Apollo 6 soaring through the sky - Image credit: NASA
Apollo 6 was launched on 4 April 1968.
It was the final unmanned Apollo mission, so a lot of things had to go right to be clear to make the next mission a success.
Expectations were very high for the mission:
- demonstrate trans-lunar injection capability of the Saturn V
- repeat demonstration of the Command Module's (CM) heat shield capability to withstand a lunar re-entry
Shortly after launch it was already clear that things weren't going to go as planned.
Rocketdyne J-2 Engine Failure
A phenomenon called pogo oscillation ruptured the internal fuel lines to the some of the Rocketdyne J-2 engines of the Saturn V rocket. That caused the second and thrid stages to shut down early.
This led to a different parking orbit than what was planned. The third stage engine also failed to restart for the trans-lunar injection.
The interstage falling - Image credit: NASA
Command and Service Module Re-entry
The second objective was to perform a high speed re-entry of the Command and Service module, in case a mission had to be aborted.
This re-entry was successful and the CSM landed 80 kilometers from the planned touchdown, in the North Pacific Ocean, north of Hawaii.
The Apollo 6 Command Module is hoisted aboard the U.S.S. Okinawa - Image credit: NASA
Despite a failure of one of the objectives, officials felt confident enough to go ahead and perform a manned mission, using the Saturn IB rocket for the Apollo 7.