Originally Posted by
Windy Militant
A lot of it was trial and error, I've read that the Pogoing on the fortunately unmanned test flight was caused by resonance in the flexible fuel lines used. This hadn't occurred during ground testing. Eventually they figured out that the fuel lines were becoming coated with ice during the ground test but as the air was less moist at altitude the pipes were free to flap about! They fitted solid lines after that was discovered.
I think they also had an engine cut out problem where sensors shut down a good engine because they'd cross wired the the looms. After this they made the looms to each engine a different length so it couldn't happen again!
The problem of the iced up propellant lines in the S-II/S-IVB and the S-IC pogo are two different (though related) problems.
Pogo was a result of harmonic resonance in the S-IC propellant tanks and supplies. In brief there was a undampened, unstable feedback loop were a change in thrust would change propellant pressure which would in turn change thrust. The problem was solved by detuning the propellant lines with dampers (basically pneumatic shock absorbers). The phenomenon had been experienced on many (if not most) earlier rocket designs, and was seen on the first test flight of the S-V (Apollo 4) but was much more severe on Apollo 5 where the oscillations were strong enough that the crew might have been injured.
The iced up propellant line problem showed up on Apollo 5. The fuel line feed line (LH2) would ice up in ground testing because the cryogenic liquid hydrogen flowing through the flexible connection to the J2 engine would freeze moisture in the air surrounding. In space there is no moisture surrounding the line so there was no ice build-up. The mass of the ice built up during ground tests of the J2 buffered the vibration of the line. The stronger vibration observed in space resulted in failure of the line. The fix was simply additional reinforcement of the flexible fuel line. During the second stage burn of Apollo 5 one of the J2 engines failed because of the fuel line problem but the 'crossed wiring' problem resulted in the shutdown of a different engine. Even so, the guidance computer compensated and achieve a sufficient, though imperfect orbit despite the fact that the two failed engines WERE ON THE SAME SIDE. This was far outside the expected performance of the guidance and control system.
Despite the anomalies observed on Apollo 5, the next flight of the Saturn V booster was on Apollo 8, the first manned mission to leave earth orbit. I can't see NASA taking that kind of gamble today.