Apollo 2 Mission
Apollo 2 is the informal name of the launch of AS-203.
The main purpose of the AS-203 flight was to investigate the effects of weightlessness on the fuel in the S-IVB tank. The reason for this was that the S-IVB would be used by Apollo astronauts to boost them from Earth orbit to a trajectory towards the moon. Engineers wanted to see what the liquid hydrogen would do in the tank - settle in one place or maybe even slosh violently. The S-IVB tank was equipped with 83 sensors and two TV cameras to record what the fuel did.
Because this was an engineering flight, there was no Command Service Module (CSM). This was also the first flight of a new type of Instrument Unit that controlled the Saturn rockets during launch and the first launch of a Saturn IB from Pad 37B.
It was found during orbit that the stage could restart and that the fuel behaved just as predicted. It was observed over four orbits and then the stage was pressurized to see how much stress it could stand. In the end this test exceeded the structural capabilities of the stage and it fragmented.