Iranan Journal of Chemcal Engneerng Vol. 6, No. 2 (Sprng), 2009, IAChE Comparatve Study on Space Qualfed Pants Used for Thermal Control of a Small Satellte A. Anvar, F. Farhan, K.S. Nak Department of Mechancal Engneerng, Iranan Research Organzaton for Scence and Technology, Tehran, Iran Abstract A satellte wth a passve thermal control system manly uses thermal coatngs and pants to mantan temperatures wthn safe operatng lmts. Satellte coatngs, exposed to harsh space envronments such as ultravolet (UV) radaton and atomc oxygen (AO), undergo physcal damage and thermal degradaton, whch must be consdered by the satellte thermal desgner for desgn optmzaton and cost reducton. In ths paper, we have brefly revewed the effects of space envronment effects on degradaton of satellte coatngs. To study the consequences of pants degradaton on the thermal performance of satelltes wth passve thermal control, a small cubcal satellte n Low Earth Orbt (LEO) has been consdered. The satellte s bottom surface faces the Earth, and the top surface faces deep space. The satellte s lateral sdes are covered wth solar panels, and the top surface, whch acts as the satellte radator, s covered wth whte pant. The satellte orbt s sun-synchronous wth an nclnaton angle of 99º. Three radator coatngs (Chemglaze A276 and SG121FD whte pants, and AZW-11LA ceramc whte coatng) have been used n turn, and the satellte has been thermally analyzed for each case. In these analyses, begnnng-of-lfe and end-of-lfe optcal propertes have been used to predct the satellte temperatures, before and after degradaton of the coatngs. The analyses results show the mportance of stablty of optcal propertes of the thermal coatngs for the long-term thermal control of satelltes. On consderng the rate of thermal performance degradaton, lower producton cost, and ease of applcaton on satellte surfaces, SG121FD whte pant s recommended as a sutable satellte radator pant for use n satellte thermal control applcatons, wth the same desgn requrements, msson lfe and orbtal parameters as the satellte consdered n ths study. Keywords: Thermal control, Radator, Degradaton, Atomc oxygen, Ultravolet radaton 1. Introducton An orbtng satellte s exposed to varyng amounts and forms of thermal energy, namely solar radaton, albedo (solar radaton reflected off the earth surface), earth-emtted nfrared (IR) rays, and the heat generated by onboard equpment. In addton, the satellte experences extreme shfts n Correspondng author: ffarhan@yahoo.com 50
Comparatve Study on Space Qualfed Pants Used for Thermal Control of a Small Satellte temperatures, as t repeatedly passes through day and nght, a scenaro referred to as thermal cyclng. A satellte n low-earth orbt (LEO), for example, must survve temperature swngs from -80 to +80 o C, approxmately every 90 mnutes. Ths happens not once, but thousands of tmes durng ts operatonal lfetme, all whle contnung to perform ts msson [1]. Hence, a well-desgned and properly operatng thermal control system s essental to ensure optmum performance when satellte equpment s operatng, and avod damage when not n use. The energy absorbed by a satellte depends on the thermal characterstcs and area of ts external surfaces, ts orentaton wth respect to the source of thermal radaton, and the characterstcs of the thermal radaton source. Snce satellte coatngs and surfaces are spectrally responsve to the radaton source, the spectral dstrbuton of the energy source becomes partcularly mportant n the desgn of a satellte thermal control system (see Fg. 1). Passve and actve methods are used for thermal control of satelltes. A passve thermal control system manly uses coatngs and pants (black & whte), multlayer nsulaton (MLI) and radators to acheve the requred control acton. Actve thermal control systems by comparson, use heaters and mechancal refrgerators for ther operatons. The passve thermal control method s used when smplcty, cost and relablty are the key desgn factors [2]. Pants are passve thermal control hardware, used to alter optcal propertes of satellte surfaces such as radators and electronc boxes, as requred by the thermal desgn. The optcal propertes of pants, namely solar absorptance (α s ) and emttance (ε), are fxed at the tme of applcaton, and are functons of the raw materals used and the processng technques employed n applyng them to the space hardware [1]. The satellte radator presents the nterface of the satellte wth the outer space, and radatvely couples t to the space envronment. To maxmze heat rejecton and lmt heat absorpton from the surroundngs, most satellte radators are coated wth whte pants of low (α s /ε) ratos, or combnatons of black and whte pants n varous proportons, to provde dfferent shades of gray wth hgh emttance and a range of solar absorptance values [3]. However, n order to select sutable thermal pants and coatngs for satellte radators, t s necessary to understand the effects of space envronment on the performance of such thermal hardware. Fgure 1. Spectral absorptance/emttance of several materals and coatngs [2] Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2 51
Anvar, Farhan, Nak The prmary change that occurs to pants n servce s degradaton of physcal and optcal propertes due to the harsh space envronment. Ths degradaton vares as functon of tme and the satellte orbt [2]. Ths s not desrable, because satellte radators must be szed to account for the substantal ncrease n absorbed solar energy due to degradaton over the msson lfe. These radators, whch are overszed to handle the hgh solar loads at the end-of-lfe (EOL), cause the satellte to run much cooler n the early years of ts msson, sometmes requrng heaters to avod under-temperatures of electronc components. For example, changng the solar absorptance from 0.2 to 0.4 over the lfe of a satellte ncreases the needed radator area from 35 m 2 to 56 m 2 for a 10 KW satellte [4]. However, emttance (ε) of the whte coatng does not change much durng the satellte lfe (remanng almost constant). Therefore, satellte thermal desgners must account for the end-of-lfe propertes by overdesgnng components such as radators. However, ncreased area translates nto extra weght. The degradaton also affects the number, szes and postons of the satellte radators for any partcular satellte applcaton [5]. Varous thermal control pants are used for satellte applcatons, ncludng polyurethanes, slcones, and slcates. Polyurethanes tend to be cheaper, but suffer from greater degradaton due to the space envronment. The slcates are more stable, but are also more expensve, more brttle, and harder to apply [3]. Pants are very susceptble to physcal damage and thermal degradaton caused by exposure to the harsh ultravolet (UV) radaton, atomc oxygen (AO), and outgassng due to outer space hgh vacuum level. Reference [3] presents a thorough study on the space envronmental effects on materals used n Low Earth Orbts (LEO). Change n the solar absorptance of thermal control pants goes from the ntal values of 0.15-0.20 to fnal values n the vcnty of 0.4, after fve to seven years of servce [1]. However, the satellte thermal control s desgned to provde full protecton to the satellte for the entrety of ts servce lfe. Therefore, satellte desgners must account for the extent to whch the protectve capabltes of the thermal control pants degrade over that tme, and sze the thermal control system based on the predcted propertes at the end-of-lfe. Ths means that for long n-servce lfe, the satellte thermal control system must be desgned for the hgher absorptance values n excess of 0.4, resultng n a thermal control desgn, whch s large, heavy and expensve to buld. The hgh ntensty UV radaton of the space can damage organc chemcal bonds. In addton, UV radaton can lead to dslocatons n polymerc materals and ceramcs (e.g. glasses), whch change the solar absorptance of materals. Ultravolet radaton s partcularly damagng to whte pants used on the external surfaces of satelltes. UV radaton causes sgnfcant ncrease n the solar absorptance of whte pants, whle t decreases the absorptance of black pants due to bleachng effects [3]. Atomc oxygen s produced n the upper atmosphere when UV radaton from the sun s absorbed by oxygen molecules, and causes them to dssocate nto negatvely charged ons. Atomc oxygen s prevalent n orbts between 100 and 650 klometers. The atmospherc densty at these alttudes depends hghly on both orbt and solar actvty. Whle the densty s small, the onc flux encountered s hgh gven the satellte's velocty of orbt. For 250 km to 300 km orbts, a densty of 10 9 atoms/cm 3 yelds a flux of 8*10 14 atoms/cm 2 s [1]. The hgh velocty of orbt also gves AO roughly 5 electron volts (ev) mpact energy per on [4]. The carbon bonds of many organc materals are susceptble to mpact energes of ths magntude. Atomc oxygen eroson of the pant bnder results n a fragmented surface, whch could cause partculate contamnaton to other areas of the satellte. It also causes the 52 Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2
Comparatve Study on Space Qualfed Pants Used for Thermal Control of a Small Satellte emttance to ncrease gradually whle the absorptance decreases gradually [3]. The degree of surface degradaton s drectly proportonal to AO fluence (total ntegrated flux), whch n turn s determned by several factors, ncludng satellte orbt alttude, orbtal nclnaton, msson duraton, and solar actvty [6]. Although the near-earth space envronment, namely AO and UV, can have extremely damagng effects on materals, more detrmental to the satellte are the synergstc effects that arse when a materal s exposed to these factors smultaneously. The thermal desgner or materal developer should keep ths n mnd, because these synergstc effects can reduce satellte performance much faster than predcted. For example, optcal propertes (solar absorptance and thermal emttance) may change due to AO bleachng or UV radaton darkenng, polymerc flms may peel due to thermal cyclng, whch n turn opens new surfaces that can be attacked by AO or AO + UV, and fnally the electrcal conductvty of a materal may be affected by AO, resultng n spacecraft chargng [3]. The extreme condtons n the space envronment and the hgh cost assocated wth placng satelltes n orbt requre that satelltes be thoroughly tested pror to deployment n ther respectve orbts. Every satellte program must address the effect of the space envronment on ther hardware. Addtonally, all new materals must be space flght qualfed pror to use. Ground-based testng [7, 8] offers the flexblty to study multple materals n smulated space envronmental condtons wthout the extreme cost and lmted avalablty of space flght testng. Reference [1] presents research work on the mprovement of performance of passve thermal control coatngs through weght reducton, mproved end-of-lfe propertes, and formulaton changes of exstng materal combnatons. Extensve work s beng done to reduce flm thckness and lower the specfc gravty (.e., densty) of coatngs. Researchers are also workng to mprove end-of-lfe solar reflectance propertes by usng materals wth lower overall solar absorptance, and good resstance to degradaton when exposed to the space envronment. Improvements n end-of-lfe propertes wll lead to addtonal satellte weght reductons, manly due to the smaller thermal radator surface area requred for a gven heat duty. In ths paper effects of degradaton of coatngs on the thermal control of a satellte wth passve thermal control have been studed through the comparson of thermal performance of some space qualfed thermal pants and coatngs, havng dfferent begnnng-of-lfe and end-of-lfe optcal propertes. The satellte consdered s a small cubcal satellte n Low Earth Orbt (LEO). The top surface of the satellte, whch acts as the satellte radator, s covered wth whte coatng. Three radator coatngs (Chemglaze A276 and SG121FD whte pants, and AZW- 11LA ceramc whte coatng) have been used n turn, and the satellte has been thermally analyzed to predct ts temperatures, before and after degradaton of the radator thermal coatngs. 2. Systems of thermal coatngs consdered n ths study Thermo-optcal propertes and the effects of space envronment on degradaton of the coatngs under study are presented n Tables 1 through 3. The Chemglaze A276 whte pant (Table 1) s a polyurethane pant, used on many short-term space mssons. It s formulated for space applcatons requrng hgh reflectvty and low outgassng, and provdes excellent gloss and color retenton. A276 s known to degrade moderately under long term UV exposure and to be susceptble to Atomc Oxygen (AO) eroson [3]. SG121FD whte pant (Table 2) provdes excellent thermo-optcal propertes and hgh resstance to space adverse envronment (AO, UV and charged partcles such as electrons and protons), and can wthstand Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2 53
Anvar, Farhan, Nak very low temperatures (-170 C) [9]. AZW- 11LA (Table 3) s a whte ceramc coatng, whch ncorporates a stablzed pgment system wth a slcate bnder. Ths coatng s sutable for use n the harshness of the space envronment, because t has a low rate of degradaton [10]. Table 1. Thermo-optcal propertes and degradaton effects for Chemglaze A276 whte pant Pant Chemcal Composton Descrpton Solar Absorptance (α s ) IR Emttance (ε) No degradaton 0.28 0.88 Chemglaze A276 whte pant Pgment: Ttanum doxde Bnder: Polyurethane After 15000 hrs exposure to UV n LEO, Wth no atomc oxygen exposure After 15000 hrs exposure to UV n LEO, wth atomc oxygen exposure 0.60 0.35 0.88 0.88 Table 2. Thermo-optcal propertes and degradaton effects for SG121FD whte pant Pant Chemcal Composton Descrpton Solar Absorptance (α s ) or Solar Absorptance Increase ( α s ) IR Emttance (ε) No degradaton 0.20 ±0.04 0.88±0.03 SG121FD whte pant Pgment: Encapsulated znc oxde Bnder: Slcon Solvent: Aromatc and alphatc Varatons of solar absorptance due to the effect of UV alone Varatons of solar absorptance due to the combned effect of UV, electrons and protons (1 year operaton n GEO orbt) Δα s = + 0.01 for 1000 esh Δα s = + 0.09 - - 54 Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2
Comparatve Study on Space Qualfed Pants Used for Thermal Control of a Small Satellte Table 3. Thermo-optcal propertes and degradaton effects for AZW-11LA ceramc whte coatng Pant Chemcal Composton Descrpton Solar Absorptance (α s ) or Solar Absorptance Increase ( α s ) IR Emttance (ε) No degradaton 0.08 ±0.02 (for thckness > 10 mls) 0.91 ± 0.02 AZW- 11LA ceramc whte pant Pgment: Stablzed pgment system Bnder: Slcate Varatons of solar absorptance due to the combned effect of UV and atomc oxygen exposure (NASA Experments) Varatons of solar absorptance after 9 months exposure to space envronment (on Optcal Propertes Montor (OPM)) Δα s = 4% for 7.4*10 20 atomc oxygen exposure and 832 hours UV exposure Δα s = + 0.02 Δε = 1% - 3. The satellte modelng To study the consequences of degradaton, a satellte wth passve thermal control has been modelled and analyzed. The satellte model consdered s cubc n shape, n whch the bottom surface faces the Earth, and the top surface faces deep space. The satellte s lateral sdes are covered wth solar panels, and the top surface, whch acts as the satellte radator, s covered wth whte thermal coatngs. The man nternal unts of the satellte nclude an electronc box (E-Box), two telemetry unts (UHF, VHF), and two battery packs. The satellte s postoned n an orbt of crcular sun-synchronous near polar type. Fg. 2 shows the satellte model consdered for analyses, and the satellte when postoned n ts nomnal orbt. The modelng process begns wth constructon of a geometrcal mathematcal model (GMM) (shown n Fg. 3). Thermal Desktop software has been used for ths purpose. The GMM conssts of sub-models such as E-Box, telemetry unts, batteres, structural elements and the solar panels. In the next step a thermal mathematcal model (TMM) has been constructed. The complete TMM conssts of a network of thermal nodes, lnear and radaton conductors between dfferent unts, and modelng of nternal heat dsspatons n the satellte. Lnear conductors transport heat n drect proporton to the dfference n nodal temperatures: Q1 2 = G( T1 T2 ), where Q 1-2 s the heat flowng from node 1 to node 2 through a conductor of value G, T 1 s the current temperature of node 1 and T 2 s the current temperature of node 2. On the other hand, radaton conductors transport heat accordng to the dfference n the fourth power of absolute temperature: 4 4 Q = G( T ) 1 2 1 T2. Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2 55
Anvar, Farhan, Nak More than 1000 thermal nodes of arthmetc and dffuson types have been consdered n these analyses. The satellte radator has been consdered wth a whte coatng, allowng t to have sutable thermal emsson to the outer space through the radatve conductors. The nternal surfaces have been consdered black to facltate radatve heat exchange between varous surfaces and the nternal satellte unts. Outputs from the GMM, ncludng vew factors of the satellte surfaces exposed to the space envronment, and the envronmental heat fluxes (Earth albedo, drect solar flux, and Earth emtted IR), have been used n the thermal mathematcal model. Fgure 2. The satellte model under consderaton (left), satellte n ts nomnal orbt (rght) (colors are for demonstraton only) Fgure 3. Geometrcal mathematcal model of the satellte (colors are for demonstraton only) 56 Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2
Comparatve Study on Space Qualfed Pants Used for Thermal Control of a Small Satellte The satellte system under consderaton s transent, and hence, a transent thermal analyss, based on numercal mplct Forward-Backward method, has been used. A standard thermal software [11] has been used to determne the temperatures of varous satellte surfaces and unts. The heat balance for a dffusve thermal node s gven as [11]: 2 C Δt N G j= 1 j= 1 N n+ 1 n ( ) n n = + ( ) + n 4 n 4 T T 2Q G T T G j ( T ) ( T ) j n+ 1 n+ 1 + + ( ) n 1 4 n 1 4 T T + G j ( T ) ( T ) j j j j j + (1) T j Temperature of thermal node j at current tme t n+1 T j Temperature of thermal node j at current tme t+δt G j Lnear conductor for connectng the dffuson thermal node j to thermal node G Radatve conductor for connectng j the dffuson thermal node j to thermal node C Q Heat capactance of the dffuson thermal node Heat Source/Heat Snk for dffuson thermal node. 4. Results and dscusson Fg. 4 presents the radator temperatures, predcted before the degradaton of the three radator coatngs (α s and ε are BOL values). As the absorptance value for A276 whte pant s hgher than the other two coatngs, and the absorptance value for AZW-11LA s the lowest among the three, the maxmum radator temperatures occur for A276 whte pant, and the lowest temperatures are predcted for AZW-11LA whte ceramc coatng. Temperatures of the radator coated wth SG121FD whte pant le between the correspondng values for the other two coatngs. Fg. 5 presents the radator temperatures, predcted after the degradaton of the three radator coatngs (α s and ε are EOL values). As shown, the temperature trends for the three coatngs reman the same, however, the maxmum temperature values are hgher than the correspondng values n Fg. 4. The reason for ths relatve ncrease n temperature values s the ncrease n the absorptance values due to the thermal degradaton of the three coatngs. Out of the three coatngs, A276 pant shows the maxmum ncrease, because t undergoes the maxmum degradaton n comparson to the other coatngs (see Tables 1 through 3). The SG121FD whte pant comes second, whle AZW-11LA whte ceramc coatng has the least degradaton, and hence, the least ncrease n temperature of the radator covered wth ths coatng. However, due to the low absorptance of AZW-11LA whte ceramc coatng, compared to the other two whte pants, the radator temperatures for the cold orbtal condton are lower than the correspondng temperatures for the other pants. As a result, the man satellte components n the vcnty of the radator (e.g., battery) wll experence lower temperatures, even needng heaters to heat them up durng certan perods n orbt. Ths s not desrable, because t results n an ncrease n the satellte power budget. On the other hand, applcaton of SG121FD whte pant does not result n crtcal thermal stuatons n ether cold or hot orbtal cases, or any ncrease n satellte power budget, makng t more sutable than AZW-11LA whte ceramc coatng for the case under consderaton. Fg. 6 presents the radator temperatures, predcted before and after the degradaton of A276 pant. As shown, the degradaton has resulted n about 12 ºC ncrease n the radator temperature, whch s not desrable. Snce the heat dsspaton from nternal satellte unts and the envronmental heat loads (solar, earth albedo and emtted IR) are reradated to the space by the satellte radator, any ncrease n the radator temperature due to ncreased envronmental Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2 57
Anvar, Farhan, Nak heat loads, wll result n reduced temperature gradent between the radator and the dsspatng unts, and hence, reduced heat rejecton capablty. The consequence s a rse n temperatures of the dsspatng unts, whch n some nstances, even exceeds ther safe operatng temperatures. Therefore, t s advsable to keep the radator temperature at a reasonably low temperature by usng coatngs wth less thermal degradaton. The effect of degradaton of A276 whte pant on the satellte battery temperature s shown n Fg. 7. The maxmum temperature of the satellte batteres, before and after the degradaton of A276 pant, shows an ncrease of about 8.5 ºC. It s mportant to note that any ncrease n the operatng temperature of the battery, whch s a crtcal unt wth tght temperature requrements, wll result n reduced effcency and reducton of the battery s useful lfe over the msson. Radator Temp. Radator Temp. Radator Temp. 2.5 A276 Temperature[C] 0.0-2.5-5.0-7.5 SG121FD AZW-11LA -10.0 33.25 33.50 33.75 34.00 34.25 34.50 34.75 35.00 Tme [hr] Fgure 4: Temperatures of radator coated wth the three selected coatngs (no degradaton) Radator Temp. Radator Temp. Radator Temp. 15 A276 10 Temperature[C] 5 0-5 -10 SG121FD AZW-11LA 33.25 33.50 33.75 34.00 34.25 34.50 34.75 35.00 Tme [hr] Fgure 5. Temperatures of radator coated wth the three selected coatngs (after degradaton) 58 Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2
Comparatve Study on Space Qualfed Pants Used for Thermal Control of a Small Satellte Radator Temp. Radator Temp. 15.0 12.5 After Degradaton 10.0 Temperature[C] 7.5 5.0 2.5 0.0-2.5 Before Degradaton -5.0 33.25 33.50 33.75 34.00 34.25 34.50 34.75 35.00 Tme [hr] Fgure 6. Radator temperatures, before and after degradaton of A276 whte pant BATTERY Temp. BATTERY Temp. 27.5 25.0 After Degradaton Temperature[C] 22.5 20.0 17.5 Before Degradaton 33.25 33.50 33.75 34.00 34.25 34.50 34.75 35.00 Tme [hr] Fgure 7. Effect of degradaton of A276 whte pant on the battery temperatures Fg. 8 shows temperatures of the telemetry unt (UHF), predcted after the degradaton of the three radator coatngs (α s and ε are EOL values). The ncrease n temperature of the telemetry unt, due to the degradaton of the three coatngs, s clearly shown n ths fgure. Agan, the maxmum ncrease s predcted for A276 pant, whch has the hghest rate of degradaton among the three coatngs. Temperatures of the electronc box (E-Box), predcted after the degradaton of the three radator coatngs (α s and ε are EOL values) s shown n Fg. 9. Here also, the coatng wth the hghest rate of degradaton (A276 whte pant), produces the maxmum ncrease n the satellte electronc box (E-Box) temperature. Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2 59
Anvar, Farhan, Nak Telemetry Temp. Telemetry Temp. Telemetry Temp. 45 A276 40 Tem perature[c] 35 30 25 SG121FD AZW-11LA 33.25 33.50 33.75 34.00 34.25 34.50 34.75 35.00 Tme [hr] Fgure 8. Effect of degradaton of the three coatngs on temperatures of telemetry unt 55 A276 EBOX Temp. EBOX Temp. EBOX Temp. 50 Tem perature[c] 45 40 35 SG121FD AZW-11LA 30 33.25 33.50 33.75 34.00 34.25 34.50 34.75 35.00 Tme [hr] Fgure 9. Effect of degradaton of the three coatngs on temperatures of the E-Box 4. Conclusons Pants and other thermal coatngs n satelltes are nfluenced by adverse space envronmental effects, namely atomc oxygen (AO) and ultravolet (UV) radaton. These nfluences become more crtcal when the synergstc effects of AO and UV are taken nto account. Consequently, thermal control pants and coatngs degrade wth tme due to ther extended exposure to envronmental effects. Ths degradaton, whch drectly affects the thermal control of senstve 60 Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2
Comparatve Study on Space Qualfed Pants Used for Thermal Control of a Small Satellte satellte unts, must be gven due consderaton when selectng thermal pants and coatngs for any partcular space applcaton. The three thermal control coatngs analyzed n ths paper show dfferent behavor when exposed to envronmental effects, resultng n dfferent rates of degradaton for each thermal coatng. The analyses results for the case of a satellte wth A276 whte pant, whch has the hghest degradaton rate (about 100%) and begnnng of lfe absorptance value, show that the satellte radator experences a maxmum ncrease n temperature of about 12 C. Ths apprecable ncrease n the satellte radator temperature suggests the need, at the desgn stage, for consderaton of an approprate margn for the begnnng of lfe absorptance value (100% or more n certan nstances), to account for the degradaton of ths pant at the end of the satellte msson. The results further show that the use of A276 whte pant on satelltes requrng precse thermal control on extended LEO mssons should be avoded as far as possble. However, other factors such as low cost and ease of applcaton, make ths pant much more desrable for boosters and upper stage rockets that do not requre long msson lfetmes. The analyses results for SG121FD whte pant, wth a degradaton rate of about 45%, show a maxmum ncrease n the satellte radator temperature of only about 4 C. Further observaton ndcates that applcaton of SG121FD whte pant does not result n crtcal thermal condtons n ether cold or hot orbtal cases. The results of analyses for AZW-11LA whte ceramc coatng, wth the lowest degradaton rate (about 25%) and begnnng of lfe absorptance value, show neglgble ncrease n the satellte radator temperature. However, for the satellte under consderaton, temperatures durng the cold orbtal condtons are lower than the allowable temperature lmts for senstve satellte unts. Ths necesstates the use of heaters to warm up the the unts to ther safe operatonal temperatures durng cold orbtal condtons. From the pont of vew of satellte power consumpton, ths s not desrable because t ncreases the satellte power budget to compensate for the ncreased power usage n the heaters. The two thermal coatngs, SG121FD whte pant and AZW-11LA whte ceramc coatng, wth hgher resstance to degradaton due to space envronmental effects, could be good canddates for use n mssons lastng 3 years or more. However, consderng the overall thermal performance degradaton, lower producton cost, and ease of applcaton on satellte surfaces, SG121FD s recommended as a sutable satellte radator pant canddate for use n satellte thermal control applcatons, havng the same desgn requrements, msson lfe and orbtal parameters as the satellte consdered n ths study. References 1. Berman, E.S. et al, Spacecraft materals development programs for thermal control coatngs and space envronment Testng, The AMTIAC Quarterly, 8(1), (2004). 2. Glmore, D.G., Spacecraft Thermal Control Handbook, Vol. I: Fundamental Technologes, The Aerospace Corporaton Press (2002). 3. Slverman, E.M., Space Envronmental Effects on Spacecraft: LEO Materals Selecton Gude, NASA Contractor Report 4661, Parts 1 and 2, TRW Space & Electroncs Group, Redondo Beach, Calforna, August (1995). 4. Gutbre, J.D. et al Testng n the Space Envronment, The AMTIAC Quarterly, 8(1), (2004). 5. Anvar, A, Nak, K.S., and Farhan, F., Numercal nvestgaton on the effect of radator poston on temperatures of small satellte components, Proceedngs of The 14 th Annual (Internatonal) Mechancal Engneerng Conf. (ISME2006), Tehran Iran (2006). 6. Doolng, D., and Fnckenor, M.M., Materal Selecton Gudelnes to Lmt Atomc Oxygen Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2 61
Anvar, Farhan, Nak Effects on Spacecraft Surfaces, NASA/TP- 1999-209260, Marshal Space Flght Center, MSFC, Alabama 35812, June, (1999). 7. Marco, J., Bhojaraj, H., and Hulyal, R., Evaluaton of thermal control materals degradaton n smulated space envronment, Proceedngs of the 9th Internatonal Symposum on Materals n a Space Envronment, Noordwjk, The Netherlands, 16-20 June (2003). 8. Stuckey, W. and Meshshnek, M.J., Ground testng of spacecraft materals, Crosslnk The Aerospace Corporaton Magazne of Advances n Aerospace Technology, 199, (2002). 9. SG121FD WHITE PAINT, Data Sheet RS127, CNES Report: DTS/AE/MTE/TH/00-141, (1998). 10. AZW11LA Inorganc Low Alpha Whte nonspecular thermal control coatng, AS1 Data sheet, Spacecraft Thermal Control (TC) Coatngs and Servces Catalog, AZ Technology Inc., Jan. (2004). 11. SINDA/FLUINT User s Manual, Verson 4.4, Cullmore and Rng Technologes, Inc., (2001). 62 Iranan Journal of Chemcal Engneerng, Vol. 6, No. 2