Emily C. Imka and Olivia C. Asmar

Hathaway Brown School, Shaker Heights, Ohio

Henry C. de Groh III

Glenn Research Center, Cleveland, Ohio

Bruce A. Banks

Science Application International Corporation, Cleveland, Ohio

Effects of Low Earth Orbit on 

Docking Seal Materials

NASA/TM—2014-218435

December 2014
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Emily C. Imka and Olivia C. Asmar

Hathaway Brown School, Shaker Heights, Ohio

Henry C. de Groh III

Glenn Research Center, Cleveland, Ohio

Bruce A. Banks

Science Application International Corporation, Cleveland, Ohio

Effects of Low Earth Orbit on 

Docking Seal Materials

NASA/TM—2014-218435

December 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

The authors wish to thank and acknowledge the guidance and help received from Kim de Groh and Patty Hunt. We would also 

like to express our sincere appreciation to Phil Jenkins of the Naval Research Laboratory and Gary Pippin (retired) of Boeing for 

providing the opportunity to À y these experiments as part of the MISSE 7 mission.

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This report is a formal draft or working 

paper, intended to solicit comments and 

ideas from a technical peer group.

This report contains preliminary ¿ ndings, 

subject to revision as analysis proceeds.
NASA/TM—2014-218435 1 

Effects of Low Earth Orbit on Docking Seal Materials 

Emily C. Imka and Olivia C. Asmar 

Hathaway Brown School 

Shaker Heights, Ohio 44122 

Henry C. de Groh III 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Bruce A. Banks 

Science Application International Corporation 

Cleveland, Ohio 44135 

Abstract 

Spacecraft docking seals are typically made of silicone elastomers. When such seals are exposed to 

low Earth orbit (LEO) conditions, they can suffer damage from ultraviolet (UV) radiation and atomic 

oxygen (AO, or monoatomic oxygen, the predominant oxygen species in LEO). An experiment flew on 

the International Space Station (ISS) to measure the effects of LEO on seal materials S0383-70 and 

ELA-SA-401 and various mating counterface materials which included anodized aluminum. Samples 

flown in different orientations received different amounts of UV and AO. The hypotheses were that most 

of the damage would be from UV, and 10 days or more of exposure in LEO would badly damage the 

seals. Eighteen seals were exposed for 543 days in ram (windward), zenith (away from Earth), or wake 

(leeward) orientations, and 15 control samples (not flown) provided undamaged baseline leakage. To 

determine post-flight leak rates, each of the 33 seals were placed in an O-ring groove of a leak test fixture 

and pressure tested over time. Resistance temperature detectors (RTDs), pressure transducers, and 

LabVIEW (National Instruments) programs were used to measure and analyze the temperature and 

pressure and calculate leakage. Average leakage of control samples was 2.6u10–7 lbs/day. LEO exposure 

did not considerably damage ELA-SA-401. The S0383-70 flight samples leaked at least 10 times more 

than ELA-SA-401 in all cases except one, demonstrating that ELA-SA-401 may be a more suitable 

sealing material in LEO. AO caused greater damage than UV; samples in ram orientation (receiving an 

AO fluence of 4.3u1021 atoms/cm2

) and in wake (2.9u1020 atoms/cm2) leaked more than those in zenith 

orientation (1.58u1020 atoms/cm2

), whereas variations in UV exposure did not seem to affect the 

samples. Exposure to LEO did less damage to the seals than hypothesized, and the data did not support 

the conjecture that UV causes more damage than AO. 

Introduction 

Motivation 

After two spacecraft dock, the safety of the crew and the success of the mission are threatened by the 

potential for air to leak from the interior of the spacecraft, where the pressure is approximately one 

atmosphere, to the exterior, a vacuum environment. A seal must be formed between the two docking 

spacecraft to prevent air from leaking out. NASA is developing advanced space-rated elastomeric seals 

for a new spacecraft docking system which will enable spacecraft to dock with the International Space 

Station (ISS) and other spacecraft (Refs. 1 and 2). The docking system seal presses and mates against 

either a metal flange (such as on ISS) or against an identical seal on another spacecraft. Candidate seal 

materials were space-exposed to determine their durability to LEO. 
NASA/TM—2014-218435 2 

Threats of the Space Environment 

Prior to docking the seals will be exposed to the low Earth orbit (LEO) space environment. There are 

risks associated with this environment such as exposure to vacuum, atomic oxygen (AO), ultraviolet (UV) 

radiation, charged particle radiation, temperature extremes, and impacts with meteoroid and orbital debris 

(Refs. 3 to 6). Exposure to LEO can cause physical damage to a seal such as embrittlement and erosion, 

thereby compromising its ability to function and threatening crew safety and mission objectives (Ref. 7). 

The extent of exposure to each threat is dependent on the orientation of the sample during flight. 

Orientations include: windward (also referred to as ram, is forward-facing), leeward (also known as wake, 

is rear-facing), zenith (facing away from Earth), and nadir (Earth-facing). In general, surfaces in the 

windward orientation receive higher levels of AO whereas zenith oriented surfaces typically receive 

relatively high UV exposure (Ref. 8). Specimen in-flight testing conditions are determined by 

measurements and modeling the exposures for the particular flight scenario and orientations for the 

specimen being considered. 

Prior Work 

In this work two silicone seal materials were studied (S0383-70 and ELA-SA-401); previous studies 

by de Groh et al. tested a similar material, S0899-50 (Refs. 9 and 10). It was found that S0899-50 had the 

highest adhesive tendencies of the three, which could not be sufficiently lowered by AO pre-treatment 

(Ref. 9), and to be badly damaged by UV (Ref. 10). The S0899-50 compound was judged to be unsuitable 

for docking seal use. Three types of metal counter-faces (bare and anodized aluminum, and nickel coated 

aluminum) were flown with the S0383-70 and ELA-SA-401 elastomers. We have not found prior 

published work presenting space exposure and testing of metal counterface materials. 

Atomic oxygen exposure has previously been used as a pre-treatment method to reduce seal adhesion 

and was found to have positive effects on the functionality of S0383-70 and ELA-SA-401 as compared to 

untreated samples. As shown by de Groh (Ref. 9), Garafolo (Ref. 7), Daniels (Ref. 11), and de Groh 

(Ref. 12), the pre-treated seals adhered less, were less damaged by UV radiation and space AO, and the 

leak rate did not increase significantly. Penney (Ref. 13) found that, in untreated samples, leak rate 

increased significantly with temperature. 

Other types of pre-treatment options being explored to control excessive adhesion include specialized 

greases applied to the surface of the seals, and vacuum ultraviolet (VUV) radiation exposure. In a study by 

Berkebile (Ref. 14), greases were used to mitigate dust contamination. K. de Groh (Ref. 15) used grease to 

prevent AO erosion, but found that the coating could be compromised by dust and scratches. Work by H. de 

Groh (Refs. 12 and 16) focused on grease outgassing and using grease to lower adhesion and prevent UV 

and AO damage through the employment of UV blocking additives to the grease. These works (Refs. 12 and 

16) found that outgassing, adhesion, and UV and AO damage were sufficiently mitigated. It was found by 

de Groh (Ref. 17) that small doses of VUV lowered the adhesive tendencies of S0383-70 in both seal-to-seal 

and seal-to-metal configurations and that leakage was not adversely affected. 

Prior work by Linton et al. (Ref. 18) found negligible change in the permeability of silicone S383 and 

Viton V747 after ground-based exposure to UV and AO. This is likely because they measured bulk 

permeability through the thickness of sheet stock. Radiation and AO cause the degradation of a seal 

primarily through damage to the seal’s surface, due to chemical changes, and mechanisms such as 

mud-tile cracking. Such surface cracking of silicone elastomers due to AO exposure is discussed by 

Banks (Ref. 19); and Chang-Su Huh et al. showed evidence of scissoring of side chain Si-CH3 and 

cross-linking on the surface of silicone rubber due to UV exposure (Ref. 6). Dever and Banks found that 

shorter (140 to 185 nm) wavelength UV damaged DC93-500 silicone rubber less than longer wavelengths 

(185 to 200 nm) due to the deeper penetration of the longer wavelengths (Ref. 20). 
NASA/TM—2014-218435 3 

Candidate Elastomers 

Two candidate silicone elastomer compounds are presented in this study: S0383-70 (Parker Hannifin 

Corp.) and ELA-SA-401 (Esterline Corp.). These were chosen because of their relative ability to remain 

flexible at low temperatures, low hardness (durometer rating), vendor recommendation, and the 

successful history of S0383-70 in NASA flight programs (Ref. 21). The compounds can be differentiated 

by their color and durometer ratings: S0383-70 is rust colored and has a Shore A durometer of 70; 

ELA-SA-401 is blond with a durometer rating of about 50. 

Involvement With MISSE 

Ground-based LEO environment testing facilities do not accurately simulate all aspects of space 

exposure (Ref. 11), so to fully understand the reaction of the seals to LEO, #2-106 size O-rings and 

counterface materials were flown as part of the Seals Experiment on the Materials International Space 

Station Experiment-7 (MISSE-7). MISSE is a series of experiments that received in-flight exposure to the 

LEO environment while mounted on the ISS (Ref. 15). MISSE-7 was launched on Shuttle mission 

STS-129 and placed on the exterior of the ISS at an altitude of approximately 190 km on November 23, 

2009. MISSE-7 was retrieved May 20, 2011 and returned on STS-134 after receiving 543 days of LEO 

exposure. Figure 1 shows MISSE-7 on ISS. 

Adhesion Mitigation 

Silicone elastomers have natural adhesive tendencies that help prevent seal leakage, but frequently 

make separating coupled parts problematic, potentially resulting in damage to the seal or an inability to 

un-dock. Application of specialized grease or exposure to vacuum ultraviolet (VUV) radiation are two 

potential solutions (Refs. 12 and 17). Another promising ground facility pre-treatment used to lower 

adhesion is AO exposure, in which highly reactive unpaired oxygen atoms combine with silicon on the 

surface of the elastomer to create a glassy, non-adhesive SiOx–rich layer (Ref. 9). Ground-based AO 

pre-treatment has affects that differ from exposure to AO in space; the former is beneficial in certain 

cases whereas the latter contributes to erosion and degradation. All flight seals included on the MISSE-7 

Seals Experiment were AO exposed pre-flight to a fluence of 1u1020 atoms/cm2 r 0.1u1020 atoms/cm2

with the expectation that such a pre-treatment could be used on the full-scale docking system seals to 

control adhesion. 

Figure 1.—Left: Astronaut placing MISSE-7 on ISS; Right: MISSE-7 mounted and open on ISS exposing various 

experiments to ram, wake, nadir or zenith exposure. 

 
NASA/TM—2014-218435 4 

Objectives 

The objective of the experiment was to characterize the effects of LEO exposure on the sealing 

performance of S0383-70, ELA-SA-401, and anodized aluminum. This includes analysis of the effect of 

different MISSE-7 flight orientations (and therefore various AO and UV exposures) on both elastomer 

and metal surfaces, and the effect of AO pre-treatment on seal durability and performance. 

Experimental Procedures 

Flight Hardware and Test Articles 

This experiment included samples mounted on three aluminum trays; each tray was mounted on ISS 

in a different flight orientation. The windward (Fig. 2) and leeward trays were identical, each containing 

two S0383-70 seals, three ELA-SA-401 seals, six aluminum flanges with various surface treatments, two 

RTV566 tensile samples, one RTV566 O-ring, and one Kapton (DuPont) AO fluence witness sample. 

One of the 3 ELA-SA-401 samples was placed underneath the Kapton, and thus was exposed to vacuum 

only. The zenith tray contained two S0383-70 seals and two ELA-SA-401 seals and is shown in Figure 3. 

All O-ring seals were size #2-106 with an outer diameter of approximately 0.38 in. The 4 flanges shown in 

 Pre-flight Post-flight

Figure 2.—Pre and post-flight images of the 4 cm wide, windward MISSE-7 Seals Experiment assemblies. 

Figure 3.—Pre-flight zenith facing samples. 
NASA/TM—2014-218435 5 

Figure 2 on the top-left were all made from 6061 T651 aluminum and electroless nickel plated. The 

flange at the top-right was 6061 T651 aluminum and MIL-A-8625 Type II anodized. The flange shown at 

the bottom right side in Figure 2 was bare, uncoated 6061 T5 aluminum. Data included in this report is 

limited to tests of the S0383-70 and ELA-SA-401 O-rings mated against the anodized leeward flight 

sample, or pristine stainless steel. 

The mold for the RTV566 tensile samples was attached to the flight assembly. The tensile specimens 

were cast directly into the mold. Half of the RTV566 O-ring was cast, allowed to cure, and then removed 

from the mold; the mold was then refilled, and the cured half placed on the uncured half in the mold, 

there-by creating the complete O-ring. 

In-Flight Exposures 

After modeling radiation received during flight, and analysis of various Kapton HN witness samples 

(Ref. 8), it was approximated that: 

x Windward orientation received an AO fluence of 4.3u1021 atoms/cm2

 and 2,400 ESH 

(Equivalent Sun Hours) of UV; 

x Leeward received an AO fluence of 2.9u1020 atoms/cm2

 and 2,000 ESH of UV; and 

x Zenith received an AO fluence of 1.58u1020 atoms/cm2

 and 4,300 ESH of UV. 

The Sun has an irradiance in the UV wavelength range (400 nm) of approximately 110.9W/m2

 (Ref. 3); 

thus an equivalent Sun hour (ESH) is equal to about 0.4 MJ/m2

; thus the exposures listed for windward, 

leeward and zenith were 960, 800, and 1720 MJ/m2

, respectively. 

Imaging and Physical Characteristics 

Pre and post-flight macro images were taken along with post-flight optical microscope (OM) images 

in order to document visual changes of all samples. Sample mass and dimension measurements were 

taken pre and post-flight. A Mettler balance was used to determine mass within r0.2 mg; additional mass 

measurement uncertainty is expected due to estimated moisture content variations of up to 0.03 percent 

(0.04 mg) in the elastomers (Refs. 22 and 23). The total uncertainty associated with mass measurements is 

0.17 percent thus changes in mass less than 0.17 percent are considered insignificant. Calipers (Inerapid 

by Brown & Sharpe) were used to measure specimen outer diameter and thickness. Total uncertainty was 

1 and 2 percent for the outer diameter and thickness, respectively. 

Post-Flight Leak Rate Measurements 

Post-flight leak testing was done using the MISSE Flow Fixture (MFF) wherein seals were placed in 

the metal O-ring groove of the MFF and compressed against a metal plate (Fig. 4). The groove was 

0.078 in. deep and the O-ring thickness was approximately 0.103 in., thus 25 percent compression was 

imposed. The metal mating surface (counter-face) was either pristine stainless steel or the anodized 

aluminum alloy flight sample (6061 in the T651 heat treated condition). Temperature and pressure were 

monitored while one atmosphere pressure differential was imposed between the air contained within the 

seal and the external atmosphere to simulate the pressure differential between the interior of the 

spacecraft and the vacuum of the space environment. The MFF system was hermetic and leak tested using 

a helium leak detection system. The temperature of the compressed gas was measured using an RTD 

(resistance temperature detector) with an uncertainty of r0.2 K. The average pressure was measured using 

two calibrated pressure transducers; the pressure transducers had a range of 0 to 20 psig and a percent of 

full scale output accuracy of r0.05 percent. The MFF system included a large reservoir of air contained in 

 
NASA/TM—2014-218435 6 

Figure 4.—Plates of the MFF, a #2-106 O-ring, and anodized 

aluminum alloy counter-face at the center of the top plate. 

a water bath. The bath helped to maintain a constant temperature, and the reservoir provided a large 

supply of air for the test, so that a moderate leak across the seal would not cause a large pressure drop in 

the system. Temperature and pressure data were collected and analyzed with LabVIEW programs 

(Ref. 24). The air mass in the system was determined using the ideal gas law: 

 m = PV/RT 

where P is pressure, V is volume, R is the ideal gas constant, and T is temperature. The leakage was 

determined by the slope of the mass verses time curve at a pressure of 1 atmosphere (14.7 psi). The MFF 

systems and LabVIEW programs were similar to those used by Daniels (Ref. 11). Leak rate uncertainty 

was determined using error analysis techniques similar to those presented by Daniels (Ref. 11) and 

Garafolo (Ref. 25). As shown by Daniels (Ref. 11) the highly accurate measurement techniques result in 

negligibly small bias and precision errors and minute error bars for an individual leak rate measurement. 

However, there was significant variability between repeat leakage tests. For these reasons, further 

discussion of the uncertainty of individual tests was omitted. For most test conditions, 2 or 3 tests were 

done; the average standard deviation of the resulting data was used to define the uncertainty (error bar). 

The stainless steel counter-face and O-ring groove were cleaned between each test. The anodized Al flight 

counterface was never cleaned. 

Results and Discussion 

Mass and Size Changes 

Mass and dimension measurements are presented in Table 1. Mass changes of less than 0.17 percent 

are within measurement uncertainty as are diameter changes of  1 percent and thickness changes 

 2 percent. Mass loss for S0383-70 was negligible for O-rings flown in the windward and leeward 

orientations but not for specimens in the zenith orientation. This can be explained by considering the 

AO and UV doses in the different orientations and the compounds expected responses: In general silicone 

elastomers are not expected to lose significant mass during AO exposure because incoming oxygen reacts 

with Si forming a stable SiOx phase; however, UV exposure can break chemical bonds in the compound 

resulting in radicals that are subsequently drawn off by the vacuum of space. This is reflected in the mass 

loss response of S0383-70 where UV exposure and mass loss were lowest for the leeward oriented 

specimens, and highest for zenith oriented. Mass loss for ELA-SA-401 was similarly highest for the 

zenith oriented samples; mass loss for ELA-SA-401 was consistently higher compared to S0383-70. Mass 

loss for all cases was less than the NASA requirement of  1 percent. Mass loss was negligible for the 

ELA-SA-401 O-rings exposed to vacuum only (zero AO and UV) and for the metal specimens. 
NASA/TM—2014-218435 7 

TABLE 1.—MASS AND DIMENSIONAL CHANGES OF ELASTOMER O-RINGS AND 

METAL DISK SPECIMEN DUE TO EXPOSURE TO LEO

[Specimens in the “Vacuum” orientation were covered by Kapton thus received no AO or UV.] 

LEO flight 

mode 

Specimen type Pre-flight 

mass, 

g 

Postflight 

mass, 

g 

Average 

mass 

change, 

percent 

Preflight 

outer 

dia., in.

Pre-flight 

thickness, 

in. 

Post-flight 

outer dia., 

in. 

Post-flight 

thickness, 

in. 

Average 

outer dia. 

change, 

percent 

Average 

thickness 

change, 

percent 

Windward S0383-70 0.1445 0.1442 

–0.1383

0.37 0.1 0.3755 0.1 

0.743 –0.150 

Windward S0383-70 0.1449 0.1448 0.372 0.1 0.372 0.0997 

Windward ELA-SA-401 0.135 0.1346 

–0.2219

0.376 0.106 0.373 0.102 

–1.243 –2.363 

Windward ELA-SA-401 0.1355 0.1353 0.385 0.105 0.3785 0.104 

Windward Bare Al 1.67 1.6698 

–0.0089

0.4935 0.1205 0.494 0.124 

–0.050 0.466 

Windward Anodized Al 1.6596 1.6595 0.4955 0.119 0.496 0.119 

Windward Ni coated Al 1.7 1.6998 0.497 0.1235 0.4965 0.125 

Windward Ni coated Al 1.6694 1.6693 0.4942 0.1203 0.4935 0.12 

Windward Ni coated Al 1.6869 1.6868 0.4948 0.1218 0.494 0.1215 

Windward Ni coated Al 1.6788 1.6786 0.495 0.121 0.4945 0.12 

Vacuum ELA-SA-401 0.1355 0.1356 0.0738 0.381 0.107 0.379 0.105 –0.525 –1.869 

Leeward S0383-70 0.1443 0.1442 

–0.1019

0.376 0.1 0.3705 0.1 

–1.266 0.495 

Leeward S0383-70 0.1486 0.1484 0.374 0.101 0.37 0.102 

Leeward ELA-SA-401 0.1373 0.1368 

–0.3295

0.386 0.106 0.3795 0.1055 

–2.089 –0.472 

Leeward ELA-SA-401 0.1357 0.1353 0.381 0.106 0.3715 0.1055 

Leeward Bare Al 1.663 1.6628 

–0.0158

0.4955 0.1195 0.496 0.1205 

0.134 1.715 

Leeward Anodized Al 1.6923 1.6919 0.4965 0.1225 0.497 0.1275 

Leeward Ni coated Al 1.6991 1.6988 0.4965 0.1225 0.497 0.1225 

Leeward Ni coated Al 1.6651 1.6649 0.496 0.119 0.4965 0.121 

Leeward Ni coated Al 1.6883 1.6881 0.4955 0.121 0.496 0.123 

Leeward Ni coated Al 1.7023 1.702 0.4975 0.1225 0.499 0.125 

Vacuum ELA-SA-401 0.135 0.1349 –0.0741 0.383 0.105 0.38 0.104 –0.783 –0.952 

Zenith S0383-70 0.1482 0.1477 

–0.3756

0.373 0.102 0.373 0.102 

–0.134 –0.495 

Zenith S0383-70 0.145 0.1444 0.374 0.101 0.373 0.1 

Zenith ELA-SA-401 0.1381 0.1374 

–0.5129

0.386 0.107 0.3765 0.104 

–2.068 –1.402 

Zenith ELA-SA-401 0.1349 0.1342 0.388 0.105 0.3815 0.105 

Control S0383-70 0.1459 0.1459 

0.0000 

0.375 0.105 0.369 0.101 

–0.733 –1.905 

Control S0383-70 0.1464 0.1464 0.371 0.101 0.3715 0.101 

Control ELA-SA-401 0.1374 0.1375 

–0.0382

0.38 0.107 0.385 0.107 

–0.786 0.000 

Control ELA-SA-401 0.134 0.1338 0.381 0.105 0.37 0.105 

Control Bare Al 1.6936 1.6936 

0.0059 x x x x 

Control Anodized Al 1.6898 1.69 x x x x 

Control ELA-SA-401 0.1347 0.1346 –0.0742 0.376 0.106 0.3705 0.105 –1.463 –0.943 

 
NASA/TM—2014-218435 8 

The outer diameter and thickness of S0383-70 O-rings did not change substantially for any of the 

orientations. By comparison ELA-SA-401 shrank more, though shrinkage was still very small with a 

maximum of –2.36 percent for the thickness of the windward ELA-SA-401 O-rings. Control 

ELA-SA-401 and S0383-70 O-rings shrank an average of approximately 0.8 and 1.3, respectively, 

indicating that some shrinkage is expected as part of the natural aging process for the compounds. 

Damage Determined by Leak Rate 

Figure 5 presents seal leakage results. Reference seals were pristine; Control seals received the 

1u1020 atoms/cm2

 AO pre-treatment; Vacuum received the AO pre-treatment and were flown in space but 

were covered by Kapton thus received no additional AO or UV; all other elastomer samples were spaceexposed and their orientation is indicated. “SS” refers to the seal being mated with the pristine stainless 

steel counterface; “Al” indicates that the seal was mated to the leeward space-exposed anodized 

aluminum specimen. The results presented in Figure 5 were from either a single test or the average of two 

or three repeat tests with the same seal and counter-face; repeats were done for 11 of the conditions 

shown in Figure 5. The average percent standard deviation for these tests was 71 percent; this value was 

used to characterize leak rate uncertainty and defined the error bars shown in Figure 5. Figure 5 shows 

that the leakage for Reference and Control samples is about the same, thus AO pre-treatment does not 

cause an increase in seal leakage. The leakage of the Vacuum ELA-SA-401 seals was about the same as 

the windward, leeward, and zenith samples, showing these doses of AO and UV to have little effect on 

seal leakage for this elastomer compound. Leakage for space exposed S0383-70 seals was much higher 

than the Control and Vacuum seals of the same type, indicating damage severe enough to compromise 

functionality. The effects of space exposure on anodized aluminum (Al) appear very small since on 

average it performed as well as pristine SS in Reference and Control tests, and in all space exposed 

ELA-SA-401 tests. Leakage of space exposed S0383-70 paired with anodized aluminum was however 

consistently higher than with SS, indicating the effects of damage on S0383-70 are exacerbated by the 

exposed Al counter-face. 

Figure 5.—Air leak rates for seals tested against the anodized aluminum flight sample (Al) and pristine 

stainless steel (SS). Dark-red bars are S0383-70; gold bars are ELA-SA-401. 

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

Reference-SS

Reference-Al

Reference-SS

Control-SS

Control-Al

Control-SS

Control-Al

Vacuum-SS

Vacuum-Al

Windward-SS

Windward-Al

Windward-SS

Windward-Al

Leeward-SS

Leeward-Al

Leeward-SS

Leeward-Al

Zenith-SS

Zenith-Al

Zenith-SS

Zenith-Al

Leakage, lbs/day
NASA/TM—2014-218435 9 

Atomic oxygen caused more damage than UV; windward and leeward samples (which received high 

AO) leaked more than those in zenith orientation (low AO), whereas variations in UV exposure did not 

seem to affect the samples. A greater sample size is necessary to confirm the consistency of these 

conclusions. Sources of error may include the differences in shape and size as compared to a full-scale 

seal for the docking system, as well as between the way these samples were tested compared to actual use 

of the docking system seal (which is not seated in an O-ring gland). Error may also have been caused by 

the small number of samples analyzed. Prior work has shown significant damage of silicone rubber 

resulting from small doses (1 MJ/m2

) of UV (254 nm) (Ref. 16). The major differences between 

(Ref. 16) and the MISSE-7 results presented here were (Ref. 16) employed a larger seal with a rectangular 

cross section, pressed between two flat plates (no gland) and 15 percent compression rather than 

25 percent. 

The findings of this study are in disagreement with a similar study reported by Daniels et al. (Ref. 11). 

Daniels et al. flew S0383-70 and ELA-SA-401 #2-106 O-rings on MISSE-6, exposing them to 18 months of 

windward and leeward orientated LEO and found the performance of ELA-SA-401 to be worse compared to 

S0383-70. The reasons for this inconsistency are unknown. One contributing factor may have been in the 

process by which O-rings were chosen for use. The quality of the ELA-SA-401 O-rings was not consistent, 

so in this work the O-rings were examined and only the best were included in the study. 

Conclusions 

Leakage rates of Reference elastomer samples were very similar to the leakage rates of the Control 

samples indicating that AO pre-treatment did not cause a significant increase in seal leakage. Thus, AO 

pre-treatment appears to be an effective method of reducing adhesion without compromising the integrity 

of the seal. A comparison of the leakage rates of the ELA-SA-401 seals that were in a Vacuum and the 

ELA-SA-401 specimens that were exposed to LEO conditions suggests that the varying doses of AO and 

UV as the result of a windward, leeward, and zenith orientations on the ISS had little effect on leak rate. 

However, the leakage rates of LEO exposed O-rings made of the S0383-70 elastomer compound were 

markedly higher than the S0383-70 Control specimens. This indicates that the S0383-70 compound was 

damaged as a result of LEO exposure and that the damage was severe enough to compromise the 

functionality of the seal. The anodized aluminum exposed to LEO did not show signs of significant 

damage or compromised functionality, as it performed nearly as well as pristine stainless steel when 

mated to ELA-SA-401 test specimens. Despite this apparent lack of damage, the leak rates of the 

S0383-70 O-rings were dramatically higher when paired with the aluminum rather than stainless steel 

mating surface. This indicates that the effects of damage due to LEO exposure to the S0383-70 elastomer 

compound were worsened by being paired with the exposed anodized aluminum counterface. The results 

of this experiment indicate that ELA-SA-401 is more durable to LEO exposure, however additional 

testing is needed due to the presence of contradicting data in the literature. 

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