S-IVB

S-IVB

S-IVB-206 which was used for the Skylab 2 flight
Manufacturer	Douglas
Country of origin	USA
Used on	Saturn IB (stage 2) Saturn V (stage 3)
General characteristics
Height	58 feet 5 inches (17.81 m)
Diameter	21 feet 8 inches (6.60 m)
Gross mass	253,000 pounds (115,000 kg)
Propellant mass	229,000 lb (104,000 kg)
Engine details
Engines	1 J-2 engine
Thrust	232,250 pounds-force (1,033,100 N)
Specific impulse	421 s (4.13 km/s)
Burn time	475 seconds
Fuel	LOX/LH2

The S-IVB (sometimes S4b, always pronounced "ess four bee") was built by the Douglas Aircraft Company and served as the third stage on the Saturn V and second stage on the Saturn IB. It had one J-2 engine. For lunar missions it was fired twice: first for the orbit insertion after second stage cutoff, and then for translunar injection (TLI).

The S-IVB evolved from the upper stage of the Saturn I rocket, the S-IV, and was the first stage of the Saturn V to be designed. The S-IV used a cluster of six engines but used the same fuels as the S-IVB — liquid hydrogen and liquid oxygen. It was also originally meant to be the fourth stage of a planned rocket called the C-4, hence the name S-IV.

Eleven companies submitted proposals for being the lead contractor on the stage by the deadline of 29 February 1960. NASA administrator T. Keith Glennan decided on 19 April that Douglas Aircraft Company would be awarded the contract. Convair had come a close second but Glennan did not want to monopolize the liquid hydrogen-fueled rocket market as Convair was already building the Centaur rocket stage.

In the end the Marshall Space Flight Center decided to use the C-5 rocket (later called the Saturn V), which had three stages and would be topped with an uprated S-IV called the S-IVB which instead of using a cluster of engines would have a single J-2 engine. Douglas was awarded the contract for the S-IVB because of the similarities between it and the S-IV. At the same time it was decided to create the C-IB rocket (Saturn IB) that would also use the S-IVB as its second stage and could be used for testing the Apollo spacecraft in Earth orbit.

Douglas built two distinct versions of the S-IVB, the 200 series and the 500 series. The 200 series was used by the Saturn IB and differed from the 500 in the fact that it did not have a flared interstage and had less helium pressurization on board as it would not be restarted. On the 500 series, the interstage needed to flare out to match the larger diameter of the S-IC and S-II stages of the Saturn V. The 200 series also had three solid rockets for separating the S-IVB stage from the S-IB stage during launch. On the 500 series this was reduced to two, and two small APS (auxiliary propulsion system) thruster modules were added as ullage motors for restarting the J-2 engine and to provide attitude control during coast phases of flight.