Editing AS-203 (section)

==Flight==
The rocket launched on the first attempt on July 5. The [[S-IVB]] and [[Saturn V instrument unit|Instrument Unit]] (IU) were inserted into a {{convert|100|nmi|km mi|adj=on}} circular orbit.<ref name=":0" />

The S-IVB design test objectives were carried out on the first two orbits, and the hydrogen was found to behave mostly as predicted, with sufficient control over its location and of engine temperatures required for restart. The next two orbits were used for extra experiments to obtain information for use in future cryogenic stage designs. These included a free-coast experiment to observe and control the negative acceleration of the fuel caused by the small amount of aerodynamic drag on the vehicle; a rapid fuel tank depressurization test; and a closed fuel tank pressurization test.<ref name=":1" />
[[File:AS 203 separation of S-IVB stage.webm|thumb|Video of the S-IVB separation]]
The closed fuel tank experiment involved pressurizing the hydrogen tank by closing its vents, while depressurizing the oxygen tank by allowing it to continue venting. It was expected that the pressure difference between the two tanks (measured as high as {{convert|39.4|psi|kPa}} would collapse the common bulkhead separating them, as happened in a ground test. The rupture occurred during the two-minute loss of signal between the [[Lyndon B. Johnson Space Center|Manned Spacecraft Center]] and the [[Trinidad]] [[Missile Test Project#Tracking stations|tracking station]]. The Trinidad radar image indicated the vehicle was in multiple pieces, and telemetry was never re-acquired. NASA concluded that a spark or impact must have ignited the propellants, causing an explosion.{{citation needed|date=October 2024}}

Despite the destruction of the stage, the mission was classified as a success, having achieved all of its primary objectives and validating the design concept of the restartable S-IVB-500 version. In September [[Douglas Aircraft Company]], which built the S-IVB, declared that the design was ready for use on the Saturn V to send men to the Moon.