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Great flight!

Elon tweeted that the header tank pressure was low:

https://twitter.com/elonmusk/status/1336809767574982658

There was also a substantial green flame coming from the raptor engine used for landing. Group consensus over on /r/SpaceX is that it was copper burning. We're amazed the engine didn't explode sooner.

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as soon as I saw the green I thought "uhoh". That's called running "engine rich". still, so amazing to see the belly flop and relight.

I’m a great fan of all of the “this is fine” sort of phrases for engineering failures. “engine rich” is a new one for me today, joining things like letting the magic blue smoke out of electronics and rapid unplanned disassembly.

While it isn't humorous, "delta P event" is so dryly understated that IMO it belongs (when not involving human tragedy). Delta P events would be explosions if they happened in air. As violent as an atmospheric shockwave is, the speed of sound is >4x higher underwater, in a medium that is >800x heavier. The amount of casual violence implied in a "delta P event" is profoundly unnerving.

https://www.youtube.com/watch?v=flzpRbWl8bQ https://www.youtube.com/watch?v=fzRb2vMfGyk

Oh yes, today I learned that RUD (rapid unscheduled disassembly) is apparently an established term, but I didn't get what "engine rich" means until now.

Huh, I assumed it was TEB to keep the reignition stable but apparently raptors have electronic ignition: https://www.youtube.com/watch?v=pqCatb2oApU

For those who don't want to click the tweet:

> Fuel header tank pressure was low during landing burn, causing touchdown velocity to be high & RUD, but we got all the data we needed! Congrats SpaceX team hell yeah!!

> RUD

“Rapid Unplanned Disassembly”?

I love reading Musk’s tweets :)

RUD is very much a euphemism that has I guess drifted a bit from it's initialism; I'm pretty sure in this case the disassembly was planned and the survival case would have been unplanned.

I take your point, but they're not planning for a launch failure even if they're pretty sure that's what's going to happen.

What happened when most of the mass of a Space Shuttle launch hits the ocean: that's an RPD.

If you’re not planning for failure modes you’re not engineering correctly.

Sure but "lithobraking" takes too much space in a tweet.

”Unscheduled”, I think

I like reading Musk's tweets when he talks about something he knows (e.g. engineering). I don't much like them when they're ignorant and irresponsible commentary on Covid-19 (something he knows less about than even most ordinary but interested people, seeing as he wouldn't have the time to follow much expert commentary), or when he's sharing his puerile taste in humour or music or literature etc (although, to be fair, it's his personal account so he can share what he likes).

So does that mean that if things had gone to plan the flip would have been even more dramatic/extreme?

Starship is supposed to be able to carry passengers, not just cargo. How would the human body hold up to a flip like that?

I think it was purging leftover hypergolic fuel, which is used to ignite the engines. Or, maybe the engine sensed the engine was abnormal (because of low fuel), so it tried to re-ignite.

https://en.wikipedia.org/wiki/Triethylborane

Its spark ignited. No hyperglos

TEA-TEB is not used on Starship. What you saw was an oxygen rich burn due to the low pressures in the header tank, the engine was eating itself and burning copper in the process. Copper burns green.

> TEA-TEB is not used on Starship

This is correct. The Raptor uses an electric spark igniter.

Yo what's going on here? Is this some sort of misinformation campaign? To what end?? There's been a number of comments making this claim even though there's no reason to suggest this was the case.

I don't think it's malicious, it's just a lot of amateur speculation in the absence of an official report.

Definitely not misinformation, just misinformed armchair enthusiasts. SpaceX did the hard thing using spark ignition which goes against the industry norms. Even the everyday astronaut YouTube guy said tea tab on his stream and he’s a space nerd.

He just made a video pointing out how they don't use it. But as a SpaceX fan for 10+ years, green flame is just totally associated with TEA-TEB.

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