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Comment by rrmm | Hacker News Reader

If you catch the audio on this from the nasa space flight forum (I think?), it was pretty funny. They were speculating if it was a lox purge (vs a methane purge). They assumed lox, since if it were methane, the flare would..... BOOOM.

The timing was very perfect.

EDIT: https://twitter.com/NASASpaceflight/status/12664420878489600... has the audio with it.

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A bit later they run the tape back frame by frame and determined the fire definitely didn't come from the flare stack. (Though in realtime it happens so fast they weren't sure.)

Yup. but the timing was just too good. It was like the methane wanted to have a say in the conversation: "Acccctuaally,....BOOM"

That video was almost certainly taken with a telephoto lens a long distance away, and telephoto lenses tend to compress distance. The flare could’ve been a long way away from the rocket, and a telephoto lens would make them look much much closer.

Yes you can see also that the shockwave arrives at the camera more than a second later.

Best illustrated with dolly zooms. Even side-by-side stills don't quite capture how significant the effect is.

https://www.youtube.com/watch?v=MeyjyZ6UZII

"I don't know how nominal that is"

"That was not nominal"

I thnk they slipped a 'norminal' in on us. (A John-Insprucker-ism).

> I thnk they slipped a 'norminal' in on us. (A John-Insprucker-ism).

They did. Its become a meme apparently. I'm looking at Todd's stream (Everyday Astronaut) and he has a hat with that on it.

"More data is more better" (beginning of the video) is my favourite new engineering meme.

What's the purpose of the methane purge?

It was not intentional. There must be some mechanism to disconnect the external propellant lines before lift-off and they obviously disconnected unintentionally after the static fire. The best guess based on the way the flame spread was that both lines disconnected and there was a mixture of methane and oxygen just waiting for the right fuel-oxygen ratio before it was ignited on any number of post-burn hot spots. (I haven't seen any new info recently so this explanation may have been already superseded by more recent analysis.)

From just the video it didn't look like a very fast burn. I wouldn't be surprised if it was just methane, or at least a non-stoichiometric mixture. (Could also just be poorly mixed).

Scott Manley posted on Twitter [0] that it looks like it was a proper detonation (supersonic flame front, etc.) based on some math given the known size of the test rig and rocket.

[0] https://twitter.com/DJSnM/status/1266586181392674817?s=20

Cool! I missed the shock front. I guess the cloud was a lot bigger than I was giving it credit for.

thanks for the update.

EDIT: oh man I just saw another view of it in the twitter thread. It was proper!

Elon Musk confirmed on Twitter that they were testing the "quick disconnect."

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