Effects of Low Earth Orbit on Docking Seal Materials

Spacecraft docking and hatch seals are typically made of silicone elastomers. Such seals exposed to low Earth orbit (LEO) 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 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 in LEO would badly damage the seals. Eighteen seals were exposed for 543 days in ram (windward), zenith (way from Earth), or wake (leeward) orientations, and 15 control samples (not flown) provided undamaged base-line leakage. To determine post-flight leak rate, each of the 33 seals were placed in an o-ring groove and pressure tested over time. Resistance temperature detectors (RTDs) pressure transducers, and LABView programs were used to measure and analyze the temperature and pressure and calculate leakage. Average leakage of control samples was 2.6 x 10^-7 lbs/day. LEO exposure didn’t 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 ELA-SA-401 to be a more suitable sealing material in LEO. AO caused greater damage than UV; samples in ram orientation (receiving an AO fluence of 4.3 x 10²¹ atoms/cm²) and in wake (2.9 x 10²⁰ atoms/cm²) leaked more than those in zenith orientation (1.58 x 10²⁰ 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