space resources RICHARD M. LAWRENCE WILLIAM H. BOWMAN'
for teachers
8011 State University Munde. Indiono 47306
Optical Surfaces for Temperature Control of Spacecraft The problem of maintaining the temperature of a spacecraft and its components is complex. The compartmental temperatures required for a typical space mission vary from the extremely low temperatures needed for storage of cryogenic rocket propellants to those found in the combustion chambers of rocket engines (1). Once the compartmental temperat,ures are determined, a temperature history for each component in the spacecraft is calculated. Heating or cooling devices, conductive heat paths, and heat shielding are introduced as required. The problem of maintaining and regulating the temperat,ure of a spacecraft is influenced by two factors. First, any major heat sources that are carried in the spacecraft provide a considerable weight penalty for the mission. Secondly, space is characterized by extremely low gas pressures, and consequently, the transfer of heat between a spacecraft and its surroundings is essentially by radiation. Within the solar system a spacecraft receives electromagnetic radiation directly from the sun (2) and indirect,ly, in the form of reflected sunlight (29, from nearby planets. The fraction of the radiation absorbed by the spacecraft depends on the nature of its surface and on the nature of thc incident radiation. The problem is made more complex by the fact that the intensity and The qualit,y of the radiation environment can vary considerably during the course of a mission. For instance, the prelaunch environment is drastically different from that of space. In space, the radiation environment can vary as a result of the spacecraft's being eclipsed from the sun by a planet. High temperature pulses that occur when a spacecraft passes through planetary atmospheres also contribute to the complexity of the problem. The energy received by a spacecraft whether from its surroundings or from nuclear, chemical, mechanical, and electrical processes occurring within the spacecraft, is dissip,rtrd into space in tlir form of infrared wdi,ition. The r.unnrit\. :tnd ou~litvofthis emission arrdrrt.rmincil by the nature of the surfaces of the spacecraft and by This article is one of the series of articles based on resource units in Lawac~cn,R. M., A N D Bo\\-MAN,W. H., "Sp~ceIZesources for Teachers: Chemistrv." " , NASA EP-87. ~, 1971. availzhle throueh the Superintendent of Documents, Government Printing Office, Washington, D. C. 20402 (82.50). Present address: Laboratory of Biochemical Pharmacology, National Institute of Arthritis and Metabolic Diseases, Bethesda, Md. 20014. ~
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their temperatures. It is not unexpected, therefore, that an important method of temperature regulation is the use of surfaces that have appropriate light absorbing and emittingcharacteristics (1,4,6). The absorbing qualities of a surface are described in terms of its solar absorptivity, a,,the fraction of solar radiation which is absorbed. An ideal black body would have an a, value of unity. The infrared emitting qualities of a particular surface are given in terms of its emissivity, e. Emissivity is the ratio of the energy emitted by a surface to that which would be emitted by an ideal black body a t the same temperature. Surfaces and surface coatings are grouped on the basis of their relative values of a,and r as shown in the table (6). Solar Reflectors
Solar reflectors are characterized by small values of the ratio or,/e. They are substances which reflect most incident radiation and readily emit energy in the form of infrared radiation. Such surfaces are useful for coating compartments which require low temperatures such as storage tanks for cryogenic fuels. Two types of materials that have these properties have been tested, white paints and secondary mirrors. The nature of the emissivity (emittance) of these solar reflectors is compared in Figure 1 (I) with that of a material considered an ideal solar reflector. White paints can be made by the addition of stable white pigments to organic vehicles. However, the op-
TABOR SURFACE
WHITE PAINT &OPTIMUM MIRROR
~ . g y ~z ~ SOLAR ABSORBER
S
,
ALUM~NU~
0
,sOY..RE.YOO.,
0
2
10 WAVELENGTH ( P )
20
2
1
20
10 WAVELENGTH ( y )
-
Figure 1 . Representdive spectral emitlance curves. The wove length dirtribvtionsof light emitted by octual substances and ideal materials are given for four typesof optical surfmser.
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tical properties of paints made with organic vehicles are especially susceptible to alteration in space where the intensities of ultraviolet and soft X-ray radiation are relatively high. These components of t,he solar spectrum have sufficient energy to rupture chemical bonds in organic vehicles usually causing an increase in the value of m J r . Paints containing organic vehicles are also more readily degraded by high temperature pulses that occur during passage of spacecraft through planetary atmospheres. Silicone paints wibh white pigments, such as TiOzand ZnO, are more stable to high temperatures and to the short wavelength radiation from the sun hut are generally more difficult to bond to a spacecraft. Ceramics, on the other hand, have good adhesion to common structural materials as well as superior temperature and radiation st,ahilit,y. Ceramics are formed from white pigments mixed with vehicles such as water or aqueous alkali metal silicates or phosphates. Suitable pigments are finely powdered lit,hium aluminum silicates, silica, alumina, and zinc oxide. Ceramic optical coatings, however, are difficult to apply. Secondary mirrors are another type of solar reflector. A secondary mirror is formed by coating a shiny metal surface with a material which has a high infrared emissivity and is transparent to solar radiation. The solar absorptivity of the mirror is nearly that of the metal backing, and the infrared emissivity is dependent on the transparent substance. For instance, the Vanguard satellite was coated with a secondary mirror prepared by evaporating a film of aluminum on the base metal and covering the film with a 0 . 6 5 ~thick layer of silicon monoxide, SiO. Flat Reflectors
Surfaces which reflect solar radiation but emit only a small amount of infrared radiation are called flat reflect,ors (see the table). Materials having these optical Characteristics of Thermal Control Coatings Ranges
Tvoe
Solar Absorptivity
Emissivity
Solar Reflector 0.1-0.3 Flat Reflector 0.1-0.3 Fl?t Absorber 0.8-0.9
Ratio a./r
a~
Examole
0.8-0.9 0.1-0.4 White paint 0.1-0.3 0.3-3 Aluminum psint 0.8-0.9 0.9-1.1 Black psint
solar
Absorber
0.2-0.5
0.03-0.3
2-15
Polished metal
properties are not common, and flat reflectors are presently obtained with highly polished metals or with paints pigmented with metal flakes. The emissivities of polished silver and of aluminum paint are shown in Figure 1. The optical properties of these coatings are susceptible to alteration as a result of corrosion of the polished metal or through chemical changes in the vehicle of the paint, respectively. Flat Absorbers
Flat absorbers, a third class of optical surfaces, differ from flat reflectors in that they have relatively high solar absorptivities and high emissivities (see the table). Paints which are flat absorbers can be made from black pigments such as the oxides or mixed oxides Cr30r, FezO,. NiO, FesOn, or n'In,O,. NO. The pig606 / Journal o f Chemicol Education
Figure 2. Twelve-foot diameter Explorer IX ~ o f e l l i t ewith 3600 white dots for temperature control. The shell war con~tructed of M y l a r polyester fllm and aluminum foil and w o i dotted wit11 white epoxy covering 17% of its surfme.
ments are ground and dispersed in silicone elastomers or alkali metal silicate vehicles and applied to the base structure. An alternate approach is to plate the substrate material with metals, such as copper or nickel, followed by oxidation of the coating. Because the base materials are often alloys of aluminum or magnesium, experimental work also is being done with anodizing. Through anodizing one might achieve protection from corrosion along with a coating having the desired optical properties. Because the optical qualities of a flat absorber are easily obtained from any rough black matte surface, Figure 1, the choice of a particular coating material is usually governed by other factors. These factors include the coating's temperature resistance, mechanical strength, abrasion resistance, adhesive strength, flexibilit,~,cost,, and ease of application. Solar Absorbers
The final class of thermal coatings, characterized by the highest values of or,/c, are called solar absorbers (see the table). Such materials absorb moderate amount,s of solar energy striking their surfaces but emit very small amounts of infrared radiation, Figure 1. High values of a , / r can be achieved from polished metal surfaces, metal films, or thin films of metal oxides. These surfaces appear black yet have the infrared emitting qualities of the substrate. In actual practice a variety of surface t,reatmentsmay be used in the control of spacecraft temperatures (7). "or instance, the outer surface of Explorer I, the first 'United States spacecraft, lyas stainless steel which was both oxidized to a straw color and striped with aluminum oxide. The aluminum surfaces of some later Explorer satellites were dotted with vhite paint, Figure 2. More sophisticated temperature control for the Mariner spacecraft, Figure 3, mas achieved by a large number of surface treat,ments (8). Heat shielding on the Mariner IV was provided on the upper deck, the side maintained toward the sun, by thirty layers of crinkled, aluminum-coated Mylar covered by black Dacron. Aluminized Teflon covered the lower deck and exposed cable harnesses and wiring. The panels of solar cells were hacked with black paint to dissipate most of t,he solar energy as it was absorbed. The an-
surface of TiOzsilicone base white paint (solar reflector) beneath. Each mission in space requires a completely new set of data which NASA scientists must consider in selecting appropriate thermal controlling optical surfaces. In practice, thermal controlling surfaces are combined with conductive heat paths, heat shields, heat pumps, and heat sources to maintain the numerous temperature environments necessary for successful completion of a mission. Literature Cited
Figure 3. T h e M o r n e r IV lpocecroft using porr:vc on. octive method% of t e r n p e r ~ t ~ reg~lalion. re Tne rhermol-rontrollovrer~con oe seen on the sioes of t n e lower portion of t h e spocecroft.
tenna disk was painted green. Some components were plated with gold. In addition to these passive methods, the Mariner I1 and IV spacecraft used an active temperature controlling device involving optical surfaces. The electronic gear compartment was maintained in the proper temperature range by regulating the position of six sets of louvers by means of coiled bimetallic strips. The louvers had outer surfaces of polished aluminum (solar absorber) hut when opened exposed a radiating
(1) VA~T*, T. F., in"Spaoe Materials Handbook" (Editow GOETZEL, C. G., RITTENROUBE, J. B., A N D SIN(ILET*RY, J. B.) Addison-Wdey Publishing Co.. Reading. Mass., 1965, Chapter 10. (2) Joxnsor. P. S., in "Soaoe Materials Handbook" (Editors: GOETZEL. C. G., RrTTmnouae. J. B.. A N D S~XOLETARI, J. B.) Addison-Wesley Publishing Co.. Reading, Msss., 1965, Chapter 5. (3) Cnar*cx, W. G., in " S ~ s o e,Materials HandbooW' (Editors: G o m z m , C. G.. RITTENHOUBE. J. B., A N D SINOLETART. J. B.) Addison-Wesley Publishing Co., Reading, Mass., 1965. Chapter 6. (4) P L V N K E J. ~D.., "NASA Contribntions t o the Teohnology oi Inorganic Coatings." NASA SP-5014, 1964, pp. 19-105. 15) RITTENXOUBI;. J. B.. A N D SINGLETART. J. B.. "Soace Materials Handbook,'' ~up&emeit 1 to the 2nd Ed.. NASA
[email protected], pp. 588 to S102. (6) GLLLLOAN, J. E.. SIBERT,M. E., A N D GREEN IN^, T. A,. "P&89iveThermal Control Coatinzs." Lookheed Missiles and Sosoe Co.. Pslo Alto. Chlii.. 1963, p. 3. ( 7 ) RITTENHOUSE. J. B., A N D S~NCLETARI, J. B., "S~aceMateriala Handbook," Supplement 1 to the 2nd Ed., NASA SP-3025, 1966, pp. S-18 to S33. (8) "Report from Msrs: Mariner IV 1964-1965," NASA EP-39. 1966, pp. 15-16.
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