Computer models of environmental conditions in Earth orbit are needed for the following reasons: (1) derivation of material performance parameters from orbital test data, (2) evaluation of spacecraft hardware designs, (3) prediction of material service life, and (4) scheduling spacecraft maintenance. To meet these needs, Boeing has developed programs for modeling atomic oxygen (AO) and solar radiation exposures. The model allows determination of AO and solar ultraviolet (UV) radiation exposures for spacecraft surfaces (1) in arbitrary orientations with respect to the direction of spacecraft motion, (2) overall ranges of solar conditions, and (3) for any mission duration. The models have been successfully applied to prediction of experiment environments on the Long Duration Exposure Facility (LDEF) and for analysis of selected hardware designs for deployment on other spacecraft. The work on these models has been reported at previous LDEF conferences. Since publication of these reports, a revision has been made to the AO calculation for LDEF, and further work has been done on the microenvironments model for solar exposure

Bourassa, R. J.

Pippin, H. G.

English

NASA Technical Reports Server

N94- 31021
MODEL OF SPACECRAFT ATOMIC OXYGEN AND SOLAR
EXPOSURE MICROENVIRONMENTS*
R. J. Bourassa, and Dr. H. G. Pippin
Boeing Defense & Space Group
P. O. Box 3999, MS 82-32
Seattle, WA 98124
Phone: 206/773-8437; Fax: 206/773-4946
INTRODUCTION
Computer models of environmental conditions in Earth orbit are needed for the following
reasons: (1) derivation of material performance parameters from orbital test data, (2) evaluation of
spacecraft hardware designs, (3) prediction of material service life, and (4) scheduling spacecraft
maintenance. To meet these needs, Boeing has developed programs for modeling atomic oxygen
(At) and solar radiation exposures. The models allow determination of At and solar ultraviolet
(UV) radiation exposures for spacecraft surfaces (1) in arbitrary orientations with respect to the
direction of spacecraft motion, (2) over all ranges of solar conditions, and (3) for any mission
duration. The models have been successfully applied to prediction of experiment environments on the
Long Duration Exposure Facility (LDEF) and for analysis of selected hardware designs for
deployment on other spacecraft.
The work on these models has been reported at previous LDEF conferences (refs. 1 through
5). Since publication of these reports, a revision has been made to the At calculation for LDEF, and
further work has been done on the microenvironments model for solar exposure.
OBJECTIVE
The objective of this report is to present the results of a revised calculation for At exposure
of LDEF experiments and to describe a newly developed microenvironments model for predicting
solar exposure of spacecraft.
ATOMIC OXYGEN EXPOSURE
Primary Atomic Oxygen Model
Since 1986 Boeing has been developing predictive models for determining the exposure of a
spacecraft surface to At. The first program developed is referred to as the primary At exposure
* Includes work done under NAS 1-19247, Task 8.
__lG P_AGE BLANK NOT FILMED 105
https://ntrs.nasa.gov/search.jsp?R=19940026516 2020-06-16T12:21:49+00:00Z
model. The primary model is used to determine the At flux (atorrdcm2-s) and fluence (atoms/cm2)
to flat surfaces. The model includes the effects of thermal motion of ambient At atoms and co-
rotation of the atmosphere in addition to the ambient atmospheric density and the velocity of the
spacecraft. The model treats noninterfering surfaces at arbitrary, but definite, orientations with
respect to the direction of spacecraft motion. Orbit parameters and mission duration are defined by
the user. The NASA MSIS-86 Model Atmosphere (ref. 6) is used to establish atmospheric
densities as a function of time. Solar conditions required by the model atmosphere are input as
functions of time.
Details of the primary model were presented at the First LDEF Post-Retrieval Symposium
and at the LDEF Materials Workshop '91 (refs. 1,2) and its application to LDEF is reported in
NASA CR 189627 (ref. 3). Following publication of reference 3, a revised prediction of At fluences
to LDEF experiments was completed. Results of the revised calculation are shown in Figure 1. The
revision calculation differs from that given in reference 3 because of a correction made to the
atmospheric co-rotation programming code. The actual differences between the two calculations is
not large (3 to 4 percent) for leading surfaces of the spacecraft (rows 7, 8, 9, 10, and 11). For rows
12 and 1 on the side of the vehicle, the revised calculation shows an increase of about 25 percent
relative to the previous calculation. A greater relative increase was seen on trailing edge rows 4 and
5, but the total fluence values for these rows are still very low. The total fluence values for rows 1, 2,
and 3 are unchanged because fluence to these rows was dominated by the brief unplanned At
exposure during retrieval of the LDEF.
Atomic Oxygen Microenvironments Model
A second, more detailed At exposure model has been developed over the last 2 years to
account for interference, or shadowing of surfaces, by the three-dimensional structure of a spacecraft.
This model, termed the "At microenvironments model," also accounts for specular and diffuse
reflectance from surfaces exposed to either primary or secondary impacts, and accounts for the
potential of individual atoms to recombine on, or react with, the impacted surface. The secondary
scattering processes are determined by a Monte Carlo routine which follows an individual particle
until it either reacts on a surface or is scattered back into the ambient environment.
The microenvironments model is described in the proceedings of the LDEF Materials Work-
shop '91 (ref. 2) and the proceedings of the Second LDEF Post-Retrieval Symposium (ref. 4). Com-
parisons of observed effects of At on materials flown on LDEF with results predicted by the
microenvironments model are presented in references 4 and 5. The At microenvironments model
predicted exposure effects to within the uncertainty of the corresponding experimental
measurements.
The model predictions are sensitive to the relative contribution of specular and diffuse
scattering and to recombination efficiency. The surface property data we have used are estimates.
We have made preliminary determinations of recombination efficiency for copper, silver oxide, gold,
and anodized aluminum in the laboratory using calorimetric measurements. Further research is
needed to establish the methodology for laboratory measurement of molecular reflective properties
and recombination efficiency of spacecraft materials.
106
SOLARRADIATION EXPOSURE
Solarradiation exposureof LDEF is reportedin reference7 which gives exposurein equiva-
lent Sunhoursfor eachsurfaceof theLDEF vehicle.Like the At exposureshownin Figure 1, the
solarexposuredatareportedin reference7 arelimited to flat or convex,noninterferingsurfaces.To
overcometheselimitations, Boeinghasdevelopeda "solar exposuremicroenvironmentsmodel" to
accountfor shadowingandscatteringof solarradiationcausedby complexsurfacegeometry.The
model is similar to the "At microenvironmentsmodel." The effectson solar exposurecausedby any
arbitrary surfacesizeor shapemay bemodeled,including protrusions,indentations,andcurvature.
Figure2 illustratestheeffectsonshadowing,specularreflection,anddiffuse reflection of solar
exposureof an indentedsurface.
The "solar microenvironmentsmodel" accountsfor both directand for Earth-reflectedsolar
radiation.Entry times are randomlyselectedfor solarephemeralcalculations.Satellitepositionsare
determinedusing anorbital mechanicsroutine.For theselectedpositionof the Sun,rays are traced
to nodeson thespacecraftsurface.The MonteCarloroutinefollows individual rays asthey reflect
from surfaceto surface.Oncea ray is eitherabsorbedon a surfaceor is scatteredback into the
ambientenvironment, the processis repeatedfor anotherSunposition.Earth-reflectedradiation is
handledin a similar mannerexceptthat the sourceof Earth-reflectedradiation is takenasa location
on Earth determinedby weighted-randomselection.The attributesof the solar microenvironments
modelaresummarizedin Table 1.
Equivalent Sunhoursexposurecalculatedby theMonteCarlo solarmicroenvironmentsmodel
areshownonTable 2 comparedwith resultsreportedfor LDEF in reference7. The resultsreported
in reference7 arebasedon a deterministicanalysis.Deterministicanalysesof solar exposureare
limited in applicationto simplegeometriesandgenerallydo notaccountfor reflectedradiation and
shadowing.Thedatareportedin reference7 arevalid for exposureof noninterfering,flat planarsur-
faces.The Monte Carlo calculationwasappliedto the samegeometry.Even thoughthe procedures
usedfor the two calculationsare totally different, resultsare in satisfactoryagreementfor the six
locationson LDEF for which comparisonsweremade.However,thecomparisonis for the flat sur-
faces.Routinesin the microenvironmentsprogramfor shadowingandfor scatteringof radiation
betweensurfacesmust be verified by othermeans.
The solar microenvironmentsmodelhasbeenusedto predict solarexposureat the edgefold
of a silverized/fluorinatedethylenepropylene(FEP)thermalcontrol blanket.The fold analyzedwas
at the trailing edgeof Experimenttray D1.Theblanketedgeis identified in Figure 3. The mostfre-
quentazimuthof the Sun:visible from theexperiment,tendsto bewestof thevehicle.This point is
important in understandingthe resultscalculatedby themicroenvironmentsprogramfor exposureof
the blanketedge.
The geometryof the blanketedgeattachmentis shownin Figure4. Figure 5 showshow dis-
tanceis defined.Distancesshownin Figure 5 aremeasuredfirst along the surfaceof the blanketand
thencontinuealong the aluminumframeof theexperimentray. Theedgeof the FEPblanketwas
designatedas45 millimeters. The origin (zerodistance)for themeasurementsis a point on the fiat,
FEP-coveredsurfaceof theexperimenttray. Pointsalongthe surfaceof the aluminum frameareat
distancesgreaterthan45 mm from theorigin. The Sunat any randomlychosentime canbe in any
direction from an examinedpoint (node)on theexperimentsurfaceasdeterminedby solar ephemeral
calculations.The Sunmay not bevisible from a nodeposition.For aft-facing trays,the most
107
frequentlyobservedSundirectionstendto beaft (to theright for tray D1 asshownin Figure 4) of
the tray surfacenormal becausethe pathof the Suntendsto beoverheadandis not alwaysvisible
from the side of thevehicle.This causestheradiusbetweendistancesof 20 to 30mm along the FEP
blanket to be the most directly exposed surface of the blanket. Also, reflections from the FEP side of
the attachment notch would be directed toward the aluminum. Much less reflection from the
aluminum to the FEP would be expected than from the FEP to the aluminum.
These expectations are verified by the results shown in Figure 6. The area of highest solar
exposure corresponds to the radius of the blanket fold (from 20 to 30 mm). The area of highest inci-
dence of reflected radiation is on the aluminum side of the edge notch (from 45 to 60 mm). Exposure
of the surface between 0 and 20 mm is the same as between 74 and 100 mm as would be expected
because these surfaces are flat and parallel, thus they always make the same angle with a ray from
the Sun.
Figure 6 shows that the plotted results of the Monte Carlo calculation are jagged. This may
be due in part to statistical variations. The selection of node spacing and the number of Sun positions
for the test case may not have been optimum. Overall, the calculated exposures behave as expected.
Even with the variations noted, the results would be accurate enough for many engineering uses.
The model is new and improvements will be incorporated as they are identified. The test calculation
shown in Figure 6 does not provide verification between observed and predicted exposure of the
materials on LDEF because the effects of solar exposure on FEP materials are not readily quanti-
fied.
Another factor in estimating solar exposure effects should be noted. The model yields equiw..
lent Sun hours of exposure, the same as the deterministic model (ref. 7). It is radiation in the vacuum
UV range that significantly affects exposed materials. The intensity of UV radiation at wavelengths
shorter than 1,800/_ in the solar spectrum is a function of solar activity (ref. 8). This variation can be
taken into account in future versions of the Monte Carlo model.
Applications of the environmental models are listed in Table 3. The primary At program has
been the most widely used thus far. The new microenvironments programs will greatly extend the
application range of the modeling work in the future. We are now working on more flexible spacecraft
orientation routines which will facilitate the determination of materials exposures on spinning and
maneuvering satellites.
CONCLUSIONS
The latest revision of the At exposure calculation reported herein should be used for
analysis of LDEF results. The newly developed solar exposure microenvironments model produces
results that are consistent with the deterministic model for flat surfaces exposed in orbit. Solar
exposures calculated for surfaces of complex geometry using the microenvironments model are con-
sistent with expected radiation intensity and reflection patterns.
108
REFERENCES
.
.
.
.
.
.
.
.
Bourassa, R.J., Gillis, J.R., and Rousslang, K.W.: "Atomic Oxygen and Ultraviolet Radiation
Mission Total Exposures for LDEF Experiments." LDEF 69 Months in Space, First Post-
Retrieval Symposium, NASA Conference Publication 3134, 1991.
Bourassa, R.J. and Gillis, J.R.: "LDEF Atomic Oxygen Fluence Update." LDEF Materials
Workshop '91, NASA Conference Publication 3162, 1991.
Bourassa, R.J. and Gillis, J.R.: "Atomic Oxygen Exposure of LDEF Experiment Trays." NASA
Contractor Report 189627, 1992.
Bourassa, R.J., Pippin, H.G., and Gillis, J.R.: "'LDEF Microenvironments Observed and Pre-
dicted." Presented at the Second LDEF Post-Retrieval Symposium, San Diego, CA, June 1992.
Pippin, H.G., and Crutcher, E.R.: "LDEF Materials Data Analysis: Representative Examples."
Presented at the Second LDEF Post-Retrieval Symposium, San Diego, CA, June 1992.
Hedin, A.E.: "MSIS-86 Thermospheric Model." Journal of Geophysical Research, vol. 92, No.
A5, May 1, 1987, pp. 4649--4662.
Bourassa, R.J., and Gillis, J.R.: "Solar Exposure of LDEF Experiment Trays." NASA Contrac-
tor Report 189554, 1992.
Jursa, A.S., Scientific Ed.: "Handbook of Geophysics and the Space Environment." Air Force
Geophysics Laboratory, 1985, pp. 2-19.
109
Table 1. Solarexposuremicroenvironments model.
Monte Carlo calculation of solar exposure to complex spacecraft surfaces.
• Includes direct-solar and Earth-reflected radiation.
• Multiple scattering of radiation from surface to surface.
• Weighted-random selection of specular reflection, diffuse reflection, or
absorption with probabilities dependent on surface properties.
• Flexible input allows modeling for long or short missions.
Precision proportional to square root of number of Sun and satellite position pairs.
Code is computationall), intensive.
Table 2. Comparison of Monte Carlo and deterministic models for solar exposure.
Monte Carlo, NASA CR- 189554, Difference,
Surface Sun Hours Sun Hours Percent
Space End
Earth End
Row 3
Row 6
Row 9
Row 12
14,200 14,547 -2.4
4,400 4,472 -1.6
11,900 11,100 +7.2
6,690 6,400 +4.5
10,900 11,200 -2.7
6,900 6,800 +1.5
Standard Deviation 4.1
LDEF Mission
1,000 Sun Positions
Earth Albedo = 0.246
Surface Grid: 10×10
ii0
Table 3. Applicationsof theenvironmentmodels.
Atomic OxygenExposure
LDEF
• Fluenceasa functionof timeby tray.
• Thermalcontrol blanketedgeattachment,traysB7 and D11.
• Angle bracket,tray F9.
• Coppergroundingstraps.
• Specimencoverplatefor ExperimentA0171.
• Space-endtray clamp.
SpaceStation Freedom
• Fluence as a function of time and incidence angle.
EOIM-3 Orbital Test
• Indirect exposure experiment.
TRMM--Tropical Rain Forest Measurement Mission
• At fluence for mission.
Solar Exposure
LDEF
• Comparison of Monte Carlo model with analysis by Berrios and Sampair.
• Thermal control blanket edge attachment, trays D1 and C5.
TRMM--Tropical Rain Forest Measurement Mission
• UV exposure for mission.
111
YAW: 8.1 Degrees
PITCH: 0.8 Degrees
ROLL: 0 Degrees 7.27E÷21
Ram
Vector_._..
8.43 E+21
9.02E+21
8.1 D_
BZ'Axis 8.99E+21
Row
10
Row
9
8.36E+21
7.15E+21
I REVISED: 9/29/92 I
Ram Direction •
9.09E+21 Atoms/CM 2
3.56E+21 1.33E+21
Row
Row 12 Row
11 1
LDEF
Earth End: 3.33E+20
Space End: 4.59E+20
.22E+19
2.92E+17
1.54E+17
Row
2
Row Row
8 4
Row
3
.54E+17
.43E+17
2.66E+03
2.31E+05
Row Row
5.45E+21 7 Row 6.84E+08
3.39E+21 9.60E+12
1.16E+21 4.94E+19 7.33E+16
Figure 1. AO fluences at end of mission for all LDEF row, longeron, and end-bay locations
including the fluence received during the retrieval altitude excursion.
X/INCIDENT
RADIATION
DIRECT EXPOSURE
AND SHADOWING
INCIDENT INCIDENT
,__ON RADIATION
SPECULAR DIFFUSE
REFLECTANCE REFLECTANCE
Figure 2. Effects of local geometry on solar exposure of spacecraft surfaces.
112
Thermal Control
Fold, D1
Longeron
Row
Numbers
Row 2
East Heading _+28.5 deg
Yaw, 8.1 de_l
z- axis
2
) 3
8 4
The most frequent azimuth
of the Sun as seen from
trailing surfaces of the
opaque vehicle is aft of the
surface normal.
Figure 3. Location of the trailing side of LDEF Experiment tray D 1.
SUN
DIRECTIONS
\ li' 
VELCRO
R=7.75 mm R=10.32 mm
Figure 4. LDEF Experiment tray D1 thermal control blanket showing the blanket edge folded into a
notch next to the experiment tray frame.
113
D1 near row 2 edge FEP Blanket
Aluminum
Surface Properties
Specular Diffuse Absorption
83% 10% 7%
6% 60% 34%
Figure 5.
- FEP Blanket Aluminum
0 20 30 45 60 74 100
Distance along Surface (mm)
Definition of distance measurements for the microenvironments model study of the
thermal control blanket edge fold, Experiment tray D 1.
Figure 6.
8O0O
m 7000
o
6000
Z
5000
Z
w
< 4O00
O
m 3000
W
2000
1000
O
0
!
1
I
I
!
I
PRIMARY RADIATION
SPECULAR REFLECTION
DIFFUSE REFLECTION
TOTAL EXPOSURE
= i
0 10 20 30 40 50 60 70 80 9
DISTANCE, MM
Calculated solar exposure for the Experiment tray D1, thermal control blanket fold.
114


Model of spacecraft atomic oxygen and solar exposure microenvironments

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server