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 test 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 spacecraft, and to repeat demonstration of the Command Module’s (CM) 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 Command/Service Module’s (CSM) main engine, with a total flight time of about 10 hours.
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. The vehicle’s onboard guidance system was able to compensate by burning the second and third stages longer, though the resulting parking orbit was more elliptical than planned. 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 (SM) 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.