1,720,991 research outputs found
On the critical parameters for feasibility and advantage of air-breathing electric propulsion systems
No full textIn recent years, air-breathing electric propulsion emerged as a potential enabling technology for long-duration space missions in Very Low Earth Orbit (VLEO). In this work, we show how the complex relation between mission environment, spacecraft configuration, and propulsive performance can be associated to a single requirement for full air-breathing drag compensation, which becomes less stringent for higher VLEO orbits. The impact of the spacecraft shape and size on the performed analysis is then evaluated and the results compared with conventional electric propulsion solutions with stored propellant, showing how the adoption of air-breathing propulsion becomes more advantageous, from a volume and mass fraction perspective, when the spacecraft scale is reduced
Atmospheric propellant fed Hall thruster discharges: 0D-hybrid model and experimental results
Full text linkAs part of on-going efforts in advancing air-breathing electric propulsion, the HT5k Hall thruster was characterized in six operating conditions, ranging from 5 mg s(-1) to 7 mg s(-1) of 0.56N(2) + 0.44O(2) mass flow rate and 225 V to 300 V of discharge voltage. The cathode was operated with N-2 at mass flow rates between 0.5 mg s(-1) and 0.7 mg s(-1). This paper presents a 0D-hybrid model for atmospheric propellant fed Hall thruster discharges. Verified performance ranged between 30 mN to 120 mN in thrust, 1.2 kW to 5.2 kW in discharge power, and 8% to 18% in anodic efficiency. Calibrated model comparison against experimental data resulted in a mean absolute error of 3.7% in thrust and 7.6% in discharge power
Atmospheric propellant fed Hall thruster discharges: 0D-hybrid model and experimental results
No full text
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
Development status of the HT5k high voltage model
No full textSince 2013, SITAEL has been developing its 5kW-class Hall thruster unit, the HT5k-TU, comprising the HT5k thruster and the HC20 hollow cathode. Through several supporting projects, the thruster unit has undergone three design iterations and is now in the Engineering Model definition phase. As the design of the thruster unit evolved towards higher levels of maturity, multiple test campaigns were performed including several characterization tests, magnetic field topology optimization for life extension, tests with alternative propellants, assessment of operation in direct-drive mode, facility background pressure effects tests at two different pressure levels, thermal cycling and endurance test. As a result, the HT5k-TU thruster family cumulated more than 700 hrs of firing, demonstrating stable and efficient thruster operation in the power range of 3 to 7 kW. One important modification in the design of the HT5k thruster was the implementation of magnetic shielding of the discharge channel walls, eliminating in this way one main failure mode of the thruster. This improvement results in increasing the thruster lifetime by at least two orders of magnitude, making the thruster design further apt for station-keeping and deep-space robotic exploration missions of long lifetime. These scenarios typically necessitate operating the thruster at relatively-high specific impulses. However, operation of the thruster unit in the high-voltage range (500-800 V) require updating several design solutions, particularly from the perspective of voltage isolation and heat management. In this respect, to better understand the implications and requirements of high-voltage Hall thruster operation, the HT5k-TU models, both unshielded and shielded, were tested in this voltage range. The aim was to investigate the performance and stability of the thruster, in addition to better understanding the impacts from the design compatibility point of view. This paper presents an overview of the development activities performed on the various models of the HT5k-TU, focusing on the characterization results in the high-voltage regime. Furthermore, we also discuss the solutions devised for addressing the technological requirements of high-voltage Hall thruster's operation
L'azione di danno temuto promossa dal custode giudiziario
No full textSommario: 1. Premessa. – 2. La legittimazione processuale del custode giudizia rio a promuovere le azioni che spetterebbero al debitore esecutato. – 3. L’azione
di danno temuto: profili processuali. – 4. Rilievi conclusivi: la possibile concorrente legittimazione del debitore e la necessità di previa autorizzazione giudiziale
all’azione
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