General conceptual design problems of a parabolic solar sail structure

• Autor(a) principal: Potes, Filipe Couceiro
• Data de Publicação: 2012
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.6/2001
• topic: Vela solar; Estruturas espaciais; Coletor parabólico; Pressão solar; Distância focal; Mancha de luz

Solar sails represent a promising and practicable solution for future long-duration space missions involving deep space travel. Since their source of propulsive thrust derives from the energy exerted by solar radiation pressure, they do not depend on any on-board propellant, which thus minimizes energy consumption during a mission and consequently its final costs. However, because in Earth´s vicinity solar pressure presents a very low intensity, and in order to provide the necessary amount of thrust this type of solar-powered spacecraft typically requires an enormous surface area whilst at the same time maintaining the lowest possible mass. Such requirements oblige the adoption of very large structures usually consisting of ultra-thin membranes supported by long slender booms, in the order of tens of meters. This causes some difficulties on the structural design process since the deployment and maneuvering capabilities have to be optimized without compromising the mechanical performance of the whole solar vehicle. Among the various types of solar sail structural configurations, there is a very ambitious concept considered to be highly efficient, which consists on the parabolic solar sail. In this type of solar-propelled spacecraft, a small mirror designated as the director reflects the sunlight collected from a larger parabolic structure which in turn remains constantly oriented towards the incoming direction of the sunbeams so as to always guarantee the maximum available thrust. Since in this case maneuverability is achieved exclusively by tilting the director component, this allows the entire structure to operate without having to change its inclination in relation to the Sun. The scope of this work is to explore different possibilities for the conceptual design of a parabolic space structure intended to act as a solar sail collector, with special emphasis on the parabolic shape maintenance issue, since this is crucial for the maximization of the sunlight concentration into its focal point, as well as on its overall structural behavior. On a first approach, a structure which consists solely of an initially flat circular membrane simply supported around its perimeter and without any type of load supporting elements along its radials is considered. In this case, the possibility for the structure to make use of the solar pressure itself as a form of obtaining the desired parabolic curvature is investigated. Assuming the membrane as a perfectly reflecting surface, a parametric study is conducted in order to determine several parameters of interest, as a function of its radius size and thickness. Although the resulting deformed shapes prove to be very similar to parabolic ones, since the obtained focal distances are considerably high, this structural concept does not become feasible for solar sail applications. Consequently, a parabolic boom-based type of structural configuration resembling an open umbrella is considered afterwards. Here, the booms are responsible for guaranteeing the membrane´s parabolic curvature therefore enabling the focal distance to be initially established during the design phase of the structure. In order to assess its behavior due to solar pressure action, a parametric study is performed to investigate the effects of several design features such as the collector radius, membrane thickness, number of booms, number of supporting points of the membrane along the booms, as well as the effect of a possible angular velocity around the structure´s symmetry axis, with regard to the impact that these have on the resulting loadings and maximum deformations of the entire structure. In the end, the structure´s natural frequencies and mode shapes are also determined. The obtained results are of upmost importance to the design of an optimized structure which provides greater efficiency in terms of control thrust of the vehicle, thus forming the basis for further more detailed structural and orbital control analysis of this solar propelled parabolic sail spacecraft.