For close proximity flying on the order of 10-100 meters, Coulomb thrusting presents a promising alternative to other methods of propulsion. This clean and fuel-efficient propulsion method is being investigated for use in formation flying and virtual structures. In the latter application, the individual spacecraft assume fixed positions relative to each other through the use of Coulomb forces. In the work presented here, an analytical and numerical analysis is performed on such virtual structures. In the analytical portion, the constant, open-loop charges necessary to maintain a Hill-frame-static formation are determined for the cases of linear two- and three-spacecraft formations and for the case of equilateral triangular formations with spacecraft of equal mass. In addition, analysis is provided for the N-craft case so that the inter-craft charge products can be determined for any static formation. In the numerical portion, a genetic algorithm is employed to support the analytical results by determining formation geometries and charging schemes such that the formation craft remain static in the Hill frame in the absence of perturbation. The results of the numerical analysis include examples of static two-craft through nine-craft formations, including several formations that display a broader range of configurations than considered in previous works. Issues encountered during the numerical analysis are discussed, as well as the course of action taken to overcome these issues. Finally, a method is presented by which the genetic algorithm could be extended to take advantage of cluster computing. / Master of Science

Zugl.: München, Univ., Diss., 2008

No description available.

No description available.

No description available.

No description available.

Recently, it has become increasingly clear that boundaries play a significant role in the understanding of the non-perturbative phase of the dynamics of strings. In this thesis we propose to study the effects of boundaries in non-critical string theory. We thus analyse boundary conformal field theories on random surfaces using the conformal gauge approach of David, Distler and Kawai. The crucial point is the choice of boundary conditions on the Liouville field. We discuss the Weyl anomaly cancellation for Polyakov's non-critical open bosonic string with Neumann, Dirichlet and free boundary conditions. Dirichlet boundary conditions on the Liouville field imply that the metric is discontinuous as the boundary is approached. We consider the semi-classical limit and argue how it singles out the free boundary conditions for the Liouville held. We define the open string susceptibility, the anomalous gravitational scaling dimensions and a new Yang-Mills Feynman mass critical exponent. Finally, we consider an application to the theory of non-critical dual membranes. We show that the strength of the leading stringy non-perturbative effects is of the order e(^-o(1/βst)), a result that mimics those found in critical string theory and in matrix models. We show how this restricts the space of consistent theories. We also identify non-critical one dimensional D-instantons as dynamical objects which exchange closed string states and calculate the order of their size. The extension to the minimal c ≤ 1 boundary conformal models is also briefly discussed.

Stuttgart, Univ., Diss., 2001.

University, Diss., 2004--Freiburg (Breisgau).

University, Diss., 2005--Heidelberg.