1,721,029 research outputs found
Refined Solar Sail Force Model with Mission Application
No full textThe aim of this paper is to propose a refined mathematical model for describing the acceleration experienced by a
solar sail. Unlike the conventional model characterized by constant coefficients, the force coefficients of the sail are
now assumed to depend on the light incidence angle, the sail surface roughness, and the sun–sail distance. The new
model is elaborated with the support of experimental data that show how the main variable affecting the force
coefficients is the light incidence angle. To emphasize the differences between the refined force model with respect to
the conventional one, a comparison is established through the analysis of a circle-to-circle interplanetary rendezvous
problem between coplanar orbits. The problem is solved using an indirect approach and the optimal steering law is approximated in polynomial form. A number of optimal trajectories toward Mars and Venus are simulated and the
results obtained are discussed as a function of the dimensionless sail loading parameter and the sail surface
roughness
Attitude and orbital modeling of an uncontrolled solar-sail experiment in low-Earth orbit
No full textSolar sails are big and lightweight structures that exploit the radiation from the Sun to propel the vehicle. They are very different from usual satellites since they yield a large moment of inertia, highly flexible parts and coupled attitude dynamics, orbital dynamics and structural vibrations. To these days studies that confirm the importance of investigating the elasticity of the sail in gossamer structures are available, but a full dynamics modeling that considers the coupling among the structural elasticity, orbital motion and attitude motion is still absent. The thesis is going to cover this subject. The thesis concentrates on Gossamer-1, a technology demonstrator that aims to test the deployment mechanism of a sail in LEO. To keep the satellite as light, simple and cheap as possible, no ADCS is mounted. The main question is then whether a completely arbitrary initial attitude and rotation will ensure that the sail can indeed complete its mission or whether there are some configurations to avoid. Within this work, the structural coupling is tested by means of a 3D model for the sail, to understand whether the modelling of the elasticity is necessary or negligible. To obtain significant results a 6-DoF model has been considered with drag, SRP, J2 and gravity gradient as perturbations. A grid search technique is used to investigate different initial values for the rotational behavior. There are three main results to be underlined from the outputs of the simulation: the non-negligible influence of the elastic behavior of the sail on the rotational motion of the satellite, the rotational rate evolution during de-orbit and the lifetime of the satellite that underlines the presence of a region of initial values that prevents the sail from re-entering in a small amount of time. Finally the problem of the self-shadowing, given the concave shape of the structure, is also addressed. Lastly, non-nominal deployment of the sail, resulting in particular shapes is also analysed to understand the influence of the shape on the satellite motion.Astrodynamics and Space MissionsAerospace Engineerin
Parametric model and optimal control of solar sails with optical degradation
No full textSolar-sail mission analysis and design is currently performed assuming constant optical and mechanical properties of the thin metalized polymer films that are projected for solar sails. More realistically, however, these properties are likely to be affected by the damaging effects of the space environment. The standard solar-sail force models can therefore not be used to investigate the consequences of these effects on mission performance. The aim of this paper is to propose a new parametric model for describing the sail film's optical degradation with time. In particular, the sail film's optical coefficients are assumed to depend on its environmental history, that is, the radiation dose. Using the proposed model, the optimal control laws for degrading solar sails are derived using an indirect method and the effects of different degradation behaviors are investigated for an example interplanetary mission
Optical solar sail degradation modelling
No full textWe propose a simple parametric OSSD model that describes the variation of the sail film's optical coefficients with time, depending on the sail film's environmental history, i.e., the radiation dose. The primary intention of our model is not to describe the exact behavior of specific film-coating combinations in the real space environment, but to provide a more general parametric framework for describing the general optical degradation behavior of solar sails
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Impact of optical degredation on solar sail mission performance
No full textThe optical properties of the thin metalized polymer films that are projected for solar sails are likely to be affected by the damaging effects of the space environment, but their real degradation behavior is to a great extent unknown. The standard solar sail force models that are currently used for solar sail mission analysis and design do not take these effects into account. In this paper we use a parametric model to describe the sail film's optical degradation with its environmental history to estimate the impact of different degradation behaviors on solar sail mission performance for some example interplanetary missions: Mercury rendezvous missions, fast missions to Neptune and to the heliopause, and artificial Lagrange-point missions
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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