1,720,964 research outputs found
Performance modulation of colloid thrusters by the variation of flow rate with applied voltage.
No full textThe effect of voltage on flow rate within cone jet mode electrospraying has been investigated, with particular emphasis on the effect of emitter geometry. Using a high fidelity flow meter a set of experiments investigated the effect of the emitter geometry on the flow rate relationship to voltage, in cone jet mode electrospray. It is demonstrated that there are various parameters that influence the flow rate sensitivity to voltage, including the inner and outer diameter of the emitter, and the emitter to extractor distance. By a simple derivation,the latter two parameters relationship is explained by the variation of theoretical electric pressure with voltage, as the geometry is varied. The theoretical and experimental agreement has important implications for variable throttling of thrust in colloid thrusters, and could lead to the optimization or selection of a particular thrust variation
Design and fabrication of the thruster heads for the MicroThrust MEMS electrospray propulsion system
No full textMicrofabricated electrospray thrusters are widely acknowledged as one of
the most promising technologies for the propulsion of small spacecraft. Their relative simplicity, high efficiency (>70%), low footprint (M <500g, V <10cm3) and large potential specific impulse (>3000s) enable the creation of a miniature system capable of providing up to 5km/s∆ V to 3U CubeSats.
We report here on our latest efforts in the development of such a thruster system, completed within the MicroThrust (www.microthrust.eu) project. While a companion paper will present early test results of the thrusters, this paper will focus on their design and fabrication.
We use MEMS microfabrication to manufacture internally fed capillary emitters from silicon. This permits the high fluidic impedance required to get the necessary low flow rates associated with pure ionic mode operation, in addition to allowing the fabrication of large arrays of perfectly aligned, nearly identical emitters. We present for the first time the wafer-level integration of an acceleration stage, with individual electrodes operating on up to 127 emitters on a single chip. By adding the accelerator, we increase both the specific impulse and thrust generated by the emitters, while also increasing the thrust efficiency
by electrostatic focusing the spray.
We have fabricated chips with varying emitter density (213 and 125 emitters per
cm2) and have successfully tested passively fed emitter arrays, obtaining up to 35μA of current at +875V for a 91 emitter arra
MicroThrust MEMS electrospray emitters - integrated microfabrication and test results
No full textWith the growth of interest in small satellites (<10kg), there is a particular need to provide a propulsion element for this class of spacecraft. Microfabricated electrospray thrusters offer a solution to this problem. By using ionic
liquids as the propellant solely ions can be emitted, resulting in a large specific
impulse[1] . The thrust from an individual emitter is though a fraction of a N. However by using well-established MEMS technologies thousands
of capillary emitters can be manufactured within an area of a few cm2, increasing the thrust to the mN level. We report on results from the Microthrust FP7 Project1,where the aims are to manufacture and test a complete breadboard thruster system based upon microfabricated thruster chips, alongside the design of a flight system that could enable a CubeSat to leave earth orbit. Prior to this
project we had developed a number of manufacturing processes for specific thruster elements[2,3] .
We report here on a new generation of microfabricated emitters, and their relative performance. The emitters consist of 70μm high etched-Silicon capillaries with outer diameters tapering to less than 10μm. Previous designs included 5μm silica microspheres within the 18 to 24μm internal diameter of the emitter to increase the hydraulic impedance[4]. However the filling factor of these microspheres in individual emitters differed; therefore a new generation of emitters having more similar impedance and with 5 -10μm internal diameters and hole depths of 100μm have been manufactured. Previously the etched-Silicon extractor chip was aligned to the emitter chip using 200μm ruby spheres
[2] . Due to assembly difficulties this has been replaced with a polymer-
based wafer bonding interface, allowing for simplified assembly and a wafer
-scale fabrication process.
These emitters have been tested in both uni-polar and bi-polar mode, using the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4). The tests
herein have been achieved without an acceleration stage. The Time-of-Flight data shows a mixed ion-droplet regime, approaching a Purely Ionic Regime (PIR) at low flow rate
The influence of the gas surface interaction on spacecraft orbital motion
No full textCharacterisation of the interaction between the neutral atmosphere in Low Earth Orbit and spacecraft surfaces is required for a broad range of applications to system and mission definition. For the prediction of spacecraft trajectories in this Free Molecular regime, the nature of the interaction must be accurately modelled. Accurate definition of the interaction is also required for the design of the attitude control system for vehicles such as Space Station which will generate large aerodynamic moments. The methods available to determine this interaction indirectly from observation of the motion of a satellite are reviewed. The orbital analysis technique is chosen for this study. Two methods of modelling complex spacecraft configurations are developed. The first, a Monte Carlo Test Particle approach, is able to account for all the phenomena characteristic of Free Molecular Flow. The second, adopting a panel method approach, accounts only for the possible shielding of surfaces from the flow. Discrepancies between the two modelling programs are identified but for the limited flow range relevant to the ANS-1 mission, good agreement is found and the computationally more efficient panel method program adopted. A new set of momentum accommodation coefficients are introduced to characterise the interaction. These are resolved in an aerodynamic frame. The new coefficients are found to be more robust than classical surface-resolved models and to have greater sensitivity to the nature of the interaction. The theory required to represent the relationship between the changes in the spacecraft trajectory and the gas-surface interaction is then developed. The sensitivity of the derived momentum accommodation to the parameters used in the modelling process is then determined. The most sensitive parameters are found to be the accuracy of the orbital elements semi-major axis and inclination, the rotation rate of the atmosphere, the temperature of the spacecraft surface and the complexity of the spacecraft configuration model. This theory is then applied to the orbital data derived for ANS-1 (1970-70A) by ESOC. The results suggest that the nature of the interaction is close to the classical diffuse case but the accuracy of the results are limited by uncertainties in the measured changes in orbital inclination
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
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