50   Figure 9. Laser -modified surfaces for increased friction coefficient  

Lifetime   The lifetime requirement of the Scanner mechanism is 201 million revolutions; the requirement for  the  Derotator mechanism is 101 million revolutions . Both values include on-ground operation and margins as  per ECSS -E-ST-33-01C.  The design operating range of  both mechanisms was specified as  10°C to 34°C.   Due to local dissipation, bearing temperatures were calculated to reach more than 50°C  during operation.   Lifetime analysis  per ISO 281 of the standard drive unit concept (hyperstatic bearing layout) with the bearing  size required for the Derotator (pitch diameter 124 mm) yielded that this design was not feasible for the  given temperature environment and lifetime requirement.  Due to its design principle, the Derotator has to  accommodate the conic optical path ( 105 mm to 89  mm in diameter) and the bearing size could not  be  reduced.  Therefore, the Derotator bearing layout was changed to an isostatic one (one bearing pair in back - to-back configuration), the contact angle was increased to >30 deg, and the preload was decreased by  approx imately  25%. This yielded a drastic improvement in calculated life:  60 billion revolutions  for the isostatic design vs. 62 million revolutions for the original hyperstatic design.  The Derotator performance  requirements, which are less  stringent  than the Scanner ones, were still met.  

Long- term storage   The specification required that the mechanisms be designed, manufactured and qualified to sustain at least  15 years of storage on satellite level plus 5 additional years on- ground lifetime.  This requirement imposed   careful selection of materials and processes with respect to corrosion, stress corrosion cracking, long- term  stability, creep, etc. The following major design choices were made:   • Nickel plating of motor rotor yoke and stator sheet metal stack   • Selection of  SmCo magnets  (instead of NdFeB )  Nickel plating of steels is a widely used low-risk process. However, the plating of the stator sheet metal  stacks was a major concern. Protrusions of t he bonding varnish between the metal  sheets cannot be  avoided during stator baking. The protrusions can be removed on the outer diameter of the stator  by turning.  On the inner diameter and in the stator slots, the bonding varnish cannot be removed. As  nickel does not  adhere to bonding varnish, a continuous nickel layer cannot be achieved on the inner diameter of the stator  and in the stator slots  (refer  to Figure 10, left) . This was confirmed in a damp- heat test  (7 days, 50°C, 95%  relative humidity ): the stator sample showed significant levels of corrosion at these locations, whereas the  outer diameter and the entire rotor yoke did not show corrosion at all. Corrosion at the inner diameter of the  motor stator, which is only 1 mm away from the rotor magnets, could not  be accepted. In order to increase  the corrosion resistance  of the stator while maintaining the robust nickel layer, it was decided to add a  secondary moisture barrier on top of the nickel layer. Parylene, a polymer primarily used as moisture barrier  on printed circuit boards , was selected. Paryl ene is less robust with respect to handling than nickel, but  adheres well to many substrates, including the bonding varnish of the stators. Hence, the chosen approach  came with the following advantages:   • At least one continuous moisture barrier  available  at all locations on the stator, even at spots  where the nickel did not adhere well   Lasered titanium surface Blank titanium