## 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, of-10 to +50 °C and a minimum non-operating 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 5-year 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 low-Earth 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 sub-solar 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. (11-17) and (11-18) 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 5-mil thick silver teflon surface finish commonly used on radiators in low-Earth orbit, the radiator will have an emittance of 0.78 and a minimum beginning of life solar absorptance of 0.05. Because the radiator absorp-tance will increase over its life, however, we must also consider an end-of-life absorptance value of 0.15 to account for the degradation that will occur over the 5-year mission. The results of these calculations are shown in Table 11-48B.

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 orbit-average fluxes. From

TABLE 11-48B. Earth IR and Albedo Heat Loads Absorbed on an Earth Facing Radiator In a 500 km, 90 deg Inclined Orbit ß Is the angle of the Sun out of the orbit plane.

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