I've been playing this game for several years and I think I have a pretty good grip now on orbital mechanics, i.e. I can predict how the shape of any orbit is going to change with a burn in any direction, and vice versa. However I still struggle with predicting what the result of gravitational assists in encounters is going to be. Recently this has led to situations like this http://forum.kerbalspaceprogram.com/index.php?/topic/132476-help-get-jeb-and-bill-home/ I have given it some