Columbus on orbit test: HCU set-point change

This paper describes the test performed on orbit on Columbus on the Heater Control Unit (HCU), the unit that commands the temperature control of the module external shell. Nominally, the HCU on/off set points are 20°C/23°C, while the cabin set point is 23°C. In 2016, upon temporary cabin set point change (from 23 to 19°C), it was pointed out an increase of power consumption by the HCU-controlled heaters. Based on this observation, Flight Control Team (FCT) required to verify the relationship between HCU set points, cabin air temperature and heater power consumption. Since the latter cannot be exactly predicted analytically (too many variables should be considered, e.g. cabin temperature set point, internal power dissipation, beta angle, ISS attitude variation...), an on orbit test was proposed by Engineering Control board (ECB) to integrate the available data. The on orbit HCU limits have been decreased to 18°C (low) and 21°C (high) from beginning of June 2017 to mid of January 2018. This test allowed collecting data over a significant range of beta angles and provided the necessary information to decide how to proceed with the nominal HCU set points. Available operational products allowed to execute this test and to protect against condensation issue if needed. Prior to the test, it was not known whether after decreasing the HCU limits the cabin temperature set point could be guaranteed for any combination of on-orbit conditions (beta angle) and cabin temperature set points (crew preference), therefore the cabin temperature trend was carefully monitored throughout the whole test in order to restore the previous HCU limits, if needed. The test has been successfully completed and preliminary results confirm a remarkable heater power saving and some cabin temperature issues, while the potential condensation issue during Condensing Heat eXchanger (CHX) dry-out is avoided applying the available operational countermeasures.