Apollo 4, which launched on November 9, 1967, was the first unmanned test flight of NASA's Saturn V rocket, which was developed to bring astronauts to the moon.
The mission was the first-ever launch from the Kennedy Space Center. It was a success for NASA, as it proved that Saturn V worked. At the time, the 363-foot-tall vehicle was the largest spacecraft to ever attempt flight.
Apollo 5 launched a few months later, on January 22, 1968. The mission successfully tested the ability of the Apollo Lunar Module — the spacecraft designed to land on the moon's surface — to ascend and descend.
The Apollo 6 launch followed on April 4, 1968. The mission aimed to show that the Saturn V rocket was capable of trans-lunar injection, which puts a spacecraft on its path to the moon. But the system quickly ran into problems: Two of the five engines shut down unexpectedly, and the spacecraft could not be propelled into orbit.
Despite the issues with Apollo 6, NASA pushed ahead with plans for its first manned launch.