It isn't a game about throwing stuff together, at least for me. Since physics amd aerodynamics are important factors in ship design, one's craft may not work. Thrust misalignment, drag, misplaced center of mass and/or pressure... all these things make for an engineering challenge. Which is why the game is fun for me. Getting, as you say, that last 10% is the most satisfying thing I've come across in a game.