Autonomous Guidance Method For Lift Transportation Vehicle

Now, lift transportation vehicle similar to the X-37B which uses multi stagesto orbit with vertical takeoff and horizontal landing is the focus research anddevelopment in many countries. Space applications have raised the demand onautonomy, security and reliability for them and vehicle must have the ability toland safely to the designated area when there are faults or the flight conditions arechanged, which requires strong robustness and adaptability of vehicle’s guidance.This paper takes the lift transportation vehicle X-37B as the research backgroundand focuses on its autonomous guidance method for ascent and entry.Currently, open loop is used in the ascent guidance of lift transportationvehicle while inside the atmosphere, and another optimal trajectory has to bereplaned for any change in mission and system parameters. The key ofendoatmospheric guidance for the vehicle is to solve the problem of rapidgeneration of the endoatmospheric optimal trajectory. Existing direct method issensitive to initial value and its solution speed is slow, indirect method has fastsolution speed, high accuracy, but it is more difficult to deal with the processconstraints. In this paper, the indirect method is adopted to get ascent optimaltrajectory without process constraints using semi-analytical expressions ofendoatmospheric ascent optimal trajectory without process constraints, and adouble iterative algorithm is constructed, atmospheric influence is introducedgradually with homotopy density. Then the direct method is adopted to get optimalascent trajectory with process constraints using the trajectory without processconstraints as initial value.Perturbation guidance method based on normal trajectory has low accuracyand poor autonomy. Traditional iterative guidance is based on a small yaw angleand is not applicable to large yaw angle mission. Three autonomous iterativeguidance methods with different iterative models and suitable for large attitudeangles are proposed in this paper. Their analytic forms of the Jacobian matrix arederived. Comparative analysis between three methods and precision analysis foriterative guidance method with the inner update iterative model for large yawangle mission are performed, and simulations results show that the iterativeguidance method with the inner update iterative model is suitable for large attitudeangles and high precision exoatmospheric autonomous guidance for lifttransportation vehicle.Entry trajectory optimization analysis is the basis for entry guidance design.Sequential gradient-restoration algorithm is an indirect method for solving optimal control problem. The algorithm uses the uniform optimality conditions, whichreduces complexity of process for deriving adjoint differential equations andtransversality conditions and is suitable to be implemented in a modular way, andthe algorithm is not sensitive to initial values. This paper studies applications ofthe algorithm in entry trajectory optimization of lift transportation vehicle.Transformation process of control constraints and trajectory constraints ispresented. Considering the strong nonlinearity of entry dynamics, state integral isintroduced and update method of the algorithm is improved. Practicability ofimproved algorithm for quasi-smooth trajectory optimization is comparativelyanalyzed. Simulation results demonstrate that the improved algorithm is betterthan the original one in convergence speed and optimization results and can obtaina quasi-smooth trajectory of lift transportation vehicle which satisfies trajectoryconstraints and boundary conditions in a short period of time. Coveragecapabilities of quasi-smooth trajectory and skip trajectory for the vehicle areanalyzed. The simulations results show that the maximum downrange of skiptrajectory can be increased significantly, and the normal load, dynamic pressure,heat rate of skip trajectory are overall smaller than quasi-smooth trajectory, which,as a result, has lower requirements on structural strength and thermal protectionsystem. However the skip trajectory has higher requirements on control system, aswell as stability and aerodynamic characteristics of vehicle.Taking drag acceleration profile as the design basis to study autonomousentry guidance method for lift transportation vehicle is most prospective andpractical in engineering application value, and trajectory planning method is themajor problem needed to be solved. A new autonomous entry guidance methoddesigned based on drag acceleration versus energy profile is proposed. The dragacceleration versus energy profile is obtained using interpolation between upperand lower boundary of entry corridor, and bank angle uses two reverse mode, andthe requirements of range and terminal heading can be met by adjustinginterpolation factor and reversal point of the bank angle considering longitudinaland lateral movement in trajectory planning, and the planning method only need tointegral lateral motion model.