twice that of the laser-irradiated surace and neglects all conductive losses. Solving the above for the irradiance E required to bring the aluminum up to its 3 melting point (773 K) and setting e - 0.90 for a good radiator yields a minimum 2 threshold absorbed irradiance for melting of Ea ~ 3.6 W/cm . The requirement for incident irradiance can be determined from Figure C.3-4, which shows coupling coefficients (ratios of absorbed to incident irradiance) for the wavelength regime of interest. As the antenna face is designed to be a good thermal radiator, its coupling can be approximated by the black paint line for low laser irradiance. Using the high value of 0.9, the minimum threshold incident 2 irradiance for melting is about 4 W/cm . The above minimum irradiance requirement must now be related to energy density 2 (J/cm ) and time requirements. The incident energy density to melt a typical 2 aluminum alloy is about 3300£ J/cm , where £ is the thickness in cm, and the 2 coupling is 0.9. At 4 W/cm the time required for melting of a 0.5-cm-thick aluminum plate is about 400 seconds, which is quite long. More typical times for spacecraft components to reach thermal equilibrium, through a combination of radiation and the conduction neglected in this analysis, are in the range of 50 to 100 seconds. Using this range as a time criterion leads to a minimum incident 2 irradiance requirement of about 15 to 30 W/cm for a front face of 0.5-cm-thick aluminum. This incident irradiance requirement would scale approximately linearly for other thicknesses. A typical SPS antenna system would therefore be invulnerable to the nominal laser threat from an Earth-based laser but could be vulnerable to a space-based laser. Solar Cell Arrays. The solar cell arrays are a critical portion of the SPS. This section evaluates the vulnerability of the solar cells in terms of expected configurations for commercially available cells. The assessment and resulting vulnerability levels include the assumption of prudent design practices, but do not include the effects of possible countermeasures such as filtering or hardened material development. A typical solar cell consists of a protective front cover, the cell itself, the attachment of electrical connectors to the cell, a bottom substrate, and adhesives to bond at least one of the interfaces. The vulnerability level of a composite cell is essentially a function of the failure temperature for one or more
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