Although this mod is great, I have found some bugs:
Firstly, the engine heat damage hitbox for the
Merlin Vacuum engine is too tall for the height of the exhaust for this engine; I think it is even taller than the Titan Engine's Hitbox (Please reduce the height of the hitbox of Merlin Vacuum engine).
Secondly. there is no
functional Solid Rocket Booster separator during stage separation for the
Second Stage Tank (I think the SRB separator as separate parts would be a good idea):
Finally,
I believe that the
Piston that pushes up the upper stage should transfer some momentum to the upper stage (If you can't due to how SFS works it's ok, as long as the interstage SRB is implemented)
But overall, great job on the mod.