Try looking through the archives over at StarshipModeler. I can't remember when I saw it or which vehicle it was (I think it was a shuttle) where the model builder used cotton batting stretched out out in a realistic dispersal pattern from the rocket motors. For further realism, he painted the inside of the smoke plume orange and yellow only near the rocket motor, then darkened the outer area of the smoke plume with gray's and blacks. He might have even added LED's inside the rocket motor but I can't remember. I wish I had bookmarked it, the effect was very realistic.