217 DAY IN THE LIFE: EMPTY HOUSE—DECREWING THE INTERNATIONAL SPACE STATION CHAPTER 12 Emergency Response As discussed above, the crew takes a leading role in responding to emergency events on board the ISS. The team thoroughly reviewed emergency responses to develop a strategy for the flight control team to respond to each kind of emergency without a crew available. Rapid Depress As stated above, prior to leaving, the crew would close hatches between modules while the IMV was left enabled. If on-board sensors detected a depressurization, automatic software would close the IMV valves, thus isolating the modules. If a depressurization is too slow to trigger automatic software, the flight control team would manually command the Rapid Depress emergency response— an action usually taken by the crew. This would limit the depressurization to the leaking module or modules. Without a crew on board, the leaking module could not be repaired. At that point, the flight control team would unpower equipment in the affected module. If the affected module contained critical equipment, the flight control team would attempt to keep the critical equipment operational on a best-effort basis. Toxic Atmosphere As discussed in Chapters 11 and 19, one of the most dangerous events that can potentially occur on the ISS is the rupture of a TCS heat exchanger between the external and internal TCS loops. If this were to happen, the ammonia in the external loop would flood the internal loop and quickly fill the ISS atmosphere to lethal levels. Additionally, the pressure from ammonia entering the atmosphere could overcome ISS design limitation. However, a system of valves provides Positive Pressure Relief (PPR) by venting excess pressure overboard. This PPR is available only in certain modules. Many redundant levels of hardware design and hardware and software active controls prevent the heat exchangers from freezing, which could cause them to rupture. However, since the potential is so dangerous, the crew and ground teams train extensively to respond to a heat exchanger rupture. When crew members are on board, their response is to evacuate the United States On-orbit Segment (USOS), which is the only segment directly impacted by the ammonia. They do not have time to close hatches between USOS modules. However, the automatic toxic atmosphere software response does close IMV valves between modules. With the hatches left open, all USOS modules have PPR available. In a decrew configuration, the USOS hatches would be closed and automatic software that closes IMV valves would leave some USOS modules isolated with no access to PPR. If one of those modules contained the ruptured heat exchanger, that module would be exposed to pressures that could catastrophically damage the ISS. The team decided the best course of action was to inhibit the toxic atmosphere emergency software automatic response, which would leave the IMV valves open even if an ammonia leak was detected. This would provide PPR to the entire USOS, and would be similar to the crew response of not closing hatches. If the flight control team observed other potentially toxic substances via downlink cabin video or telemetry, the affected modules would be isolated by closing IMV to those modules. Fire Automated software response to a fire is to shut down both intra- modular ventilation and IMV fans (see Chapter 19). This is based on the fact that there is no convection in microgravity. Without convection or forced ventilation, fires will consume the locally available oxygen and then extinguish due to a lack of additional oxygen (see “Convection—Gravity’s Cooling Mechanism” in Chapter 11). Following ventilation shutdown, a crew would use air sampling equipment to pinpoint the location of a fire. Once the location was identified, equipment in that location would be unpowered to remove possible ignition sources. The crew would then investigate this equipment further to determine the cause of a fire, and set up equipment to remove smoke from the atmosphere. Flight controllers would not be able to pinpoint fire sources or perform detailed equipment inspections without a crew on board. In this scenario, the team determined the best fire response would be to allow the automated software to terminate ventilation. The ground team would then review available telemetry, including cabin video, to determine whether any equipment show off- nominal signatures. If telemetry pointed to a potential fire source, that equipment would be unpowered. Whether or not a potential source