During Starship's launch on November 18, 2023, the two stages successfully executed hot staging and separated from each other. In the seconds leading up to ignition of the upper stage engines, the booster shut down all but three of its engines. The tracking camera images showed a spidery cloud centered on the stack. My question is: what is the cloud made of and why is it there? It doesn't look at all like the high altitude exhaust plumes I've seen on other launches. Perhaps they are pre-cooling the upper stage engines by dumping one of the propellants through the injectors and out the nozzles?
There are quite a number of gases that are vented out the engine section:
After the fire in the engine bay which caused a lot of problems during IFT-1, there is a CO₂ purging system for the engine bay. This CO₂ gets vented out the side of the engine skirt.
Raptor uses regenerative cooling of the nozzle and combustion chamber, where liquid propellants are flowed through cooling channels and extract heat as they turn to gas. Directly after shutdown, those would be still be very hot, so it makes sense to leave the engine cooling running for a bit. Especially since 10 of those engines are going to re-light in a few seconds.
There will generally be some unburnt gases or liquids flowing through the engines even after combustion is extinguished. You wouldn't want to do a hard close of all values, in order to avoid fluid hammering.
In a test firing on Earth, both on a test stand at McGregor and in static fires at Starbase, you can see a large orange flame emerge from underneath the test stand after engine shutdown. So, this seems to be normal, and it seems to be mostly methane: since we see no flames in this particular case, that would indicate there is no oxygen present and we only get the flame down on Earth when the methane burns with atmospheric oxygen.
We also see those flames during pre burner tests. Assuming SpaceX extinguishes the main combustion chamber first, then the pre burner, we would expect the last phase of an engine shutdown to look like a pre burner test, but again without atmospheric oxygen.