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The RocketLab YouTube video STP-27RD Launch - 05/05/2019 shows the exhaust plume from the second stage engine evolve as the atmospheric pressure drops. During this time there are frequent outbursts of presumably hot glowing particles in the exhaust, some of them appear to be moving far slower than the exhaust velocity.

The engine burns RP-1 and LOX. Usually when we talk about soot for example, it appears to be a continuum of so many fine soot particles (I'm guessing microns in size or smaller), but these have got to be much larger in order to be so individually bright and distinct.

What are these exactly, and why are the so prevalent from the Rutherford engine?

Electron rocket 2nd stage exhaust GIF

Electron rocket 2nd stage exhaust

Electron rocket 2nd stage exhaust

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    $\begingroup$ Asked myself the same when I just saw the launch. From my expirience with model rockets I'm pretty sure these are metal sparks and since the rutherford engines are 3d printed my guess is that some of the powder used ablates from the chamber walls. If this is true it would be quite interesting for the reusability thread. $\endgroup$
    – Christoph
    May 6, 2019 at 6:25

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Tim Dodd, the Everyday Astronaut, just uploaded an interview with Peter Beck from Rocket Lab where he asked exactly this question. His answer was that soot build up at the injector and breaks lose from time to time. So basically its just carbon that lights up due to the heat.

Unofficial transcription based on listening and watching YouTube's inacurate closed-captions:

Dodd: Second stage has a bunch of sparks that always shoot out the nozzle. What’s that from again?

Beck: Yeah, so the Rutherford engine is really high performance, and what that actually is, I know it’s quite funny, you see people talking “that must have been an ablative nozzle”, despite the fact that it’s glowing bright red! But no, they’re little soot deposits.

So the soot deposits kind-of build up on the injector face like stalactites, and then they get to a certain size and break free and then it’s soot entrained — carbon entrained in the exhaust plane.

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  • $\begingroup$ Thanks for your answer and then remembering the question! I've given the transcription another go, please feel free to edit further. I see that they mentioned your ablative comment also ;-) $\endgroup$
    – uhoh
    Dec 19, 2019 at 16:40
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    $\begingroup$ @uhoh thanks for improving the transcript! Had a really hard time understanding ablative. I understood "a blighted" nozzle all the time but that didn't make sense .. guess that's what happens if you learn english mostly from textbooks ;-) $\endgroup$
    – Christoph
    Dec 19, 2019 at 16:50
  • $\begingroup$ Ha! btw here's an example of a blighted nozzle $\endgroup$
    – uhoh
    Dec 19, 2019 at 16:52

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