Even while Gemini spacecraft were flight-testing the techniques for journeys to the moon, the hydrogen-fueled S-IVB stage built by McDonnell Douglas for the Saturn V launch vehicle was being designed and tested. S-IVB pushed the Apollo spacecraft into Earth orbit and then re-ignited to accelerate the spacecraft toward the moon. The power of the S-IVB came from super-cold liquid oxygen kept only a bulkhead away from even colder liquid hydrogen and brought together in a rocket engine producing 200,000 pounds of thrust.
S-IVB performed Apollo's first trans-lunar insertion when it ignited to push Apollo 8 toward a Christmas Eve 1968 rendezvous with the moon. It repeated its flawless performance in the continuing series of Apollo lunar flights, most notably the Apollo 11 mission which was highlighted by man's first landing on the moon on July 20, 1969.