1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-09a71 Center Point of Contact: GSFCa71 Submitted by: Wil HarkinsSubject: Earth Orbit Environmental Heating Practice: Use the currently accepted values for the solar constant, albedo and earth radiation when calculating the
2、 heat balance of earth orbiters. This practice provides the heating rates for the black body case without consideration of spectral effects or collimation.Programs that Certify Usage: N/ACenter to Contact for Information: GSFCImplementation Method: This Lesson Learned is based on Reliability Guideli
3、ne Number GD-AP-2301 from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Consideration of the solar, albedo, and earth radiation thermal inputs, including seasonal variation with tolerances, is required to accurately predict the thermal environment
4、of orbiting devices.Implementation Method:SOLAR CONSTANTProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-The nominal solar constant value is 1367.5 W/m2. The variation of the earth-sun distance causes a 3.5% seasonal variation from nominal. The accura
5、cy of the solar constant is taken as 0.5%. The following are the values for various seasons in the northern hemisphere.NOMINAL1367.5 W/m2WINTER1422.0 W/m2(NOM + 4.0%)SUMMER1318.0 W/m2(NOM - 4.0%)ALBEDO FACTOR*The nominal albedo factor is 0.30. The variation around the nominal should be 0.05. No vari
6、ation during the sunlit portion of a given orbit should be assumed unless extremely light weight items are being considered. Programs that compute albedo energy should use 0.35 (hot case), 0.30 (nominal case), and 0.25 (cold case), respectively.*Note: Since earth temperature and albedo vary with lat
7、itude, as the orbit approaches either extreme of a polar or equatorial orbit, further study of the literature should be made. (see AIAA-87-1596)EARTH EMITTED ENERGY *The nominal earth temperature for earth emitted IR energy is 255 degrees K. This temperature produces a heating rate of 241 W/m2. A re
8、asonable variation can be obtained by maintaining consistency using the following relationship between Solar, Albedo, and Earth Emitted Energy:Earth Emitted Energy = (1-Albedo Factor)x Solar Constant / 4.0Table 1 shows the variations in Earth Emitted Energy that result from using the above recommend
9、ed Solar and Albedo ranges.Software programs that compute Earth Emitted Energy should use the appropriate hot, nominal, or cold case Solar and Albedo values; and the corresponding black body Earth temperature to achieve an energy balance.REFERENCES FOR QUICK CHECKS OR SIMPLE CALCULATIONSHand calcula
10、tions should be made to verify that computer outputs of heating values for flat surfaces of known orientation and minimal reflected inputs from other surfaces are reasonable. Hand calculations also may be necessary when time does not permit a computer study. A check of incident Provided by IHSNot fo
11、r ResaleNo reproduction or networking permitted without license from IHS-,-,-Albedo energy to a flat plate at various altitudes and orientations can be made by using TN-D 1842 “Earth Reflected Solar Radiation Incident Upon an Arbitrary Oriented Spinning Flat Plate,“ by F. Cunningham. Figures 1 throu
12、gh 9 show the orbit-averaged incident Earth and Albedo energies to an Earth-oriented flat plate at various altitudes and orbit/sun angles. Eclipse factors for elliptical orbits are provided in “Calculation of the Eclipse Factor for Elliptical Satellite Orbits“, by F. Cunningham. A hand calculation o
13、f incident Earth Emitted Energy to a flat plate at various attitudes and altitudes also is possible. Figure 10 shows the instantaneous geometric shape factor for a planar surface as a function of altitude and attitude (h/R is the ratio of the orbit altitude to the Earth radius). The earth radius is
14、6,365 km. The incident Earth Emitted Energy is found by multiplying the shape factor times the black body emissive power at the earth temperature. For an altitude of 1,000 km and a flat plate whose normal is 90 degrees to the nadir (l= 90); h/R = 0.157, which gives a shape factor of 0.19. The Earth
15、Emitted Energy incident on the plate is 0.19 x 241 W/m2or 46 W/m2.*NOTE: Since earth temperature and albedo vary with latitude, as the orbit approaches either extreme of a polar or equatorial orbit, further study of the literature should be made (see AIAA-87-1596).RECOMMENDED SOLAR AND ALBEDO RANGES
16、*SOLAR CONSTANT (W/m2)ALBEDO FACTOREARTH EMITTED ENERGY (W/m2)EQUIV. EARTH TEMP (K)NOMINAL 1368 0.25 0.30 0.35256 239 222258 254 250WINTER SOLSTICE14220.25 0.30 0.35267 249 231262 258 253SUMMER SOLSTICE13180.25 0.30 0.35247 231 214256 251 246*For use in Orbit Average AnalysesEQUIVALENT SINK TECHNIQU
17、E The equivalent sink technique can be used by replacing all surrounding surface radiant interchanges and the absorbed Solar and Earth energies to node i with a single radiation coupling to a single node at temperature T sink.Provided by IHSNot for ResaleNo reproduction or networking permitted witho
18、ut license from IHS-,-,-To derive the equation for this sink temperature, first consider an energy balance at node i where all the inputs are treated as gross inputs and node i has a view to space of 1.0.(1) refer to D description DFrom planetary flux program (TRASYS or SSPTA) From thermal program (
19、SINDA) results obtained from Geometric Math Model (GMM) radiation exchange program Where:refer to D descriptionD Next consider the equivalent sink energy balance situation:refer to D descriptionD(2)refer to D descriptionDSolving (1) for refer to D descriptionD and setting equal to the right side of
20、(2) gives:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-(3)refer to D descriptionDThe equivalent sink for node i may be determined from the detailed thermal math model by determining the adiabatic temperature of node i when node i is disconnected f
21、rom internal heat paths and heat dissipations. For a transient situation, node i must be an arithmetic node or a low mass node.Technical Rationale:Thermal analysis of an earth orbiting spacecraft requires the accounting of incident thermal energy from all external sources. The most significant exter
22、nal sources of energy incident on the spacecraft are the sun, the thermal radiation of the earth, and the solar energy reflected from the earth (albedo). The modification of the energy incident on the spacecraft due to the earth-sun distance variation, and the accuracy of the measurements of the sol
23、ar constant, are of sufficient magnitude to be important parameters in performing a thermal analysis.Impact of Non-Practice: Not considering the variations in the environmental thermal effects as described in this guideline will result in an incomplete thermal analysis. The temperature variation of
24、the spacecraft could be grossly underestimated, thereby reducing its reliability.Related Practices: N/AAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2000-03-09a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
