1,721,164 research outputs found
A simple procedure to improve the pressure evaluation in hydrodynamic context using the SPH
No full textIn literature, it is well know that the Smoothed Particle Hydrodynamics method can be affected by numerical noise on the pressure field when dealing with liquids. This can be highly dangerous when an SPH code is dynamically coupled with a structural solver. In this work a simple procedure is proposed to improve the computation of the pressure distribution in the dynamics of liquids. Such a procedure is based on the use of a density diffusion term in the equation for the mass conservation. This diffusion is a pure numerical effect, similar to the well known artificial viscosity originally proposed in SPH method to smooth out the shock discontinuities. As the artificial viscosity, the density diffusion used here goes to zero increasing the number of particles recovering consistency and convergence of the final numerical scheme adopted. Different artificial density diffusion formulas have been studied, paying attention to prevent unphysical changes of the flows. To show the improvements of the new scheme proposed here, a suitable set of examples, for which reference solutions or experimental data are available, has been tested
Integrated vibration suppression attitude control for flexible spacecrafts with internal liquid sloshing
Full text linkVibration suppression during attitude control is a fundamental research topic whenever control of the rotational motion of a spacecraft with flexible appendages and internal liquid sloshing is of interest. The proposed method is based on an attitude control system with centralized sensors and actuators, without the usage of collocated devices for vibration management. In this way, it is possible to develop and implement a computationally efficient real-time control system that is suitable for any kind of spacecraft, even with advanced control capabilities. An integrated vibration suppression attitude control is designed and analyzed, exploiting also a numerical simulation verification procedure based on validated code. The developed attitude control system applies two fundamental control schemes: classical proportional-derivative (PD) control, with nonadaptive band-stop filters, and wave-based control. The proposed wave-based control implementation allows managing three-dimensional attitude dynamics in steady state pointing, without cross-coupling between the separate body axes. To overcome this limitation, the paper presents the integration of the wave-based control with the filtered PD control scheme, allowing us to have a complete three-dimensional real-time MIMO controller, with vibration suppression capabilities and robustness to system uncertainties. The paper also presents the development of an accurate dynamical model of a generic flexible spacecraft with internal liquid sloshing based on a multibody formulation
Enhancing Technologies and Operations for Service Transportation in Cislunar Environment
Full text linkFuture space exploration missions are intended to exploit cislunar environment as effective outpost to advance technology readiness in view of human presence beyond Earth. The forthcoming space projects entail modular large space infrastructures to be available in non-Keplerian orbits, in the Moon vicinity, to run manned and robotic activities. The realization of such a complex space system will require enhanced technologies and operations for the service transportation vehicles, which will be involved as cargo and Earth-Moon transfer spacecrafts. The paper discusses the peculiarities and the novelties of service missions in Cislunar space, compared to analogous service missions in Low-Earth Orbits (LEO). In fact, the operational orbit in the new space scenario is going to be a Near Rectilinear Halo Orbit (NRHO), which is dynamically distinct from any existing Keplerian trajectory. Thus, figures of merit of the future service missions are much different from the ones that are known from International Space Station (ISS) heritage. The discussion will be particularly focused on the proximity phases of service transportation missions, including the phasing with the target s staging orbit. Time and DV budgets will be presented and compared to those of LEO missions, such as Soyuz, ATV and Dragon. The terminal rendezvous phase will be described, highlighting the sequence of required operations to approach the Cislunar space station. A section of the paper will be analysing the undocking phase, with the subsequent departure from the target and its NRHO. This sequence of proximity operations is peculiar and somehow different from the approaching one, especially considering the natural Cislunar dynamics that may be leveraged to support the undocking and departure operations. The discussion is also considering the enabling technologies to support the proposed Cislunar operations. The service transportation system architecture and design are considered, discussing some preliminary requirements for the GNC and the Propulsion subsystems
Dynamics and Control of Modular and Extended Space Structures in Cislunar Environment
No full textProposed future space programmes, which, among others, include a space station in lunar vicinity, pose some interesting research problems in the field of non-Keplerian dynamics. This paper investigates the orbit-attitude dynamics and the control of rotational motion of an extended space structure in cislunar environment. The paper presents a fully coupled model for orbit-attitude dynamics, which is based on a Circular Restricted Three-Body Problem formulation. The equations of motion take also into account the most relevant perturbing phenomena, such as the Solar Radiation Pressure (SRP), the fourth-body (Sun) gravity and the variation in the gravitational attraction due to the finite dimension of the large space structure. Preliminary results exploiting efficient control methods are presented. Single and dual-spin stabilisation are compared and the results are carefully analysed to highlight a control strategy that is less resource consuming. The space of orbit-attitude solutions is studied to highlight possible stable conditions that may be exploited to host the cislunar station with minimum control effort. The outcomes of the research presented in this paper are intended to highlight drivers for the lunar outpost design and station-keeping cost minimisation. Furthermore, a case study for a large space structure in selected non-Keplerian orbits around Earth-Moon collinear Lagrangian points is discussed to point out some relevant conclusions for the potential implementation of such a mission
Autonomous Fault Management in Attitude Determination and Control Subsystems: Hardware and Processor in the Loop Testing
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