ISS Ammonia Pump Failure, Recovery, and Lesson Learned –   A Hydrodynamic Bearing Perspective   Robert J. Bruckner*and Richard A. Manco II** Abstract The design, development, and operation of long duration spaceflight hardware has become an  evolutionary process in which meticulous attention to details and lessons learned from previous experiences play a critical role. Invaluable to this process is the ability to retrieve and examine spaceflight hardware that has experienced a premature failure. While these situations are rare and unfortunate, the failure investigation and recovery from the event serve a valuable purpose in advancing future space mechanism development. Such a scenario began on July 31, 2010 with the premature failure of an  ammonia pump on the external active thermal control system of the International Space Station. The ground-based inspections of the returned pump and ensuing failure investigation revealed five potential bearing forces that were un-accounted for in the design phase and qualification testing of the pump. These forces could combine in a number of random orientations to overload the pump bearings leading to solid-surface contact, wear, and premature failure. The recovery plan identified one of these five forces as  being related to the square of the operating speed of the pump and this fact was used to recover design  life through a change in flight rules for the operation of the pump module. Through the course of the  failure investigation, recovery, and follow-on assessment of pump wear life, design guidance has been developed to improve the life of future mechanically pumped thermal control systems for both human and robotic exploration missions. Introduction The International Space Station (ISS) is an engineering marvel. This micro-gravity laboratory was the result of decades of engineering, design, international cooperation, construction, and operation. One of the most significant engineering accomplishments of this laboratory has been the power generation and active thermal control system. These two systems must be considered in the same context because of their integral relationship in the operation of the ISS. Both of these systems contain large-scalemechanisms and control schemes to continuously optimize the performance of the system. The Photovoltaic Arrays (PVA) pivot and rotate in order to maximize the capture of energy from the sun, while the heat rejection radiators rotate to seek shade in order to maximize heat rejection to space through radiation. In addition the active thermal control system utilizes a high-speed centrifugal pump to maintain adequate flowrate of the heat transfer fluid throughout the system. In highly simplified terms, the thermal  control of the inhabited volume of the ISS is a precise balancing act between electrical power input from the PVA and waste heat rejection through the radiator panels. An additional requirement of the active thermal control system is to maintain a uniform temperature on critical parts of the ISS structure such as habitat interfaces and airlocks. The energy balance of the system is illustrated in Figure 1. The thermal  control system is symmetric having independent port and starboard loops. Although flight rules dictate  that both systems shall run at all times, there is limited redundancy on the ISS such that limited operation  is possible with only one functional loop. The working fluid in the heat rejection system is high-pressure liquid, anhydrous ammonia at a nominal system pressure and temperature of 2.62 MPa and 275 K (380  psia, 36/g113F). Additional components of the system are shown in Figure 2. The specific arrangements of the  components of the active thermal control system are also shown in the figure. The pump module is an  Orbital Replacement Unit (ORU) designed so that it could be exchanged with spare units during an  *NASA Glenn Research Center, Cleveland, OH, USA **Sierra Lobo Inc. at the NASA Glenn Research Center, Cleveland, OH, USA /g51/g85/g82/g70/g72/g72/g71/g76/g81/g74/g86/g3/g82/g73/g3/g87/g75/g72/g3/g23/g21/g81/g71/g3/g36/g72/g85/g82/g86/g83/g68/g70/g72/g3/g48/g72/g70/g75/g68/g81/g76/g86/g80/g86/g3/g54/g92/g80/g83/g82/g86/g76/g88/g80/g15/g3/g49/g36/g54/g36/g3/g42/g82/g71/g71/g68/g85 /g71/g3/g54/g83/g68/g70/g72/g3/g41/g79/g76/g74/g75/g87/g3/g38/g72/g81/g87/g72/g85/g15/g3/g48/g68/g92/g3/g20/g23/g16/g20/g25/g15/g3/g21/g19/g20/g23/g3 451