There seems to be a compatability issue with Principia. With (only) Principia and KerBalloon both installed simultaneously, you can still inflate and deflate a balloon, with the corresponding animation displaying correctly. However, once the balloon has inflated the vessel is momentarily lifted by a couple of centimeters, only to immediately descend back to the ground. It then oscillates between barely aloft and landed. Once deflated, the balloon settles down.  The issue occurs