AEON designed and manufactured Optical Ground Support Equipment (OGSE) for RAL Space’s STC-2 and STC-3 simulation chambers. All internal surfaces required a low-outgassing high absorptivity black coating to minimise the reflection of stray light during optical calibration of satellite instrumentation. Aeroglaze Z306 paint was specified as it met optical, cleanliness and outgassing requirements; and it has a proven track record of successfully being used in previous space applications.
Following application, it is necessary to forcefully remove most organic and inorganic condensable volatiles from the surface treatment. This type of process is known as a ‘bakeout’ and requires for the finished product to be subject to heat and vacuum, this environment forces volatile molecules to be ejected from the substrate through diffusion and desorption mechanisms; this is achieved through the application of heat and ultra-high vacuum to the Item Under Test (IUT).
It is important to note that materials with a high volatile mass are significant sources of contamination which can easily degrade the performance of spacecraft hardware. For components being used in high vacuum and ultra-high vacuum applications failing to outgas components before installation will not only contaminate the vacuum system with unwanted molecules, but it will also affect the pump-down profile likely leading to a much longer pump-down duration to achieve the required vacuum.
For this project the IUT was installed in the chamber with ancillaries (heaters and thermocouples) and was subject to a temperature of approximately 120°C (a limit set by the coating manufacturer’s maximum duty temperature) and a pressure of 1×10-6 mBar over a period of 96 hours.
To monitor the rate of volatile molecule outgassing, AEON’s engineers used an instrument known as a Thermally-controlled Quartz Crystal Microbalance (TQCM). The TQCM is a large quartz crystal oscillator cooled to a specific temperature that promotes the condensation of ejected volatile molecules; once these molecules are adhered to the crystals surface tiny changes to the oscillators resonant frequency can be detected, therefore allowing AEON’s engineers to calculate exactly how far through the bakeout process hardware is at any given time. The TQCM can be regenerated by heating the crystal, ejecting the collected molecules then cooling back down, typically this is performed several times over the course of a bakeout campaign.
AEON successfully performed the bakeout process on four OGSE windows, including protection covers and ancillary equipment. Process and environmental variables were fully monitored throughout the process and a detailed bakeout report delivered alongside hardware to empirically verify the completion of the activity to RAL Space and ECSS bakeout standards (ECSS-Q-ST-70-02C).