faced by the pump designer is to increase the life of these pumps as space missions become more challenging. The operating life requirements for micro-gravity operation of high-speed centrifugal pumps  has increased from days, to weeks, to years with the expectation of extending pump life to decades. The  pump module failure of 2010 has provided invaluable experience in damage initiation and the progression of very light wear accumulation over a long period of time. The main complication of this type of pump is the dual use of the working fluid as the hydrodynamic  bearing lubricant. Characteristics of good heat transfer fluids and good lubricants are not complimentary. For high heat capacity at modest temperatures and low pressure drops through pumped loops, heat transfer fluids required high fluid density and low absolute viscosity. However, hydrodynamic bearings perform best with fluids having a low density and high absolute viscosity. For reference, typical fluid  properties for anhydrous ammonia, SAE10 lubricating oil, and Carbon Dioxide gas are shown in Table 1. The technical challenge of using the process fluid as the hydrodynamic bearing lubricant often leads to a low bearing load capacity, low vibrational damping characteristics, and wear in the bearings. This bearing  wear then becomes the primary life limiting factor for the pump. The canned motor type of pump is ubiquitous in terrestrial operations especially in applications where explosive, corrosive, and caustic liquids are processed such as petroleum refineries and chemical plants. In such ground-based applications, additional instrumentation is utilized to monitor machine health, rotor vibrations, and bearing  wear, which permits maintenance intervals to be scheduled accordingly. However, for operation on  spacecraft additional instrumentation and frequent maintenance intervals are not a reasonable solution. Therefore, design details that impact the wear life must be considered as important design parameters  and qualification tests must be conducted to flesh out these details prior to mission launch.  The bearings used to support the rotor in this pump were rigid, hydrodynamic bearings. Two journal  bearings supported the rotor in the radial direction, while a single sided thrust bearing positioned the rotor axially. A thrust washer was used to limit the axial rotor motion in the negative thrust direction, but in  normal use the single sided thrust bearing was biased against the impeller inlet suction. During low speed  operation and on startup and shutdown the thrust bearing was loaded by a skewed magnetic force generated by the electric rotor. The rotor-bearing system was a rigid shaft design with the permanent magnet motor straddle mounted between the journal bearings and the pump impeller was cantilevered.The journal bearings were 9 mm and 18 mm in diameter on the rear and front bearing respectively and  separated axially by 144 mm. The clearance ratio of the journal bearings was 0.26% for both bearings. This created a static angular misalignment at zero speed of 79 microradians. The stationary sleeves of  the bearing were fabricated from a special ammonia tolerant grade of graphitic carbon for low startup friction and low wear during solid surface contact events. Each bearing sleeve had four axial grooves machined at roughly equal spacing around the circumference to reduce the probability of half frequency  whirl, impede the onset of turbulent flow, and to provide additional area for the secondary flow. A secondary flowpath from the pump discharge provides lubricant flow to both journal bearings and the thrust bearing. The secondary flow also passed between the rotor-stator gap to provide cooling for the eddy current losses. The typical operating speed range of the pump is 11,000 to 14,700 revolutions per minute. Hydrodynamic Bearing Performance The hydrodynamic performance of the journal bearings is challenged in terms of fluidic instabilities and  turbulence due to the relatively large clearances, high rotor speed, and low absolute viscosity of the  lubricant. Figure 4 demonstrated this feature by overlaying the Taylor number region for the two journal  bearings of the pump on top of experimental data taken for journal bearings operating with lubricants having very low kinematic viscosity. The data shown in Figure 4 used various high pressure gases as the  lubricant in a concentric cylinder type of journal bearing. Despite the challenging conditions for the journal  bearings, they are still able to generate meaningful film pressures and load support. The main reason that  these bearings do not lose all film pressure to turbulence is the four axial grooves machined into the carbon sleeves. These grooves serve to reduce the turbulent length scale and maintain load support  across each sector of the bearing. In order to calculate hydrodynamic load capacity for any journal  454