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The BBC's Watching North Korea from a supersonic fighter jet (screenshot from YouTube) shows the launch of a South Korean rocket.

I notice that it has both a very long, nicely expanded blue exhaust plume straight down as well as a large, lower velocity and wider bright yellow-white exhaust plume surrounding the top several meters of the blue one. I don't see any strap-on boosters that could account for the bright plume.

What's going on here?

In my experience rockets without SRBs come in either "blue" or "yellow/white" only

Question: What is this South Korean rocket and why does it have separate yellow and blue exhaust plumes?

screenshot from The BBC's Watching North Korea from a supersonic fighter jet

screenshot from The BBC's "Watching North Korea from a supersonic fighter jet" https://www.youtube.com/watch?v=ZHQMPbMOxUM

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That's KSLV-2 aka Nuri. The first stage uses kerolox gas generator engines so the brighter flame is almost certainly ignited fuel-rich gas generator exhaust.

https://en.wikipedia.org/wiki/Nuri_(rocket)

This image of a KRE-075 first stage engine test shows the dark, fuel rich gas generator exhaust at the left. (image source https://www.reddit.com/r/SpaceXLounge/comments/h0jxqm/korean_engine_and_a_rocket_inspired_by_spacex/)

enter image description here

Good video of an engine test here: https://www.reddit.com/r/engineteststands/comments/kq2j7w/kre075_test_firing/

A higher-quality launch picture from here https://www.nasaspaceflight.com/wp-content/uploads/2021/10/NSF-2021-10-21-09-08-43-346.jpg shows the gas generator exhaust plumes exiting into the bright cloud of flame.

enter image description here

enter image description here

The informative paper DEVELOPMENT OF A RELIABLE PERFORMANCE GAS GENERATOR OF 75 TON-CLASS LIQUID ROCKET ENGINE FOR THE KOREA SPACE LAUNCH VEHICLE II

shows a schematic of the engine

enter image description here

and confirms the fuel-rich gas generator exhaust.

In the rated condition of the 75tGG, only about 10 % oxidizer is supplied to gas generator compared with propellant flow ratio at the stoichiometric condition. So, most of oxidizers participates in the combustion and reacts with the fuel.

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    $\begingroup$ The Wikipedia article (linked above) has a photograph of the engine. A spectator has his hand on what is likely the tail pipe from the gas generator. This is likely the discharge site of the yellow flame from one of 4 first stage engines. The fuel is Jet-A which has a carbon chain length of 8-16. When Jet-A is burned rich (like in a gas generator) it produces soot and yellow flame. At stoichiometric mixture (like in the main engine), it is much hotter and produces H2O and CO2 instead of soot. $\endgroup$
    – Woody
    Commented Jan 18, 2022 at 22:10
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    $\begingroup$ Organic Marble, You said the generator is hydrogen/LOX? Wiki says main engine fuel is Jet-A. Does the engine have separate fuel for the generator? $\endgroup$
    – Woody
    Commented Jan 18, 2022 at 22:13
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    $\begingroup$ @Woody you're right, in my rush I typed hydro for hydrocarbon. Will fix, thanks. $\endgroup$
    – Organic Marble
    Commented Jan 18, 2022 at 22:15
  • $\begingroup$ Interesting! The primary exhaust plume is much bluer than what's seen from most LOX/Kerosene first stages featured in popular media; and the color & brightness seem to be related to a choice of mixture? How did the kerosene-burning Black Arrow have transparent exhaust? (seems to just "hover" in photos) $\endgroup$
    – uhoh
    Commented Jan 18, 2022 at 23:25
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    $\begingroup$ @uhoh I think the Black Arrow used hydrogen peroxide for its oxidizer. There was a quote I so wish I could find about how it took off without any fuss, not like the messy rockets of those uncouth colonials. $\endgroup$
    – Organic Marble
    Commented Jan 18, 2022 at 23:48

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