"... though the tail surface provides stability in pitch and yaw, no purely aerodynamic means has been found to achieve roll stability, since the airflow remains symmetrical about the axis of rotation. The coupling between roll and yaw becomes more severe as vertical-tail size increases, and it has presented a multitude of problems to designers of high-speed aircraft. "The solution to the stability-and-control analysis is the development of an adequate mathematical model. But such an analysis also requires a mathematical model for the pilot..."
roll stability "... The X-15's maximum altitude was extended to 354 200 feet, but not until after much trial and error..."