed into a place where humans would live. S-IVB serial number 212 was converted into the hull of Skylab, the first American space station. Launched on the 14th of May 1973 atop a Saturn V, Skylab housed three successive crews before reentering the atmosphere on the 11th of July 1979.

  A second stage, S-IVB serial number 515, underwent the same conversion as a backup Skylab, designated Skylab B. It was proposed multiple times as a candidate for an actual launch, but funding never materialized. That backup station never flew and is now preserved at the National Air and Space Museum.

  The stages intended for Apollo 18, 19, and 20 -- missions canceled before they launched -- also found themselves on display rather than in service. S-IVB-513 is at Johnson Space Center; S-IVB-514 is at Kennedy Space Center. Their Saturn V stacks were repurposed: the other stages of the SA-513 rocket launched Skylab itself to orbit.

• 7. Legacy in Later Rockets

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  Designers of rockets that came after the Saturn V looked back at the S-IVB as a reference point. The second stage of the Ares I rocket and a proposed vehicle called the Earth Departure Stage would both have drawn on the S-IVB's basic architecture. Both designs called for an uprated version of the J-2 engine, designated the J-2X. The Earth Departure Stage in particular was meant to perform the same two functions as the 500-series S-IVB: inserting a payload into orbit and then firing again to push it toward the Moon or beyond.

  A more ambitious idea, the MS-IVB, was a proposed modification of the S-IVB intended for a Mars flyby mission. It was never produced. The broader concept of a restartable upper stage fueled by liquid hydrogen and liquid oxygen, proved out by AS-203 in 1966 and first used for a lunar mission on Apollo 8, remained influential long after the Saturn program ended. The S-IVB-S, the earliest test stage, built with thicker stainless steel tanks for propellant loading and engine tests and assembled in mid-1964, survived until 2024, when it was finally scrapped after spending years stacked on top of S-IB-11 at the Alabama Welcome Center in Ardmore, Alabama.