Apollo 6

==Aftermath==

==Aftermath==

[[File:Asi-assy-and-main-injector.jpg|thumb|Apollo J-2 engine Augmented Spark Igniter (ASI). Rupture of an ASI fuel line caused an engine to shut down early on Apollo 6.]]

[[File:Saturn J-2 ASI H2 line problem Apollo 6.jpg|thumb|Liquid air damped bellows vibrations when the J-2 engines were tested at ground level. On Apollo 6, undamped vibrations in space caused a fuel line to faill]]

In a post-launch press conference, Apollo Program Director [[Samuel C. Phillips]] said, "there's no question that it's less than a perfect mission", but added that the launch vehicle's reaching orbit despite the loss of two engines was "a major unplanned accomplishment". Mueller called Apollo 6 "a good job all around, an excellent launch, and, in balance, a successful mission ... and we have learned a great deal", but later stated that Apollo 6 "will have to be defined as a failure".

In a post-launch press conference, Apollo Program Director [[Samuel C. Phillips]] said, "there's no question that it's less than a perfect mission", but added that the launch vehicle's reaching orbit despite the loss of two engines was "a major unplanned accomplishment". Mueller called Apollo 6 "a good job all around, an excellent launch, and, in balance, a successful mission ... and we have learned a great deal", but later stated that Apollo 6 "will have to be defined as a failure".

[[File:Fernbank-07.jpg|thumb|upright|Apollo 6 command module on display at the [[Fernbank Science Center]] in [[Atlanta]], Georgia]]

[[File:Fernbank-07.jpg|thumb|upright|Apollo 6 command module on display at the [[Fernbank Science Center]] in [[Atlanta]], Georgia]]

The problems with the S-II and the S-IVB were traced to the J-2 engines, present in both stages. Tests showed that the propellant lines leading to the spark igniters could fail in low atmospheric pressure or in [[Vacuum#Outer space|vacuum]]. The propellant lines had [[metal bellows]] to allow for thermal expansion. In ground testing the cold propellants passing through the propellant lines would form a layer of frost on the LOX line and [[liquid air]] on the LH₂ line, damping out vibrations. In the vacuum of space, there was no such protection from vibrations. On S-II stage engine number 2, the LH₂ line failed, the LOX line continued to feed liquid oxygen to the igniter. The excess oxygen in the hydrogen-rich combustion chamber produced very high temperatures in the igniter area, causing a burn-through of the engine dome. The resulting loss of pressure caused the IU to send an engine shutdown signal. Due to a wiring error, the shutdown signal caused engine number 3 to shut down. Its loss of pressure initiated another shut down signal which, due to the same crosswiring, shutdown engine number 2. The remaining three engines continued to operate.{{rp|p.6-1}}{{cite web|url=http://heroicrelics.org/info/j-2/augmented-spark-igniter.html|title=J-2 Rocket Engine Augmented Spark Igniter|publisher=Heroicrelics.org|access-date=April 7, 2026}} A similar failure of igniter propellant lines likely prevented the single J-2 engine on the S-IVB from restarting.{{rp|p.7-1}}

The problems with the S-II and the S-IVB were traced to the J-2 engines, present in both stages. Tests showed that the propellant lines leading to the augmented spark igniters could fail in low atmospheric pressure or in [[Vacuum#Outer space|vacuum]]. The propellant lines had [[metal bellows]] to allow for thermal expansion. In ground testing the cold propellants passing through the propellant lines would form a layer of frost on the LOX line and [[liquid air]] on the LH₂ line, damping out vibrations. In the vacuum of space, there was no such protection from vibrations. On S-II stage engine number 2, the LH₂ line failed, the LOX line continued to feed liquid oxygen to the igniter. The excess oxygen in the hydrogen-rich combustion chamber produced very high temperatures in the igniter area, causing a burn-through of the engine dome. The resulting loss of pressure caused the IU to send an engine shutdown signal. Due to a wiring error, the shutdown signal caused engine number 3 to shut down. Its loss of pressure initiated another shut down signal which, due to the same crosswiring, shutdown engine number 2. The remaining three engines continued to operate.{{rp|p.6-1}}{{cite web|url=http://heroicrelics.org/info/j-2/augmented-spark-igniter.html|title=J-2 Rocket Engine Augmented Spark Igniter|publisher=Heroicrelics.org|access-date=April 7, 2026}} A similar failure of igniter propellant lines likely prevented the single J-2 engine on the S-IVB from restarting.{{rp|p.7-1}}

The bellows were replaced with rigid bends and the lines strengthened.{{sfn|Brooks 1979|pp=251–252}} In Apollo 6's wake, NASA engineers debated whether to configure the spacecraft's emergency detection system to automatically abort in the event of excessive pogo; this plan was opposed by Director of Flight Crew Operations [[Deke Slayton]]. Instead, work began on having a "pogo abort sensor" to allow the flight crew to judge whether to abort, but by August 1968, it had become clear that pogo could be dealt with without such a sensor, and work on it was abandoned.{{sfn|Orloff & Harland 2006|p=172}}{{sfn|Brooks 1979|pp=251–252}}

The bellows were replaced with rigid bends and the lines strengthened.{{sfn|Brooks 1979|pp=251–252}} In Apollo 6's wake, NASA engineers debated whether to configure the spacecraft's emergency detection system to automatically abort in the event of excessive pogo; this plan was opposed by Director of Flight Crew Operations [[Deke Slayton]]. Instead, work began on having a "pogo abort sensor" to allow the flight crew to judge whether to abort, but by August 1968, it had become clear that pogo could be dealt with without such a sensor, and work on it was abandoned.{{sfn|Orloff & Harland 2006|p=172}}{{sfn|Brooks 1979|pp=251–252}}