Physical Modeling and Simulation of Reusable Rockets for GNC Verification and Validation

Reusable rockets must rely on well-designed Guidance, Navigation and Control (GNC) algorithms. Because they are tested and verified in closed-loop, high-fidelity simulators, emphasizing the strategy to achieve such advanced models is of paramount importance. A wide spectrum of complex dynamic behaviors and their cross-couplings must be captured to achieve sufficiently representative simulations, hence a better assessment of the GNC performance and robustness. This paper focuses on of the main aspects related to the physical (acausal) modeling of reusable rockets, and the integration of these models into a suitable simulation framework oriented towards GNC Validation and Verification (V&V). Firstly, the modeling challenges and the need for physical multibody models are explained. Then, the Vertical Landing Vehicles Library (VLVLib), a Modelica-based library for the physical modeling and simulation of reusable rocket dynamics, is introduced. The VLVLib is built on specific principles that enable quick adaptations to vehicle changes and the introduction of new features during the design process, thereby enhancing project efficiency and reducing costs. Throughout the paper, we explain how these features allow for the rapid development of complex vehicle simulation models by adjusting the selected dynamic effects or changing their fidelity levels. Since the GNC algorithms are normally tested in Simulink®, we show how simulation models with a desired fidelity level can be developed, embedded and simulated within the Simulink® environment. Secondly, this work details the modeling aspects of four relevant vehicle dynamics: propellant sloshing, Thrust Vector Control (TVC), landing legs deployment and touchdown. The CALLISTO reusable rocket is taken as study case: representative simulation results are shown and analyzed to highlight the impact of the higher-fidelity models in comparison with a rigid-body model assumption

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

Planetary Defense Ground Zero: MASCOT's View on the Rocks - an Update between First Images and Sample Return

At 01:57:20 UTC on October 3rd, 2018, after 3½ years of cruise aboard the JAXA spacecraft HAYABUSA2 and about 3 months in the vicinity of its target, the MASCOT lander was separated successfully by from an altitude of 41 m. After a free-fall of only ~5m51s MASCOT made first contact with C-type near-Earth and potentially hazardous asteroid (162173) Ryugu, by hitting a big boulder. MASCOT then bounced for ~11m3s, in the process already gathering valuable information on mechanical properties of the surface before it came to rest. It was able to perform science measurements at 3 different locations on the surface of Ryugu and took many images of its spectacular pitch-black landscape. MASCOT’s payload suite was designed to investigate the fine-scale structure, multispectral reflectance, thermal characteristics and magnetic properties of the surface. Somewhat unexpectedly, MASCOT encountered very rugged terrain littered with large surface boulders. Observing in-situ, it confirmed the absence of fine particles and dust as already implied by the remote sensing instruments aboard the HAYABUSA2 spacecraft. After some 17h of operations, MASCOT‘s mission ended with the last communication contact as it followed Ryugu’s rotation beyond the horizon as seen from HAYABUSA2. Soon after, its primary battery was depleted. We present a broad overview of the recent scientific results of the MASCOT mission from separation through descent, landing and in-situ investigations on Ryugu until the end of its operation and relate them to the needs of planetary defense interactions with asteroids. We also recall the agile, responsive and sometimes serendipitous creation of MASCOT, the two-year rush of building and delivering it to JAXA’s HAYABUSA2 spacecraft in time for launch, and the four years of in-flight operations and on-ground testing to make the most of the brief on-surface mission

Navigation System for Reusable Launch Vehicle

PHOENIX is a downscaled experimental vehicle to demonstrate automatic landing capabilities of future Reusable Launch Vehicles (RLVs). PHOENIX has flown in May 2004 at NEAT (North European Aerospace Test range) in Vidsel, Sweden. As the shape of the vehicle has been designed for re-entry, the dynamics are very high and almost unstable. This requires a fast and precise GNC system. This paper describes the navigation system and the navigation filter of PHOENIX. The system is introduced and the high requirements are shown. The sensor configuration to fulfill the requirements is presented. Special focus is laid on the development and the structure of the navigation filter. The flight results of PHOENIX are shown and the performance of the navigation system is analyzed, which includes sensor failure scenarios

On-Ground Path Planning Experiments for Multiple Satellites

This paper presents a path-planning and collision avoidance method for satellite formations and swarms and its validation in simulation and in a testbed on ground. The pathplanning technique uses the definition of several behavior functions like Gather, Avoid and Dock, which form a virtual potential from which desired velocities are computed. A controller is used to achieve these velocities by commanding the onboard thrusters. This method has been validated in simulation and on the Test Environment for Applications of Multiple Spacecraft (TEAMS), a test facility for satellite formations and swarms based on air cushion vehicles. The vehicles are floating on two granite tables with a total experiment area of 5m x 4m. Each vehicle has a thruster system and its own onboard computer on which the presented path-planning algorithm is running together with a Kalman-Filter for state estimation, an attitude control and a thruster actuation algorithm. Experiments performed in simulation and on the testbed include formation acquisition and reconfiguration as well as collision avoidance. Experiment results are presented and show the performance and the robustness of the implemented guidance algorithm, as well as the test readiness of the TEAMS facility

Testbed for on-orbit servicing and formation flying dynamics emulation

The testbed for on-orbit servicing and formation flying dynamics emulation is a facility to emulate the force and momentum-free dynamics of multi-spacecraft missions on ground. The facility consists of a large granite table and several air cushion vehicles that float on this table. The granite table has a size of 4m x 2.5m and a thickness of 0.6m with a very high evenness. The air-cushion vehicles consist of two parts: a lower translation platform and an upper attitude platform. Both platforms are connected by a spherical air bearing. The translation platform carries the flat air bearings which enable the vehicle to float on the table, high pressure air tanks to support the flat and the spherical air bearings and a vertical linear actuator. With this actuator it is possible to adjust the altitude of the spherical air bearing and thereby the altitude of the attitude platform. All components needed for a control loop to actuate the air cushion vehicles are mounted on the attitude platform. These are 12 proportional cold gas thrusters with high pressure air tanks, 3 reaction wheels, an onboard computer with real-time operating system and Wireless LAN for communications and software upload, an inertial measurement unit (IMU) and a sensor for an infrared tracking system. In addition the attitude platform carries a power system with batteries to support all components with electrical energy and a balancing system to move the center of mass in the center of the spherical air bearing. Due to the linear air bearing the system can emulate 2 translational degrees of freedom. The third translational degree of freedom (parallel to gravity vector) can be actuated by the vertical linear actuator. As the attitude platform can tilt and rotate, 3 rotational degrees of freedom are emulated. So there is a total of 5+1 degrees of freedom that can be emulated by the testbed. One purpose of this testbed is the testing of control algorithms for multi-spacecraft missions. It can be used for precise formation control as well as path-planning and formation acquisition. In addition to the usage of real hardware as in Hardware-in-the-Loop simulations, the force and momentum-free dynamics are not simulated but are real (emulation). This is also a foundation for the second task of the testbed: the emulation of contact dynamics for Rendezvous and Docking missions. Models of docking adapters can be mounted on the attitude platform of the air-cushion vehicles and docking maneuvers with real contact dynamics can be tested on ground. A further purpose of the testbed is the qualification of dedicated relative navigation sensors and inter-spacecraft communication systems. These components can also be mounted on the attitude platform and tested in an agile environment. This paper describes the design and configuration of the testbed for on-orbit servicing and formation flying dynamics emulation. Results showing the quality of the force and momentum-free environment are presented. First control, navigation and thruster actuation algorithms have been developed and their design and the result of their usage within the testbed are also shown. Finally possible scenarios for the usage of this testbed are presented and an outlook on further developments and improvements is given

EXPERIMENTAL IMPLEMENTATION OF SDRE METHOD FOR AUTONOMOUS RENDEZVOUS AND DOCKING MANEUVERING

In this paper a nonlinear controller technique, referred to State-Dependent Riccati Equation (SDRE), is exploited to handle relative position tracking and attitude synchronization problem involving in docking manoeuvring operations between two Earth orbiting satellites. More specifically a testbed developed by DLR-Institute of Space Systems is used for testing the proposed control algorithm. The experimental results show the effectiveness of SDRE controller for proximity operations problem and its feasibility for real-time implementation on the hardware