1,037 research outputs found
A 3D hybrid simulation study of the electromagnetic field distributions in the lunar wake
No full textAs a consequence of its lack of a thick atmosphere and an ionosphere,
the interaction of the solar wind with the Moon is characterized
by the direct impact of the solar wind on its sunward
hemisphere. This absorption effect produces a near-vacuum in the
wake immediately behind the Moon. The absence of a global magnetosphere
and the low electrical conductivity further leads to the
free passage of the interplanetary magnetic field (IMF) through the
lunar interior. This classic scenario of the solar wind�Moon interaction
was established by the very first plasma measurements in the
lunar environment made by the Explorer 35 spacecraft (Lyon et al.,
1967; Schubert and Lichtenstein, 1974). The wake region is gradually
filled in by the diffusion of solar wind protons into the zone of
density depletion. As described in the early analyses by Michel
(1968) and Whang and Ness (1970), the expansion of the solar wind
plasma into the wake is accompanied by rarefaction waves. At the
same time, the magnetometer experiment on Explorer 35 detected
the existence of a field reduction zone at the wake boundary surrounding
the central region, with magnetic field enhancement up
to a factor of 1.4 in comparison to the value in the ambient solar
wind (Colburn et al., 1967). Some of these features were repeatedly
observed at different distances in the lunar wake by the Win
A handle on charge reorganization
No full textGreen Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.ChemE/Opto-electronic Material
Marginalization and contemporaneous aggregation in multivariate GARCH processes
Full text linkTime Series;GARCH Models
Steady-state behaviour of a solar array system with elastic stops
No full textIn recent years a method was developed by the authors for efficient analysis of the long term behaviour of mechanical systems with local nonlinearities under periodic excita-tien. In this method the linear parts of the system are modelled using the finite element method. In order to keep the cpu-time for the nonlinear analyses acceptable, the number of degrees of freedom (dof) of the linear part of the system is reduced using a compo-nent mode synthesis (cms) technique. The cms technique used is based on free-interface eigenmodes and residual flexibility modes. Eigenmodes are kept up to a user-defined cut-off frequency. Subsequently, the reduced linear model is coupled to local nonline-arities, such as nonlinear springs and dampers, dry fiction elements, backlash etc. The model obtained in this way is analysed using a nonlinear dynamics toolbox, which among others contains solvers for the calculation of periodic solutions and their stability and a path following method. The approach outlined above is described in Fey (1992) and Fey et al. (1996) and was integrated in the finite element package DIANA (1997). Until now, this approach was applied to rather academie, archetypal problems in order to verify its value. The approach turned out to be very successful: numerical resuits were compared with experimental resuits and a Bood correspondence was achieved (van de Vorst, 1996, van de Vorst et al., 1996a, van de Vorst et al., 1996b)
Materialien für Münzgesetzgebung und dabei entstehende Erörterungen ...
No full textAttributed to F.C. Cleynmann.Mode of access: Internet
Improved Collocation Methods to Optimize Low-Thrust, Low-Energy Transfers in the Earth-Moon System
Full text linkModern and near-future Solar Electric Propulsion capabilities enable many new missions that were inconceivable using chemical propulsion systems. Many of these involve highly complex trajectories that are very challenging to design. New tools are needed that effectively utilize the rapidly growing parallel processing capabilities of modern computers. This research improves Gauss-Lobatto collocation methods, which are known to perform very well for low-thrust trajectory optimization, by formulating them as massively parallel processes. The parallelized elements of the problem formulation execute up to 11 times faster, depending on what force model is used and when evaluated by themselves. When accounting for the operations of the nonlinear programming solver, this translates to up to 3.7 times faster performance for solving a complete trajectory optimization problem, again depending on the force model that is used. The remaining barriers to further performance improvements, and the conditions upon which these depend, are clearly identified.
The implemented methods are combined into an optimization tool named Maverick. More general improvements to the formulation of the Gauss-Lobatto collocation methods are also developed and included in Maverick, which permit a more flexible use of these optimization schemes and enable them to find more complex solutions. One example of this is Maverick's ability to autonomously introduce gravity assists into trajectories, which greatly increases the utility and convergence radius of these methods.
In order to demonstrate the benefit of this work, three applications are studied. The first are transfers between halo-like orbits in the Earth-Moon system, which shows this is likely an unattractive region for missions like the New Worlds Observer. The second application investigates stabilization maneuvers in lunar distant retrograde orbits. This work demonstrates the feasibility of these stabilization transfers for a variety of sample return missions, such as the upcoming Asteroid Redirect Mission. The final application discussed is a series of multi-body low-thrust transfers from the Earth to the Moon that efficiently utilize highly variable dynamics to reduce propellant consumption, which is relevant for a variety of future mission concepts. These are computed for a wide range of flight times, showing that reductions up to 45% of the transfer time can be achieved with a propellant consumption as little as 0.5% of the total spacecraft mass. Up to 90% of the flight time can be eliminated for a propellant cost of 4% of the total spacecraft mass, or up to 83% for a propellant cost of less than 2%. The developed algorithm seamlessly transitions its solutions from full low-thrust, low-energy trajectories to the `pure' low-thrust trajectories that define the shortest transfer trajectories, validating its robust performance. Beyond these quanti_able results, these examples illustrate the complexity of the solutions that can be identified with these improved implementations of Gauss-Lobatto collocation methods, with many instances where the optimization method autonomously introduces powered gravity assists, an unusual capability that has the potential for useful application to many other trajectory optimization problems
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