The concept of a "first object" is kind of annoying to deal with. I have a rover carrier with a few rovers on rockets. I want to check the center of mass/thrust to make sure they will fly straight when separated. Each rover has a Stayputnik on it that will allow it to be controlled independently. I can make it a subassembly but for some reason I can't use a subassembly to start with, nor can I switch the first object to something else, even if something else is capable. Is there anything I can do?