During Servicing Mission 3B (SM3B) for the Hubble Space Telescope (HST) the Power Control Unit (PCU) will be replaced. The PCU was not originally designed to be replaced on orbit but was later identified as having the capability of being removed and replaced on orbit. The PCU has many connectors and bolts, some of which are difficult for the astronauts to reach. Due to the added difficulty, the replacement will take an entire six hour Extra Vehicular Activity (EVA) day. For four of these six EVA hours the door of the Support Systems Module (SSM) Equipment Section (ES) bay where the PCU is found will be open to allow the astronauts access to the PCU and its connectors. This bay, SSM ES Bay 4, also contains the four Power Distribution Units (PDUs), which house the busses, switching, fusing, and monitoring circuits that distribute power to the telescope. These PDUs are attached to the bay door and will be seeing a much colder than normal environment with the door open.
The PCU controls power for the entire telescope and therefore when it is replaced all power to the telescope will be turned off. With power removed and the bay door open the PDUs will cool rapidly. In order to be sure that the PDUs remain above their turn-on and operating temperature limits during the change out, a detailed open bay door analysis was conducted. For this analysis a Thermal Synthesizer System (TSS) model was created of Bay 4 and it’s components. The model was used to calculate radiation couplings and heat rates with the bay door open. The FORTRAN code used to analyze the Servicing Mission timeline was modified and the new radiation couplings and heat rates added.
This paper deals with the creation of the detailed TSS model as well as the modifications made to the timeline code in order to run the analysis. It will also present the temperature predictions for the PDUs during the PCU change out.