Apollo 6, launched on April 4, 1968, was the Apollo program's second and last A type mission—unmanned test flight of its Saturn V

Saturn V

The Saturn V was an American human-rated expendable rocket used by NASA's Apollo and Skylab programs from 1967 until 1973. A multistage liquid-fueled launch vehicle, NASA launched 13 Saturn Vs from the Kennedy Space Center, Florida with no loss of crew or payload...

 launch vehicle. It was intended to demonstrate full lunar injection capability of the Saturn V, and the capability of the Command Module's heat shield to withstand a lunar re-entry. Fuel line failures in several Saturn V second and third stage engines prevented it from achieving lunar injection, but it was able to get close to lunar return velocity by using the Apollo spacecraft's engine, as was done on Apollo 4

Apollo 4

Apollo 4, , was the first unmanned test flight of the Saturn V launch vehicle, which was ultimately used by the Apollo program to send the first men to the Moon...

, the first Saturn V test.