R, C. Ried, S. Jacobs/ Structures and Mechanics Division IV-C-9. Thermal Control The thermal characteristics of the antenna have been investigated in some detail by the Raytheon Company and the Grumman Aerospace Corporation (reference 1). Current results tend to reinforce previous findings and support the contention that thermal control of the antenna can be achieved in a passive manner. However, the requirement for a very closely integrated design of the microwave generators, waveguide system, electronics, each level of structure, and materials all center around the thermal characteristics of the antenna. There are no apparent insurmountable thermal problems which cannot be solved through technology development programs and testing prior to SPS initiation. However, three significant thermal design considerations are apparent: 1. Rejection of waste heat. 2. Minimization of therma’i distortions of the primary structure due to the daily solar orientation cycle. 3. Accommodation of thermal strain and elimination of excessive thermal stress occurring as a result of occultation by the earth. The rejection of waste heat from the antenna requires thermal radiation from the rear of the antenna. Although operating temperatures could be reduced through high thermal emission from the waveguides, the waveguide configuration is a rather efficient thermal radiation shield. In addition, the waveguide efficiency requirement of high electrical conductivity on all exterior surfaces significantly limits the thermal emissivity obtainable. Fortunately, about half of the waste heat generated by a Klystron is produced at the anode, thereby affording a relatively simple path for thermal radiation rejection. The other half of the waste heat must be transmitted through radiation, conduction, or heat pipe to the radiator. The representative thermal profile shown in figure IV-C-9-1 was obtained by assuming that 90 percent of the waste heat can be rejected out the back of the antenna with an additional 10 percent through the waveguides. The effect of antenna taper is to produce maximum temperatures at the center of the antenna. In addition, the operating temperature of the antenna is a strong function of the amount of waste heat. The operating temperature and variations in operating temperatures are potentially very important to the phasing electronics and phasing system as well as to the microwave generation and waveguide efficiencies, equipment lifetime and reliability. The other major heat rejection problem is the rotary joint. Sufficient surface area must be provided to provide adequate thermal emission, and some means for transporting this heat to the exterior surface is required. To allow adequate heat flow and still maintain sufficient electrical insulation between the departing and returning current may require independent heat rejection systems. This problem has not been treated in depth; however, it does appear to be a potential area for heat pipe application. If it is assumed that thermal distortion between an operating and non-operating antenna system can be accommodated through a calibrated alignment, then the thermal distortion problem is reduced to the effect of the daily solar orientation cycle. If a low thermal expansion coefficient material, such as a graphite composite, is utilized for all levels of structure, then thermal distortions are of significance only for the prime structure influence on the subarray orientation. The antenna tends to curl away from the sun with the maximum effect at the edges. The lower temperature regions are more strongly affected by the solar flux
RkJQdWJsaXNoZXIy MTU5NjU0Mg==