Apollo 6

Apollo 6, launched on April 4, 1968, was the Apollo program's second and last unmanned test flight of its Saturn V launch vehicle.
This was the final qualification flight of the Saturn V before its first manned flight (Apollo 8) (While Apollo 7 was the first manned Apollo mission, it used the smaller Saturn IB, not the Saturn V.) It was also the first mission to use High Bay 3 in the Vertical Assembly Building (VAB), Mobile Launcher 2 and Firing Room 2. Another objective was testing the Command Module re-entry system under extreme conditions simulating a worst-case return from the Moon. This objective was not met due to J-2 engine failures.
The S-IC first stage arrived by barge on March 13, 1967 and was erected in the VAB four days later, with the S-IVB third stage and Instrument Unit computer arriving the same day. The S-II second stage was two months behind them and so was substituted with a dumbbell shaped spacer so testing could proceed. This had the same height and mass as the S-II along with all the electrical connections. The S-II arrived May 24. It was stacked and mated into the rocket on July 7.
Testing was slow as they were still checking out the launch vehicle for Apollo 4, a limitation of the system where there wasn't two of everyone and everything. The VAB could handle up to four Saturn Vs but could only check out one at a time.
The Command and Service Module arrived September 29 and was stacked December 10. It was a hybrid, featuring the Command Module Number 20 and Service Module Number 14 after SM-020 was destroyed in a tank explosion and Command Module Number 14 was dismantled as part of the investigation into the Apollo 1 fire. After two months of testing and repairs the rocket was moved to the pad on February 6, 1968.
Unlike the near 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 pogo, the spacecraft adaptor that attached the CSM and mockup of the Lunar Module to the rocket started to have some structural problems. Airborne cameras recorded several pieces falling off it at T+133s.
After the first stage was jettisoned at the end of its task, the S-II second stage began to experience its own problems. Engine number two (of five) had performance problems from 206 to 319 seconds after liftoff and then at 412 seconds shut down altogether. Then two seconds later Engine Number Three shut down as well. The onboard computer was able to compensate and the stage burned for 58 seconds more than normal. Even so the S-IVB third stage also had to burn for 29 seconds longer than usual.
The S-IC first stage impacted the Atlantic Ocean east of Florida , while the S-II second stage impacted south of the Azores.
Due to the less than nominal launch, the CSM and S-IVB were now in a 178 by 367 km orbit instead of the planned 160 km circular orbit. But after two orbits of checking out the spacecraft and rocket stage the S-IVB failed to restart to simulate the Trans Lunar Injection burn that would send the astronauts to the moon.
It was decided to use the Service Module engine to raise the spacecraft into a high orbit in order to complete some of the mission objectives. It burned for 442 seconds, longer than it would ever have to on a real Apollo mission and raised the apogee of the orbit to 22,200 km. There was now however not enough fuel to speed up the atmospheric reentry and the spacecraft only entered the atmosphere at a speed of 10,000 m/s instead of the planned 11,270 m/s. This meant it landed 80 km from the planned touch down point.
Ten hours after launch it was lifted on board the USS Okinawa.
S-IVB reentered on April 25, 1968.
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 dampen pressure oscillations in the fuel and oxidzer 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.
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 three 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 shutdown command signal for engine three was cross-wired to engine two. Engine two shut down and in turn its pressure sensor sent a shutdown signal back to engine three.
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.
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.
The problems of the Apollo 6 test would have resulted in an abort of a manned Apollo flight. However, the booster rocket shakedown on this mission was invaluable, as none of the eleven subsequent Saturn V flights experienced any serious problems.
Documentaries often use footage of a Saturn V launch, and one of the most used pieces shows the interstage between the first and second stages falling away. This footage is usually mistakenly attributed to the Apollo 11 mission, when it was actually filmed on the flights of Apollo 4 and Apollo 6.
A compilation of original Nasa footage shows the jettisoning of the first stage (S-IC) and the interstage ring as seen from the bottom of the second stage (S-II), followed by the separation of the S-IVB third stage as seen from the top of the S-II. The hot, invisible hydrogen-oxygen flames of the J-2 engines on the S-II can be seen impinging on the S-IC and the ring. The S-II/S-IVB separation footage shows S-IVB ignition, and both films show the more conspicuous plumes of the solid lower stage retrorockets and upper stage ullage motors as they pull the stages apart.
The cameras filmed at high speeds causing an estimated 15 times slow-motion view of the sequence when seen in a documentary. The camera capsules were jettisoned soon after the first stage separation, and, though at about 200,000 feet in altitude, were still below orbital velocity. They then reentered the atmosphere and parachuted to the ocean, where they floated waiting for recovery. Only one of the two S-II cameras on Apollo 6 was recovered.
Another launch shot often attributed to Apollo 11 and other launches was shot on this day: it shows a view of the rocket lifting up, positioned relatively close up and dead center. The shot can be identified as Apollo 6 by examining the Apollo service module on the launch; Apollo 6 was the only Saturn V-launched Apollo craft with a white SM; all others were silver.
The Apollo 6 Command Module is on display at the Fernbank Science Centre, in suburban Atlanta,Georgia.