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Here is a GIF that I made of the recent Electron Launch from frame grabs in the Rocket Lab video ELaNa 19 Launch - 12/16/2018 It is about 4M, so it won't fit in SE's normal image import.

Frames are every 2 seconds in the GIF which plays at 8 fps so this is 16x faster than normal.

You can also watch the video for about four minutes to see the same thing

What causes those curtain-like hot and cool spots in the 2nd stage nozzle to drift around randomly like that? Is it crud or carbon on the inside of the nozzle, or shadowing of exhaust from material farther up in the throat?

Cued at 20:50 Play at maximum speed to see the changing hot/cold pattern on the 2nd stage nozzle

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(Image Credit: Medium / Rocket Lab)

The engine in question, Rocket Lab's Rutherford Engine is using a technique named regenerative cooling. It passes the fuel through the nozzle wall before it reaches the combustion chamber, cooling the nozzle in the process. The big pipe going down to the bottom of the engine is responsible for feeding the fuel into the nozzle. I could not find a whole lot of information about the inside of the engine, but it probably looks similar to this:

(Image Credit: u/christophertacon on Reddit)

There the fuel gets piped up in vertical or slightly spiraling lines. We also don't know a whole lot about the fuel injector. (If anybody finds anything useful about any part of that engine: please let me know!). The fuel probably gets injected into the engine quite soon after the cooling process.

This is where the speculation begins:

I have two theories. Both of these could be true, maybe they are even linked:

  • I could imagine the streaks being the result of some sort of oscillating feedback system: A cooling channel gets hot and expands. It subsequently lets through more fuel causing it to cool again and shrinks back down. It is important to mention here, that the bit of the engine changing color is the vacuum nozzle, which is attached below the cooling system, but would still get the effects of the hot gases downstream from that assembly.
  • Fuel injectors have a habit of creating visible patterns even outside the engine. In the picture below is the exhaust of a Falcon 9 second stage single(!) Merlin Engine. The Pattern is produced by the injectors being single holes rather than concentric rings: There are points in the combustion chamber where this is an imbalance between fuel and oxidizer because it is closer to an injector of that specific resource and there are points with a better mixture ration. This results in varying temperature and composition of the exhaust yielding visual patterns.

(Image Credit: Peter Thoeny on Flickr, Emre Kelly / FLORIDA TODAY)

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  • $\begingroup$ The image of the exhaust pattern from F9 2nd stage (single engine!) is really helpful, I see what you mean, the exhaust isn't perfectly uniform, and the pattern of non-uniformity can be stable over several seconds or longer. Thanks for this nice explanation! $\endgroup$
    – uhoh
    Commented Dec 18, 2018 at 14:28
  • $\begingroup$ However the injector pattern should remain constant over time and not vary over time, because the injectors do not move. $\endgroup$
    – Hans
    Commented Dec 18, 2018 at 14:41
  • $\begingroup$ What about coking? I can imagine some kind of instability where surfaces (not necessarily injectors) get coated with deposits and then they get eroded, and it's the deposits that either shape the flow, or shape the heat transport through the nozzle wall. $\endgroup$
    – uhoh
    Commented Dec 18, 2018 at 14:46
  • $\begingroup$ Do you have any references or calculations to back up what seems like an answer based on guesswork? $\endgroup$
    – Organic Marble
    Commented Dec 19, 2018 at 2:29
  • $\begingroup$ In addition, looking at the background, the vehicle appears to be manoeuvring, so will also be some inertial input into the chaotic combustion and cooling. $\endgroup$
    – user20636
    Commented Feb 4, 2019 at 12:11
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Not a full answer, but by looking through the soot-less LH2/LOX exhaust from a Delta IV Heavy's RS-68 engine from underneath, we can see the inside of a nozzle and that the same type of striations appear here. This suggests that the effect is not limited to the Electron's Rutherford engine engine or even RP-1/LOX.

Images from Teslarati tweet

twitter.com/Teslarati/status/1028997673024671744/photo/1

twitter.com/Teslarati/status/1028997673024671744/photo/1

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