considered due to their poor fabrication capabilities. While strength rupture capabilities of the nickel and cobalt base alloys have shown only a modest advance in the past 25 years, significant improvements in thermal fatigue, oxidation resistance, and stability characteristics have been achieved." They then conclude that: • There is little or no improvement trend in the cobalt base alloys. • Nickel base alloys have been improving at the rate of approximately 6.2°F (3.4 K) per year. • Introduction of a new alloy type, e.g., columbium-based B-66, can cause the most dramatic increase. b. Fastening/Joining Techniques Welding seems to be a preferred approach, but few details were found on this. Ref. E4 proposes a thermite-type fusion welding process. Laser welding was also mentioned as a possibility in a more general document. Ref. All discusses a screw-jack fastening method for the waveguide assembly, and proposes that this approach could also be used for alignment purposes. c. Thermal Torques The magnitudes of thermal torques in large structures are unknown at this point, but it is known that they can produce significant distortions, and, on entering and leaving eclipse, possibly vibrations, since the natural frequency of the very large structure is extremely low. Ref. All proposes to investigate these thermal transients, and the results of some thermal analyses are reported in Ref. E4. These results indicate that thermal gradients can create distortions, but that they can be corrected by an active microwave pointing system. d. Gravity Gradient Torques It is a well-known fact that gravity gradient torques will occur in structures of the size being considered. (See, for instance, Ref. E6.) Ref. E4 reports examining these for an aluminum structure, and
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