177,100 research outputs found
Luciano Rossi et Richard Straub, éd. trad. introd. — Fabliaux erotiques. Textes de jongleurs des XIIe et XIIIe s. [Postface de R. Howard Bloch]. Paris, Livre de Poche, 1992 (" Lettres Gothiques ", 4532)
No full textNoomen Willem. Luciano Rossi et Richard Straub, éd. trad. introd. — Fabliaux erotiques. Textes de jongleurs des XIIe et XIIIe s. [Postface de R. Howard Bloch]. Paris, Livre de Poche, 1992 (" Lettres Gothiques ", 4532). In: Cahiers de civilisation médiévale, 38e année (n°150), Avril-juin 1995. pp. 192-193
Analytical low-thrust transfer design based on velocity hodograph
No full textShape-based models can be used to approximate low-thrust transfer orbits between celestial bodies. Here, a new model is proposed, which is based on simple analytical base functions that together represent the velocity of the spacecraft. After integration, these base functions also yield analytical expressions for distances traveled. As a result, both the velocity and the trajectory of a transfer can be modeled analytically with a series of such base functions, which can be chosen and scaled at will. Constraints (i.e. conditions on initial and final position and velocity) can be satisfied directly, and a constraint on the final polar angle can be met with a straightforward, fast numerical integration. The technique allows for direct solutions with no degrees of freedom, but also facilitates a more extensive analytical modeling where certain aspects of the resulting transfer trajectory (e.g. required?V , maximum acceleration) can be optimized. The main characteristics of the technique are illustrated in a number of cases: transfers to Mars and Mercur
Computational Methods for the Long-Term Propagation of Space Debris Orbits
No full textSpace debris poses a significant problem for the space sector. This problem relates to potential collisions of debris objects with active satellites, which in many cases will lead to catastrophic damage. Due to the absence of natural decay mechanisms in the higher regions of space, debris objects in these regions have very long orbital lifetimes. In order to assess the hazards posed to active satellites, it is relevant to be able to predict how the orbits of these debris objects behave on long timescales. A simulation code in C++ has been created for this thesis project, capable of efficient propagation of space debris trajectories over long periods of time (typically a century or more), while taking into account various relevant perturbing forces. The simulation code can be applied to simulate the orbits of debris objects with a wide range of area-to-mass ratios, from intact satellites to tiny flecks of paint. The results produced with the simulation code have been verified to be consistent with results presented in recent research papers on space debris. An extensive performance comparison has been made regarding the efficiency of different computational methods for carrying out accurate, long-term integrations of space debris orbits. Both traditional integration methods and symplectic integration methods were tested, the latter of which are interesting because of their energy conservation properties. All methods were also combined with different formulations of the equations of motion. Of the methods tested, the Dormand-Prince 8(7) integration method combined with Gauss' form of Lagrange's planetary equations in modified equinoctial elements was found to be the most efficient. The performance of the symplectic integration methods was markedly less for this application than for the integration of completely Hamiltonian systems, though it was certainly acceptable. The simulation code was also applied to predict the long-term orbital evolution for debris objects in GEO and GNSS graveyard orbits. While proposed GEO graveyard orbits were found to be safe, graveyard orbits in the GNSS region were found to be susceptible to resonances induced by the luni-solar perturbations, and hence, require a careful selection of the initial orbital parameters. In all cases, debris objects with high area-to-mass ratios were determined to be dangerous to active satellites, regardless of the initial conditions of the graveyard orbit.Astrodynamics and Space MissionsAerospace Engineerin
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe 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
Optimisation Strategies for Galilean Moon Tours: Low-Thrust Multiple Gravity-Assist Trajectory Design for GTOC6
No full textAround 1610 Galileo Galilei made his discovery of the four large moons orbiting Jupiter which are referred to now as the Galilean moons. The moons Europa and Ganymede attract a significant amount of scientific interest due to potential present subsurface oceans. As consequence, the design of missions to go there and explore the moon system are increasing. This culminated in the sixth edition of the Global Trajectory Optimisation Competition (GTOC6) which is focussed on solving low-thrust multiple gravity-assist trajectories to map the Galilean moons. The aim of the thesis is to understand the complexity of the GTOC6 problem and to explore and evaluate the quality of various optimisation strategies to solve flyby sequences with low-thrust arcs. First, insight to the complexity of the problem was gained by analysing the best solution to GTOC6 so far by the Chinese Centre for Space Utilisation (CSU). From the results a clear picture was drawn from what the trajectory model should be capable of. The low-thrust trajectory model is based on the spherical shaping method that is part of the Tudat astrodynamics toolbox. A full analysis of the shaping method was performed to identify the capabilities and shortcomings of the algorithm. One of the main shortcomings is the limited accuracy for trajectories where the departure and arrival conditions differ with several degrees and more for the right ascension of the ascending node (RAAN). For optimisation use was made of differential evolution (DE). An extensive test was performed to determine the optimal settings. The result was that defining the control parameters randomly during the evolution was the best option with respect to quality and convergence. What followed was defining the optimisation model for a variable number of flybys. Furthermore, a framework was developed with six different optimisation strategies. A sequence of maximum five flybys was set to test the strategies. The strategies define the amount of freedom around the epochs of the flybys for the optimisation. Also the number of flybys that are influenced by this freedom is defined by the strategy. The goal was to optimise for ?V for a main sequence of five flybys. Here the main sequence was divided into smaller problems (subsets with less flybys). The optimisation of the main sequence was guided by the solutions of the preceding smaller subsets. Results showed that the initial subset of two flybys did not influence the optimisation of the subsequent subsets at all. Furthermore, two sequences were tested. The first sequence showed large ?V due to thrust constraint violations and limited accuracy of the spherical shaping method. On the other hand the second sequence showed ballistic solutions to go through all five moons in the sequence. Finally, from the previous test resulted an optimal strategy that was applied to a sub-problem of GTOC6. Optimisation was set to map the most interesting surfaces of the moons and to minimise ?V. The resulting trajectories were able to map the surfaces of interest. However, at the cost of more ?V compared to the previous test which only optimised for ?V.Space ExplorationAstrodynamics and Space MissionsAerospace Engineerin
Application of the Spherical Shaping Method to a Low-Thrust Multiple Asteroid Rendezvous Mission: Implementation, limitations and solutions
No full textSince the development of the exponential sinusoid for low-thrust trajectory design by Petropoulos and Longuski [2004], more shape-based methods have emerged that ease up the work of a mission analyst. These methods are the ideal tool to generate a quick and almost complete overview of a large search space and are useful for producing first estimate trajectories. The spherical shaping method by Novak [2012] is one of the more recently developed methods capable of shaping a transfer in three dimensions, satisfying constraints on initial and final position and velocity, and capable of satisfying a time of flight constraint; all at the same time. This makes it a particularly interesting method for application to rendezvous missions. In this thesis, the spherical shaping method will be used to generate first estimate trajectories for the GTOC2-mission (a multiple asteroid rendezvous mission). The goal is to find out whether or not the spherical shaping method is capable of producing sub-optimal trajectories. Implementing the spherical shaping method turned out to be a more massive job than anticipated. The documentation in the PhD thesis by Novak [2012] appeared not to be fully sufficient to use the method. Additional derivations were necessary to grasp the meaning of some functions. In this thesis, this process is explained and corrections of some typos found in Novak’s PhD thesis, are given. The spherical shaping function relations are re-ordered in a step-wise implementation scheme. This helps to get an overview of the implementation and will make future applications of this method easier. The implemented method is fully validated, both with respect to general two- and three-dimensional trajectories and with respect to the same test cases as used by Novak [2012]. The applicability of any method is dependent on its limits. For the spherical shaping method, these limits were not given unambiguously. Novak [2012] speaks of low and high inclinations but does not specify what is meant by "low" and "high". To find out what the limits for the inclination of the orbit are, several trajectories were computed for a range of inclination angles, for both trajectories at a constant inclination and trajectories with inclination changes. It was found that up until an inclination of 15 degrees, the relative difference between the deltav for a transfer at this constant inclination and the deltav for a same transfer at a zero inclination remains below 1 percent. For higher inclinations the difference rises quickly. At high inclinations of about 50 degrees the method breaks down. Also the Keplerian arc can not be reconstructed at inclined orbits. A way to solve this problem is to perform a reference frame transformation. A transformation involving a rotation over the line of nodes to remove the initial inclination was developed. This transformation works perfectly when the right ascension of both the initial and final orbit is equal and removes the effects caused by the high inclination. Also for high inclinations of above 50 degrees the spherical shaping method with reference frame transformation keeps producing feasible trajectories. The Keplerian arc can now be reconstructed at any inclination. When the right ascension of both orbits differs, the rotation is performed over the initial line of nodes, which makes it a less interesting rotation for the final orbit. To find out up to which right ascension difference the transformation can be used, several trajectories were computed for a range of right ascension differences (keeping other characteristics equal). It was found that up to 10 degrees the reference frame transformation can be useful. For higher differences in the initial and final right ascension of ascending node, the reference frame transformation could do more harm than good. To solve for high inclination changes, a solution was proposed to solve for multiple smaller inclination changes and sum the results. A promising result is obtained when combining this summation with the reference frame transformation. A last verification is performed by applying the implemented spherical shaping method to multiple test cases. Good results are obtained and therefore the implementation is considered fully validated. Finally the spherical shaping method is applied to the GTOC2 problem, using the top 3 asteroid combinations found by GTOC2, Heiligers [2013] and Secretin [2012]. Monte Carlo simulations with 100,000 samples were run and for each asteroid combination feasible trajectories are obtained, within the constraints set by GTOC2. The initial search space was set too broad which resulted in less feasible trajectories for GTOC2. Additional runs for a smaller search space are needed. There is however no time left to do this as well. It is recommended that the search is continued and a more thorough optimisation is performed. For the objective function of the GTOC2 problem a best value of 71.12 kg/yr was obtained for the asteroid combination equal to GTOC2 rank 2. Also a minimum value of deltav was obtained, equal to 20.04 km/s, for the asteroid combination of Secretin [2012] rank 3. Better results for both the objective function values or deltav for all asteroid combinations are expected when a more extensive optimisation is performed for the implementation of the spherical shaping method.Orbits & MissionsAstrodynamics and Space MissionsAerospace Engineerin
Impact of Satellite Fragmentations in GEO Graveyard Orbits
No full textAerospace EngineeringSpace Engineerin
Optimal Translunar Lagrange Point Orbits for OLFAR
No full textInvestigation of the feasibility of using translunar Lagrange point orbits for low-frequency radio wave astronomy, in particular for Orbiting Low-Frequency Antennas fro Radio astronomy (OLFAR). Furthermore suggesting swarm and orbit configurations that lead to the best possible results.Astrodynamics & Space Missions (AS)Aerospace Engineerin
Optimization of interplanetary trajectories with deep space maneuvers - Model development and application to a Uranus orbiter mission
No full textAstrodynamics and Satellite SystemsAerospace Engineerin
Optimization of Space Trajectories Including Multiple Gravity Assists and Deep Space Maneuvers
No full textThe optimization of high-thrust interplanetary trajectories continues to draw attention. Especially when both Multiple Gravity Assists (MGA) as well as Deep Space Maneuvers (DSMs) are included, the optimization is typically very difficult. The search space may be characterized by a large number of minima and is furthermore very sensitive to small deviations in the decision vector. Various options are available to model these high-thrust trajectories. The trajectory may be modeled using a simple MGA trajectory model as well as using models including DSMs. Both a position and a velocity formulation variant may be adopted and also unpowered or powered swing-bys may be used. These trajectory models were implemented to study the effect of both DSM as well as powered swing-bys. Especially the option to perform DSMs proved to be vital for obtaining good trajectories. Also powered swing-bys may improve the efficiency of the trajectory. The velocity formulation variant proved to be much easier to optimize than the position formulation model. By analyzing the sensitivity and dependency of the various parameters in both models, a proposal for an even better trajectory model is suggested. Also regarding the optimization of these trajectories many options are available. Especially metaheuristics have proven to be very successful in optimizing these trajectories. Various studies have shown the importance of proper tuning of the basic versions of these metaheuristics, which is however often overlooked. This study applied a very rigorous tuning scheme to find the optimal settings for DE, GA and PSO. The results clearly reveal the superiority of DE above other methods. The tuned variants of DE outperformed other settings by one or multiple orders of magnitude, revealing the importance of this tuning scheme. The tuned variants of DE helped to improve a large number of instances in the Global Optimization Trajectory Problem (GTOP) database of ESA. Also the efficiency of these DE variants was shown to be competitive with, and sometimes better than, the best algorithms encountered in literature.Astrodynamics and Space MissionsAerospace Engineerin
- …
