Apollo 6

Apollo 6 was the last unmanned test of the Saturn V launch vehicle. The mission had many significant problems, but rendered enough information to convince NASA to go ahead with manned missions. While Apollo 7 was the first manned Apollo mission, it used the smaller Saturn IB. Apollo 8 was the first manned mission for the Saturn V. Apollo 6 was 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. J2 engine failures caused this goal to not be met.

Apollo 6 was launched April 4, 1968, 07:00:01 a.m. EST from Launch Complex 37B, Eastern Test Range, Cape Canaveral FL. Apollo 6 experienced problems right from the start. A short two minutes into the flight, the rocket experienced severe Pogo oscillations for about 30 seconds. Pogo oscillations are a type of combustion instability in liquid fuel rocket engines. Because of 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.

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 cavities 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 a ruptured fuel line that fed the engine igniters. 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 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.

NASA has a page on the Apollo 6 mission here. There is a good article on the mission here. On Universe Today we have a great article about how to build your own Apollo landing computer. Astronomy Cast offers a sweet episode on spacesuits.

References:
http://science.ksc.nasa.gov/history/apollo/apollo-6/apollo-6.html
http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1968-025A

Tagged as: Apollo 6, apollo missions, apollo program, astronauts, NASA