207 SYSTEMS: THERMAL CONTROL—THE “CIRCULATORY SYSTEM” OF THE INTERNATIONAL SPACE STATION CHAPTER 11 single-loop configuration. Operating in this fashion means only one pump is running at a given time, and is therefore experiencing wear and tear. Heat is removed from some of the hardware using coldplates, as shown in Figure 9. A rack can house several coldplates, in which case the whole rack is treated as one heat load with a rack FCV that functions in the same way as TWMVs to allow for temperatures to be adjusted, depending on the need of what is in the rack (Figure 10). Conclusion The thermal environment of space is extremely challenging to manage. The ISS has multiple thermal systems to help keep the space station from getting too hot or too cold. The systems employed on the ISS are actually similar to those used in terrestrial buildings. Fiberglass insulation in the walls or paint on a house are examples of passive thermal systems used every day. Heaters are also employed on the station and in the home to prevent areas from getting too cold. Most buildings and homes in hot climates have air conditioning, where heat is transferred to a fluid that, in turn, is radiated outside of the vehicle or building. One major difference exists between terrestrial heating and cooling systems and those employed on the ISS: when an air conditioner on Earth fails, a repairman can order parts and come fix it. When cooling fails on the ISS, every other system—from power generation, to computer controls, to the astronauts themselves—are put in jeopardy. Only the materials that are already in space can be used to make the repair. In the end, the ultimate goal of both Earth-based systems and the ISS thermal systems is the same: to keep the structure and occupants safe and comfortable inside. The engineering challenge with the ISS was to have a thermal system that worked in the extremes of the space environment, worked with the first elements of the ISS, and was able to adapt as the structure grew and matured. The flight control team carefully watches and manages the various cooling loops as the heat loads (e.g., different experiments or numbers of astronauts) vary and as the space station orbit changes (e.g., when the beta angle gets above 60 degrees). The flight control team has also had to deal with significant failures in the system, such as when the pump that controlled ETCS Loop A failed, essentially removing half of the ISS-critical systems. Or when the FCV in the ETCS failed, which removed the ground’s ability to control the loop temperature and thus provide adequate cooling to other ISS systems. Any number of failures elsewhere on the ISS could have meant the loss of the crew or spacecraft (for more details, see Chapter 20). As with so many aspects of life in space, something taken for granted most of the time on Earth requires thoughtful design and focused attention to detail in operation to enable humans to live in the utter darkness and extreme brilliance of space.