Commanded Drag Guidance for the Entry Phase of Winged Entry Vehicles

- A dual mode guidance algorithm developed for the high mach entry phase of winged entry vehicles is presented in this paper. The vehicle is guided to meet the desired downrange and cross range requirements. The vehicle has to be transferred from high energy state to low energy state. The excess energy is dissipated as heat and the rate of dissipation is limited by the constraints on heat flux, dynamic pressure and structural load on the vehicle. The guidance strategy is to plan and track commanded drag deceleration. All constraints are mapped on to the drag-energy plane. Nominal profiles of drag and drag derivatives are stored as a function of energy using cubic spline functions. The range covered during entry phase is a function of drag. A novel development is that to meet the downrange requirements, reference drag is analytically modulated online using a continuous, inverse parabolic multiplicative modulation factor. Modulated drag is tracked using dynamic inversion with desired dynamics. To meet the cross range requirements, bank reversal is commanded based on the azimuth error. Simulation studies carried out show that the guidance algorithm meets the requirements even for large dispersions in initial conditions.