The space environment effects on the silver teflon covering (solar panels) of two SOLRAD SATELLITES AND NTS-Z are examined to analyze the design requirements for the Shuttle Launch Dispenser (SLD)

Betz, Frederick E.

Hauser, Joseph A.

English

NASA Technical Reports Server

F/OME EXAMPLES OF "['HED_GRADATION
OF PNOP_REIES OF MATERIALS IN SPACE
N89- 23535
Frederick E. Betz
and
Joseph A. Hauser
Naval _esearch laboratory
Washington, D.C. 20375-5000
PRECEDING PAGE BLANK NOT FILMED
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https://ntrs.nasa.gov/search.jsp?R=19890014164 2020-03-20T03:15:06+00:00Z
SOLRAD ii SPACECRAFT
An artist's conception of one of the two Naval Research Laboratory (NRL)
SOLRAD ii satellites that were launched on 14 March 1976 from Cape Canaveral
is shown in figure i. These spacecraft, the 65th and 66th launched by NP_,
were in a 63,000 nautical mile circular orbit, spin stabilized and sun
pointing. The prime power was provided by four deployed solar panels and four
body mounted panels insulated from the body with a multi-layer insulation
blanket.
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
Figure I
110
SOLAR CELL AND SILVER TEFLON PANEL LAYOUT
One of the four body mounted panels is deDicted in figure 2. Overall
dimensions of the panel were 25.4 cm (I0.0 in) by 33.0 _n (130 in). On the
panel was mounted an 80 cell series circuit of 2 c_n by 2 on solar cells and
the non-cell area was covered with .13 mm (.005 in) thick silver teflon for
themnal control. The solar cells, terminals, etc., accounted for 42.1%,
and silver teflon 57.9% of the pansl area. As I recall, the design
o_rating temperature was about 50 C.
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SOLAR CELL CIRCUIT
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Figure 2
111
gDLP_ID lIB SOLAR PANEL TEMPEP_I7J_E
Figure 3 is a smoothed plot of the insulated solar panel temDerature
versus time in orbit. The "waves" are causal bF the annual variation in
solar intensity due to the F_arth's slightly elliptical path around the sun.
Solar panel electrical output was reduced due to a qon-optimum operating
voltage resulting Fr(_n the elevated t_nperature. Mission i,_oact was
minimal since higher voltage was only needed to rechaLge batteries after
eclipses, which occulted about 30 times each year. Understanding the
increase in tem[>erature was a greater concern.
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Figure 3
112
ESTIMATED OqANGE IN SILVER-TEFLON SOLAR ABSORBTANCE
It is reasonable to assume that the temperature increase is driven by
an increase in the solar absorbtance oF. the silver teflon. I am not aware
of changes occurring in material _nmittance. Inorganic silicon solar cell
absorbtance, already high, should not increase; and the reduction in solar
cell efficiency is negligible. A least squares fit of the absorbtance
value, derived from panel temperature, is shown in figure 4 over a peri(x_
of 1500 days in orbit.
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113
NAVIGATION TECHNOLOGY SATELtITE-2 (NTS-2)
While data was accu_]lating from SOLRAD ii, NRL was preparing NTS-2
for launch. Them_al design and engineering was contracted to the A.D.
Little, Inc. Both Optical Solar B_flections (OSR) and silver teflon were
incorporated for tl]ec,nal contcol. Additionally, NASA Goddard Space Flight
Center had a thermal control coatings experiment aL_)ard with eight sample
materials including OSR's and silveL" teflon. Internal deck temperatures
exceeds] the woLst case temperature based on p_edicted degraded optical
properties and maximum _]lac constant after less than two months on orbit.
Figure 5
114
ORIGINAL PAGE
_',.ACK AND WFHTE PH'O.O_:]RAPH"
MEASURED DEGRADATION IN SOLAR ABSO_TANCE
OF OSR AND SILVER TEFLON ON NTS-2
Figure 6 combines data from the NASA-GSFC experiment and thermal
analysis of NTS-2 spacecraft equipment, deck, and radiator temperatures.
D.W. Almgren of A.D. Little, Inc., concluded that the nonlinear nature of
the dat-a suggested contamination and, in an internal technical report,
described four conceivable conta,nination mechanisms. Three mechanisms were
based principally on "desorptive transfer". The solar panels were folded
around the vehicle for launch and the first 14 days on orbit and provided
the intermediate transfer surface. (The Figure is from D.W. Almgren's
report).
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COMPUTER MODEL OF NTS-2
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DAYS FROM FIRST EXPOSURE
Figure 6
115
TEMPE_RE OF A SILVER TEFLON COVERED,
EAJ_fH FACING, RADIAt[OR FOR SEVEN YEA_S IN [f)W EARI_ ORBIT
l_ermal analysis of the change in ten_oerature of an E.arth facing
radiator over a seven year period, as shown in Figure 7, result_ in an
estimated change in solar absorbtance of 0.27. Assuming an initial 0.08
absorbtance, the final value totaled 0.35. A similar vehicle with only 4.5
years in orbit has shown an increase in absorbtance of only 0.13 based on
thermal analysis. Interestingly, the thermal analyst findin_ these results
displeasisj, performed a "sensitivity" analysis, adjusting heat flow and
t_r_oerature change interpretation to reduce the cha_je in absorbtance by
more than a factor o_ two.
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Figure 7
116
EXPERIMENTS 3 AND 4, ,SOLAR CELL _O_i_ CIRCUIT CURRENT DEGRADATION
NTS-2 also carried solar cell e×per[me.nts. Shown in [igure 8 is the
difference in percent of initial on orbit short circuit current of
identical solar cell and cover slide circuits. The only dif[erence was
that experiment 3 coverslide had anti-reflective coating a_ ultra-violet
filters and was bonded with space quality organic adhesive DC 93-500, while
experiment 4 cover slide had no coatings or filter and was bonded with
fluorinated ethylene propylene (FEP) teflon. The data is from NRL
M6_orandum Report 4580 (ref i).
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Figure 8
117
EXPERIMENTS 1 AND 13 SOLAR CELL $HO_f CIRCUIT CURRENT DEGRADATION
Similarly, experiments 13 and I, also contained identical cells.
Experiment 1 had its cover slides bonded with organic adhesive R63-489
while experiment 13 had its cover slides electrostatically bonded without
any other adhesive. Is it possible that we are throwing away 5% to 10% of
available solar power because of degrading adhesive on solar array covers?
It is noted that these data (ref i) contradict data taken on the Applica-
tions Technology Satellite (ATS)-6 solar cell experiment in geostationary
orbit (ref 2).
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Figure 9
118
SHUTTLE LAUNCH DISPENSER (SLD) SPACECRAFT SHOWING PROPELLANT TANKS
Although, to our knowledge, NRL has not experienced debris or meteoroid
damage, the threat is a design consideration for our space platforms.
As an example, when designing the Shuttle Launch Dispenser (_LD), shown in
figure I0, the design requirement was that there be greater than 0.99
probability that the propellant tanks would survive the expected meteoroid
environment. This was accomplished by performing an analytical study that
addressed the problem. First, using perforation eq,]atio_s, the minimum
size particle that would penetrate the tank was determined. Then using a
flux equation for our orbit, the probability of encountering a particle of
this size or larger was determined. Using this approach the tank wall
thickness was evaluated. Also, because Multi Layer Insulation thermal
blankets were being used, it was decided to use a one-inch standoff fo_ the
blankets which produced a "Whipple Meteoroid Bumper" effect and provided
additional protection for the tanks. Based on this analysis our tanks have
a survival probability of greater than 0.999.
Figure I0
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119
RELATIVESOLARPANELANDSOLIDROCKETMOTORPOSITIONS
A rough correlation of the contamination of solar panels from solid
rocket motoc plume has been attempted. A ccmparison is ,nadefrom
pre-launch solar panel calibration output and post pl_ne exposure data.
Data samples are from several satellites. Whenthe data was consolidated
and no_lized, panel i, located behind the nozzle, was assigned 100%.
Panel 2 averaged 97.0%and panel 3 averaged 97.9%of the panel 1 value.
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TOP VIEW
51DE VIEW
Figure 11
120
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REFERENCES
Walker, D.H. ; Performance of The Solar Cell Experiments Aboard the
NTS-2 Satellite After Three Years in Orbit. NRL Memocandum Report
4580, July 30, 1981.
Goldhammer L.J. ; ATS-6 Solar Cell Experiment/Improvement. Final
NeDort , NAS Contract NAS 5-22873, 31 January 1977.
121


Some examples of the degradation of properties of materials in space

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