Apollo 6, the second and final uncrewed test of the Saturn V rocket, lifted off from Kennedy Space Center suffering severe pogo oscillations and two engine failures in the second stage. Though the mission fell short of its objectives, the flight revealed critical flaws that NASA engineers corrected before entrusting the rocket with human crews later that year.
The Saturn V, standing 363 feet tall, was the most powerful machine ever built, and Apollo 6 was meant to prove it flight-worthy for carrying astronauts to the Moon. Instead, roughly two minutes into flight the rocket began shaking violently with pogo oscillations, a longitudinal vibration named for the children's toy, caused by unstable fuel flow that could have torn the vehicle apart or injured a crew.
Things worsened further: two of the five second-stage engines shut down prematurely, and the malfunctioning guidance system had the rocket compensate by burning the remaining engines longer, throwing off the planned trajectory badly enough that the third-stage engine failed to reignite for a planned second burn.
Despite the near-disastrous performance, NASA's engineers treated the failure as an invaluable gift. They traced the pogo problem to resonance in the fuel lines and redesigned components accordingly, while the engine failures were linked to a ruptured fuel line. Confident the fixes worked, NASA controversially declared the next Saturn V flight-ready for a crewed mission, and Apollo 8 carried astronauts around the Moon just eight months later.