# Apollo 6

**Apollo 6** (designated AS-502, flying [Spacecraft](https://www.edgepedia.org/spacecraft) 020/Saturn 502) was an uncrewed test flight launched by NASA on April 4, 1968, as the second test of the Saturn V Moon rocket and the final qualification flight of the [Saturn V](https://www.edgepedia.org/saturn-v) and [Apollo](https://www.edgepedia.org/apollo-spacecraft) spacecraft before crewed missions.<sup>[1](https://www.nasa.gov/mission/apollo-6/)</sup><sup> • </sup><sup>[2](https://www.nasa.gov/the-apollo-program/)</sup> The rocket lifted off from Kennedy Space Center Complex 39A at 07:00:01.5 EST.<sup>[1](https://www.nasa.gov/mission/apollo-6/)</sup> Although the vehicle reached orbit and much of the flight plan was salvaged, the mission exposed three serious problems: violent first-stage oscillations known as pogo, early shutdown of two second-stage engines, and failure of the third-stage engine to restart.<sup>[3](https://www.nasa.gov/history/55-years-ago-nasa-administrator-webb-oks-flying-a-crew-on-the-third-saturn-v/)</sup>

| Key fact | Detail |
|---|---|
| Launch | April 4, 1968, 07:00:01.5 EST, Kennedy Space Center Complex 39A<sup>[1](https://www.nasa.gov/mission/apollo-6/)</sup> |
| Vehicle and spacecraft | Saturn V (AS-502) with Spacecraft 020, a Block I Command and Service Module carrying Block II improvements such as the new hatch<sup>[4](https://www.apolloproject.com/sp-4205/ch10-5.htm)</sup> |
| Mission role | Second uncrewed Saturn V test and final qualification of the launch vehicle and Apollo spacecraft for crewed flights<sup>[2](https://www.nasa.gov/the-apollo-program/)</sup><sup> • </sup><sup>[5](https://www.nasa.gov/history/55-years-ago-the-flight-of-apollo-6/)</sup> |
| First-stage anomaly | Significant pogo (coupled structure-propulsion) vibrations during first-stage flight<sup>[3](https://www.nasa.gov/history/55-years-ago-nasa-administrator-webb-oks-flying-a-crew-on-the-third-saturn-v/)</sup> |
| Second-stage anomaly | Two of the five J-2 engines shut down early<sup>[3](https://www.nasa.gov/history/55-years-ago-nasa-administrator-webb-oks-flying-a-crew-on-the-third-saturn-v/)</sup> |
| Third-stage anomaly | The single J-2 engine failed to restart to send the CSM onto a simulated lunar trajectory<sup>[3](https://www.nasa.gov/history/55-years-ago-nasa-administrator-webb-oks-flying-a-crew-on-the-third-saturn-v/)</sup> |
| Outcome | Fixes demonstrated before crewed flights: J-2 repair on a Saturn IB before Apollo 7, pogo and SLA repairs before Apollo 8<sup>[6](https://www.nasa.gov/wp-content/uploads/2024/01/presrep1968.pdf?emrc=69b24408869d0)</sup> |

## Purpose and spacecraft

Apollo 6 was intended as the final qualification flight of the Saturn V launch vehicle and Apollo spacecraft for crewed Apollo missions.<sup>[2](https://www.nasa.gov/the-apollo-program/)</sup> The mission also served to give the Pad 39 launch team another rehearsal before astronauts flew the rocket.<sup>[4](https://www.apolloproject.com/sp-4205/ch10-5.htm)</sup> The spacecraft itself had no primary objectives to accomplish; NASA allocated a Block I Command and Service Module, CSM-020, that incorporated many Block II improvements, including the new hatch.<sup>[4](https://www.apolloproject.com/sp-4205/ch10-5.htm)</sup>

The first Saturn V flight, [Apollo 4](https://www.edgepedia.org/apollo-4) on November 9, 1967, had verified spacecraft systems and tested the command module under the heat conditions expected on a lunar return.<sup>[7](https://www.ccas.us/CCAS_NASA_PressKits/Apollo_Missions/Apollo06_PressKit.pdf)</sup> Apollo 6 was to build on that success by qualifying the vehicle for crewed space flight.<sup>[8](https://www.nasa.gov/wp-content/uploads/2023/04/1968.pdf)</sup>

## In-flight anomalies

**Pogo.** The first stage experienced significant vibrations known as pogo, a coupled structure and propulsion system instability that, in the worst cases, can impair a crew, force an unplanned engine shutdown, lose the mission, or cause structural failure.<sup>[3](https://www.nasa.gov/history/55-years-ago-nasa-administrator-webb-oks-flying-a-crew-on-the-third-saturn-v/)</sup><sup> • </sup><sup>[9](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080018689.pdf)</sup> One retrospective account describes the rocket as vibrating so badly on this second uncrewed flight that it nearly shook itself to pieces.<sup>[10](https://time.com/5475697/apollo-8-50-years-later/)</sup>

**Engine failures.** Two of the second stage's five J-2 engines shut down early, and after two orbits the restart of the third stage's single J-2 engine proved unsuccessful, so an alternate mission plan was put into effect.<sup>[3](https://www.nasa.gov/history/55-years-ago-nasa-administrator-webb-oks-flying-a-crew-on-the-third-saturn-v/)</sup><sup> • </sup><sup>[6](https://www.nasa.gov/wp-content/uploads/2024/01/presrep1968.pdf?emrc=69b24408869d0)</sup> The restart failure of the third-stage J-2 resulted from the same defect in the ASI (auxiliary propulsion unit) that had affected the flight.<sup>[3](https://www.nasa.gov/history/55-years-ago-nasa-administrator-webb-oks-flying-a-crew-on-the-third-saturn-v/)</sup> Early speculation about the second-stage shutdowns included the possibility that wires carrying cutoff commands to the engines had been interchanged.<sup>[11](https://www.american-spacecraft.org/documents/sp-4204/chapter-20.html)</sup> The spacecraft lunar module adapter (SLA) also showed a structural failure, which engineers at the Manned Spacecraft Center investigated in two areas, launch vehicle oscillation and spacecraft structures.<sup>[11](https://www.american-spacecraft.org/documents/sp-4204/chapter-20.html)</sup>

## Investigation and fixes

Marshall Space Flight Center issued an initial report a week after the launch.<sup>[11](https://www.american-spacecraft.org/documents/sp-4204/chapter-20.html)</sup> Coordinated plans for resolving the Apollo 6 anomalies were presented to the Apollo Program Director in a teleconference on May 2, 1968, and included fixes related to pogo.<sup>[11](https://www.american-spacecraft.org/documents/sp-4204/chapter-20.html)</sup>

The corrective program had three parts, each tied to a crewed flight deadline:<sup>[6](https://www.nasa.gov/wp-content/uploads/2024/01/presrep1968.pdf?emrc=69b24408869d0)</sup>

1. A fix to the J-2 engine, which the [Saturn IB](https://www.edgepedia.org/saturn-ib) also carried in its second stage, demonstrated before certifying the crewed [Apollo 7](https://www.edgepedia.org/apollo-7) flight on a Saturn launch vehicle.
2. A fix to the first-stage pogo problem on the Saturn V.
3. A fix to the SLA anomaly before certification of a crewed flight on the third Saturn V, Apollo 8.

## Significance

Despite the anomalies, Apollo 6 completed the uncrewed qualification sequence: the first crewed Apollo mission, Apollo 7, lifted off on October 11, 1968, for a 10-day flight in Earth orbit.<sup>[2](https://www.nasa.gov/the-apollo-program/)</sup> The pogo experience shaped NASA's later treatment of the phenomenon; the agency's own retrospective identifies the near failure of the [Apollo 13](https://www.edgepedia.org/apollo-13) second stage in 1970 as the defining moment in that longer story, but Apollo 6 was among the Apollo-Saturn flights on which pogo appeared in first- and second-stage flight.<sup>[9](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080018689.pdf)</sup> The Saturn V launched twice uncrewed before carrying astronauts; [Apollo 8](https://www.edgepedia.org/apollo-8), the first crewed Saturn V flight in December 1968, sent the first humans toward the Moon.<sup>[12](https://apnews.com/article/c3bb9888b75e67574a1b66e643b87621)</sup>

## References

1. [Apollo 6, NASA](https://www.nasa.gov/mission/apollo-6/)
2. [The Apollo Program, NASA](https://www.nasa.gov/the-apollo-program/)
3. [55 Years Ago: NASA Administrator Webb OKs Flying a Crew on the Third Saturn V, NASA](https://www.nasa.gov/history/55-years-ago-nasa-administrator-webb-oks-flying-a-crew-on-the-third-saturn-v/)
4. [Chariots for Apollo, SP-4205, Chapter 10](https://www.apolloproject.com/sp-4205/ch10-5.htm)
5. [55 Years Ago: The Flight of Apollo 6, NASA](https://www.nasa.gov/history/55-years-ago-the-flight-of-apollo-6/)
6. [NASA Performance of the Space Vehicle, 1968 report](https://www.nasa.gov/wp-content/uploads/2024/01/presrep1968.pdf?emrc=69b24408869d0)
7. [Apollo 6 Press Kit, NASA, March 28, 1968](https://www.ccas.us/CCAS_NASA_PressKits/Apollo_Missions/Apollo06_PressKit.pdf)
8. [Astronautics and Aeronautics, 1968, NASA](https://www.nasa.gov/wp-content/uploads/2023/04/1968.pdf)
9. [NASA Experience with Pogo in Human Spaceflight Vehicles, NASA NTRS](https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20080018689.pdf)
10. [Apollo 8 Astronauts Reflect on Historic Moon Voyage 50 Years Later, Time](https://time.com/5475697/apollo-8-50-years-later/)
11. [Moonport: A History of Apollo Launch Facilities and Operations, SP-4204, Chapter 20](https://www.american-spacecraft.org/documents/sp-4204/chapter-20.html)
12. [Apollo vs. Artemis: What to know about NASA's return to the moon, AP News](https://apnews.com/article/c3bb9888b75e67574a1b66e643b87621)

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*Topic: Encyclopedia › Technology and the built world › Transport and spaceflight › Spaceflight › Spaceflight history and chronology › Spacecraft launches by year › Spacecraft launched 1963–1968*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*

License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license