KAPTON AND SOLAR CELL MATERIAL DURABILITY TEST WITH EXPOSURE TO A 10N BI-PROPELLANT THRUSTER PLUME AND VUV

Kurzfassung

It is well known that plume induced contamination from chemical propulsion systems can affect the properties and performance of functional spacecraft materials. But up to now, the rare ground testing of plume induced contamination did not consider the harsh environment when approaching the sun, i.e. especially the impact of propellant droplet residues in combination with UV exposure and heated spacecraft surfaces was never investigated. This activity introduces testing of the aforementioned scenario with a 10 N bipropellant thruster operated on MON3-MMH in a conventional vacuum chamber. The main question of this investigation is whether a UV irradiation of deposited droplets at different surface temperatures leads to a photo-fixation effect, or enables damaging chemical reactions on Kapton® surfaces, including Kapton® insulation on solar cells.

Samples were exposed to major amounts of liquid contaminants in the backflow (pulse trains with 20-100 ms thruster on-time, 5...15 pulses each) and subsequently exposed to UV for 4 h in-situ. Recovered samples were analysed by optical microscopy. Experiments showed, that with permanent droplet contamination generated at surface temperatures from 20-160 °C, neither plain Kapton® samples, nor entire solar cell elements exhibited damaged sites, craters or holes provoked by the applied conditions, when analysed ex-situ. The main observation is, that re-evaporation of a majority of deposited liquid takes place at elevated sample temperatures. An exposure to UV, conducted with three Hamamatsu® light sources, that do not accurately represent the solar spectrum in the UV range, but exhibit a strong signal of up to 20 solar constants in the VUV range from 115-200 nm with peak intensity at 160 nm but low intensity from 200-400 nm, seems to favour this re-evaporation. However, in each case, some droplets permanently remain on the surface even at elevated temperatures up to 160 °C. Microscopic imaging revealed a field line-like arrangement of droplets for samples impinged at room temperature without subsequent UV exposure, both on plain Kapton® samples and on the solar cell cover glass, which is an exceptional observation and has not yet been explained. Applying a voltage from solar cell to substrate, separated by a layer of Kapton®, did not result in a sustained measurable current (and thus a breakdown in insulation resistance) in any of the six tested solar cell assemblies.

High levels of VUV irradiation of droplets composed of MMH/MON plume products do not appear to lead to damage on Kapton® surfaces under the applied conditions. Note, however, that each firing has a unique contamination potential, depending for example on firing history, thruster settings and receiver material conditions. The influence of the UV-spectrum or -intensity on the chemical reaction or re-evaporation of droplets on Kapton® surfaces is yet to be quantified.