Design of flight control systems for RLVs with structural flexibility: application to the CALLISTO vehicle

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Abstract

The analysis of structural flexibility in Launch Vehicles (LVs) and Reusable Launch Vehicles (RLVs) is a vital aspect during the design and operation of the Guidance, Navigation and Control (GNC) subsystem. This aspect becomes even more critical as vehicles are currently becoming larger and slender due to conflicting goals: 1) needing larger payloads deployed into orbit and, at the same time, 2) reducing losses due to shape-induced drag. During its flight across the atmosphere, several forces acting interact with the vehicle’s structure i.e., those produced by the actuators (Thrust Vector Control (TVC), Reaction Control System (RCS) and/or fins) or disturbances like the aerodynamic forces created by the body itself. Addressing this problem from a Guidance, Navigation and Control (GNC) perspective requires an understanding of structural mechanics. This requires the usage of mechanical equivalent models that capture the core dynamics of the problem. The parameters used by practitioners are typically extracted from more accurate, but computationally expensive, methods like Finite Element Method (FEM) analysis. The high-frequency response of the vehicle is decomposed as the sum of the harmonic response of n so-called modes. Each mode is characterized by a natural frequency ω_n and a modal shape Φ_i, which changes depending on the viewpoint (force application location or measurement unit location). The first and natural step is to look at the positioning of the frequencies compared to the desired rigid-body bandwidth. Furthermore, in the frequency domain, the bending modes are translated into resonances with gain amplification and phase shift, which in terms of the control systems means; correct gain and phase margins in the closed-loop must be guaranteed. In our investigations, we perform the analysis for a real 40 KN-class vehicle, a Reusable Launch Vehicle (RLV) with a gimbaling engine, flying a typical Return-to-Launch-Site (RTLS) scenario trajectory and varying mass. First, we present the mathematical procedure to consolidate the models used for simulation and control design. Secondly, we present the evolution of the frequencies and modal shapes across the trajectory for the atmospheric ascent phase of the vehicle. Consequently, we present problem formulation from a control perspective, the n modes are introduced into the state-state representation from our earlier investigations, and later employed for control synthesis. In the final part, the closed-loop behavior of the flight control system for the Cooperative Action Leading to Launcher Innovation for Stage Tossback Operation (CALLISTO) vehicle is validated against the influence of structural flexibility. The performance of an H-∞ synthesized rigid body controller from our earlier investigations, has been extended accordingly, and is validated. Monte Carlo (MC) simulations are run on the 6-Degrees of Freedom (DoF) simulator for the reference mission under nominal and uncertain conditions.

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