Apollo 6

Apollo 6, launched on April 4, 1968, was the final unmanned mission of the United States Apollo Program. It was a A type mission and the second test flight for the Saturn V launch vehicle.

It was intended to demonstrate full lunar injection capability of the Saturn V with a nearly full simulated payload, and also (again) the capability of the Command Module's heat shield to withstand a lunar-speed re-entry. Again, the mission was not designed to go to the moon, but merely to achieve a translunar speed toward an imaginary point in space nowhere near the moon, then turn around and return in about 10 hours.

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, the first Saturn V test. Despite the engine failures, the flight nonetheless provided NASA with enough confidence in the Saturn V to use it for manned launches. Since Apollo 4 had already tested the heat shield at full lunar re-entry velocity, a potential third unmanned flight was cancelled.