Recovery from one failure during early assembly stages of the International Space Station

摘要

Primary source of electric power for the International Space Station (ISS) is the photovoltaic module (PVM). At assembly complete stage, the ISS is served by four PVMs. Each PVM contains two independent power channels such that one failure will result in loss of one power channel. During early stages of assembly, the ISS is served by only one PVM and one power channel failure has a more significant impact on the ISS electric power capability. In the limited context of this study, the early stages are limited to stages 4A and 5A. Power channel components are arranged in orbital replaceable units (ORU) and repairs are accomplished by replacing these ORUs. While a power channel is operating, all of its ORUs are maintained within their allowable temperature ranges by a balance between the heat generated and heat removed. Heat removal is accomplished either passively or by an active thermal control system, called photovoltaic thermal control system (PV TCS). An ORU failure may result in dormancy of one or more ORUs, and in the worst case, dormancy of all ORUs within one power channel and shutdown of PV TCS. Dormant ORUs may exceed their allowable temperature ranges and may result in failure of additional ORUs. It is very important to ensure that all dormant, but otherwise operational ORUs, remain within their allowable temperature ranges. A set of operational procedures exists that uses backfeed of electric power to monitor and control temperature of dormant ORUs in one power channel. These procedures cannot be used during the early stages without performing an extravehicular activity (EVA) to install electric jumpers. During the early stages, it may take 1 week to plan and complete an EVA. This time period is long enough that all dormant ORUs will reach their steady-state temperatures and potentially suffer damage because of thermal exceedances. This paper describes a new set of procedures that can thermally protect dormant ORUs. These procedures use the following: Supply of DC control power from the operating channel to the dormant channel via remote bus isolators (RBI). Control of the ISS orientation with respect to Sun and space within allowable ranges. Supply of secondary power via remote power controller (RPC). The paper also describes electric power capability of the PVM and how it can satisfy power demands with only one operating power channel.