Figure 2.1-14. Existing MPTS Concepts reviews indicated two major areas of concern which drove Rockwell to investigate and adopt an alternate antenna structure concept. The areas of concern are: 1. Construction Time—Both concepts required the fabrication (either on the ground or on-orbit) of thousands of individual structural beam-columns. The beam-columns would then be joined together to form a 1-km disc. This antenna structure assembly activity was estimated to take up to one year to complete. 2. Thermal Distortion—dc-to-RF conversion and transmission equipment (i.e., amplitrons/klystrons and waveguides) generates excess heat which is radiated to the antenna structure. As a result, a temperature delta exists between the two parallel surfaces of the structure. In addition, because of the Gaussian microwave distribution and resulting variation in heat flux, a temperature delta exists between the center of the structure and the circumferential perimeter. The result is structure surface distortion, and in turn, a decrease in beam efficiency. Counteracting this decrease in beam efficiency requires individual waveguide array adjustment devices which introduces additional system failures modes and system weight. With these two concerns as drivers (i.e., minimizing on-orbit construction time and structure thermal distortion), the tension web-compression frame concept shown in Figures 2.1-15 and 2.1-16 was developed. The concept as developed and analyzed is applicable to the Rockwell reference SPS configuration. Variations of this basic concept were made in the assembly operations tasks.