Notional Design Test Case Spacecraft often use reaction wheel assemblies (RWA) for motion control functions. These assemblies  consist of an electric-motor-driven heavy wheel supported on ball bearings. RWA’s are typically hard  mounted to a spacecraft structure in order to impart stabilizing forces and moments through inertial phenomena. A more complex device, the control moment gyroscope (CMG), employs multiple spinning  wheels housed in one degree of freedom swivel mounts (gimbals) and can provide stabilization as well as pointing forces to the spacecraft. Both of these devices work in conjunction to traditional thrusters  (chemical propulsion devices) to accurately position a spacecraft. To function properly, the bearings must  provide low and stable running torque for the entire mission life that can span a decade or more. Despite  modest operation loads and speeds, long life is assured only if the bearings are lubricated properly and survive the rigors of launch without damage. While spinning in orbit, bearing loads are low but during launch while the wheels are stationary the  vibration and acceleration load environment can be severe. It is during this period of high load that bearing balls can dent bearing races affecting the long-term life and smooth operation of the device. The  contact between the balls and inner race surface are particularly vulnerable due to geometrical effects.  The ball-inner race contact areas are very small because of the race curvature and this leads to high stresses even at modest bearing load. Estimating launch loads and designing robust yet lightweight mechanical systems capable of withstanding  these loads can be as much an art as a science. Each rocket launch vehicle has its own unique vibration  signature and, depending upon the payload configuration and isolation techniques used, random and steady acceleration loads transmitted to a particular payload can vary widely. These uncertainties often  lead to the requirement that both analytical and experimental methods be used to verify that a device,  such as a reaction wheel, can withstand anticipated load levels before being approved for flight. The use of superelastic 60NiTi in reaction wheel bearing applications offers the potential for increased  static load margins. To determine the potential magnitude of these improvements, a small rotor (5 kg)  supported by two duplex pairs of small (R4 size) deep groove bore ball bearings is considered. Maximum  contact stress limits for 60NiTi, 440C and the high carbide content REX20 tool steel are applied to the  ball-inner race contact to estimate the maximum allowable g-forces the assembly can withstand. By  comparing these resulting maximum load values, the benefits of using superelastic materials for races,  and balls in wheel bearings can be determined. A commercially available reaction wheel assembly shown in Figure 8 is the basis for the load modeling (Ref. 11). R4 ball bearing design and internal geometry is based upon catalogue values and are typical  for such bearings (Ref. 12).  For simplicity, the rotor is modeled as being axially and radially symmetric. Bearing preload is neglected  and the load case considered is a radial load passing through the center of mass equally counteracted by all four bearings. This calculation does not require a priori knowledge of the wheel geometry or inertia properties and thus can be undertaken without revealing any manufacturer’s proprietary information. Radial bearing load capacity is lower than thrust load capacity because less than half the rolling elements support the load whereas for thrust load support all the ball-race contacts share the load equally.Examination of the radial load capability of bearings made with varying materials helps to clearly reveal  the potential of using superelastic bearing materials. Determining a specific launch load value is difficult. Developing a more generalized range of expected  maximum loads is fairly straightforward. NASA provides a standard (GSFC-STD-7000) in which the maximum expected random vibration levels for payload components of various sizes (mass) are given(Ref. 1). By adding the random level to the steady acceleration g-forces one can establish a reasonable load level to impose when making the bearing load calculation. For payload components with masses less than 22.7 kg, the random load levels are 14.1 G rmsfor qualification and 10.0 G rmsfor acceptance.  81