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

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

Control System Design for the ALINA Lunar Lander

This paper presents the control system developed by the German Aerospace Center (DLR) for the ALINA lunar lander. The control system is part of the overall Guidance, Navigation and Control (GNC) subsystem. ALINA, developed by Planetary Transportation Systems GmbH (PTS), is a spacecraft able to semi-autonomously per-form a complete Earth to lunar surface mission and to deliver up to 200 kg of payload. The control system has successfully passed the project’s Preliminary Design Review (PDR). The vehicle, the mission phases and objectives as well as the control requirements are introduced, and the adopted control solutions presented. Of major importance is the spacecraft’s propulsion configuration: it comprises a cluster of throttlable and non-throttlable main engines, as well as attitude control thrusters, fed by several propellant tanks. Therefore, specific challenges arise, such as control allocation and propellant sloshing. The synthesis of the controllers employs optimal control techniques to design the control laws for the different GNC tasks, including detumbling, single-axis pointing, three-axis pointing, maneuver execution, and powered descent, Hazard Detection and Avoidance (HDA) and landing. Attention has been given to the translational and rotational dynamics couplings, as the individual engines do not provide thrust vector control capabilities. The control allocation problem is tackled by minimizing a cost function that takes into account the desired forces and torques, as well as the fuel consumption; the online solution is found by transcribing the problem into a linear programming form and solving it using the SIMPLEX method. Propellant sloshing can disturb thrust vector pointing and can potentially generate critical deviations from the nominal trajectory and instabilities, particularly, as initially more than70 %of the spacecraft mass consists of fuel. Therefore, the plant model includes a representation of propellant sloshing dynamics using mechanical analogies. This has been achieved using the multi-physics object-oriented modeling language Modelica, better suited for large multibody representations; the resulting implementation is consequently embedded within a high-fidelity simulation framework. For the verification process, the control system has been included in the latter. Representative Monte-Carlo campaigns were conducted, with a dedicated focus on the powered descent, HDA and landing. The analysis of the control performance throughout each mission phase shows a good overall performance, well complying with all applicable requirements

Sensor fault detection and isolation for electro-mechanical actuators in a reusable launch vehicle TVC system

This paper introduces a model-based Fault Detection and Isolation (FDI) approach for a Reusable Launch Vehicle (RLV) Thrust Vector Control (TVC) system operated by Electro-Mechanical Actuators (EMAs). The focus is on the sensors required for the EMA embedded control system to track the on-board computer control commands. The nullspace FDI method is considered and applied to detect and isolate additive faults affecting the mentioned sensors. A detailed formulation of the problem and the EMA-based TVC system modelling for FDI synthesis is provided, including the mechanical load exerted by the rocket nozzle. The FDI synthesis framework is introduced and the application of the nullspace-based strategy is described, including considerations about isolability of the faults. Vehicle-induced loads can potentially disrupt the fault detection process, therefore they are included in the problem formulation to achieve decoupling from the residual generator output and not incur into false alarms. The generator performance is then assessed in fault-free and faulty scenarios using a high-fidelity TVC physical model, and successively benchmarked at the example of an RLV mission scenario

Robust Control for Reusable Rockets via Structured H-infinity Synthesis

This paper discusses the problem of synthesizing robust controllers for reusable rocketsduring the aerodynamic descent phase. Emphasis is given to a well-established subset ofmethods, specifically robust control techniques based on theH∞concept. A thoroughdescription of how this family of methods can be used for the descent phase of reusablerockets is provided, together with a comparison of the full- and structured-version ofH∞methods. The methodology, the problem faced and the performance that can be obtainedare discussed. Some results are shown for CALLISTO, a reusable rocket demonstratorjointly developed by DLR, JAXA, and CNE

Vector Field-based Guidance Development for Launch Vehicle Re-entry via Actuated Parafoil

In this paper, a launch vehicles re-entry strategy using an actuated parafoil is analyzed. In recent years, this concept is gaining new momentum: it offers a lightweight and cost-effective control solution for autonomous landing of reusable rockets to specific ground or sea coordinates, as well as for mid-air capturing. This landing maneuver requires appropriate modeling together with suitable guidance and control strategies. This work expands upon the following aspects: (1) the development of suitable models for control synthesis and verification; (2) the design of heading control system; (3) the application of a path-following guidance law capable of steering the payload (i.e. the launch vehicle) to the prescribed end-of-mission point. Three models of increasing complexity are proposed based on different assumptions and the dynamics are compared in an ad-hoc simulation environment. MATLAB-Simulink is employed to design two versions of a 6 Degrees Of Freedom (DOF) model accounting for distinct aerodynamic effects. On the other hand, the multi-physics object-oriented language Modelica is used to develop a higher-fidelity 9DOF dynamic model of the system. The latter is then compiled and embedded within MATLAB-Simulink. The same environment allows the implementation of the designed Guidance and Control (G&C) algorithms. The G&C architecture comprises both low-level control loops, regulating course andyaw angles by means of differential steering commands onto the canopy strings, and a guidance layer where the VF path-following is employed. VF methods have already shown remarkable results for fixed-wing unmanned vehicles due to the lower steady-state errors as compared to other approaches, while retaining the potential for real-time implementation. With this work, the method is extended to the application of a launcher recovery. The results of the simulations are investigated, highlighting overall satisfactory performance even in presence of wind disturbances

Advanced GNC-oriented modeling and simulation of Vertical Landing vehicles with fuel slosh dynamics

This paper introduces a flexible framework to enable high-fidelity simulations of the fuel slosh dynamics for verification of the Guidance, Navigation and Control (GNC) algorithms. The sloshing phenomenon must be tackled during the control system design phase and the GNC software run and validated within appropriate simulation frameworks. Equivalent mechanical models can well approximate sloshing under specific assumptions, allowing the derivation of the multibody vehicle’s equations of motion; however, while suitable for analysis and control synthesis purposes, this approach presents several limitations for implementation as a simulation model. In this work the multibody plant embedding slosh dynamics is modeled by means of the DLR’s Vertical Landing Vehicles Library (VLVLib) written using the object-oriented Modelica modeling language. The advantages of using Modelica are explored throughout the paper. Given that control synthesis, analyses and simulation campaigns are usually performed in the Matlab/Simulink environment, a core point of the proposed solution is to achieve a good synergy within the latter and the Modelica plant model with the least readaptation burden for the developer. The strategies to achieve a full integration and verification tool chain are illustrated. The potential of this approach is demonstrated via two different test cases: a lunar landing scenario of the ALINA spacecraft developed by PTS, and CALLISTO reusable rocket demonstrator

Guidance of fixed-wing UAVs without a priori knowledge of course dynamics and wind

The high maneuverability of fixed-wing unmanned aerial vehicles (UAVs) exposes these systems to several dynamical and parametric uncertainties, severely affecting the fidelity of modeling and causing limited guidance autonomy. This article shows enhanced autonomy via adaptation mechanisms embedded in the guidance law: a vector-field method is proposed that does not require a priori knowledge of the UAV course time constant, coupling effects, and wind amplitude/direction. Stability and performance are assessed using the Lyapunov theory. The method is tested on software-in-the loop and hardware-in-the-loop UAV platforms, showing that the proposed guidance law outperforms state-of-the-art guidance controllers and standard vector-field approaches in the presence of significant uncertainty

Addressing Unmodelled Path-Following Dynamics via Adaptive Vector Field: a UAV Test Case

The actual performance of model-based path-following methods for Unmanned Aerial Vehicles (UAVs) show considerable dependence on the wind knowledge and on the fidelity of the dynamic model used for design. This work analyzes and demonstrates the performance of an adaptive Vector Field (VF) control law which can compensate for the lack of knowledge of the wind vector and for the presence of unmodelled course angle dynamics. Extensive simulation experiments, calibrated on a commercial fixed-wing UAV and proven to be realistic, show that the new VF method can better cope with uncertainties than its standard version. In fact, while the standard VF approach works perfectly for ideal first-order course angle dynamics (and perfect knowledge of the wind vector), its performance degrades in the presence of unknown wind or unmodelled course angle dynamics. On the other hand, the estimation mechanism of the proposed adaptive VF effectively compensates for wind uncertainty and unmodelled dynamics, sensibly reducing the path-following error as compared to the standard VF

Physical modeling and simulation of electro-mechanical actuator-based TVC systems for reusable launch vehicles

This paper presents the high-fidelity physical modeling of a Thrust Vector Control (TVC) system operated by Electro-Mechanical Actuators (EMAs) for Reusable Launch Vehicles (RLVs). In contrast to simplified, often linear, models, high-fidelity physical models enable a better assessment of the actual system performance and a deeper verification of the on-board software robustness against disturbances, unmodeled dynamics or degradation. This aspect is essential to enable the reusability concept as driven by long-term reliability requirements. Moreover, critical systems like Fault Detection, Isolation and Recovery (FDIR) logic, whose robustness and efficacy are often difficult to prove, can be thoroughly tested without requiring hardware experiments with eventual invasive modifications for fault injections. This work captures the whole dynamics of the EMA and TVC components, including the power drive electronics, the electrical motor, and the mechanical transmission for the EMA, as well as the mechanical properties of the engine nozzle acting as a load. The resulting differential algebraic equation system is modeled in the Modelica acausal object-oriented modeling language. An ad-hoc framework is implemented to guarantee flexibility and modularity, which allows models with different fidelity levels to be easily exchanged to achieve the simulation objectives and needed accuracy level. The impact of the different models on the closed-loop TVC dynamics is analyzed in both the frequency and time domain, and then benchmarked with a realistic RLV mission. The results show that the high-fidelity physical models provide a better understanding of the more complex effects governing the TVC dynamics and can, in turn, effectively improve its requirement definition process