If some components will be too advanced, it shouldn't be able to add it into some old aircraft or only with any other component (which allows you to connect old technology with advanced) I think that researches of components (jets) should look like this : Standard Jets (you can buy it) -> Improved Jets (better power, more weight) -> "Minimalized" Improved Jets (less space, less weight, better power) -> "Superior Jets" (the same space, the same weight, the best power in standard jets system). After these researches you should be able to connect two jets together - double jets etc.