Unlike the virtually perfect flight of Apollo 4, Apollo 6 experienced problems right from the start.
Two minutes into the flight, the rocket experienced severe pogo oscillations for about 30 seconds.
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.

In part due to the vibrations, the spacecraft adapter that attached the CSM to the rocket and housed the mockup LM started to have some structural problems.
Airborne cameras recorded several pieces falling off it at T+133 seconds.

After the first stage was jettisoned, the S-II second stage began to experience its own problems.
Engine number two had performance problems from 225 seconds after liftoff, which abruptly worsened at T+319 seconds, and then at T+412 seconds the Instrument Unit shut it down altogether.
Then two seconds later, engine number three shut down as well.
The Instrument Unit was able to compensate, and the remaining three engines burned for 58 seconds longer than normal.
The S-IVB third stage also had to burn for 29 seconds longer than usual.
The S-IVB also experienced a slight performance loss.

The S-IC first stage impacted the Atlantic Ocean east of Florida (30°12′N 74°19′W), while the S-II second stage impacted south of the Azores (31°12′N 32°11′W).
Unlike the virtually perfect flight of Apollo 4, Apollo 6 experienced problems right from the start. Two minutes into the flight, the rocket experienced severe pogo oscillations for about 30 seconds. 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.
In part due to the vibrations, the spacecraft adapter that attached the CSM to the rocket and housed the mockup LM started to have some structural problems. Airborne cameras recorded several pieces falling off it at T+133 seconds.
After the first stage was jettisoned, the S-II second stage began to experience its own problems. Engine number two had performance problems from 225 seconds after liftoff, which abruptly worsened at T+319 seconds, and then at T+412 seconds the Instrument Unit shut it down altogether. Then two seconds later, engine number three shut down as well. The Instrument Unit was able to compensate, and the remaining three engines burned for 58 seconds longer than normal. The S-IVB third stage also had to burn for 29 seconds longer than usual. The S-IVB also experienced a slight performance loss.
The S-IC first stage impacted the Atlantic Ocean east of Florida (30°12′N 74°19′W), while the S-II second stage impacted south of the Azores (31°12′N 32°11′W).
Apollo 6