Saturn V S-IVB (third) stage - Saturn Apollo Program

This cutaway illustration shows the Saturn V S-IVB (third) stage with the callouts of its major components. When the S-II (second) stage of the powerful Saturn V rocket burnt out and was separated the remaining units approached orbit around the Earth. Injection into the desired orbit was attaineded as the S-IVB (third stage) was ignited and burnt. The S-IVB stage was powered by a single 200,000-pound thrust J-2 engine and had a re-start capability built in for its J-2 engine. The S-IVB restarted to speed the Apollo spacecraft to escape velocity injecting it and the astronauts into a moon trajectory.