Ir
244 W/m2
Soiar Absorptance IR emittance
Maximum Minimum
Minimum Maximum
See Sec. 11.5.1
Solar Absorptance IR EmissMty Effectiveness
0.55 0.67 0.01 cold side 0.03 Sun side
0.35 0.75 0.03 cold side 0.01 Sun side
Kapton outer layer Kapton outer layer Biased effective emissivity
Power Dissipation
Maximum
Minimum
Based on component estimates
Geosynchronous spacecraft can utilize average fluxes on only the north and south sides because the Sun maintains a constant angle to those surfaces over an orbit; the other four sides each get about 12 hours of Sun at varying angles of incidence during the 24 hour orbit
Example
Problem: Determine the radiator area and heater size needed for a group of electronics boxes located on the nadir face of an Earth pointing spacecraft These boxes have an allowable mounting surface temperature range, while operating, of10 to +50 °C and a minimum nonoperating temperature of 20 °C. the electronics boxes dissipate a maximum of 500 W and a minimum of 400 W when operating and 0 W when not operating. Assume a 5year mission in a 500 km altitude, 90 deg inclination Earth orbit
Solution: A nadir facing radiator will receive Earth IR and albedo heat loads along with some direct solar illumination in this lowEarth orbit The Earth IR load will be constant around the orbit since the radiator is constantly facing straight down. Albedo will be at a maximum near the subsolar point and decrease to near zero as the spacecraft crosses the terminator. Because there is only a brief period, between eclipse entrance or exit and terminator crossings, when this surface will receive direct solar illumination at a shallow angle of incidence, we will neglect the contribution of direct solar load in this calculation. (To be rigorous, one could calculate the solar load using the equations provided in Sec. 5.1.)
Using Eqs. (1117) and (1118) and the tables in Appendix D.2, we can calculate the absorbed Earth IR and albedo fluxes for a number of points around the orbit If we assume that the radiator has the 5mil thick silver teflon surface finish commonly used on radiators in lowEarth orbit, the radiator will have an emittance of 0.78 and a minimum beginning of life solar absorptance of 0.05. Because the radiator absorptance will increase over its life, however, we must also consider an endoflife absorptance value of 0.15 to account for the degradation that will occur over the 5year mission. The results of these calculations are shown in Table 1148B.
Because the thermal time constant of a radiator coupled to electronics boxes is large compared to the orbital period, we can size the radiator to orbitaverage fluxes. From
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