Apollo 6 (flown April 4, 1968) was the third and final uncrewed flight in the United States' Apollo program and the second test of the Saturn V launch vehicle. Apollo 6 was intended to show that the Saturn V's third stage, the S-IVB, could propel itself and the Apollo spacecraft to lunar distances. Once components arrived at Kennedy Space Center, testing proceeded slowly, often delayed by testing of the Saturn V intended for Apollo 4, but once that launched, there were fewer delays. NASA planned a flight of about ten hours, but during launch (pictured) vibrations damaged some of the Rocketdyne J-2 engines, and the resulting parking orbit was more elliptical than planned. The third-stage engine failed to restart, and flight controllers elected to repeat the flight profile of Apollo 4, achieving a high orbit and high-speed return. Despite the engine failures, the flight qualified the Saturn V for crewed launches, and it was first used for crewed flight in December 1968 with Apollo 8. (Full article...)