which cannot be reflected. There are two passive means for reducing this effect to within subarray structural alignment tolerance requirements (3 arc minutes on boresight). The first is to geometrically taper the antenna prime structure to be relatively thin at the outer edges. This is not inconsistent with structural load paths, stiffness requirements, and a reduction of gravity gradient torques. The second approach is to lower the emissivity of the surface and allow the waste heat to bring the operating temperatures at the large radii up to the levels at the center. There are many design advantages to operating the antenna at a uniform temperature; however, it must be emphasized that the temperature level would be established by waste heat generated at the peak power density center. The third thermal consideration of significance to the antenna design is the relatively rapid and rather extreme temperature transients associated with eclipse by the earth. The characteristic time for the antenna to cool from operating temperatures is about twenty minutes. Since eclipse can last for up to 75 minutes, the potentially large changes in temperature (almost 500°K) impose a design requirement of thermal stress or strain. The major problem here is the disparity between necessarily low thermal expansion coefficient prime, secondary and subarray support structures and the high electrical conductivity waveguides which will have a significantly higher thermal expansion coefficient. A particular design solution would be to have the waveguides for each Klystron form a determinant structure relative to each Klystron. Thus, each Klystron and its waveguide will form a structural unit, as shown in figure IV-C-9-2, with the subarray structure supporting each Klystron unit individually. The waveguides for each unit, however, must not transmit loads from one waveguide to the next. If they did, the thermal stresses associated with occultation could be excessive. The cost and weight estimates of thermal control for the transmission module are based on a conservative assumption that all exterior surface will require a thermal control coating. This is balanced by an assumption that if the production of space qualified thermal control coatings rises by several orders of magnitude the cost will drop from current values by at least one order of magnitude. Table IV-C-9-1 provides characteristics of three potential thermal control coatings. Although the metallized films are resistant to UV radiation, they are susceptable to charged particle degradation and have rather high spectral reflectance components. The white paints are fairly resistant to UV radiation; but they have significant limitations with respect to outgassing and an ability to be cleaned. Ideally coatings with little degradation from charged particles and with low (.08) solar absorptance need to be developed. Some progress along this line has been made with zinc orthotitanate pigments. Low a/e, high electrical conductivity coatings have yet to be developed. a. References 1. Microwave Power Transmission System Studies, Fourth Engineering Review, Contract NAS 3-17835, Raytheon Company for NASA Lewis Research Center, December 12, 1974.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==