Apollo 6
However problems in the fuel lines of several Saturn V second and third stage engines stopped it from reaching the speeds Apollo would need to reach the Moon. It was able to test the return speeds by using the Apollo spacecraft's engine. This had been done with Apollo 4, the first Saturn V test. Despite the engine failures, the flight gave NASA enough confidence in the Saturn V to use it for manned launches. Since Apollo 4 had also tested the heat shield at a full speed re-entry, a third unmanned flight was cancelled.