Loading...
9 results
Search Results
Now showing 1 - 9 of 9
- Dynamics of a gyrostat satellite with the vector of gyrostatic moment tangent to the orbital planePublication . Morais, Renato; Santos, Luis; Silva, André; Melício, RuiIn this paper, a gyrostat satellite in a circular orbit with its gyrostatic moment tangent to the orbital plane and collinear with the orbital speed is studied regarding its equilibria, bifurcation of equilibria, and asymptotic stability conditions. In the general case, where any gyrostat angular momentum is aligned with any of the orbital coordinate frames, interesting results arose regarding its equilibria bifurcation regarding conditions near to the ones presented in this paper, namely equilibria regions outside their main regions near to the orbital plane tangent. For equilibria and bifurcation of equilibria, a symbolic-numerical method is used to obtain the polynomial equations in function of non-dimensional parameters whose roots are equivalent to the number of equilibria positions. For the asymptotic stability, the results are tested using the Lyapunov stability theory scheme.
- Gyrostat Dynamics on a Circular Orbit: General Case of Equilibria Bifurcation and Analytical ExpressionsPublication . Santos, Luís; Melício, Rui; Silva, A. R. R.This paper is on the study of the Dynamics of a Gyrostat Satellite on a Circular Orbit, and the achievement of the analytical expressions which describes the bifurcation of equilibria. The attitude control of modern spacecraft's is essential to fulfil its missions. Every time that is necessary to point a camera or a scientific instrument the space platform must be perfectly balanced and stabilized. When a space platform loses its ability to be balanced and stabilized, which might be due to several factors, loses also its ability to carry out its mission. One approach to achieve such conditions is using Gyrostat configuration.
- Modelling a Loop Heat Pipe as Heat Switch for Transient Application in Space SystemsPublication . Castanheira, João Pedro Conceição; Dias, Nicole G.; Melício, Rui; Gordo, Paulo; Silva, André; Pereira, Roger MichaelHeat switches are devices for controlling heat flow in various applications, such as electronic devices, cryogenic cooling systems, spacecraft, and rockets. These devices require non-linear transient thermal simulations, in which there is a lack of information. In this study, we introduce an innovative 1D thermo-hydraulic lumped parameter model to simulate loop heat pipes as heat switches by regulating the temperature difference between the evaporator and the compensation chamber. The developed thermo-hydraulic model uses the continuity, energy, and momentum equations to represent the behaviour of loop heat pipes as heat switches. The model also highlights the importance of some thermal conductance parameters and correction coefficients for accurately simulating the different operational states of a loop heat pipe. The simulations are conducted using the proposed 1D model, solved through the application of the Mathcad block function. The numerical model presented is successfully validated by comparing the temperatures of the evaporator and condenser inlet nodes with those of a referenced loop heat pipe from the literature. In conclusion, in this research, the mathematical modelling of loop heat pipes as heat switches is presented. This is achieved by incorporating correction coefficients with Boolean logic that results in non-linear transient simulations. The presented 1D thermo-hydraulic lumped parameter model serves as a valuable tool for thermal system design, particularly for systems with non-linear operational modes like sorption compressors. The graphical and nodal representation of this proposed 1D thermo-hydraulic model further enhances its utility in understanding and optimising loop heat pipes as heat switches across various thermal management scenarios.
- Housekeeping System for Suborbital Vehicles: VIRIATO Mock-Up Vehicle Integration and TestingPublication . Rodrigues, Geraldo; Arribas, Beltran; Melício, Rui; Gordo, Paulo; Valério, Duarte; Casaleiro, J.; Silva, AndréThe work presented in this paper regards the improvement of a housekeeping system for data acquisition of a suborbital vehicle (VIRIATO rocket or launcher). The specifications regarding the vehicle are presented and hardware is chosen accordingly, considering commercial off-the-shelf components. Mechanical and thermal simulations are performed regarding the designed system and a physical prototype is manufactured, assembled and programmed. Functional and field test results resorting to unmanned aerial vehicles, as well as the system’s integration within VIRIATO project’s mock-up vehicle, are presented. These tests demonstrate the viability of this system as an independent data acquisition system, and simulation results show that commercial off-the-shelf components have the capability of surviving expected launch environments.
- Satellite Star Tracker Breadboard with Space Debris Detection Capability for LEOPublication . Filho, Joel; Gordo, Paulo; Peixinho, Nuno; Gafeira, R.; Melício, Rui; Silva, A. R. R.This paper evaluates the possibility of having a star tracker device running space debris algorithms. A simple star tracker breadboard was developed to evaluate the possibility of having a device running both stellar identification and space debris algorithms. The breadboard was built with commercial off-the-shelf components, representing the current star tracker resolution and field of view. A star tracker device and space debris algorithms were implemented and tested, respectively: Tetra and ASTRiDE. The device concept was tested by taking pictures of the night sky with satellite streaks. Seeking to overcome such limitations, a dual-purpose star tracker with stars detection and optical debris detection capability is proposed. Star trackers are usually used in satellites for attitude determination and therefore have a vast potential to be a major tool for space debris detection. The rapid increase of space debris poses a risk to space activities, so it is vital to detect it. Ground-based radar and optical telescope techniques used for debris detection are limited by a size threshold, detecting only a tiny amount of the total, reason why evaluating the possibility of detecting them in space is of major importance.
- Short-Term Consequences of Asteroid Impacts into the Ocean: A Portuguese Case StudyPublication . Morais, Renato; Santos, Luís; Silva, A. R. R.; Melício, RuiAsteroid impacts are a proven global threat, meaning that any location on Earth might be a subject to their consequences. Such collisions are not likely in any person’s lifetime, but their aftermath could be catastrophic. As Earth’s surface is mostly water, a water impact is more probable than a ground impact, and tsunami waves could pose a significant threat. This study expands the knowledge about asteroid impacts in the ocean and their regional environmental consequences. Three asteroids were assumed to impact the Earth: (1) the Apophis asteroid, a 370 m wide asteroid, (2) a 204 m in diameter asteroid representative of the average impactor on the near-Earth objects, and (3) a 5 km in diameter asteroid. We evaluated the consequences of all impacts for a specific case study, where the chosen impact location was the midpoint between Portugal’s mainland, Azores, and Madeira Islands. The cratering process, generated seismic shaking, overpressure, ejected material, induced thermal radiation, and tsunami waves were assessed, along with the global effects. The overpressure mainly causes structural damage. The thermal radiation can be devastating but has a short reach. The tsunami is undoubtedly the most far-reaching and threatening effect of an asteroid impact in the ocean.
- Hypothetical Apophis deep ocean impact - Energy analysisPublication . Morais, Renato; Santos, Luís; Silva, André; Melício, RuiThis paper models a hypothetical Apophis asteroid impact on an chosen deep ocean point on the globe. To assess the potential vulnerabilities of an exposed coast population, the impact effects were assessed as a function of the distance to the asteroid's impact point as well as altitude for the tsunami case. All the tools and models used are valid and can be implemented for any other point on the Earth's surface. The impact modelling includes the analysis of cratering formation, generated earthquake, overpressure, ejected material, generated thermal radiation and tsunami, as well as the global geophysical effects of the impact. The vulnerability models entail best, expected, and worst case scenarios, which allows a reasonable range of results. As the impact occurs in the ocean, and if its a great distance away from populated areas, the population is safeguarded from most of the impact effects. The tsunami is undoubtedly the most far-reaching and threatening impact effect from an asteroid impact on the ocean.
- Asteroid Impacts on the Ocean and Its Short-term Consequences: a Portuguese Case StudyPublication . Morais, Renato; Santos, Luís; Silva, A. R. R.; Melício, RuiAny location on Earth might be considered a subject to an asteroid impact because, as previous studies show, they are a proven global threat. the consequences of a collision could be catastrophic, even though such events are rare to occur in a person’s lifetime. Tsunami waves can constitute the greatest threat as the Earth’s surface is mostly water, making the probability of a water impact greater than a ground impact. This work expands the knowledge about asteroid impacts on the ocean and their short-term regional consequences. Three asteroids were assumed to impact the Earth: (1) the 2015 JJ asteroid with a 130 m diameter, (2) the 2020 FA5 asteroid with a 210 m diameter, and (3) the Apophis asteroid, a 370 m wide asteroid. We evaluated the consequences of all impacts, at a 45 degree angle, for a specific case study, where the chosen impact location was the midpoint between Portugal’s mainland, Azores, and Madeira Islands. The cratering process, overpressure, induced thermal radiation, and tsunami waves were assessed, along with the global effects. The overpressure mainly causes structural damage. The thermal radiation has too short a range to reach the studied localities. The tsunami is undoubtedly the most far-reaching and threatening effect of an asteroid impact in the ocean.
- Modelos dinâmicos de sistemas de conversão de energia eólica ligados à rede eléctricaPublication . Conceição, Mário Rui Melício da; Catalão, João Paulo da Silva; Mendes, Victor Manuel FernandesEsta tese incide sobre o tema de modelos dinâmicos de sistemas de conversão de energia eólica ligados à rede eléctrica, integrando tanto quanto possível a dinâmica relevante presente na realidade do sistema. O sistema de conversão de energia eólica é constituído por: uma turbina eólica de velocidade variável; um sistema de transmissão descrito respectivamente por um veio de uma, duas ou três massas; um gerador síncrono com excitação assegurada por meio de ímanes permanentes; um conversor electrónico de potência descrito respectivamente por um conversor de dois níveis, um conversor multinível, ou um conversor matricial. A rede eléctrica é descrita respectivamente por uma fonte de tensão sem distorção harmónica ou com distorção originada pela terceira harmónica. Os comportamentos que advêm do facto da energia eólica não ser uma fonte controlável de energia, mas também de eventuais falhas no funcionamento dos dispositivos que conduzem os sistemas de conversão de energia eólica, são estudados nesta tese recorrendo à simulação computacional. Assim, no contexto dos impactes sobre a rede eléctrica, é estudado o conteúdo harmónico injectado na rede assumindo respectivamente que a velocidade do vento não apresenta perturbações ou apresenta perturbações que originam a assimetria no alinhamento do sistema de conversão de energia eólica em relação ao vento, o efeito de sombra da torre e as oscilações mecânicas nas pás. São apresentadas novas contribuições no contexto de eventuais falhas de controlo, quer das pás da turbina, quer dos conversores de potência, sendo estudada a operação impondo transitoriamente que o ângulo de passo das pás da turbina transite para a posição de rajada de vento ou um erro nas combinações de comutação dos conversores. Em ambos os contextos, são realizados estudos envolvendo uma nova estratégia de controlo mediante o uso de controladores de ordem fraccionária, comparando com o comportamento do controlo clássico.