Game simulations of rocket exhaust are so bad, it's a serious point of loathing & despair.  

Raptor exhaust is a clean blue flame with no smoke.  The shock diamonds stay in the same place, with subtle oscillations.  There's also a startup & shutdown state.  The cloud of smoke during liftoff is water vapor from the launchpad.  The mane problem is it's not a 2D surface which can be textured with a shader but a 3D volume which must be viewable from any angle.  There are no cut & paste examples of making a 3D flame in godot.

The general way lions are aware of to get shapes like that, is to have an onion of many meshes forming a volume.  The flame is blurred by applying a fragment shader to the final output, just where the flame appears.  Godot only supports giving you the final scene in a texture SCREEN_TEXTURE & a depth buffer DEPTH_TEXTURE, but does nothing for correlating each pixel with an object or occluding it.