Hey @Aaron VFX, super cool to see one of the artists so active here in the forums. And what I've seen reading through the thread this far looks extremely promising and powerful!
I'd like to contribute something as well, all the things I've discovered researching plumes and heatblur while working as a pre-alpha tester for a heatblur mod, and as a plume maker for waterfall. I should also note that I'm the plane guy, over 3000 hours in KSP, never gotten beyond minmus without cheats. But I have build more aircraft than I can even remember. Because of this and because I'll likely do the same in KSP2, I would be forever grateful to also see jet engines get some love, not only rocket engines.
And to maybe help with this, I've written up all my knowledge about afterburners and heatblur below. It's a bit of a behemoth of a text, but I hope it will be useful regardless.
As you said, I hope I could "keep the ideas rolling".
I'll start with the many intricacies of afterburners/reheat on planes.
shown here: the F-14 afterburner through various lighting conditions and altitudes
Notes: The F-14, as most other afterburners appears blue in dim light / darkness. I am not absolutely sure why, but I suspect it is a mix of the strong infrared light overpowering the filter in the camera in the dim light and these shots showing the afterburner at almost peak efficiency, meaning very little soot in the exhaust that would glow in the yellow typical for carbon soot. The afterburner is showing some mach diamonds though, indicating that the engine is at least slightly off it's ideal design power point, as an afterburner when working exactly in the flight state range it was optimized for would ideally show no mach diamonds.
Here's an interesting contrast: Same plane, shot on the same camera, but at daylight. Also at full burners. The burners are now barely visible, and appear orange, which I suspect is a mix of the metal in the engine glowing and the flame's blue parts being swallowed by the bright day and the blue sea.
Different camera this time, but still the same plane at roughly the same time of day. However this time the glow of the exhaust overpowers the camera and completely blows the afterburner out into white, while also lighting up the deck beneath it, showing that the afterburner here is actually being that bright. This indicates that the afterburner seems to be more dense and more sooty at lower altitudes. Though it should be kept in mind that this shot was made in very dim lighting conditions, I have never seen an example of an afterburner blowing out a camera in full daylight.
And another F-14 here. This picture is rather small and old, but was likely shot on a film camera similar to the first two images I've shown. This image shows an F-14 at takeoff from a carrier, likely under full afterburner. Yet the afterburner shows no mach diamonds, indication that the engine is likely operating exactly in the environment it was tuned best for: Heavy carrier takeoffs with maximum thrust. More noteworthy things of this afterburner is that it appear orange, even though it was also shot in somewhat dim lighting. This indicates an afterburner that is not burning completely clean, but more sooty. Another noteworthy thing is that the afterburner is showing streaks in the exhaust, lines in which the afterburner appears brighter than elsewhere. These streaks appear to vary from engine to engine, but tend to stay somewhat in the same position while the engine is running. A usable reference for how these streaks look in action is this video: https://youtu.be/Oj4w7i-TqsE. Though here it should be noted that the F-16 exhaust here, at almost sea level, once again appears blue. And that while a normal F-16 afterburner appears orange under most circumstances.
Next up: Some Daylight! Afterburners are not nearly as bright as rocket engines, so in full daylight they become quite dim. Only the shock diamonds remain visible, and also only their inner rings, not the full diameter rings visible with dim light. Also note the large amount of heat blur visible, as it almost always appears with afterburners. Heatblur will have it's own section, so I will not go into further detail on that.
More F-14s, more bright daylight. This time a look up the nozzle. Visible is once again significant heatblur. Through it becomes visible how the flame behaves inside the engine, originating on the flame holder, them moving along the outside of the engine into the streaks out of the exhaust I mentioned earlier. The left nozzle here shows a good illustration of how these streaks originate in the engine and flow out of the nozzle. Also note the multi-colored flame, burning in both blue and orange within the engine.
Speaking of streaks, flameholders, and engines, this is how it looks inside the F-14's F-110 engine when it is not running. You can see the last turbine stage in the background. In front of that there is this ring of green-ish bars. As visible on the schema on the right these pans have a kind of V-shape in cross section, which creates a low pressure zone in the air stream behind them, which makes a stable combustion in the high velocity exhaust even possible. Here the fuel of an afterburner is also injected, so this part is really the start of any afterburner.
Now we've seen plenty of the F-14, and it's afterburner, though only really at full thrust. Let's take a look at an afterburner going through different power settings next. Shown below: the shutdown of an F-16 afterburner in dim daylight:
We start off with the camera once again being completely overpowered by the brightness of the afterburner in the dim light. At full power and sea level the afterburner shows no signs of mach diamonds, but as thrust decreases first an inner layer of afterburner becomes visible, that then breaks apart more and more into individual inner core mach diamonds before the engine completely shuts down. This once again highlights how mach diamonds occur when the engine is not in the state it was optimized for, in this case that being low thrust afterburner while not moving. Another interesting thing is that one can see the engine insides glowing in the cracks of the nozzle vanes.
About the SR-71, the J-58, and the Whiplash engine
The SR-71 is probably the plane that illustrates shock diamonds the best, here being used as the top image on the wikipedia article on said shock diamonds. Seen here in this night test shot where one can see the unique afterburner of the SR-71. An inner core, long, thin and orange, fanning out into turbulence at the end, with a bigger blue-ish exhaust around it that has it's own shock diamonds.
This blue exhaust becomes almost invisible at day, replaced by a large amount of heat blur coming in instead. Looking closely at the right engine there still is a faint outer exhaust core visible, but it being blue and not as bright it is mostly invisible at day.
https://i.imgur.com/uU7kWgq.gifv (KSP video from my old waterfall whiplash config)
Unlike the Whiplash in KSP the real J-58 engine also had an ignition and wasn't permanently on afterburner. It would be a neat detail to have the famous green ignition flash be present in KSP as well. This flash of TEB was required to be able to ignite the SR-71's low volatility fuel, and actually gave this engine a limited amount afterburner ignitions.
Now onto Heatblur
Heatblur is an interesting topic. We rarely ever notice it directly, but it helps convey the power and heat output an engine has, even if no flame is visible. From what I've seen in the forum thread here the heatblur effect is currently only a displacement effect, which is also what we associate most with heat blur thinking about it. However, heatblur has more than just displacement behind it. So let's have a look at what causes heatblur to look the way it does.
Displacement: As already mentioned, this is what one usually thinks about with heatblur, the air with it's turbulent temperature and therefore (optical) density differences distorting all that is behind it. A rgb map of layered noise used to displace pixels in x/y direction is a good start here
Blur: As it already says in the name, heatblur also blurs everything that is behind it. While testing the first pre-alpha versions of the heatblur mod it only had displacement. As does current day waterfall. And that just looks a bit odd, it's still too sharp. It sure looks like distortion, but not like real heat blur. And this is where blur comes in, layering a blur onto the displacement makes the effect seem way more realistic as it helps simulate all the microscopic turbulence in real air that the displacement has not considered yet.
Darkening: This one may seem a bit odd at first, but during testing on the mod we realized that the effect looks a lot better if the blur also slightly darkens that area of the screen. This makes the effect a tiny bit more visible, and makes it seem more realistic. I have also noticed that kind of darkening notice on real blur. My explanation for this is light being scattered away from the viewer, making it appear slightly darker.
About Blur: Compare this image (without blur) to the image above (which has blur) and see the difference. Displacement alone is cool, with blut it looks even better.
In the heatblur mod I was a part of the effect was achieved through world-space particles with all it's benefits and drawbacks. The blur stream was getting holes at high speed as unity's particle system couldn't keep up, but flying low over the runway left a beautiful trail of blurred air behind that lingered for a while, then faded out. This was done through particles in worldspace that inherited the speed of the plane minus the exhaust velocity. Ideally the particles would have also had drag so they slowly come to a stop while the displacement texture keeps scrolling.
The ideal implementation would probably be a trail renderer that is somewhat modified to simulate exhaust velocity, meaning it would deform to a certain point so it follows the path the particles mentioned above would have followed. Mesh based blur like in waterfall sadly is not really a solution as the beautiful lingering heat blur trails get lost. An effect like this must be world based. A trail renderer alone would look odd from behind though, so my proposal would include few particles pinned to certain mesh intervals in the trail renderer.
Another possibility may be with the effect shown early on in the thread of the engine exhaust bending to outside influences. A mesh that bends like that could handle the heatblur close to the engine where the the trail is thin and concentrated, which would be an issue for particles. Further down the line particles could then take over to produce the lingering effect.
Oh yeah and non-afterburning and even turbofan engines still produce heatblur!
This took me about three hours to write and is basically a knowledge dump of all the things I have researched over the last three years. I hope my knowledge can help at least a tiny bit to making KSP2 even better....
And please: give air breathing engines some love in KSP2!
As an addition, I'll include a selection of my favorite shots here as further inspiration. (this will probably automerge with the above post.) With the knowledge from above, see how many details are now noticable!