Featured Article Summary #91

Apollo 6, also called AS-502, was the third and final uncrewed mission in NASA’s Apollo program and the second test launch of the Saturn V rocket. It was designed to prove that the Saturn V’s S-IVB third stage could push both the rocket stage and the spacecraft toward the Moon, helping qualify the Saturn V for crewed missions. Parts arrived at Kennedy Space Center in early 1967, but testing took time, with delays tied to work on the earlier Apollo 4 mission, and the launch was moved from March to April 1968. During the flight, vibrations damaged J-2 engines, causing some to shut down early and forcing the guidance system to compensate by extending burns, which led to a more stretched parking orbit than planned. The third-stage engine could not restart for the planned lunar injection, so controllers repeated a test flight plan from Apollo 4 to get a high orbit and fast return. Even with these problems, the mission gave NASA enough confidence to proceed with crewed Saturn V launches, and a potential additional uncrewed flight was canceled. Apollo 6 was meant to test the Saturn V rocket by sending a command and service module with a simulated lunar module into a path toward the Moon, but without reaching it. The mission planned several engine burns to simulate different phases of a real mission and a direct-return abort, including testing re-entry conditions, and would last about 10 hours. Its main goal was to fully qualify the launch vehicle—especially how it handled near-full loads and the stresses and vibrations affecting the lunar module. Apollo 6 used the Saturn V rocket AS-502 with the CSM-020 command/service module. CSM-020 was a Block I capsule modified in some ways, including a new crew hatch to be tested for lunar return conditions; the previous hatch design had been criticized after the Apollo 1 fire. The service module was SM-014, and some systems (like certain radiators) were not activated because the mission was short. Apollo 6 also carried a lunar test article, LTA-2R, a simulated lunar module inside SLA-9. It included a descent stage with specific test fluids and an instrumented ascent stage made to measure vibration, sound, and structural strength, without actually using flight or landing systems. The first stages and equipment arrived between March and May 1967, and due to the S-II second stage being late, a spacer was temporarily used for testing. Apollo 6’s work in the VAB had delays and setbacks, including problems with air conditioning, slow preparation of the launch launcher arms, and a late arrival of the CSM. After more progress following Apollo 4’s launch, the spacecraft was rolled out to the launch pad in February 1968, flight tests finished in early March, and the launch date shifted from March 28 to April 3 because of guidance and fueling issues. Despite one extra postponement, final countdown built-in holds fixed remaining problems, and they did not affect the mission. Apollo 6 lifted off from Launch Complex 39A at Kennedy Space Center on April 4, 1968, at 7:00:01 am EST. For the first two minutes the Saturn V was normal, but during the first stage burn it was shaken by pogo oscillations caused by engine thrust fluctuations interacting with the vehicle’s natural vibrations; it survived with only damage to one panel of the SLA. After the first stage separated, problems with the S-II second stage engines led to two engines shutting down due to a fault starting in engine two, while the remaining engines and later stages compensated by burning longer than planned.