The 200 Series S-IVB

There were two distinct versions: A 200 series for use on the Saturn IB, and a 500 series for the Saturn V. The diagrams below explain the major differences, the most visible being the use of a larger Auxillary Propulsion System (APS) module on the 500 series. Also, the 200 series used three solid fuel ullage motors while the 500 series had two. The APS on the latter had a ullage function since the S-IVB was required to restart to send the Apollo Spacecraft to the moon.

"Ullage" is an old brewers term meaning the air space above beer in a vat. To restart the J-2 engine, the propellants must be at the "bottom" of the tanks, with the gas above them. If they aren't in this position, the fuel pumps would "cavitate" or suck up gas and the restart would fail.

• The Evolution of the S-IV Stage
• Differences between the 200 Series and 500 Series S-IVB
• Component comparisons
• The 500 Series S-IVB
• In-flight photos from NASA