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 crewed launches. Since Apollo 4 had already demonstrated S-IVB restart and tested the heat shield at full lunar re-entry velocity, a potential third uncrewed flight was cancelled.