The work considers passive dynamic stability of sail craft, propelled by microwave beams. Such systems are under consideration as low-cost means of interstellar travel. Sail-craft shapes offering passive dynamic (neutral) stability are identified. Passive stability is a key mission-enabling attribute. Very large, umbrellalike structures made of strong, lightweight materials, such as carbon-fibers, attached aft of the payload can provide sufficient propulsive power by reflecting incident microwave energy. Not all reflector shapes are stable, however. The key contribution of this work is the identification of reflector shapes which possess passive dynamic stability in translation and attitude. Reflector substructure must be concave to incident radiation and must be located aft of the vehicle center of gravity, c.g. (i.e., the incident radiation encounters c.g. first). Critical parameters for passive dynamic stability are identified. A simulation/analysis tool is also developed which can be used to adequately address the stability issue of other potential sail-craft configurations.
This work was performed by Gurkirpal Singh of Caltech for NASA's Jet Propulsion Laboratory. To obtain a copy of the report, "Stable Microwave Beam-Riders," access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category
NPO-21035
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Stable Microwave-Beam-Riding Spacecraft
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Overview
The document is a technical memorandum from the Jet Propulsion Laboratory (JPL) detailing research on the dynamic stability of a microwave sail, which is being considered for conceptual interstellar missions. The work, conducted under NASA's contract, focuses on the modeling, stability analysis, and simulation of a sail constructed from lightweight carbon fiber strands. These strands are only a few microns thick and are designed to reflect microwaves with high efficiency (approximately 90% reflectivity).
The primary motivation for this research stems from the limitations of solar pressure as a propulsive means for deep-space travel. As vehicles move further from the Sun, the effectiveness of solar propulsion diminishes due to the inverse-square law of light intensity. This necessitates the development of large-scale vehicles capable of utilizing alternative propulsion methods, such as microwave energy. The document outlines the challenges of identifying vehicle shapes that maintain dynamic stability when exposed to microwave radiation.
The research identifies specific shapes and configurations that exhibit passive dynamic stability, which is crucial for the successful operation of these sails. The study emphasizes the importance of characterizing stability in relation to the vehicle's dimensions and initial conditions, aiming to define a "region of stability" where the vehicle's motions remain bounded. The findings suggest that neutrally-stable, concave, umbrella-like structures can be effective, particularly when a large cross-section reflector is positioned behind the vehicle's center of mass.
Additionally, the document discusses the introduction of mechanisms for passive damping of dynamic motions, which are essential for maintaining stability during operation. Spring-dashpot mechanisms are proposed as effective solutions, with critical locations for their implementation identified.
Overall, this memorandum presents a comprehensive analysis of the potential for microwave sails in interstellar travel, highlighting innovative materials and designs that could revolutionize deep-space exploration. The research not only contributes to the understanding of vehicle dynamics in a microwave propulsion context but also sets the stage for future advancements in aerospace technology. The work is positioned as a pioneering effort in the field, with implications for the design and operation of future space missions.
