Rampant speculation: why did the Falcon 9 blow up?

I am not a rocket scientist, but I do like to think about engineering problems.

Here are the facts as we know them:

  • A Falcon 9 rocket blew up on the pad on September 1, 2016.
  • The rocket was undergoing a pre-launch static test, when it exploded.
  • According to SpaceX, the explosion originated in the second-stage liquid oxygen tank.
  • SpaceX uses a fancy super-cooled LOX mix, which allows more fuel in a given tank volume, which allows better performance.
  • Last summer, SpaceX had another rocket fail. The CRS-7 mission disintegrated in flight after the upper stage LOX tank burst. The internal helium tank (to maintain tank pressure) failed because of a faulty strut.

Now, for a rocket to fail during fueling, before engine firing—as the most recent Falcon failed—is very unusual. To my engineer’s mind, it suggests a materials problem in the LOX or liquid helium tanks, something failing in an unexpected way when deep-chilled. Crucially, the Falcon 9’s LOX tank experiences the coldest temperature (for a LOX tank) in the history of rocketry. Take that in combination with the failure on the CRS-7 mission: after their investigation, SpaceX switched to a new strut, which means new failure modes.

Mark my words (and feed them to me along with a heaping helping of crow, when I turn out to be wrong): this is another strut issue, be it faulty or just unsuited for the deep-cryo fuel in some other way.

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