Single segment approach and landing guidance and control for an unpowered reusable launch vehicle

Abstract

Various guidance schemes have been implemented in the past to guide an unpowered reusable launch vehicle safely to the runway satisfying the constraints on touchdown. All these schemes rely on a minimum of two or more segments to completely determine the landing reference geometry and generally involves several iterations with numeric integration for multiple segments to achieve continuity between the segments. In contrast, this paper proposes a path planning algorithm during approach and landing making use of a single segment reference path connecting the approach and landing interface to the desired touchdown point on the runway. The approach used in this paper is a novel attempt to use a single segment altitude profile for approach and landing based on a quintic polynomial that relates altitude with downrange. The main advantage is that a solution satisfying the initial as well as final boundary constraints on the vehicle states are attained at a lesser computational complexity. The nominal states obtained are compared with an existing three segment scheme designed based on an on-board trajectory planning algorithm. The three-segment scheme attains the desired touchdown specifications making use of steep glide slope computation and dynamic pressure matching using backward trajectory propagation while the proposed scheme achieves the objective using a forward propagation algorithm for the single segment. Closed loop control is implemented using both PID as well as Fuzzy Logic Control and the numerical results are compared. Simulations are carried out with wind, energy as well as drag variations to establish the robustness of the proposed single segment algorithm.