The Soyuz-style tapered side booster tank (FL-C1000) has an error in its config file.  The stock Size1p5_Tank_05.cfg (in GameData/SquadExpansion/Making History/FuelTank) contains the lines: CoMOffset = 0.2, -1.0, 0.0 CoPOffset = 0.25, 0.5, 0.0 Presumably to put the center of mass and (I assume) center of pressure away from whatever the model origin is.  The result is this: And when attached Soyuz-style to a rocket and staged away, the behavior is as follows: I wish I c