Comparative study of different methods used to stabilize lateral-directional dynamics of a typical reusable launch vehicle

Initial entry phase of all the winged body vehicles, encounter ballistic flight in the absence of aerodynamic forces in rare atmosphere at high re-entry altitude. Lifting vehicles enter the atmosphere at high angle of attack to increase the drag to reduce the orbital velocity and to increase the lift to balance the weight for equilibrium glide flight. Ineffectiveness of rudder makes the control law design very challenging due to shadow effect of fuselage at high angle of attack. Problem gets further magnified due to increase in dynamic pressure with the reduction in altitude and increase in density. This paper presents a comparative study of three methods to stabilize the highly coupled lateral-directional dynamics of a typical reusable launch vehicle at the time of re-entry. This study clearly brings out the point that just placing the eigenvalues and eigenvectors at the desired locations is not sufficient to ensure good robustness of a closed-loop system. It is seen that the intuitive physics based design methodologies result in simpler and better control solutions.