[image-51][image-67]International Space Station temperature control is a good deal more important than keeping things inside comfortable. It is a critical system that keeps the station livable and enables important scientific equipment to conduct the microgravity research that is the station’s primary reason for being.
The station has two external cooling loops. The two loops circulate ammonia outside the station through giant radiators to keep the station cool. When major abnormalities develop, some equipment is moved to the other loop for cooling and other equipment powered down. Such a failure doesn’t place the crew in any immediate danger nor does it necessarily disrupt activities. It does remove redundancy, which could make a second failure a far more serious concern. Loop A and Loop B are supported by Control Pump Modules that regulate the flow and temperature of the ammonia as it circulates.
The station orbits the Earth in about 92 minutes at an altitude of around 260 statute miles. It experiences large fluctuations in temperatures. They range from around 200 degrees Fahrenheit when the station is exposed to the sun to about 200 below zero over the night side of the planet. Heat also is generated by various equipment on board.
Temperature is controlled by passive measures – insulation, thermal coatings, heaters and heat pipes -- and by an active thermal control system.
The system is the main permanent active heat rejection system on the station. It acquires, transports and rejects excess heat from all U.S. and international partner modules except the Russian modules. The two loops circulate ammonia to transport heat and cool equipment. Pump modules for the two loops circulating that ammonia are situated outside the pressurized part of the station, on the station’s main truss. The ammonia is pumped through radiators to remove heat.
The station’s radiator system, a critical component of the active system, consists of seven panels (each about 6-by-12 feet) designed to deploy in orbit from a 2-foot-high stowed position to a 50-foot-long extended position. Tubes are routed throughout the radiators, and ammonia is circulated through the tubes. The ammonia collects heat from the space station's electronic equipment and module cooling components and transfers it to the radiator panels to be dissipated into space. Ammonia was selected because it is the best heat transport fluid that meets all of NASA's thermal performance and safety requirements (toxicity, flammability, freeze temperature, stability, cost and successful commercial and industrial use).
[image-83]Inside crew modules, the active thermal control system uses water for heating and cooling. Air passes heat to the internal system in the air conditioner, which also collects water from the air’s humidity for system reuse.
The 2010 a pump failure in Loop A involved replacement of the pump during three spacewalks that August.
“That was a failure to be able to move the ammonia,” said Kenny Todd, ISS Mission Operations Integration Manager. “Here we have a failure in controlling the temperature of the ammonia. The valve is basically a mixing valve, which helps regulate the temperature of the circulating ammonia. While it's in the same pump module, it's in a different area,” Todd said.
“Everything we can do is being done,” Todd said. “The system is good and stable. The crew is in good shape. All the right folks on the ground are looking at the problem and trying to assess exactly what the root cause is and what our options are to continue moving forward.”