Apollo 6 (AS-502) was the second and final uncrewed test flight of the Saturn V launch vehicle, and a critical step toward qualifying the rocket for crewed missions. Launched on April 4, 1968, from Kennedy Space Center’s Launch Complex 39A, the mission was designed to demonstrate the Saturn V’s ability to perform a Trans-Lunar Injection and verify the performance of all three stages under full-mission conditions.

The flight was plagued by serious anomalies. Two minutes and five seconds after liftoff, severe longitudinal “pogo” oscillations shook the vehicle during the S-IC first stage burn, with vibrations reaching dangerous amplitudes. During the S-II second stage burn, two of the five J-2 engines shut down prematurely due to faulty fuel line brazing. The remaining three engines burned longer to compensate, placing the spacecraft in a lower-than-planned orbit. Most critically, the S-IVB third stage failed to reignite for the planned Trans-Lunar Injection simulation, leaving the spacecraft stranded in Earth orbit.

Despite these failures, mission controllers improvised a remarkable recovery. The Service Propulsion System engine aboard the unmanned Command and Service Module (CSM-020) was fired for 442 seconds — far longer than any planned SPS burn — boosting the spacecraft to an apogee of approximately 13,821 miles (22,225 km). This demonstrated the SPS engine’s ability to compensate for launch vehicle failures, a capability that would later prove essential on Apollo 13.

The mission carried the Lunar Module Test Article (LTA-2R) as ballast in place of an actual Lunar Module. Although Apollo 6 fell short of its primary objectives, the engineering data collected was invaluable. NASA engineers identified and corrected the pogo oscillation problem by adding helium gas accumulators to the LOX feedlines, and the J-2 engine fuel line issue was resolved by changing the brazing process. These fixes proved successful, and the next Saturn V flight — Apollo 8 in December 1968 — carried astronauts to lunar orbit. Apollo 6 was the last uncrewed Apollo test flight.