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What is the white residue in a spray pattern seen on the inside of each nozzle of each side core of this Falcon Heavy image from the Teslarati article SpaceX reveals Falcon Heavy Block 5 in first official photo, timelapse. See also SpaceX tweet.

I'm thinking it could be related to engine shut-down, but it looks like they are assembling a Falcon Heavy for launch, so wouldn't the nozzles have at least been cleaned after the previous launch?

Falcon Heavy (cropped) from Teslarati

Falcon Heavy from Teslarati

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  • $\begingroup$ Looks like they need those little spike strips that keep pigeons from roosting! Very, very large space-pigeons. $\endgroup$ – Avi Cherry Apr 8 at 18:49
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I believe it is residue from the TEA-TEB starting fluid. Triethylaluminum combustion produces aluminum oxides, Triethylborane produces boron oxides. Both are shades of white and grey, matching the streaks. Each engine is tested on the stand at McGregor before installation in a booster, and again in the full booster checkout, so there are several opportunities to deposit the waste, even on a new booster.

They have expressed a goal of zero refurbishment before a typical reflight, which seems to include unnecessary cleaning. They likely have enough data on engine reuse to understand the rate it builds up at, and when it may become a problem.

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    $\begingroup$ This makes sense (towards zero refurbishment); and the oxides may be quite refractory. I'd wonder if the local change in emissivity could cause temperature gradients and therefore extra stress, but I assume that's been considered. $\endgroup$ – uhoh Apr 8 at 2:44
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    $\begingroup$ @uhoh The high volume of cold RP-1 flowing through the bell walls may also help reduce the strength of any temperature gradients. $\endgroup$ – Saiboogu Apr 12 at 16:46
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    $\begingroup$ I realize that what's going through my mind is images of the long, glowing red-hot 2nd stage nozzles we see in the launch videos. But these are the short, atmospheric 1st stage nozzles and they run a lot cooler than I was imagining. $\endgroup$ – uhoh Apr 12 at 22:58

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