More Than One Way To Skin A Cat

Apollo 8 Launches on 21 December 1968

Engineers are, by nature, generally cautious people. We add design factors and contingency factors and factors of safety into our design to account for any variance between design and construction, or for errors in our assumptions. Flight engineers tend to be even more cautious since they generally will have human lives at stake.

It was this cautious approach to testing the various stages of the Saturn V rocket that put the Apollo goal — reaching the moon before the end of the 1960s — at risk. In order to meet that goal, when George Mueller took over as NASA’s Associate Administrator for Manned Space Flight in September 1963 he declared that the Saturn V testing would go from a very incremental approach to an all-up approach. A full stack of stages should be tested as soon as stages are ready for testing.

But even Mr. Mueller wasn’t crazy. He knew there were risks and he wanted to mitigate those risks. One big risk is that the Saturn V rocket would never perform reliably enough to be man rated. So, he turned to the RAND Corporation think tank to come up with an alternative.

Being fiscally responsible smart engineers, E. Harris and J. Brom, came up with a backup plan that would get men to the moon using the Saturn V’s upper S-IVB stage. Here’s how it would work:

A Saturn V rocket with the command module and lunar module would launch unmanned and enter earth orbit. Then a Saturn I rocket, which had been in use since 1961, would launch with a modified command module. The manned command module would have a special mating adapter on it so that it could rendezvous with the command module already in orbit. The astronauts would move into the on-orbit command module, ditch the one they rode up in, and then light off the S-IVB to get themselves to the moon.

In April 1968, NASA launched Apollo 6, and unmanned test flight of the Saturn V system that was supposed to prove the rocket with a full payload could reach the moon. Instead of actually going to the moon, the rocket would escape earth orbit, then turn around and come back. In this way, they could also test a direct-return approach should a future mission be aborted.

About 2 minutes after launch, the Saturn V experienced “pogo oscillations”, which happen when thrust across the engines varies too much. The pogo caused damage in the upper stages. The rocket was able to achieve a semi-normal orbit, but when they tried to restart the S-IVB engines for the trans-lunar burn they failed to restart.

Apollo 6 was good and bad news for the backup plan. First, it looked like the Saturn V might fail to get its man rating due to the pogo event. However, if the S-IVB’s engines could not reignite in orbit the whole thing was moot.

Eventually, the Saturn V won out as a manned rocket and Apollo 7 and Apollo 8 went off without a hitch.

[Image Credit: NASA]

  • skitter

    A couple of times recently, I've had to explain the correct engineering use of the phrase "calculated risk". Hollywood and the man in the street look at a situation where everything has to go exactly right, and call that a calculated risk. Then they go for it, despite the slim to no chance of success. It definitely creates drama when "If X happens, people die." But if it's truly a calculated risk, then a lot of things are in place to keep "X" from happening, and people are only in danger if there's a perfect storm.

    /knuckle-dragging engineer
    //riot at parties

    • The Professor

      /and that's why we love you

  • texlenin

    When in doubt, use the Russian Engineering Matra:
    Make it Bigger
    Make it Stronger
    Make it Simple

    good to see you folks again.