Atmospheric Reentry Control for Low Lift/Drag Vehicles

The re-entry trajectory control of a low lift/drag (L/D) (L/D<1 sphere-cone shapes) re-entry vehicle is dealt with. The proposed control strategy is based on the linear quadratic regulator (LQR) design technique 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 the altitude-velocity plane (vertical plane). Terminal control is performed taking into account the range to go in the LQR and by pointing the velocity vector towards the target via a VSS strategy. A single control variable is considered, namely the bank angle, while the angle of attack is kept constant to assure the required L/D. The proposed technique is applied to the assured crew return vehicle re-entry mission in presence of off-nominal aerodynamic and atmospheric and initial conditions. Extreme cases results and a Monte Carlo simulation are presented 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 exceeded with probability larger than 0.9.