I think that you are getting disoriented by the mixed effect of multiple factor. Let me try to explain clearly.
first of all:
My "understanding" upon reading the documentation was that GrowFactor_size would affect Srcsize and GrowFactor_rate would affect Growthrate to each have an altitude dependence. As in, at say 50km altitude the particle stream would look as if you had simply used the modified Srcsize and Growthrate all along.
First part is correct, the rest I don't know if it's exact, I'm not sure if I got correctly what I highlighted. The dependence is on the pressure, by the published formulas which of course means an altitude dependence for both factor.
The particle stream works in this way(not considering now atmo and level mapping) : at each instant the engine emits particles as per specification given. The particles emitted have a size of srcsize, a number of srcrate per second of this single particles are emitted, they stay in the simulation for the lifetime defined before dissolving. While staying in the simulation (the cloud that you see behind the vehicle) they grow in size by growthrate meters/sec.
The growing particle effect allows to change in time, depending on outside pressure, two of this parameters: size of emitted particle and growthrate of the emitted particle.
So at 50 km you will see behind the rocket the particles already emitted by the engines while flying to that altitude and you will emit the particles that will have the size and growthrate applying in that moment.
To give an idea the effect is stopped when outside pressure reaches the level of 10e-5 Pa. On earth at sea level pressure is 101400 Pascal, so the formula gives at maximum a variation of 10*factor for each from original value at sealevel.
Now let's take a look at the relevant parameters for this scope which are of course:
- srcsize = size of the single particle emitted
- growthrate = measure of how the single particle emitted grows in size while rendered (it is how, once emitted the smoke grows while still "alive", before dissolve itself)
so a single particle is emitted with size = srcsize and after a while, while flying in the atmosphere it would have grown proportional to growthrate right? this is how it already works in orbiter.
If you increase the size of the particle, even if you don't touch the growthrate, you will have the feeling that also growthrate is increasing, because a bigger particle that grows with the same growthrate of the smalle particle will end up being bigger at the end: it simply started from a bigger value, grew the same amount and logically ended to a bigger value.
This is why you have the feeling of growthrate changing even if it's not.
If you instead increase just the growthrate the particles will be emitted at the same size at every altitude but they will grow much faster. This leaves the way to have a bigger cloud without having to extend the particle lifetime (which is also really performance consuming).
to see what I'm trying to explain, try this in the example of the SLS with growing particles:
you'll see regular tube of particles just growing in diameter as per altitude increase.
then if you want to see the growthrate factor working try this
Last thing: the feeling you have to see the old particle stream inside the new one is just because the texture is much denser at the center, but it's only one particle that is emitted (or 2 if 2 of them are defined).
The effects simply recalculate the parameters and once per second delete the old particle and add the new with the updated specifications.
I'm not sure if I was clear, it's not totally easy to explain just with words, drawings would have helped a lot!
Let me know if it was understandable and anyway don't hesitate to ask
---------- Post added at 01:41 ---------- Previous post was at 01:23 ----------
Just to add what might be clearer to you:
It seems to me that you think of particle streams paramters as the parameters of the cloud of smoke behind the rocket, that is not true or at least not accurate.
the particle stream parameters are the paramaters of the single particle of smoke emitted by the engines, the cloud of smoke is the result of the sum of all this particles which stay in the simulation for their "lifetime".