A liquid hydrogen / oxygen fueled booster, which acted as the third stage of the Saturn V launch vehicle and the second stage of the Saturn IB launch vehicle. It became the seasoned campaigner of the Apollo project, used more often than any other stage or spacecraft.

The S-IVB came into being as a significantly uprated version of the S-IV stage. The S-IV was the second stage of the Saturn I launch vehicle. The S-IV in turn owes much to the construction experience gained in building the Thor rocket. The Thor was built by the Douglas Aircraft Co., who were awarded the S-IV/S-IVB contract on May 26, 1960.

Initially, it was planned to use four rocket engines to power the S-IV. When problems occured in the development of these engines, NASA switched to six, less powerful RL-10 engines. The S-IV used a common bulkhead between the liquid hydrogen and liquid oxygen tanks to eliminate the weight penalty of having two completely seperate tanks (a similar design, on a larger scale, was used in the Saturn V's S-II stage).

The S-IV's life was brief, but it proved many of the construction techniques and design choices that were used in the S-IVB. It flew onboard the Block 2 Saturn I six times between January 29, 1964 and July 30, 1965

The S-IVB was bigger, in length and diameter than the S-IV. The most significant change was the replacement of the cluster of six RL-10 engines with one J-2 engine. The biggest challenge faced by the S-IVB was the requirement that the booster could be shut down after reaching orbit and then restarted to send the Apollo astronauts on their way to the moon.

Restart in orbit required solving three problems: getting an engine that could be used in less than pristine condition (punishing operating conditions means that most booster engines are good for only one use); ensuring that the cryogenic propellants did not boil away while the stage was dormant; making sure that fuel would flow to the engines after floating around in zero g.

The first problem was solved by the selection of the J-2 rocket engine and a 'chilldown' sequence prior to ignition which cooled the turbopump of the J-2 before cryogenic propellants started flowing through it. The J-2 was mounted on gimbals which allowed the flight of the S-IVB to be adjusted by pointing the nozzle in different directions. Additional small rockets mounted on the S-IVB allowed the stage's attitude to be completely controlled.

The second problem involved a careful selection of internal insulation for the liquid hydrogen tank and a painstaking production process that required 4,300 polyurethane and fiberglass tiles, each uniquely shaped, to be glued to the inside of the tank by hand.

The third problem - fuel sloshing around in microgravity - was solved by pressurizing the tank with helium from an external tank and firing two small engines briefly before restart to force the fuel to the bottom of the tank, a procedure known as ullage control.

The S-IVB first flew as part of a Saturn IB test flight on February 26, 1966 and last flew as an active Saturn V stage during the Apollo 17 mission on December 6, 1972. This was not it's last flight though, flying several more times as part of a Saturn IB to Sklyab and finally as part of the Apollo-Soyuz mission on July 15, 1975.

Proving you can teach an old dog new tricks, the S-IVB was transformed one last time after the Apollo missions. NASA's Skylab space station was constructed from a converted S-IVB stage and was launched into orbit on May 14, 1973 aboard a modified Saturn V.

The S-IVB liquid oxygen tank held 72,700 liters, while the liquid hydrogen tank held 229,000 liters. The S-IVB was 17 meters high and 6.6 meters in diameters. It could deliver 890,000 newtons of thrust for a total of 500 seconds

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