Two Axis Antenna Pointina Mechanisms (APM's) with RF Feedthrouah  There are a number of currenthear future applications for two-axis APMs that require integrated R/F  rotary joints within their azimuth actuators. The applications cover both commercial LEO constellations  through to dedicated science missions such as the Bepi-Colombo mission to Mercury. These applications  need to accommodate internally mounted rotary RF joints that call for high precision and repeatable co-  alignment of R.F. choke faces. The bearings themselves are required to be stiff during launch to support  the antenna but must provide low friction torques once on-orbit even when subjected to inverted  temperature gradients caused both by R.F. losses at the shaft and in some cases by the operational  environment.  The major risks associated with these mechanisms are their sensitivity to thermal gradients and  installation tolerances given the lack of elastic preloading. In the unique Bepi-Colombo case, an  operational temperature range of +50"C to +250"C is envisaged with 70% of the expected life at the hot  case and with adverse temperature gradients which could be up to 30°C across the bearings.  It is foreseen that the use of a BAPS for these applications would significantly enhance performance and  reduce risk. It may be an advantage for applications to control bearing stiffness at actuator level in order to  maximize the overall structural efficiency of the combined APM and Antenna Launch Configuration, with  the Antenna in its stowed configuration. To implement BAPS in for example the Bepi-Colombo application,  a pair of angular contact bearings would replace the baselined hard-preloaded super-duplex bearings  (without significant impact on radial envelope) and titanium housing. For launch, the BAPS could be set to  provide a high-preload in excess of the currently specified 1200 N (enhancing launch stiffness, eliminating  bearing gapping during launch and further minimizing risks of fretting damage to gears). On-orbit the  preload could be reduced, and more importantly the preload-stiffness achievable with BAPS is low,  compared to a conventional hard-preloaded system thus rendering the bearings much less sensitive to the  significant and adverse thermal environment.  Medium Power SADM (BAPTA)  The BAPTA was developed for early satellite programs, e.g., OTS, ECS, Marecs. It became the baseline  SADM for a later series of defense satellites transferring up to 900 W of array power and associated  signal lines.  The SADM design uses a pyrotechnically released internal bearing off-load system providing bearing  protection and adequate launch stiffness to the solar array. In orbit, the bearings are compliantly  preloaded.  Use of a BAPS for this type of SADM would replace a complicated system of compliant preloading, hard  bearing-offload and pyrotechnic release mechanisms. Final preload could be set on station for required  torque performance and maximum power efficiency. Such a device would be suitable for small to medium  mass arrays on small satellites.  Reaction/Momentum W heel/Control Moment Gvros  Reaction/momentum wheels and Control Moment Gyros run continuously and allow transfer of  momentum/torque as required by the orbital control system. Ideally the operating case requires a  minimum of bearing preload to minimize power loss, torque disturbances and motor mass/volume. The  rotating elements supported on bearings have significant mass and require correspondingly high stiffness,  both to prevent damage during launch and in the case of the CMG to provide structural efficiency during  slew maneuvers. Cage instability and wheel whirling instability are also factors to be considered.  Use of BAPS for wheels would prevent gapping on launch yet allow minimum preload and hence torque  loss value for in-orbit operation thus improving power consumption and wheel performance per unit mass.  Such a capability is particularly important for solid-lubricated wheels sometimes used in Smallsat and  Microsat applications for which life benefits may be significant. With a variable type BAPS, preload could  be varied to alleviate cage or resonant frequency problems in reaction/momentum wheels or to permit  tuning of CMG stiffness characteristics and energy dissipations.  139