It’s a generally accepted rule of thumb that if you do something and it explodes into a colossal fireball, it’s best if that doesn’t happen the next time you try. My grandma has that cross-stitched into a pillow. It’s also the situation that SpaceX finds themselves in tomorrow, when the first Falcon 9 launch since their rocket exploded last October. They have to get this one right.

I’m pretty confident they will. The launch record of the Falcon 9 is very good: out of 28 launches over the past six years, 26 have been totally successful, one was partially (a secondary payload didn’t reach the intended orbit), and one was a disaster, the October launchpad explosion. The odds are with them.

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Even better, they actually believe they know what happened to their rocket, as of about two weeks ago:

The accident investigation team worked systematically through an extensive fault tree analysis and concluded that one of the three composite overwrapped pressure vessels (COPVs) inside the second stage liquid oxygen (LOX) tank failed. Specifically, the investigation team concluded the failure was likely due to the accumulation of oxygen between the COPV liner and overwrap in a void or a buckle in the liner, leading to ignition and the subsequent failure of the COPV.

Each stage of Falcon 9 uses COPVs to store cold helium which is used to maintain tank pressure, and each COPV consists of an aluminum inner liner with a carbon overwrap. The recovered COPVs showed buckles in their liners. Although buckles were not shown to burst a COPV on their own, investigators concluded that super chilled LOX can pool in these buckles under the overwrap. When pressurized, oxygen pooled in this buckle can become trapped; in turn, breaking fibers or friction can ignite the oxygen in the overwrap, causing the COPV to fail. In addition, investigators determined that the loading temperature of the helium was cold enough to create solid oxygen (SOX), which exacerbates the possibility of oxygen becoming trapped as well as the likelihood of friction ignition.

To try and put it simply, it looks like the inner aluminum liners of their pressurized liquid helium storage tanks developed some buckles in their surfaces. These pressurized tanks are actually inside the liquid oxygen storage tanks, and in these buckles frozen bits of liquid oxygen accumulated.

The little rocks of frozen oxygen were ignited by friction from parts of the tank, which is what set off the explosion.

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SpaceX now feels ready to leap off Earth again, and tomorrow they’re scheduled to launch ten Iridium NEXT communications satellites into low-earth orbit. Hopefully, the launch will go well, because SpaceX has a pretty full plate in the near future. They want to launch their heavy-lift variant of the Falcon, the Falcon Heavy, very soon, and they’re scheduled to start using their Dragon capsules to send astronauts into orbit in 2018.

Those crewed Dragon capsules will be launched on Falcon 9 rockets, so every successful launch is one step safer to proving these launch vehicles can be human-rated.

It’s also worth noting that, like other SpaceX Falcon 9 missions, they will attempt to land the spent launch vehicle on a floating barge. It’s been done before, but it’s always impressive.

Good luck tomorrow, rocketeers! If you want to watch the launch, you should be able to see it on the SpaceX webcast page, at 12:54 p.m. EDT.