Many pictures of the Soyuz-2 show the inside of the nozzles painted with a brilliant red. Is there any function of the paint during ignition and operation of the rocket engines, or is it just a traditional decoration? The paint will burn away very quickly.
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They aren't painted red. If you zoom in on the picture, you'll see that the red parts are actually protective inserts clipped onto the nozzles to keep debris out of them during transport.
It's a widespread aerospace-industry convention to make such "remove before flight" items bright red so it's obvious if you've left one on.
Apparently the covers stay in place until very late in the launch process. In a description of the lead-up to a Soyuz launch, "Soyuz: A Universal Spacecraft" says:
The crew start the onboard tape recorder at T-15 min[utes], and ensure that their safety harnesses are tightened as the automated flight sequencer takes over. Combustion chamber covers on the first and second stages are blown off with nitrogen, while topping off of the liquid oxygen tanks is completed...
Here they're referring to the boosters and core as first and second stages.
answered Jun 6, 2018 at 22:13
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3$\begingroup$ Very good answer, thank you! Your zoom of the picture show the "remove before flight" items very precise. So there is a function of the bright red. $\endgroup$– UweJun 6, 2018 at 22:28
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$\begingroup$ I cannot find the exact moment when they are removed, that should be some time before refuelling I guess. No pictures or videos of this event for what you know? $\endgroup$ Jul 3, 2018 at 10:30
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$\begingroup$ The protective covers should be mounted in the assembley building and during horizontal transport to the launch pad. Also when the rocket is turned from horizontal to vertical at the launch pad. If the launch pad is evacuated just before fuelling, the covers should be removed before. If launchpad security regulations allow human access to the nozzles of a tanked rocket, they may be removed short before launch. $\endgroup$– UweJul 3, 2018 at 14:25
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$\begingroup$ @VeronicaRemondini I ran across a line in "Soyuz: A Universal Spacecraft" that suggests they're removed only 15 minutes before launch, but it's not 100% clear. $\endgroup$ Sep 5, 2018 at 14:53