I have noticed that the exhaust flames for rockets can be either everywhere, like in this image or it can be hardly noticeable, like in this video. I am just curious, what makes this difference? Is one better than the other?
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$\begingroup$ Did you research different fuels? what about the power output needed affecting the design? $\endgroup$– Solar MikeCommented Oct 21, 2023 at 7:03
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$\begingroup$ @SolarMike I didn't research as it was just a general curiosity. I think the exhaust where there isn't much to be seen would be hydrogen based, right? The output is just water/steam? $\endgroup$– John SnowCommented Oct 21, 2023 at 7:31
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$\begingroup$ Some similar questions from another SE 1, 2. $\endgroup$– AJNCommented Oct 21, 2023 at 7:35
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1$\begingroup$ Exhaust temp, mostly. Once you are in deep space, you can have very cold stable hypergolic fuels. On the lander for the little asteroid, the exhaust temp was something like -200C, so no glow. Obviously, any heat in the exhaust is wasted energy that could have been used to get a higher impulse. The best mass driver is hydrogen, and it doesn't have much of a glow. But first stage rockets tend to use less hydrogen proportionately. There is some discussion of this over on Space Stack. $\endgroup$– Phil SweetCommented Oct 21, 2023 at 9:35
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The side boosters in the first image are solid fuel rockets, which generate a lot of particles (soot and unburned fuel) in their exhaust plumes. These solid particles glow orange and white hot, creating a "fiery" plume. Kerosene liquid fuel engines like the Saturn V first stage or Falcon 9 first stage also generate sooty flames.
In contrast, the center engines in your first image, and main engine in your second image, are hydrogen or clean burning methane fueled. These generate much less soot, so what you see is the color of the gaseous combustion combustion itself (usually pink for hydrogen and blue for methane).
Regarding the size and shape of the exhaust plume, this depends on the exit pressure. Some engines (usually first stage like the outer boosters in your first image) are optimized for near-sea level thrust. So they try to make the nozzle exit pressure match or slightly exceed sea level pressure, making the plume roughly cylindrical or spread out slightly.
In contrast, engines that are optimized for higher altitude flight like the Space Shuttle main engines, Artemis core stage engines, and seemingly the center engines in your first example, have a lower exit pressure. This causes the plume to contract inward at sea level from the higher atmospheric pressure around it. If you observe high altitude flight photos of the Saturn V, or any upper stage in vacuum, the plume looks different and very spread out.
