Columbus Engineering Activities for NASA OCAD 124967 Implementation

This paper describes the activities performed to implement the new operational hazard control issued by NASA for the International Space Station, namely OCAD 126182, to be executed during a startup of the external active thermal control system from a previous failure or a planned activity. The OCAD aims to prevent a failure of the Interface Heat Exchanger (IFHX) due to water freezing in the IFHX core with consequent release of hazardous ammonia concentrations into the habitable volume. Basically the OCAD requires that the water flow provided by Columbus to the IFHX under such contingency conditions is at least 1700 lbm/hr: NASA thermal analysis verified that a water flow of 1700 lbm/hr is enough to prevent freezing. This specific flow requires setting Columbus in an off nominal configuration that was never tested before, neither on orbit nor on ground. The first implementation of this OCAD should occur during the EATCS restart of loops A and B as part of the loops refill currently scheduled in January 2025. These activities consisted, first, in identifying the proper Columbus configuration while satisfying the OCAD request, still taking into account all possible drawbacks and payload science needs; then in thermal analysis runs and in an on orbit test definition and execution. As a conclusion the selected configuration has been included in the relevant procedures and flight rules to support the planned EATCS loop refill. The tasks have been conducted in the frame of the Columbus Engineering Support Team (EST) Off-Site responsibilities.