The Vertical Landing Vehicles Library (VLVLib): a Modelica-based approach to high-fidelity simulation and verification of GNC systems for reusable rockets

Abstract

Vertical Landing (VL) Reusable Launch Vehicles (RLVs) must rely on performing and robust Guidance, Navigation and Control (GNC) algorithms, that are normally tested and verified in closed-loop high-fidelity simulators. This paper introduces the Vertical Landing Vehicles Library (VLVLib), a new Modelica-based tool for the advanced physical modeling and simulation of VL RLV dynamics. Modelica, an acausal open-source object-oriented modeling language, is exploited to produce a multibody vehicle model where multiple effects can be easily added, modified or switched in complexity. At the same time, it offers a modular modeling methodology to quickly adapt to vehicle changes or potential new features during the development process, with benefits in terms of project speed and costs. The way Modelica features are exploited is detailed throughout the paper. At its current development status, the library is able to account for, in more detail, four main effects relevant for the GNC robustness tests: (1) propellant slosh dynamics; (2) Thrust Vector Control (TVC) dynamics;(3) landing legs deployment disturbances; (4) touchdown dynamics. They are modeled and integrated so to allow an easy generation of a vehicle model that holds a prescribed fidelity level for each sub-system, and can be compiled and exported and harmoniously integrate other simulation environments (e.g. Simulink). The potential of VLVLib is demonstrated with representative examples as applied to CALLISTO reusable rocket demonstrative mission.