The Integrated Spacecraft Environments Model (ISEM) was used to model the LDEF induced neutral molecular environment at several different times and altitudes during the mission. The purpose of this effort was to provide the community with an estimate of the neutral molecular environment to assist in phenomenology studies

Gordon, Tim

Rantanen, Ray

English

NASA Technical Reports Server

N93"12775
LONG DURATION EXPOSURE FACILITY (LDEF)
CONTAMINATION MODELING
Tim Gordon
Applied Science Technologies
P.O. Box 621134
Littleton, CO 80162
Phone: (303) 973-7708
Ray Rantanen
ROR Enterprises
4043 South 1-25
Castle Rock, CO 80104
Phone: (303) 688-9428
SUMMARY
The Integrated Spacecraft Environments Model (ISEM) was used to model the LDEF
induced neutral molecular environment at several different times and altitudes during the mission.
The purpose of this effort was to provide the community with an estimate of the neutral molecular
environment to assist in phenomenology studies.
INTRODUCTION
The objectives of this modeling effort were twofold. First, to model the overall vehicle
induced neutral environment and to determine the flux of various molecular species on different
surface locations. Secondly, to use the overall modeling results as input for the modeling of the
molecular flux through a small aperture (vacant screw hole) into the vehicle interior. This second
modeling effort was of interest because of very noticeable brown deposition patterns on interior
surfaces in close proximity to the aperture. It was believed that understanding the molecular
141
https://ntrs.nasa.gov/search.jsp?R=19930003587 2020-03-17T10:35:42+00:00Z
environment in the vicinity of the aperture would help in determining the mechanism which
produced the deposition pattern.
INTEGRATED SPACECRAFT ENVIRONMENTS MODEL (ISEM)
ISEM is a collisional molecular transport code which computes the molecular density and
flux in a three dimensional modeling volume for any number of user defined molecular species.
MODELING PARAMETERS
Three different periods in the LDEF mission were modeled to obtain representative results
over the mission lifetime. These periods were representative of the beginning, middle and end of
the mission timeline and corresponded to orbital altitudes of 463 km, 417 km, and 333 km
respectively. Table 1 shows the ambient values for the six different ambient molecular species
modeled at the three periods. The values were obtained using the atmosphere-predicting model
MSIS86 and represent annual and orbital position averaged values for the periods modeled.
Table 2 shows the outgassing and erosion rates used for the modeling. External surfaces
were modeled as having an average uniform outgassing rate which decreased with time. The initial
outgassing rates were based on test data and the percentages of various materials present.
Outgassing from internal surfaces was allowed to escape to the external environment via the
numerous holes around the experiment trays. The external outgassing rate was assumed to
decrease with an e folding time of 6000 hours. The internal outgassing rate was assumed to
decrease with an e folding time of 7000 hours. The e folding times were based on Skylab
measurements, taking into account differences in materi_s and materials control between the two
programs. The average erosion rate was assumed to be 15% of Kapton for all the surfaces. The
erosion rate given in Table 2 is for a surface normal to ram; a cosine dependence (relative to the
velocity vector) was assumed for non-normal surfaces.
!
L
E
=
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Z
_-5
=
142
!i
GEOMETRY MODEL
LDEF was modeled as the geometric structure shown in Figure 1. Based on data at the
time of the modeling, the geometric structure was rotated 10 degrees relative to ram as shown in
Figure 2.
GENERAL MODELING RESULTS
Density
ISEM was used to compute the density of every tracked species throughout the three
dimensional modeling volume for the mission beginning, middle, and end cases described
previously. Figures 3, 4 and 5 show the total iso-density contours for a plane of values from the
three dimensional modeling volume. The total density value is the sum of ambient species, surface
reemitted ambient species, internal and external outgassed species, and the scatter portions of all
species. The contour values have been normalized to the total undisturbed ambient density at the
respective altitude. Figure 3 shows the total iso-density contours for the early mission case at an
altitude of 463 km. A slight ram buildup can be seen in front of the vehicle (velocity vector from
left to right), but the density around the vehicle is dominated by the outgassing. Figure 4 shows
the total iso-density contours for the middle mission case at an altitude of 417 km. In this figure
one can see a significant ram buildup and a distinct wake region. The density in the wake region is
dominated by the outgassing. Figure 5 shows the total iso-density contours for the late mission
case at an altitude of 333 km. There is a strong density buildup in front of the vehicle due to
ambient and erosion products. The wake is very well defined and although the densities are much
less than on the ram side the density in the wake region is still dominated by the outgassed species.
Figure 6 is an iso-density'contour plot of only the erosion products. The plot shows a strong ram
angle dependence.
143
Flux
From the standpoint of surface materials interaction with the molecular environment,
molecular flux of the different species is much more important than density. Flux of each tracked
species was computed to each of the LDEF facets. Figures 7 through 10 show the surface incident
flux at the three modeled altitudes for O, 02, N, and N2 respectively. In the figures the surface
incident flux is plotted as a function of incidence angle as measured from the ram direction. The
term "direct" on the plots refers to flux of molecules which have not had a collision; they still retain
the kinetic energy of the orbital velocity (in the spacecraft reference frame). Figure 11 shows the
flux of outgassed and erosion products at the three modeled altitudes. Note that there is no direct
flux in these plots because only transport via scattering can produce the return flux of these species
to the external surfaces (this is not necessarily true on the scale of individual trays).
SMALL SCALE MODELING RESULTS
The second portion of the modeling effort was to model the molecular flux through a small
aperture and the resulting incident flux on an internal surface (the side of an experiment tray).
Figure 12 shows the geometrical relationship of the aperture and the internal surface. Figure 13
shows the energy distribution in the spacecraft reference _eof atomic oxygen and nitrogen in
terms of electron volts. Figure 14 shows the angular distribution in the spacecraft reference frame
of atomic oxygen due tO the ambient thermal velocity distribution. Figure 15 shows the incident
flux distribution of atomic oxygen on the internal surface due to flow through the small aperture.
The flux distribution on the surface is due primarily to the thermal distribution of atomie oxygen.
The dotted lines indicate the approximate cone angle of the observed deposition pattern.
CONCLUSIONS
We believe that the internal deposition modeled was due to atomic oxygen fixing of
internally outgassed contaminants present on internal surfaces. The pattern observed is consistent
with the thermally _stributed flux of ambient atomic oxygen in the spacecraft reference frame.
144_
The atomic oxygen erosion rates at the end of the mission were comparable to initial
outgassing rates of LDEF surfaces. Return flux of erosion species near the end of the mission
were an order of magnitude greater than the return flux of outgassed products early in the mission.
ACKNOWLEDGMENTS
We would like to acknowledge the financial support of Dr. Ann Whitaker at NASA
Marshall Space Flight Center. We would also like to thank Ralph Carruth at MSFC for his
technical and logistical assistance.
145
Table 1. Average Ambient Atmosphere Density Values
(MSIS 86)
Species
#/cm3
O
02
N
N2
He
H
Total Density
Date
4/84 4/87 1/90
2.59x 107 3.48x 107 9.03x 108
7.52x 103 1.43x ! 04 6.06xl 0 s
6.65x10 s 7.44x10 s 3128x107_
4.23x 10 s 7.26x 10s 2.03x 106
3,47x 10 s 3.85x 106 5.07x 10s
1.63xl 0 s 2.30x I 0s 2.66x 104
3.06x107 4.04x107 1.15x109
Temperature
O Flux/cm 2
(K)
920 829 1303
2,0x1013 2.x1013
7.0x1014
Table 2. Outgassing and Erosion Rates
Rate 463 km 417 km 333 kmg/cm2/sec 4/84 4/87 1/90
146
r
External
Internal
Erosion
2.0 x 10-9
2,0 x 10-1o
6,3 x 10-11
2.6 x 10-11 1.4 x 10-12
5.6 x 10-12 4.8 x 10-13
8.5 x 10-11 2,2 x10-9
|
i
|
|
!
f
ZFigure 1. LDEF geometry model.
RAM
12 1
9 3
7 6
Figure 2. LDEF facet identification (from Earth end).
147
148
/
\
I i I " I I I I
, [
x Ambient (3.065 x 107 moleculeslcm3)
I . I I I -t
I I
I I
Figure 3. Total density at 463 km.
I I I I | I ! I I
| =_ •
Figure 4.
x Ambient (4.033 x 107 moleculeslcm 3)
I I I I L I
Total density at 417 km.
I I I I I I I I I
02
.1
lO
! I 1
Figure 5.
! I
x Ambient (1.150 x 10 g molecules/cm 3)
I ] ],._ I
Total density at 333 km.
I
lx10 4
100
1.0
! ! 1 I I I I ]
xl0s
IxlO 7
lX10 e
2X10 e
7x10 7
3x107
lx10 4
I I
Figure 6.
lx101
Molecules/cm 3
1 1 I ! I t !
Density of erosion products at 333 km.
149
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l.E+12
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U
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! .E+9
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× I .E+7
1._+6
100080
10000
o
RTOMIC OXYGEN FLUX ON LDEF SURFRCES RS R FUNCTION OF RNGLE
RLTITUDE - 4G3 km
!
-_.j. i DIRECT PLUS scarer FLUX .....
: _ "'i _c_TTZR FLUX oNLY
: ! :l
:t
i ; :t
! :t:.t
: : 1
: : : t
: : t
• ._ _ : i
LDEF FACET
, I , I , I , ! , I J ! A 1 ._ l . I , I . I . I , I , I • I • I , l .
3B G8 90 120 150 IB0
RLTITUDE - 417 km
_ _ .-- _ -....... :__ _ __...... ! DIRECT PLUS SCRTTER FLUX
i "'_ SCRTTER FLUX ONLY
h
:I
;I
:l
• I
- t
! i
l0 l, tz s i
DEF FRCET
_L. I • I , I , I , I , , , I . I , I , ; , I • I • I , I • I
38 B8 90 128 158
RLTITUDE = 3_] km
180
_._ DIRECT PLUS SCRTTER FLUX .....
4:_ SCRTTER FLUX ONLY
LDEF FRCET
= 1 , I , I , I , I , I , I . I ..l_ I • t • ] , I = [ . , , , . , .
= SURFRC[ RNGLE RELRTivE TO RRM
Figure 7. Atomic oxygen flux on LOEF surfaces.
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188
18
8
OXYGEN (02) FLUX ON LDEF SURFRCES RS A FUNCTION OF RNGLE
ALTITUDE - 4S3 km
............ :__ _ :....... _ :
: : :''_: DIRECT PLUS SCATTER FLUX .....
: : }
i ill SCRTTER FLUX ONLY
i :
%
LDEF FACET
• 1 , I . I , I _ ! , l , I _ I • I , I , I _ I I ! , ! , ! , I I I I
38 68 98 128 158 lBB
RLTITUDE = 417 km
i DIRECT PLUS SCATTER FLUX
_'..; SCRTTER FLUX ONLY
i i :'
• : %
c ; ' I
• , : " : I
: : . " : 1
: z : : : %
!iiiii
LDEF FRCET
, I • I , I , I , I , I , I • I , I , I , I , [ , I , I , I
38 GO 90 128 158
AL_TITUDE = 333 km
_'.''.i DIRECT PLUS SCRTTER FLUX .....
: : ._
:_. SCRTTER FLUX ONLY
w_
- . ! i !\ ]
: : : : . : _ i !
: : : • : .. % : :
ii ii ii ii
DEF RCE
. ' • 1 ..I , I , I , I • I .LI . _', _', , . , . , . , . I , , , .
30 SO 90 128 158
a I i
188
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SURFACE RNGLE RELRTIVE TO RAM
Figure 8. Oxygen flux on LDEF surfaces.
151
152
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RTOMIC NITROGEN FLUX ON LDEF SURFRCES RS R FUNCTION OF RNGLE
RLTITUDE - 4G3 km
I .E+I I _i..... 4" I... iI DIRECT PLUS SCRTTER FLUX .....
: "_ SCRTTER FLUX ONLY
:t
l.E+lO : i :l
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--__ : _ i
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1.E+7
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LDEF FRCET
|000 I ,_|_J I , I , I , I , I , ; • L, J * i , | , I , ! , I , t , l ,
0 30 GO 90 120 150
RLTITUDE - 4 7 km
I.E+12 l .................... -'" :
I. E+I ! "'i DIRECT PLUS SCRTTER FLUX .....
SCRTTER FLUX ONLY
:L
I .E+IO :I
:1
:I
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I.E+@
: t
i i i ! _ .
1000
LDEF FRCET
L I t ! A | i I _ | , I , | * | , | , I , ] I I J | ' l , I • I , •
I.E+S
100000
10000
IOO
I.E+I4
180
0 30 GO 90 120 150
ALTITUDE - 333 km
180
1.E+13
I.E+I2
1.E+II
I.E+IO
1.E+@
I.E+B
I.E+7
I.E+G
100oo0
10000
1000
Figure 9
--_.- ! DIRECT PLUS SCRTTER FLUX .....
! i i "'!, SCRTT'ER FLUX ONLY
: : : :% ......
: " ; :t : : : : : "
I T : { : : I : : : : " "
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: . : • I : : : : : :
: : • :. % - . : : : -
: : :' % : : : : : •
: : : : % : : : : • :
: : : • I .... : :
LDEF FRCET
, I , I , I , ; , I , I , L_--L--_.L_J____L_LJ , I , I • I . I • I ,
30 60 90 120 150 180
SURFRCE; RNGLE RELRTIVE TO RRM
Atomic nitrogen flux on LDEF surfaces.
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I. E+9
I. E+B
I. E+7
1. E+8
188888
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1888
188
18
8
NITROGEN (N2) FLUX ON LDEF SURFRCES RS A FUNCTION OF ANGLE
ALTITUDE " 4S3 km
i "'_'i DIRECT PLUS SCATTER FLUX .....
! 4 SCATTER FLUX ONLY
i
i i 1
d L8 L _ n2 s
LDEF FACET
38 88 98 120 158 IB8
RLTITUDE- 417 km
i
r.......
..... i---F ....... .:
"_i DIRECT PLUS SCATTER FLUX .....
"_ SCRTTER FLUX ONLYi i :,
[ : :l 1
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: : : : : 1
: : : : : ',
LI]EF FRCET "_
! • ! _ ___ l • ; • l • I , I , | a J_.-_ , l • ; • I , I • •
38 $8 98 n28 158 188
RLTITUDE- 333 km
:::---,,-... i i DIRECT PLUS SCATTER FLUX .....
_''-_ SCATTER FLU)( ONLY
: • : : % :
, l . I , I I ] , __CE] i , L ,-J. .... L I I , ' A I ,_
30 60 gO 128 158 188
SURFACE RNGLE RELATIVE TO RAM
Figure 10. Nitrogen flux on LDEF surfaces.
153
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OUTGRS RND EROSION PRODUCT FLUX ON LDEF SURFRCES RS R FUNCTION OF RNGLE
RLTITUDE - 463 km
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'--_--- ...... --'-."--, • . EXTERNRL OUTGRSSED RETURN FLUX .....
'_,'_-_ _ INTERNRL OUTGRSSED RETURN FLUX
! ! ! __,: T_E_OSION PRODUCTS RETURN FLUX
i i i i -L Y, !i
• LDEF FRCET _
, I • I • I , l ,_ , I , I , _!_L_I , I I , I ,
@ 3IB G@ 98 128 158 188
FILTlrTUDE - 417 km
Figure 11.
i
: : IN'TF-.RNRL 0UTGRSSED RETURN FLUX
i
; EROSION PRODUCTS RETURN FLUX
i i ! "_-i'"- !
,,X'-
I I l 1 _
LDEF FRCET _ _
J I • I • I , I • I __ I . I , I ,\_._ _,1_,
IB 38 68 9_ 120 158
RLTITUDE = 333 krn
E3crERNP, L OUTCRSSED RETURN FLUX .....
3
| , I , i
180
• EXTERNRL OUTCRSSED RETURN FLUX .....
---F--:... ..... i ! : INTZRNRL OUTGRSSED RETURN FLUX
"T'-'_'-.., : EROSION PRONCTS RETURN FLUX
: !\i :i
: i : "_..._i : ' ?
"-T'--......_---L i i: "\ ::
! : : ! : ..... _-. _ i
i3i ! i i _ ! i ! ', "i. .
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@ 3B S@ 98 12_ 158 1e@
SURFRCE RNGLE RELRT_VE TO RFIH
Outgassed and erosion products flux on LDEF surfaces.
!
=
f
_=-
+X
12 in.
12 in.
+Y
APERTURE LOCATION
/4 inch diameter
+Z
MODELED SURFACE
DEPOSITION
PATTERN
Figure 12. Internal surface deposition model geometry.
155
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i56
EV DISTRIBUTION OF INCIDENT RMBIENT RTOMIC OXYGEN
0
VELOCITY INCREMENT (BY)= 58
INTEGRRTEB CURVE- .999923848312TEMPERRTURE (K)- 1383
TY (m/see)- 7008
h , , , I , , , _.. I , , \, , I _ ---J J ,
5 10 15
ELECTRON VOLTS (EV)
EV DISTRIBUTION OF INCIDENT RMBIENT NITROGEN
20
8
Figure 13.
VELOCITY INCREMENT (DV)= 50
CURVE= .999923848328
TURE (K) = 1303
D
i i
5 10 15 28
ELECTRON VOLTS (EV)
Energy distribution of atomic oxygen and nitrogen.
7
I-
0
0 10
Figure 14.
RLTITUDE-4S3 km
20 30 40 50 68 70 80
RNGLE OF INCIDENCE (DEGREES) FROM VELOCITY VECTOR
90
Atomic oxygen angular distribution from ram direction.
03
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-6 -5 -4 -3 -2 -i 0 [ 2 3 4 5 6
SPECIES=
RTOMIC OXYGEN
RLTITUDE = 333 km
CONTOUR VRLUES
Mol/cm^2/sec
'_'_"1.E+13
.E+12
_ I.E+12
,_.. \t.E+ll
't.E+10
INCHES
Figure 15. Iso-flux contours of atomic oxygen on internal surface.
157
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i
i1
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=
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!
-___-
R


Long Duration Exposure Facility (LDEF) contamination modeling

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