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

Objectives were to demonstrate trans-lunar injection capability of the Saturn V with a simulated payload equal to about 80% of a full Apollo lunar spacecraft, and to repeat demonstration of the Command Module's heat shield capability to withstand a lunar re-entry. The flight plan called for following trans-lunar injection with a direct return abort using the Apollo Command/Service Module's main engine, with a total flight time of about 10 hours.

The launch phase was marred by a phenomenon known as "pogo" oscillation, which ruptured ignitor fuel lines in J-2 engines in the second and third stages. The guidance system shut down two second stage engines early, and the S-IVB third stage engine operated at less than optimum performance, preventing the vehicle from achieving a nominal parking orbit. The damaged third stage engine also failed to restart for trans-lunar injection. Flight controllers elected to repeat the flight profile of the previous Apollo 4 test, achieving a high orbit and high-speed return using the Service Module engine. Despite the engine failures, the flight provided NASA with enough confidence to use the Saturn V for manned launches. Since Apollo 4 had already demonstrated S-IVB restart and tested the heat shield at full lunar re-entry velocity, a potential third unmanned flight was cancelled.