temperature is not straightforward. The thermal heat capacity of the overall antenna system must be known. The eclipse time is on the order of half an hour. Although the thermal analysis is not presented here, a detailed analysis is necessary. The above discussion briefly covered both extreme thermal cases, hot and cold. The final aspect to be considered concerning the antenna is the thermal cycling. The components used in the antenna structure must all have thermal expansion characteristics which are similar. This is to assure that components do not crack (worst case) and that structural distortion is limited. Thermal cycling needs to be studied in a detailed simulation. The allowable thermal distortion is determined by the required pointing accuracy of the whole beam and by the beam dispersion caused by surface errors. As a general rule the thermal deformation should not exceed wavelength divided by 10 (= 12 mm) from the nominal flat antenna surface. Taking into account the surface defects, this a severe constraint, because with the beam pointing system selected the defects cannot be compensated by phase shifting. This compensation is only possible if a retrodirective system is used. The 20 m long cable used to connect the Mir power source to the antenna (see section on “Power interfaces”) is a potential source of concern. The cable has a diameter of 17.2 mm and is assumed to be made of copper for preliminary dimensioning purposes. The power loss is assumed to be 1%. For this case the equilibrium temperature of the cable was computed by the same method used above. It was furthermore assumed that the cable can only radiate heat directly to space, i.e. the angle of view was taken to be 180°. The surface of the cable was assumed to be covered with white epoxy (data: see above text). The results are summarized in Table 10.2.1: Table 10.2.1 Power Cable Equilibrium Temperature The results are not very conservative because the cable insulation has not been taken into account. The insulation would lead to a much higher core temperature of the wire since the thermal conductivity of electrical insulators is generally very low. Mechanisms and Structures The phased array antenna and rectenna are to be folded in half. They are mounted on the outside of Progress. Care must be taken that they are inside the dynamic envelope of the Soyuz launch vehicle fairing as can be seen in Figure 10.2.10. In order not to modify the Progress spacecraft too much, use is made of existing mechanical interfaces. These interface points are normally used to support solar arrays. The distance between these two mechanical interfaces is approximately two meters in the longitudinal direction of Progress. The support points of both the antenna and rectenna must be strong enough to withstand the axial launch loads. To avoid damage to the antenna and rectenna they must be secured safely during the launch. State of the art thermal knife technology as developed by Fokker Space Systems will be used to hold down the panels. This thermal knife system is based on Kevlar cables being degraded by two heating elements and has numerous advantages over using pyrotechnic devices. The thermal knife has already been used in 1988 on a CNES antenna release experiment at the Soviet space station Mir. Both the antenna and rectenna panels should have sufficient lateral stiffness so as to fulfill the dynamic requirements of the Soyuz launcher (Ariane 4 requires that the first lateral mode is more than 55 Hz). No information was available about the thermal and acoustic levels in the Soyuz fairing. The deployment mechanism of the rectenna must be simple and automated, so that there is no need for additional EVA. The thermal knives will release the panels. Torque springs will guarantee that the deployment of the rectenna is successful. This operation is followed by a rotation of 90 degrees about the hinge of the rectenna as shown in Figure 10.2.11. The antenna will be supported in a similar manner. However it should be easily removable by cosmonauts performing EVA. During EVA the cosmonaut(s) will first remove the antenna from Progress. After that they will remount it on the outside of Mir. Use will be made of either the additional solar array mounting points or the additional working platform.
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