Apollo
Apollo 6
Apollo 6 was the second and final uncrewed test flight of the Saturn V rocket. Although the mission experienced severe engine failures and dangerous pogo oscillations, NASA successfully demonstrated the Apollo spacecraft's ability to return safely from a simulated lunar mission and gathered critical engineering data that paved the way for the first crewed Saturn V flight.
Launch
April 4, 1968
Duration
9 hours, 57 minutes
Crew
Uncrewed
Mission record
Apollo archive entry · Partially Successful
Mission Overview
The mission
Apollo 6 was the second and final uncrewed test flight of the Saturn V rocket. Although the mission experienced severe engine failures and dangerous pogo oscillations, NASA successfully demonstrated the Apollo spacecraft's ability to return safely from a simulated lunar mission and gathered critical engineering data that paved the way for the first crewed Saturn V flight.
Mission Details
Program
Apollo
Launch
April 4, 1968
Duration
9 hours, 57 minutes
Status
Partially Successful
Mission by the Numbers
Mission Statistics
Mission Type
Uncrewed Lunar Qualification Flight
Launch Vehicle
Saturn V AS-502
Crew
0 astronauts
Status
Partially Successful
Mission Goals
Mission objectives
- 01
Perform the second full-scale flight test of the Saturn V launch vehicle.
- 02
Verify performance of all three Saturn V stages.
- 03
Evaluate structural loads during powered ascent.
- 04
Measure pogo oscillation effects on the launch vehicle.
- 05
Place the Apollo spacecraft into Earth orbit.
- 06
Restart the S-IVB third stage in orbit.
- 07
Simulate the Trans-Lunar Injection burn.
- 08
Demonstrate the Apollo Guidance Computer during a lunar mission profile.
- 09
Evaluate Service Propulsion System performance.
- 10
Test spacecraft navigation during high-energy flight.
- 11
Validate Command Module environmental systems.
- 12
Demonstrate high-speed atmospheric reentry.
- 13
Verify Command Module heat shield performance.
- 14
Recover the spacecraft after splashdown.
- 15
Collect engineering data before the first crewed Saturn V mission.
Mission Timeline
1967
Assembly Begins
Saturn V AS-502 and Apollo spacecraft CSM-020 were assembled for the final uncrewed qualification flight of the Apollo Program.
Late 1967
Integrated Testing
Extensive electrical, propulsion, structural, and systems testing prepared Apollo 6 for launch.
Early 1968
Launch Preparations
Engineers completed final countdown demonstrations and integrated vehicle testing at Kennedy Space Center.
April 4, 1968 · 7:00 a.m. EST
Liftoff
Apollo 6 launched from Launch Complex 39A aboard Saturn V AS-502, beginning the rocket's final qualification flight before astronauts would fly aboard.
First Stage Flight
Unexpected Pogo Oscillations
Powerful longitudinal vibrations developed during ascent, shaking the launch vehicle much more violently than engineers expected and revealing a serious engineering concern.
Second Stage
Engine Failure
One of the five J-2 engines shut down prematurely, forcing the Saturn V guidance system to compensate by extending the remaining engines' burn time.
Moments Later
Second Engine Failure
A second J-2 engine also shut down early, creating one of the most significant in-flight failures ever experienced by the Saturn V while still leaving enough performance to achieve orbit.
Earth Orbit
Parking Orbit Achieved
Despite multiple engine failures, the Saturn V successfully placed Apollo 6 into Earth orbit through automatic guidance corrections.
Planned Translunar Simulation
S-IVB Restart Attempt
Mission controllers commanded the S-IVB third stage to restart in orbit for the simulated Trans-Lunar Injection burn. The engine failed to reignite, preventing the mission from following its planned lunar-return trajectory.
Mission Replanned
Alternate Flight Profile
Rather than ending the mission, NASA quickly developed a new flight plan using the Apollo Service Module's Service Propulsion System (SPS) engine to accomplish the mission's most critical remaining objective.
High-Energy Burn
Service Propulsion System Ignition
The SPS engine fired successfully, accelerating the spacecraft onto a high-energy trajectory that would closely simulate the speed of a return from the Moon.
Coast Phase
Spacecraft Systems Evaluation
Engineers monitored guidance, navigation, electrical systems, communications, thermal control, and propulsion while the spacecraft coasted along its modified flight path.
Atmospheric Entry
High-Speed Reentry
Apollo 6 reentered Earth's atmosphere at nearly lunar-return velocity, placing enormous thermal loads on the Command Module's heat shield.
Reentry Success
Heat Shield Validated
The Command Module's ablative heat shield performed successfully, confirming it could protect astronauts returning from lunar missions.
Pacific Ocean
Splashdown
Apollo 6 safely splashed down in the Pacific Ocean after nearly ten hours in space, completing the Saturn V's final uncrewed qualification flight.
Postflight Investigation
Failure Analysis Begins
NASA engineers immediately began analyzing the pogo oscillations, engine shutdowns, and failed S-IVB restart to determine their root causes.
Engineering Improvements
Saturn V Modified
Investigators corrected fuel-line issues, engine ignition problems, and structural vibration concerns before the first crewed Saturn V mission.
December 1968
Apollo 8 Benefits
The lessons learned during Apollo 6 allowed Apollo 8 to become the first crewed Saturn V mission and the first human flight to the Moon.
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