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Purpose -The purpose of this paper is to study the closed-loop guidance algorithm for launch vehicles in an atmospheric ascent phase and present a numerical trajectory reconstruction algorithm to satisfy the real-time requirement of generating the guidance commands. Design/methodology/approach -An optimal control model for an atmospheric ascent guidance system is established directly; following that, the detailed process for necessary conditions of the optimal control problem is re-derived based on the calculus of variations. As a result, the trajectory optimization problem can be reduced to a root-finding problem of algebraic equations based on the finite element method (FEM). To obtain an accurate solution, the Newton method is introduced to solve the roots in a guidance update cycle. Findings -The presented approach can accurately and efficiently solve the trajectory optimization problems. A moderate number of unknowns can yield a good optimal solution, which is well suited for the open-loop guidance. To meet the requirements of the rapidity and accuracy for the close-loop guidance, the fewer number of unknowns is artificially chosen to reduce the calculation time, and the on-board trajectory planning strategy can increase the precision of the optimal solution along with the decrease of time-to-go. Practical implications -The closed-loop guidance algorithm based on an FEM can be found in this paper, which can solve the optimal ascent guidance problems for launch vehicles in the atmospheric flight phase rapidly, accurately and efficiently. Originality/value -This paper re-derives the necessary conditions of the optimal solution in a different way compared to the previous work, and the closed-loop guidance algorithm combined with the FEM is also a new thought for the optimal atmospheric ascent guidance problems.

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Summary Motivated by new aerospace applications, the condition of minimum‐energy achieving high accuracy of both terminal orbital injection and attitude angle is required for better navigation and observation. The stability of guidance command also needs to be improved considering the control system. In this article, an optimal guidance algorithm with an advanced numerical method of spacecrafts is proposed. In order to ensure the continuity of the attitude angle and its terminal constraints, the thrust vector is treated as the state variable in the optimal control problems, and the rate of attitude angle change is regarded as the control variable. Then the optimal ascent problem of spacecrafts based on a nondimensional dynamical model is derived in detail, including performance index considering energy consumption, optimal conditions, and terminal conditions. To improve the computational efficiency of optimal ascent problems, a numerical method and a solution strategy are proposed. Simulation results show that the terminal attitude angle error of proposed method is much less than that of the traditional guidance method, and the continuity and stability of the guidance command is also better, which demonstrates the high accuracy and strong adaptability of the guidance algorithm developed in this article.