ATMOSPHERIC REENTRY CONTROL FOR LOW LIFT DRAG VEHICLES

The re-entry trajectory control of a low lift/drag (L/D) (L/D < 1 sphe re-cone shapes) re-entry vehicle is dealt with. The proposed control s trategy is based on the linear quadratic regulator (LQR) design techni que and on the variable structure systems (VSS) mathematical machinery . The LQR results from the solution of a differential Riccati equation based on the linearization of the equations of motion along a nominal re-entry trajectory. Its objective is the control of the vehicle in t he altitude-velocity plane (vertical plane). Terminal control is perfo rmed taking into account the range to go in the LQR and by pointing th e velocity vector towards the target via a VSS strategy. A single cont rol variable is considered, namely the bank angle, while the angle of attack is kept constant to assure the required L/D. The proposed techn ique is applied to the assured crew return vehicle re-entry mission in presence of off-nominal aerodynamic and atmospheric and initial condi tions. Extreme cases results and a Monte Carlo simulation are presente d to test the robustness of the proposed control strategy, which show that the target point is reached with an accuracy of 1 km in more than 50% of the cases and the required 7.6-km maximum error is not exceede d with probability larger than 0.9.