High Speed Bearing Wear Rate Measurements for Spacecraft Active Thermal  Control Fluid Pumps with a Novel Pin on Disk Apparatus  Robert J. Bruckner* and Richard A. Manco II**  Abstract A novel pin on disc tribometer was designed and constructed to generate a high-speed, wear coefficient  database for hydrodynamic bearings that are typically used in canned motors found in the active thermal  control circuits of robotic and inhabited spacecraft. The primary motivation for this work was the premature  failure of the active external thermal control pump on the International Space Station in 2010. During the  failure investigation of this incident, the root cause was postulated to be high-speed wear of the bearings.  Although a detailed forensic analysis gave credibility to this theory, the lack of wear coefficient data at  relevant conditions prevented validation of this finding. The database generated from the new Extreme  Environment Tribometer (EET) enabled a closure calculation within 5% of the observed wear from  inspections of the failed hardware. Testing in anhydrous ammonia and surrogate fluid was performed to  provide a means for simplified testing in the future and to populate a preliminary database for the design of  future active thermal control systems on spacecraft. The EET and test techniques developed for the  measurement of high-speed wear coefficients are available to future system designers.  Introduction  Active thermal control systems on spacecraft, both robotic and inhabited, most commonly rely upon a highspeed canned motor pump to circulate the heat transfer fluid between heat exchangers and radiators. Three  notable NASA examples of spacecraft with such pumps are the Parker Solar Probe, the Orion Crew Module,  and the International Space Station [1-10]. A graphical representation of a canned motor pump is shown in  Figure 1 [11,12]. The working fluid in Figure 1 is colored light blue and highlights the most unique feature  of the pump. This feature includes the use of the primary working fluid as the hydrodynamic bearing  lubricant and the heat transfer medium to remove eddy current losses from the rotating group. The primary  reason for the use of canned motor pumps in spacecraft is the elimination of high-speed shaft seals, which  are the most likely cause of failures in traditional pump designs. The canned motor pump has become  ubiquitous not only in spacecraft active thermal management but also in ground-based applications where  high reliability and an intolerance to working fluid contamination are key design constraints. While these  pumps are elegant in their dual use of working fluid, this feature bring with it two key complications. First,  the entire motor (rotor and stator) must be internally shrouded from the working fluid to protect  electromagnetic components from the primary working fluid. Secondly, the characteristics of an excellent  heat transfer fluid are in stark contrast to the optimum rheology required of lubricants for hydrodynamic  bearings. A third complication arises from the use of these pumps in micro-gravity applications, such that  the primary force on the bearings is not determined by gravity and the weight of the rotor. In ground-based  applications this preferential load direction is used to identify the critical bearing locations and to incorporate  health monitoring instrumentation to determine incipient failure of the bearings due to high wear.  *NASA Glenn Research Center, Cleveland, Ohio ** HX5 Sierra at NASA Glenn Research Center, Cleveland, Ohio Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018  NASA/CP—2018-219887 499