I never use math in KSP, either. It seems like things like delta V change depending on the specific spacecraft you're building, and I may be wrong, but it seems like the game doesn't tell you the delta V of your spacecraft. So, amount of fuel and which engines to use seem to be just winging it, maybe with some educated guesses based on previous experience thrown in. I have two "sins" to confess, although I have some justification for them in my mind. I get the basic concepts for how orbits and transfer burns work, but I use MechJeb for everything. I can go to the Mun with just SAS (and have done it) but I figure that NASA has flight computers and automated modes for various stages of flight. Yes, they know how to do things if the computers die (Apollo 13), but they have the computers. Second sin: I use Impossible Innovations deuterium tanks. I'm not sure, but it kind of feels like it could be a sin. Slap the largest deuterium tank on there, and you can get anywhere and back....twice.