ASTM E491-73 (Reapproved 2015)

Designation: E491 − 73 (Reapproved 2015)Standard Practice forSolar Simulation for Thermal Balance Testing of Spacecraft1This standard is issued under the fixed designation E491; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (´) indicates an editorial change since the last revision or reapproval.1. Scope1.1 Purpose:1.1.1 The primary purpose of this practice is to provideguidance for making adequate thermal balance tests of space-craft and components where solar simulation has been deter-mined to be the applicable method. Careful adherence to thispractice should ensure the adequate simulation of the radiationenvironment of space for thermal tests of space vehicles.1.1.2 A corollary purpose is to provide the proper testenvironment for systems-integration tests of space vehicles.Anaccurate space-simulation test for thermal balance generallywill provide a good environment for operating all electrical andmechanical systems in their various mission modes to deter-mine interferences within the complete system. Althoughadherence to this practice will provide the correct thermalenvironment for this type of test, there is no discussion of theextensive electronic equipment and procedures required tosupport systems-integration testing.1.2 Nonapplicability—This practice does not apply to orprovide incomplete coverage of the following types of tests:1.2.1 Launch phase or atmospheric reentry of spacevehicles,1.2.2 Landers on planet surfaces,1.2.3 Degradation of thermal coatings,1.2.4 Increased friction in space of mechanical devices,sometimes called “cold welding,”1.2.5 Sun sensors,1.2.6 Man in space,1.2.7 Energy conversion devices, and1.2.8 Tests of components for leaks, outgassing, radiationdamage, or bulk thermal properties.1.3 Range of Application:1.3.1 The extreme diversification of space-craft, designphilosophies, and analytical effort makes the preparation of abrief, concise document impossible. Because of this, variousspacecraft parameters are classified and related to the importantcharacteristic of space simulators in a chart in 7.6.1.3.2 The ultimate result of the thermal balance test is toprove the thermal design to the satisfaction of the thermaldesigners. Flexibility must be provided to them to trade offadditional analytical effort for simulator shortcomings. Thecombination of a comprehensive thermal-analytical model,modern computers, and a competent team of analysts greatlyreduces the requirements for accuracy of space simulation.1.4 Utility—This practice will be useful during space ve-hicle test phases from the development through flight accep-tance test. It should provide guidance for space simulationtesting early in the design phase of thermal control models ofsubsystems and spacecraft. Flight spacecraft frequently aretested before launch. Occasionally, tests are made in a spacechamber after a sister spacecraft is launched as an aid inanalyzing anomalies that occur in space.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E259 Practice for Preparation of Pressed Powder WhiteReflectance Factor Transfer Standards for Hemisphericaland Bi-Directional GeometriesE296 Practice for Ionization Gage Application to SpaceSimulatorsE297 Test Method for Calibrating Ionization Vacuum GageTubes (Withdrawn 1983)3E349 Terminology Relating to Space Simulation2.2 ISO Standard:ISO 1000-1973 SI Units and Recommendations for the Useof Their Multiples and of Certain Other Units41This practice is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21.04 on Space Simulation Test Methods.Current edition approved Oct. 1, 2015. Published December 2015. Originallyapproved in 1973. Last previous edition approved in 2010 as E491 – 73(2010). DOI:10.1520/E0491-73R15.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at service@astm.org. For Annual Book of ASTMStandards volume information, refer to the standard’s Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.4Withdrawn.Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States12.3 American National Standards:5ANSI Y10.18-1967 Letter Symbols for Illuminating Engi-neeringANSI Z7.1-1967 Standard Nomenclature and Definitions forIlluminating EngineeringANSI Y10.19-1969 Letter Symbols for Units Used in Sci-ence and Technology3. Terminology3.1 Definitions, Symbols, Units, and Constants—This sec-tion contains the recommended definitions, symbols, units, andconstants for use in solar simulation for thermal balance testingof spacecraft. The International System of Units (SI) andInternational and American National Standards have beenadhered to as much as possible. Terminology E349 is also usedand is so indicated in the text. Table 1 provides commonly usedsymbols.3.2 Definitions:3.2.1 absorptance (αe, αv,α )—ratio of the absorbed radiantor luminous flux to the incident flux (E349)(Table 1).3.2.2 absorptivity of an absorbing material—internal ab-sorptance of a layer of the material such that the path of theradiation is of unit length (E349).3.2.3 air mass one (AM1)—the equivalent atmospheric at-tenuation of the electromagnetic spectrum to modify the solarirradiance as measured at one astronomical unit from the sumoutside the sensible atmosphere to that received at sea level,when the sun is in the zenith position.3.2.4 air mass zero (AM0)—the absence of atmosphericattenuation of the solar irradiance at one astronomical unitfrom the sun.3.2.5 albedo—the ratio of the amount of electromagneticradiation reflected by a body to the amount incident upon it.3.2.6 apparent source—the minimum area of the final ele-ments of the solar optical system from which issues 95 % ormore of the energy that strikes an arbitrary point on the testspecimen.3.2.7 astronomical unit (AU)—a unit of length defined asthe mean distance from the earth to the sun (that is,149 597 890 6 500 km).3.2.8 blackbody (USA),Planckian radiator—a thermal ra-diator which completely absorbs all incident radiation, what-ever the wavelength, the direction of incidence, or the polar-ization. This radiator has, for any wavelength, the maximumspectral concentration of radiant exitance at a given tempera-ture (E349).3.2.9 collimate—to render parallel, (for example, rays oflight).3.2.10 collimation angle—in solar simulation, the angularnonparallelism of the solar beam, that is, the decollimationangle. In general, a collimated solar simulator uses an opticalcomponent to image at infinity an apparent source (pseudo sun)of finite size.The angle subtended by the apparent source to thefinal optical component referred to as the collimator, is definedas the solar subtense angle and establishes the nominal angle ofdecollimation. A primary property of the “collimated” systemis the near constancy of the angular subtense angle as viewedfrom any point in the test volume. The solar subtense angle istherefore a measure of the nonparallelism of the beam. Toavoid confusion between various scientific fields, the use ofsolar subtense angle instead of collimation angle or decollima-tion angle is encouraged (see solar subtense angle).3.2.11 collimator—an optical device which renders rays oflight parallel.3.2.12 decollimation angle—not recommended (see colli-mation angle).3.2.13 diffuse reflector—a body that reflects radiant energyin such a manner that the reflected energy may be treated as ifit were being emitted (radiated) in accordance with Lambert’slaw. The radiant intensity reflected in any direction from a unitarea of such a reflector varies as the cosine of the anglebetween the normal to the surface and the direction of thereflected radiant energy (E349).3.2.14 dispersion function (X/λ)—a measure of the separa-tion of wavelengths from each other at the exit slit of themonochromator, where X is the distance in the slit plane and λ5Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036, http://www.ansi.org.TABLE 1 Commonly Used SymbolsSymbol Quantity Definition Equation or Value Unit Unit SymbolQ radiant energy, work,quantity of heatjoule JΦ radiant flux Φ =dQ/dt watt (joule/second) W, Js−1E irradiance (receiver) fluxdensityE =dΦ/dA watt per square metre W·m−2M radiant exitance (source) M =dΦ/dA watt per square metre W·m−2I radiant intensity (source) I =dΦ/dω watt per steradian W·sr−1ω = solid angle through which flux from source is radiatedL radiance L =dI/(dA cosθ ) watt per steradian =square metreW·sr−1·m−2θ = angle between line of sight and normal to surface dAτ transmittance τ = Φ, transmitted/Φ, incident noneτ(λ) spectral transmittance τ(λ)=Φ(λ), transmitted/Φ(λ), incident noneρ reflectance (total) ρ = Φ, reflected/Φ, incident noneεH emittance (totalhemispherical)εH = M, specimen/M, blackbodyα absorptance α = Φ, absorbed/Φ, incident noneαssolar absorptance αs= solar irradiance absorbed/solar irradiance incident noneE491 − 73 (2015)2is wavelength. The dispersion function is, in general, differentfor each monochromator design and is usually available fromthe manufacturer.3.2.15 divergence angle—see solar beam divergenceangle(3.2.60).3.2.16 electromagnetic spectrum—the ordered array ofknown electromagnetic radiations, extending from the shortestwavelengths, gamma rays, through X rays, ultravioletradiation, visible radiation, infrared and including microwaveand all other wavelengths of radio energy (E349).3.2.17 emissivity of a thermal radiator ε, ε =Me,th/Me(ε = 1)—ratio of the thermal radiant exitance of the radiatorto that of a full radiator at the same temperature, formerly“pouvoir emissif ” (E349).3.2.18 emittance (ε)—the ratio of the radiant exitance of aspecimen to that emitted by a blackbody radiator at the sametemperature identically viewed. The term generally refers to aspecific sample or measurement of a specific sample. Totalhemispherical emittance is the energy emitted over the hemi-sphere above emitting element for all wavelengths. Normalemittance refers to the emittance normal to the surface to theemitting body.3.2.19 exitance at a point on a surface (radiant exitance)(M)—quotient of the radiant flux leaving an element of thesurface containing the point, by the area of that element,measured in W·m−2(E349)(Table 1).3.2.20 field angle—not recommended (see solar beam sub-tense angle).3.2.21 flight model—an operational flight-capable space-craft that is usually subjected to acceptance tests.3.2.22 flux (radiant, particulate, and so forth)—for electro-magnetic radiation, the quantity of radiant energy flowing perunit time; for particles and photons, the number of particles orphotons flowing per unit time (E349).3.2.23 gray body—a body for which the spectral emittanceand absorptance is constant and independent of wavelength.The term is also used to describe bodies whose spectralemittance and absorptance are constant within a given wave-length band of interest (E349).3.2.24 incident angle—the angle at which a ray of energyimpinges upon a surface, usually measured between the direc-tion of propagation of the energy and a perpendicular to thesurface at the point of impingement or incidence.3.2.25 infrared radiation—see electromagnetic spectrum(E349).3.2.26 insolation—direct solar irradiance received at asurface, contracted from incoming solar radiation.3.2.27 integrating (Ulbrecht) sphere—part of an integratingphotometer. It is a sphere which is coated internally with awhite diffusing paint as nonselective as possible, and which isprovided with associated equipment for making a photometricmeasurement at a point of the inner surface of the sphere. Ascreen placed inside the sphere prevents the point underobservation from receiving any radiation directly from thesource (E349).3.2.28 intensity—see radiant intensity.3.2.29 irradiance at a point on a surface Ee,E;Ee=dΦe/dA—quotient of the radiant flux incident on an element of thesurface containing the point, by the area of that elementmeasured in W·m−2(E349)(Table 1).3.2.30 irradiance, mean total (E¯)—the average total irradi-ance over the test volume, as defined by the followingequation:E¯5 *vE~r,θ,z!dV/*vdV (1)where:E¯(r,θ,z) = total irradiance as a function of position (Table1).3.2.31 irradiance, spectral [Eλor E(λ)] —the irradiance at aspecific wavelength over a narrow bandwidth, or as a functionof wavelength.3.2.32 irradiance, temporal—the temporal variation of in-dividual irradiances from the mean irradiance. The temporalvariations should be measured over time intervals equal to thethermal time constants of the components. The temporalstability of total irradiance can be defined as:Et56100@~∆Et ~min!1∆Et ~max!!/2E¯# (2)3.2.33 irradiance, total—the integration over all wave-lengths of the spectral irradiance.3.2.34 irradiance, uniformity of—uniformity of total irradi-ance can be defined as:Eu56100@~E~min!1E~max!!/2E¯# (3)where:Eu= uniformity of the irradiance within the test volume,expressed as a percent of the mean irradiance,E(min)= smallest value obtained for irradiance within thetest volume, andE(max)= largest value obtained for irradiance within the testvolume.Uniformity of irradiance values must always be specifiedtogether with the largest linear dimension of the detector used.3.2.35 Lambert’s law—the radiant intensity (flux per unitsolid angle) emitted in any direction from a unit-radiatingsurface varies as the cosine of the angle between the normal tothe surface and the direction of the radiation (also calledLambert’s cosine law). Lambert’s law is not obeyed exactly bymost real surfaces, but an ideal blackbody emits according tothis law. This law is also satisfied (by definition) by thedistribution of radiation from a perfectly diffuse radiator and bythe radiation reflected by a perfectly diffuse reflector. Inaccordance with Lambert’s law, an incandescent sphericalblackbody when viewed from a distance appears to be aE491 − 73 (2015)3uniformly illuminated disk. This law does not take into accountany effects that may alter the radiation after it leaves thesource.3.2.36 maximum test plane divergence angle—the anglebetween the extreme ray from the apparent source and the testplane. This applies principally to direct projection beamswhere it is equivalent to one half the projection cone angle (seeFig. 1).3.2.37 natural bandwidth—the width at half height of aradiation source emission peak. It is independent of instrumentspectral bandwi