Analytic Approach and Landing Guidance through a Novel Time-Varying Sliding Mode Control Method

In this paper, a nonlinear guidance law is developed for the approach and landing (A&L) phase of a reusable launch vehicle (RLV). First, a novel time-varying sliding mode control (TVSMC) technique is introduced. The main feature of this technique is that it enables the system states to converge to zero at the desired finite time. Then, the developed technique is applied to the A&L guidance law design. The guidance law possesses the capability of generating trajectories online according to the current system states and the terminal constraints. Therefore, there is no need to design any reference trajectories in advance. In addition, the analytic solutions of the altitude and flight path angle can be obtained in advance by solving a first-order linear differential equation. These analytic solutions can also be used to estimate the boundary of the normal load factor of the RLV. The effectiveness of the proposed guidance strategy and the accuracy of the analytic solutions are demonstrated through numerical simulations.