So i was trying to send a whole sea dragon to orbit to prove a point but i realized that the interstages are just too weak
i came with a craft to show this bug, just put a lot of big fuel tanks (arround 200 kilotons) on top of an interstage.
my ideas on how to adress this bug are several
-slider that ajust interstage strength, changing it's price and mass
-xml value that when true makes them indestructible
-xml value to anust strength