1,720,961 research outputs found
Analysis of innovative scenarios and key technologies to perform active debris removal with satellite modules
Full text linkIt has surely happened sometimes to look at the night sky and catch sight of a small, brighting spot moving like an airplane, but without ashing as usually airplane lights do: it was a satellite. It is always amazing to think that there are objects that continously orbit around Earth, so far from us. Maybe, it is not well-known that they are a lot...thousands...more or less 17,000 those closer to the Earth, to be precise. It is difficult to imagine how so much objects can orbit without touching or colliding each other.
From the beginning of the space activities in 1957, an enormous number of objects have been sent or released in space, and thinking that space is so limitless that it could contain everything, all these objects have been abandoned in orbit. The result has been the creation of a great quantity of debris that have began to represent a serious threat for future space missions. Recent studies revealed that if no countermeasures are going to be adopted to reduce the generation of debris in orbit, like for example spent spacecraft and orbital stages, the space population could remain stable only for the next 20 - 30 years.
Beyond that, collisional cascade events between objects already in orbit will cause a rapidly increase of debris, even in case of a complete hault of launches. This self-sustained phenomenon, known as Kessler Syndrome, would prevent any other human access to space. Although from '90s, mitigation measures have been proposed to limit the generation of debris in orbit, they appeared to be insufficient to guarantee a long term stability of the space environment. The only way to intervene would be the implementation, in parallel, of active debris removal (ADR) missions. The effectiveness of such kind of missions has been
demonstrated, but there are numerous aspects that have to be solved yet in order to make them feasible in the near future. On one hand new mission studies and analyses are required to identify the most efficient ADR scenarios. On the other hand, there are several technological issues that are particuarly critical, espacially as regards the capture of space debris, that are essentially un-cooperative objects and so, not-specially-prepared to be grasped.
The research developed in this thesis deals with two of the main aspects related to active debris removal: I) ADR mission analysis, II) developement of an enabling technology for ADR; in the specific case, the development of a morphing adhesive interface to capture uncooperative ojects. A multiple vehicles scenario, where orbital transfers and de-orbiting operations are accomplished by distinct vehicles (a space tug for the former, proper de-orbiting kits for the latter), is selected for the
analysis. The innovative solution proposed is the employemnt of modular structures as de-orbiting kits, constructed by assembling a certain number of microsatellite elementary units, equipped with proper de-orbiting devices; the number of assembled units depends on the characteristics of the debris, in terms of mass and initial orbit, as well as to the specic de-orbiting technology selected. An increased mission efficiency, reliablity and exibility are expected from the adoption of such approach. Costs and mass savings can be also expected, as well as economy of scale, thanks to the standarization of the units employed.
Four de-orbiting solutions are implemented in the analysis: drag sails, electric propulsion, electrodynamic tethers, hybrid propulsion. Different mission scenarios are analysed, to determine the performances of each solution, in terms of de-orbiting kit mass and total de-orbiting time. For each de-orbiting technlogy, a proper base unit is determined. An optimization procedure to perform multiple orbital transfers and, hence, minimize the mass of the propulsion system of the space tug, is also performed. Five orbital bands are identified as priority regions, where debris have mass between 800 kg and 11 tons and orbits between 800 km and 1000 km.
The analyses revealed that, although drag sails are low-cost and simple solutions, they are not suitable to de-orbit massive debris from high altitudes since very large sails could be required, with consequent high risk of collisions during the de-orbiting phase. Electric propulsion and electrodynamic tethers are both promising solutions for ADR: for debris with mass 2000 kg they are comparable, both in terms of de-orbiting kit mass and total de-orbiting time. For more massive debris, mass 2000 kg, electrodynamic tethers performes better from the remover mass point of view, but higher de-orbiting time is required
compared to electric propulsion. Risks assessment evaluation revealed that in this case the probability for the electrodynamic tethers to be damaged in consequence of collisional events with debris up to 10 cm exceeded the limit of 0.001 indicated in the NASA-STD-8719.14. The risk analyses conducted for electric propulsion, on the other hand, did not reveal any risk of catastrophic collision during the deorbiting manoeuvre. Hybrid propulsion resulted the most massive solution among those implemented, but it represents the fastest solution in terms of de-orbiting time.
The second part of the research activity is focused on the development of a morphing adhesive interface to be integrated as end effect of a robotic mechanisms to allow the capture of uncooperative objects. Two technologies are investigated in the realization of the interface: shape memory polymers, for the morphing behaviour, and electroadhesion for the adhesion capabilities. Two prototypes are then developed and tested. It is observed that mechanical pre-load as well as electrostatic force increase the normal adhesion performances of the realised interface. Normal adhesion pressures can vary between 0.55
and 1.4 kPa without the contribution of electrostatic forces, as the mechanical pre-load is varied between 1.5 N and 10 N. The adhesion pressure increases in presence of electrostatic forces, varying between 1.40 kPa and 1.80 kPa for different mechanical pre-load and voltage conditions. The forces achievable range between 3.5 N and 11.5 N. Morphing tests are also performed to verify the morphing-adhesive capabilities of the developed interface. The tests demonstrates taht the presence of a foam substrate could be advantageous as regards the capture of uncooperative objects, allowing a good compliance between two contact surfaces even in presence of macroscopical irregularities, enhancing the adhesion between them. The effectivness of the proposed morphing-adhesive interface is then demonstrated
Economic benefits for LEO telecom constellations due to modular spacecraft architecture
No full textThe realization of future satellites based on the aggregation of modular units offers several advantages compared to the traditional approach, consisting of monolithic, highly customized spacecraft. In fact, the aggregation of multiple, standard, self-consistent units with the capability of cooperation would lead to the realization of platforms with equivalent performance compared to traditional satellites, but with increased fault tolerance and flexibility. Such approach would also lead to a significant increase of the platform lifespan by replacement of malfunctioning, damaged or obsolete units with spare or updated units.
The major drawback of this concept is represented by the increase of inert mass of the single unit, which is inevitable in the process of fragmentation of the monolithic platform, leading to increase of costs. While this is certain for a single satellite, this may not apply if we consider a constellation based on dozens of satellites. If each spacecraft is fragmented in a cluster of modular, standard units, their amount becomes so large that economy of scale can be applied, causing decrease of both costs and production time of the single unit. In addition, the fragmentation approach makes it possible to deploy a reduced capability constellation in a short time simply by launching platforms composed by half of the total units; such initial constellation could be further, gradually expanded by launching additional units.
In this work, the economic benefits deriving from the adoption of a modular design applied to LEO telecom constellations is analysed. The overall cost of constellations based on monolithic satellites and modular platforms with equivalent telecom capabilities and performance are compared, considering different constellation architectures and reference monolithic satellites. For each architecture, the mass of the monolithic satellite is determined exploiting historical data, as well as the mass of the modular units as a function of the number of units the monolithic satellite is fragmented. Then, the overall cost of the fragmented platforms is calculated using cost estimating relationships (CERs) and compared to the cost of the monolithic satellites. The analysis shows that in most of cases there is a fractioned configuration that significantly reduces the cost of the whole constellation
INNOVATIVE TECHNOLOGIES FOR NON-COOPERATIVE TARGETS CLOSE INSPECTION AND GRASPING
No full textClose inspection and grasping of space objects and local adhesion to spacecraft external surfaces represent key technologies for several on-orbit servicing operations, such as components repair or refurbishment, tanks refuelling and automatic assembly. Furthermore, the capability of capturing and controlling the motion of non-cooperative targets having irregular shape is becoming a crucial issue for active debris removal.
This paper presents the conceptual development and numerical evaluation of an innovative system for attaching to non-cooperative objects, based on (1) a simple and compact infrared sensors assembly for target location and (2) an adhesive hand grip that conforms to the shape of the target interface.
On one hand, the location assembly is based on infrared lasers which send impulses towards the target and an array of infrared sensitive elements to detect the back-scattered radiation. By taking differential measurements of the detector outputs generated by the reflected impulses with high accuracy, it is possible to determine the target object distance and the target surface orientation in the grasping system reference frame. The instrument is compact, made of low-cost components and requires lower computational capabilities than camera-type sensors. To assess the sensors system performance, a numerical model has been realized for the instrument and the environmental noise. Simulation results are finally presented to evaluate the uncertainty of the target orientation and distance estimates.
On the other hand, the hand grip employs a flexible electro-adhesive layer mounted on a tripod mechanism, which provides the requested interface rigidity and is capable of adapting to different target shapes. Moreover, it benefits from the presence of a damping joint that manages the relative motion between target and grasping system, reducing the reactions transmitted to the base. The passive joint dissipates part of the total kinetic energy possibly reducing the power budget requirements for the active control. A simplified simulation tool is finally presented to assess the gripper operations and performance, as well as the transient disturbances transmitted to the base during graspin
Morphing adhesive interface for space robotic applications
No full textOver the last years there has been a growing interest in the development of robotic technologies for attaching to, grasping and manipulating a wide range of objects, regardless of their shape, material and the presence of specific features on their surface. This issue has become of particular interest in the space field as regards the execution of ‘on-orbit servicing’ operations and the active removal of space debris, where the presence of non-cooperative objects requests the employment of grasping systems that are effective when the properties of the target object are unknown. In this framework, the paper deals with the study of an interface to be employed in a gripping device where morphing and adhesive capabilities are combined in a single smart system. On one hand, the morphing behaviour is provided by a foam support that could be easily deformed at ambient temperature, enhancing a good compliance with the target surface, even when macroscopic irregularities that obstacle the contact over the entire surface, like screws, nuts or reinforcements, are present. On the other hand, the adhesion is provided by means of electrostatic forces (electro-adhesion) generated when a high voltage, on the order of 1 – 5kV, is applied to a pair of electrodes embedded inside an insulating material. Two different solutions are considered to make an electro-adhesive layer with flexible electrodes: one consists of using a conductive fabric, from which the electrodes are cut with the desired geometry; the second one consist of fabricating a conductive silicone by mixing 20% conductive particles (carbon black particles) in a 10:1 PDMS silicone matrix. In both cases, the electrodes are embedded inside an insulating layer of TC-5005. The electro-adhesive layer obtained is then bonded with the foam support. Two sets of tests are performed as regards the adhesion capabilities and the morphing behaviour, aiming to evaluate (1) the force level reached with the two different types of electrodes and (2) the morphing capabilities of the entire substrate, i.e. its ability to conform to objects of different shapes or with macroscopic irregularities on the surface, and to determine how the compliance aids the adhesion behaviour
CONCEPTUAL DESIGN OF SMALL SPACECRAFT DOCKING MECHANISM ACTUATED BY ELECTROACTIVE POLYMERS
No full textIn recent years there has been a continuously growing interest in space missions conducted with miniaturized spacecraft, down to the nanosatellite class and beyond. However, these satellites are severely constrained by the small size and the consequent limitations of on-board resources make them inadequate to the vast majority of missions. In this context, we propose the realization of a standard docking mechanism for nanosatellites based on ElectroActive Polymers (EAPs) technology, that provides CubeSats with the capability to aggregate in orbit to form larger structures; at today, there is no qualified docking system for such category of satellites. This would enable scenarios where CubeSats can join together to generate multipart systems with the possibility to rearrange, be repaired or updated. The joining mechanism is designed to combine in a simple way the advantages of androgynous systems and the simplicity of probe-drogue configurations, through the docking ports shape-shifting capabilities, avoiding the need of a large amount of actuators usually employed in larger docking ports. The mechanism design and preliminary validation through numerical simulations is presented including 3-D modeling, kinematic, dynamic and structural analysis
SOFT-DOCKING SYSTEM FOR CAPTURE OF IRREGULARLY SHAPED, UNCONTROLLED SPACE OBJECTS
No full textOver the last decades, the interest around Active Debris
Removal missions grew considerably due to the increasing
threat represented by the space debris populating near-Earth
orbits. This paper presents the concept of a soft docking
system suitable for capture of non-cooperative, large objects
regardless of shape, surface features and motion. The innovative
concept exploits a compliant electro-dry adhesive
surface for mating and a robotic support structure composed
by smart-material actuators. The adhesion surface combines
electrostatic attraction and van der Waals forces; it is highly
flexible and compliant to local irregularities of the target surface.
The proposed capture concept increases the operative
flexibility, tolerating critical scenario uncertainties (i.e. target
shape, motion, mass). The active support structure can
be controlled to adapt to the target object external geometry
and to damp relative motion between target and chaser. Preliminary analysis was conducted both assessing the expected
capture loads and the available adhesion forces
An engineering model to describe fragments clouds propagating inside spacecraft in consequence of space debris impact on sandwich panel structures
No full textAll spacecraft in Earth orbit are exposed to the risk of impact with micrometeoroids and orbital debris. When such
particles have enough energy to penetrate the hull of the vehicle, clouds of fragments are ejected into spacecraft and
they can eventually compromise the functionality of various components encountered in their flight path. Knowledge
of the clouds' properties (e.g. fragments mass and velocity) is therefore a key factor to obtain accurate predictions of
the response of interior equipment to space debris threat. However, generation and evolution of debris clouds from
hypervelocity impact is a complex phenomenon governed by a large number of parameters, and existing models
mostly refer to fragments originated by impact on simple aluminium plates only, while the few models available for
sandwich panels do not provide information on the fragments mass. In such context, this paper presents an
engineering model describing debris clouds created by space debris impacts on honeycomb sandwich panels
representative of satellites structural bodies. The model consists of a set of empirical equations providing three pieces
of information, i.e. the geometric description of the cloud, the velocity distribution and the mass distribution of the
fragments. The proposed equations are derived from analogous formulas for debris clouds originated by impacts on
simple aluminium plates, by applying proper corrections to account for different materials effects and different
behaviour of sandwich panels compared to plates of same material. The model is finally evaluated by comparing its
predictions with few experimental data on triple wall structures (sandwich panel plus internal equipment cover
plate), where debris clouds exiting the panels’ rear skin are used to assess the failure of the third wall. In this latter
case, the proposed model is also compared with the SRL equations
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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