In an earlier thread, I questioned Jay Utah when he claimed that the Saturn V launch vehicle had a problem with "pogo" in its first stage, the S-IC.

"Pogo" is an engineering term for a specific kind of dynamic instability - vibration - encountered in some rockets. It consists of vibrations along the longitudinal axis of the vehicle. The resulting shock is similar to that experienced by someone using the old toy called a "pogo stick", hence the name.

My recollection was that the pogo noted in the Saturn V was produced by the second stage, the S-II. I was wrong, and Jay was right. The problem was in the S-IC.

It was first encountered during the second, unmanned test launch of a Saturn V. The pogo began about 125 seconds into the flight and lasted until staging at 150 seconds. It was severe -- so bad that had there been a human crew, the mission would certainly have been aborted. In the event, however, the flight controllers held off on their abort switches and the flight was allowed to continue. The vehicle survived and, despite further problems with the second and third stages, made it into orbit successfully.

The pogo problem was soon resolved. It was traced to a resonance effect. One natural vibration frequency of the F-1 engine's combustion chamber was about 5.5 Hz; the natural resonance of the Saturn V stack was about the same. In effect, the F-1 engines were "pumping" the vibration of the rocket, much as you may have pumped yourself to great heights on a swing.

The problem was fixed before any human beings flew on a Saturn V. (In the time-compressed world of the Apollo program, that was the very next launch of a Saturn V, namely, Apollo 8!) The engines were damped by a system of shock absorbers to decouple them from the structure. After that, the pogo problem was reduced to insignificance.

I find this to be another little gem of information about the incredible number of technical challenges faced and met by the people who put men on the Moon.