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.

Objectives

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 3in 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.

Flight

Launch
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.

Cameras

Still from footage of Apollo 6's interstage falling away (NASA)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.

http://en.wikipedia.org/wiki/Apollo_6