Something Nertea invented was the “engine core” mesh that allowed for the inside of the engine to glow brightly when the throttle was on.

You can see how in this shot, the Buzzard engine at the top has a brightly glowing purple interior thanks to this engine core mesh effect. The Swallow on the bottom didn’t have this effect, hence why the inside of its engine nozzle remains quite dark:

To add the effect yourself, create a mesh inside the nozzle of your engine and name it EngineCoreGlow. It doesn’t have to conform to the inside perfectly and can be quite low-poly, but it’s best to match the number of sides to prevent any strange gaps. Ensure that the normals of the mesh are facing correctly (i.e. downwards), and parent this mesh to the gimbal empty alongside the engine nozzle.

Create a new material, and set its KSP shader to ksp alpha transluscent additive. UVMap the mesh to a colour gradient of your choice—preferably a colour that matches that of the engine plume—and you’re good to go! The animation transition will be defined in the config file.