Apollo 6 was the third and final uncrewed flight in the United States' Apollo Program, and the second test of the Saturn V launch vehicle. It qualified the Saturn V to be used on crewed missions, as happened for the first time on Apollo 8 in December 1968.
Apollo 6 was intended to demonstrate the ability of the Saturn V's third stage, the S-IVB, to propel itself and the Apollo spacecraft to lunar distances. Its components began arriving at the Kennedy Space Center in early 1967. Testing proceeded slowly, often delayed by testing of the Saturn V intended for Apollo 4. Once that uncrewed mission launched in November 1967, there were fewer delays, but there were enough so that the flight was postponed from March to April 1968.
The flight plan called for following trans-lunar injection with a direct return abort using the service module's main engine, with a total flight time of about 10 hours. Instead, a phenomenon known as pogo oscillation damaged some of the Rocketdyne J-2 engines in the second and third stages by rupturing internal fuel lines, causing two second-stage engines to shut down early.