Skip Entry Trajectory Planning And Guidance For Lunar-Return Vehicle

Lunar exploration is always a focus of aerospace reseach. For unmanned lunar exploration missions, sampling and returning is a very important technique. For manned missions, point return is one of the key technology which is essential for the safety of astronauts. For entry vehicles with relatively low lift-to-drag ratio(L/D), a known strategy for achieving long downrange and decreasing aerodynamic load peak is to allow the vehicle to skip out of the atmosphere. This dissertation studies the reentry corridor problem, the reachable sets problem, the onboard trajectory planning method and the reentry guidance. The main results achieved in this dissertation are summarized as follows:The characteristics of skip entry trajectory are analyzed. 1) Characteristic of path constraints and their relationship in each phase of skip entry trajectory are analyzed. The upper limit expressions of heating rate and dynamic pressure are derived when load does not violate its constraint. These may reduce the number of path constraints for skip entry trajectory optimization. 2) An optimization model of skip entry corridor problem considering multiple constraints is established, and Multi Start algorithm is used to solve the problem. 3) An optimization model of reachable sites(RS) problem for skip entry is established, and Gauss Pseudospectral Method(GPM) is used to solve the problem. Then, the influences on RS by initial conditions are analyzed.Three different methods for solving on-board skip entry trajectory planning problem are proposed. 1) The reference vertical L/D in the first entry phase is obtained using an iteration method, based on analytical solution of the motion equation using matched asymptotic expansion method. Then the reference trajectory is generated by augmenting the reference vertical L/D according to the error between the real state and the state calculated using analytical solution. The reference trajectory is generated using Apollo final phase guidance. 2) Assuming the reference drag profile as a polynomial expression of the velocity in the first entry phase, linear Equtions of polynomial coefficients are established using the initial conditions and exit condition which are guessed for the first time. Drag profile in the first entry phase is obtained by solving the equations, and the reference trajectory in the first entry phase is generated by tracking the drag profile, and the reference trajectory in the final phase is generated using Apollo final phase guidance. 3) The trajectory planning problem is converted to optimal control problem and solved by State-Dependent Riccati Equation(SDRE) method. Because the state-dependent matrix is hard to creat, an online method is used to compute it in each step. An approximate analytical approach is used to solve the SDRE. The obtained suboptimal trajectory is corrected using feedback linearization(FBL) strategy in the first entry phase, and is corrected using Apollo final phase guidance in the final phase.Skip entry guidance using a reference trajectory is researched. 1) The frame of skip entry guidance using a reference trajectory is established. Drag-vs-velocity of drag-vs-energy profile is tracked in the first entry phase, and the Apollo final phase guidance is used in the final phase. 2) The FBL algorithm for tracking drag-vs-velocity in the first entry phase is researched and proposed. The drag and drag rate are both tracked. The error dynamics is asymptotic stabe. The control gains used in high velocity phase and the low velocity phase are different due to the curvature of the drag profile. 3) The nonlinear predictive control algorithm for tracking drag-vs-energy in the first entry phase is researched and proposed. The predicted tracking error is expressed as a truncated Taylor series dependent on the control. The control is then selected to minimize a cost function related to the predicted error. The executed command is obtained by correcting the optimal control according to the trajectory length error.Skip entry guidance using numeric predictor-corrector is researched. 1) For trajectory prediction, the magnitude of the bank angle in the skip phase is parameterized as a linear function of the range-to-go until a specified threshold, a constant bank angle is used below the threshold. The secant method is used to correct the prediction error. The secant iteration stops when the miss distance to the parachute deployment condition is achieved. 2) The Apollo-like lateral guidance is improved using an automated targeting bias strategy. The coordinates of the bias landing site are used for targeting in the first entry phase to ensure that the crossrange at the point where the final phase begins is inside the “funnel”. 3) Two predictive load relief methods are researched and proposed, which are used respectively outside and inside the guidance cycle. The former method dose not condider load constraint in trajectory planning, it predicts the load peak using analytical formula and augments the bank command according to the prediction results. The latter method considers load constraint in trajectory planning, augmentment on bank command is made to avoiding infeasible trajectory.The skip entry trajectory and guidance are researched in this desertation. The research results have some application value for future skip entry mission overall design.