Development of an Integrated Design Resolution Plan for the International Space Station Active Internal Thermal Control System Coolant Stability Concerns

The International Space Station (ISS) Internal Active Thermal Control System (ITCS) for the United States Laboratory (USL), and follow-on pressurized elements, uses a mechanically-pumped fluid in closed-loop circuits. The ITCS utilizes a specifically formulated heat transfer fluid (essentially de-ionized water with special additives). Silver ions are specified in the formulation to assist in the control of microorganisms. The potential for microorganisms to form biofilms known to cause heat transport, corrosion problems, and subsequent hardware deterioration has raised concerns among the ISS engineering community.

This paper provides the integrated design resolution to: 1) ensure the coolant meets the required fluid specification; 2) obtain long term “fleet leader” data; 3) allow sampling of the ITCS coolant on-orbit; 4) return the coolant to the ground for complete chemical analyses including microbial parts count; and 5) add silver ions to the coolant stream(s) for on-orbit microbial growth control.