Apollo 6 - Causes and Fixes of Problems

Causes and Fixes of Problems

The cause of the pogo during the first stage of the flight was well known. However, it had been thought that the rocket had been 'detuned'. To further damp pressure oscillations in the fuel and oxidizer pumps and feed lines, cavities in these systems were filled with helium gas from the propulsion system's pneumatic control system, which acted to attenuate the oscillations like a shock absorber.

The failure of the two engines in the second stage was traced to the rupturing of a fuel line that fed the engine igniters. The igniter was essentially a miniature rocket motor mounted in the wall of the J-2 engine's pressure chamber. It was fed by small-diameter flexible lines carrying liquid hydrogen and liquid oxygen. During the S-II second stage burn, the hydrogen line feeding the engine number two igniter broke due to vibration. As a result, the igniter fed pure liquid oxygen into the pressure chamber. Normally the J-2 engine burns a hydrogen-rich mixture to keep temperature down. The liquid oxygen flow caused a much higher temperature locally and eventually the pressure chamber failed. The sudden drop in pressure was detected and caused a shutdown command to be issued. Unfortunately, the command signals for engine three were partially cross-wired with engine two, so that the shutdown of engine two caused a liquid oxygen valve for engine three to close, resulting in a shutdown of that engine, as well.

The problem in the igniter fuel lines was not detected during ground testing because a stainless steel mesh covering the fuel line became saturated with liquid air due to the extreme cold of the liquid hydrogen flowing through it. The liquid air damped a vibration mode that became evident when tests were conducted in a vacuum after the Apollo 6 flight. This was also a simple fix, involving replacing the flexible bellows section where the break occurred with a loop of stainless steel pipe. The S-IVB used the same J-2 engine design as the S-II and so it was decided that an igniter line problem had also stopped the third stage from reigniting in Earth orbit. Ground testing confirmed that the slight underperformance seen in the first S-IVB burn was consistent with damage to the igniter line.

The spacecraft adapter problem was caused by its honeycomb structure. As the rocket accelerated through the atmosphere, the cells expanded due to trapped air and water. This would cause the adapter surface to break free. To stop this occurring again, small holes were drilled in the surface to allow for expansion.

While the engine failures experienced on Apollo 6 would have resulted in an abort of a manned lunar mission, NASA considered the flight an invaluable engineering shakedown of the launch vehicle, and no similar major failures occurred on any of the eleven subsequent Saturn V flights. However, the pogo vibration problem had only been reduced, not completely solved, as it reappeared on AS-508, causing premature shutdown of the S-II center engine on the Apollo 13 flight.