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"Hot case: EOL absorptance of 0.15 and maximum albedo (0.30) and Earth IR (244 W/m2) constants bCold case: BOL absorptance of 0.05 and minimum albedo (0.23) and Earth IR (218 W/m2) constants

Table 11-48A we can see that the hottest case occurs at end of life for an orbit fi angle of 0 deg. If we allow for 10 °C of analysis uncertainty margin, we can use Eq. (11-15) and set the sum of the maximum electronics waste heat and the heat load absorbed from the environment under hot case conditions to equal the radiator heat rejection capacity at 40 °C (313 °K):

500 W + (182 W/m2) A = (0.78) (5.67 x 10"8 W/m2K4) A (313 °K)4 (11-19)

Solving for A, we get an area of 2.06 m2.

To determine if any heater power is required during normal operations, we again use Eq. (11-15) to calculate the minimum temperature under cold-case conditions. We use the above area with the cold case power dissipation of 400 W and cold case environmental loads from the 90 deg /¡angle orbit to solve for temperature:

400 W + (148 W/m2) (2.06 m2) = (0.78) (5.67 x 10"8 W/m2!?4) (2.06 m2) (T)4 (11-20)

Solving for T, we get a cold case minimum temperature of 297 °K = 24 °C. Subtracting 10 °C for analytical uncertainty gives a minimum operating radiator temperature of 14 °C, which is well above the minimum allowable operating termperature of -10 °C.

To determine the heater power required to keep the electronics above their minimum non-operating temperature limit, we add 10 °C of uncertainty margin to the minimum allowable non-operating termperature and use Eq. (11-15) to solve for the power required to keep the radiator above -10 °C under cold case conditions:

(XW) + (148 W/m2) (2.06 m2) = (0.78) (5.67 x 10"8 W/m2!?») (2.06 m2) (263 °K)4 (11-21)

Solving for X, we get a heater power of 131 W needed to keep the electronics above their non-operating minimum temperature limit under worst-case conditions.

11.5.6 Mass, Power, Telemetry Estimates

Mass

Historically, the TCS mass loosely correlates to the spacecraft dry mass or total spacecraft power generation capability. Typically the thermal control hardware mass is 2 to 10% of the spacecraft or instrument dry mass. A purely passive thermal control approach tends toward the lower end of the range. If active control techniques are used, the mass tends toward the higher end of the range.

As the complete thermal control approach is formulated and a hardware list is developed component-level estimates can be performed to provide a more accurate assessment. For low-power spacecraft, the bulk of the thermal mass is usually the MLI, but if radiator panels or heat transport systems are to be used, their mass should be estimated early. Any heater controller electronics, as well as all heaters, wiring, and thermostats, should be included in the thermal control subsystem mass estimate.

TABLE 11-49. Thermal Hardware Mass and Power Estimates. The estimates of mass and power for typical components of the thermal control system are shown here.

Thermal Component

Mass

Power

Comments

Multi-Layer Insulation

0.73 kg/m2

0 0

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