A proposed lightweight, off-axis reflector structure for a microwave communication antenna would be made of a carbon/carbon composite material. The structure was conceived for use aboard the Solar Probe spacecraft, where it would also serve as a shield to protect the spacecraft against solar radiation at perihelion. The basic concept of the carbon/carbon reflector structure could be also adapted to design lightweight, strong, off-axis reflector structures for antennas to be used on Earth.
Carbon/carbon was chosen as the class of structural materials because such materials offer a combination of light weight, high strength, good radio-frequency (RF) reflectance properties, and low mass loss at high temperatures. Results of tests of candidate materials suggest that the proposed shield/antenna structure would function well at a temperature greater than 2,000 K. The major drawback of materials in this class is that they are expensive.
In the original Solar Probe application, the dual use of the structure as a solar shield and antenna reflector was made possible by a fortuitous combination of optimum shield and antenna shapes that was effected by designing the spacecraft trajectory to obtain Sun/spacecraft/Earth quadrature at spacecraft perihelion. The combined shield/antenna would also enable a reduction of overall spacecraft diameter: According to an older design concept, the solar shield would be a separate, conical structure and the antenna reflector would lie within the shadow of the shield. The overall spacecraft diameter according to that concept would be 4 meters. The overall diameter according to the proposed simplification would be reduced to 1 meter, and the overall mass and cost of the spacecraft would be concomitantly smaller. Of course, whether or not such simplification and reduction in size could be effected in other applications would depend on the geometries and design and operational requirements specific to those applications.
This work was done by James Randolph of Caltech for NASA's Jet Propulsion Laboratory. NPO-20318
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Carbon/carbon shield/antenna structure
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Overview
The document discusses advancements in the development of a carbon/carbon shield and antenna structure for NASA's Solar Probe mission, which aims to study the Sun's outer atmosphere. The focus is on the unique challenges posed by the extreme temperatures and radiation encountered near the Sun, particularly at perihelion, where temperatures can reach up to 1400K.
Key highlights include the testing of high-temperature solar arrays designed to operate effectively close to the Sun. A solar cell testing program initiated in 1995 at JPL, in collaboration with Rockwell International, evaluated the performance of gallium arsenide (GaAs) photovoltaic cells at high solar incidence angles (SIAs). The findings indicated that while high temperatures significantly reduce power output, usable power remains possible even at extreme SIAs, suggesting that high-temperature solar arrays could function effectively closer to the Sun.
The document also details the development of a heat shield technology, which involved testing various carbon-carbon composite materials to assess their mechanical and thermal properties. The results indicated that certain fibers, such as P-30 and K-321, exhibited superior performance. The chemical vapor deposition infiltration (CVI) process was noted for providing excellent thermal and mechanical properties, while pitch densification improved optical properties, enhancing the shield's effectiveness against solar radiation.
Additionally, the document outlines the design of an antenna feed system capable of operating at high temperatures. This system includes a high-temperature dipole antenna connected to a waveguide that leads to a transponder operating at room temperature. The development of a high-temperature coaxial cable that maintains conductivity under extreme conditions is a critical aspect of this design.
Overall, the document emphasizes the innovative approaches taken to address the challenges of operating spacecraft in extreme environments. The combination of advanced materials, testing programs, and design strategies aims to ensure the success of the Solar Probe mission, enabling unprecedented close-up observations of the Sun and contributing to our understanding of solar phenomena. The research and development efforts highlighted in this document represent significant strides in aerospace technology, with potential applications beyond the Solar Probe mission.
