The launch of the Apollo 6 (CSM-020/SA-502) unmanned space mission occurred at 07:00:01.5 (EST). "The five F-1 engines of the huge Apollo/Saturn V space vehicle's first (S-IC) stage leave a gigantic trail of flame in the sky above the Kennedy Space Center seconds after liftoff."
"For the first two minutes, the Saturn V launch vehicle behaved normally. Then, as the Saturn V's S-IC first stage burned, pogo oscillations shook the vehicle. The thrust variations caused the Saturn V to experience a g-force of ±0.6 g (5.9 m/s2), though it had only been designed for a maximum of 0.25 g (2.5 m/s2). The vehicle suffered no damage, other than the loss of one of the panels of the Spacecraft-Lunar Module Adapter (SLA).
NASA Associate Administrator for Manned Space Flight George Mueller explained the cause to a congressional hearing:
Pogo arises fundamentally because you have thrust fluctuations in the engines. Those are normal characteristics of engines. All engines have what you might call noise in their output because the combustion is not quite uniform, so you have this fluctuation in thrust of the first stage as a normal characteristic of all engine burning.
Now, in turn, the engine is fed through a pipe that takes the fuel out of the tanks and feeds it into the engine. That pipe's length is something like an organ pipe so it has a certain resonance frequency of its own and it really turns out that it will oscillate just like an organ pipe does.
The structure of the vehicle is much like a tuning fork, so if you strike it right, it will oscillate up and down longitudinally. In a gross sense it is the interaction between the various frequencies that causes the vehicle to oscillate."
Photographed from a chase plane.
