N89- 23535 F/OME EXAMPLES OF "['HED_GRADATION OF PNOP_REIES OF MATERIALS IN SPACE

Frederick E. Betz and Joseph A. Hauser Naval _esearch laboratory Washington, D.C. 20375-5000

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I09 SOLRAD ii SPACECRAFT

An artist's conception of one of the two Naval Research Laboratory (NRL) SOLRAD ii 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 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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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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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

ORIGINAL PAGE _',.ACK AND WFHTE PH'O.O_:]RAPH" 114 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 d at-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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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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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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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 (ATS)-6 solar cell experiment in geostationary orbit (ref 2).

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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.

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, _.='..... e_ 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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Figure 11

120 REFERENCES

Io 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.

o Goldhammer L.J. ; ATS-6 Solar Cell Experiment/Improvement. Final NeDort , NAS Contract NAS 5-22873, 31 January 1977.

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