Advanced modeling and trajectory optimization framework for reusable launch vehicles

Launch vehicle dynamics modeling, simulation, and trajectory optimization within a single modeling tool is a challenging task due to the highly interconnected disciplines involved such as propulsion, aerodynamics, structures, mechanisms, and GNC, amongst others. In particular, changing environmental conditions and perturbations have to be considered throughout the ascent of expendable launch vehicles (ELV) as well as in the more complex scenario of the ascent and descent of reusable launch vehicles (RLV). Both the multidisciplinary design approach and the vehicle's mission definition can have considerable consequences for the overall modeling and optimization strategy. Therefore, a standardized modeling tool able to meet design requirements for a broad range of mission scenarios from vertical takeoff vertical landing (VTVL) to winged horizontal takeoff horizontal landing (HTHL) configurations is needed. Dedicated developments of multidisciplinary frameworks for launch vehicle modeling and preliminary design optimization have been presented in the relevant literature and also developed in the industry. It is common that the modeling of launch vehicles is performed by several independent, discipline-specific tools. With such an approach, only a limited amount of interactions of the involved disciplines with the overall system dynamics can be accounted for. Therefore, we propose a new multidisciplinary modeling framework considering all relevant effects on the system dynamics of launch vehicles using the object-oriented, equation-based, multi-physical, and acausal modeling language MODELICA. By capitalizing MODELICA's modeling capabilities, the framework enables the object-oriented and mathematically efficient modeling of subsystems and components related to most of the key disciplines of a launcher system. Another objective of this paper is to present a subset of the modeling framework for expendable and reusable launch vehicles regarding Functional Mock-up Units (FMU) and to demonstrate the advantages and capabilities of such a modeling approach within a combined trajectory optimization of the ascent and descent phases of launch vehicles. The modeling framework is shown for a standardized three degrees of freedom (3-DOF) model, covering the kinematics and dynamics formulation, environmental effects, aerodynamics, and propulsion models for system dynamics and subsequent trajectory simulations. The 3-DOF launch vehicle model is integrated as an FMU into the Trajectory Optimization Package trajOpt of DLR-SR's multi-objective optimization tool MOPS. The benefits of our modeling framework are discussed in terms of future rigid and flexible multibody modeling capabilities as well as GNC design and trade-off studies.

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