Pierpaolo Viggiano
Thank you first of all for your answer.
My idea is to set a source on the surface of the heat shields. The source generates heat through an electric field. Again, I would have to enter a UDS describing the trend in electrical conductivity. 
The problem is that it seems to me that you can only set a source on a volume and not on a surface.
Is there a method for setting it on a surface?
In my case study I do not need the body of the spacecraft, so through a subtractive boolean I eliminate the volume of the spacecraft and therefore cannot apply the source as I would like.