Modelling, Simulation, and Control of a Flexible Space Launch Vehicle

TL;DR

This paper models, simulates, and controls a flexible space launch vehicle, specifically Falcon 9, addressing the challenges of structural flexibility on stability and control through advanced simulation and classical control methods.

Contribution

It develops high-fidelity nonlinear and linearized models of a flexible SLV and designs robust attitude controllers with filters to mitigate flexible mode interactions.

Findings

Flexible modes significantly affect SLV stability.

Designed controllers effectively suppress flexible dynamics.

Monte-Carlo simulations demonstrate robustness of control strategies.

Abstract

Modern Space Launch Vehicles (SLVs), being slender in shape and due to the use of lightweight materials, are generally flexible in nature. This structural flexibility, when coupled with sensor and actuator dynamics, can adversely affect the control of SLV, which may lead to vehicle instability and, in the worst-case scenario, to structural failure. This work focuses on modelling and simulation of rigid and flexible dynamics of an SLV and its interactions with the control system. SpaceX's Falcon 9 has been selected for this study. The flexible modes are calculated using modal analysis in Ansys. High-fidelity nonlinear simulation is developed which incorporates the flexible modes and their interactions with rigid degrees of freedom. Moreover, linearized models are developed for flexible body dynamics, over the complete trajectory until the first stage's separation. Using classical control methods, attitude controllers, that keep the SLV on its desired trajectory, are developed, and multiple filters are designed to suppress the interactions of flexible dynamics. The designed controllers along with filters are implemented in the nonlinear simulation. Furthermore, to demonstrate the robustness of designed controllers, Monte-Carlo simulations are carried out and results are presented.