390 In order to keep the development risk (both techni cal and regarding the schedule) low, two different  breadboard models and one Qualification Model have been built and tested; two Flight Models and one  Flight Spare will be deliv ered to the customer. 

Key Equipment Design and Performance Requirements   The Refocusing Mechanism of the NIRSpec instrument provides the focusing func tion of the instrument  by means of two corner mirrors changing the opt ical path length by movement of the mirrors.    Three titanium blades guide the RMA upper sled, which is driven by an eccentric drive requiring a maximum torque of about 0.55 Nm. Considering ECSS margins, the actuator has to generate a worst case torque of 1.21 Nm at ambient and 1.4 Nm at 30K  driven with a rated current of 120 mA. This is  realized by a stepper motor attached to a planetary gear wi th a gear ratio of 184:1 distributed to 3 stages.    The most important requirement is the large oper ational temperature range from 30K to 323K. The  observations at the first actuator demonstrate the high importance of both the selection of a proper dry  lubrication as well as a CTE-consistent design en suring constant tolerances within the operational  temperatures for both the bearings and gears.  According to the expected in-orbit cycles as well as the Flight Model test campaign, the Qualification  Model has to be loaded in different cycles from the Midstroke Position resulting in a total of about 400,000  motor revolutions (including ECSS margins) without significant performance reduction. As learned from the first actuator, special attention has to be paid to  the metal-to-metal contact due to coating wear  between sliding parts after life time that has to be prevented in any case. Therefore, a redundant  lubrication design should be considered, especially taking into account the RMA being a Single Failure  Object; that means a breakdown of the subunit results in the inability to re-foc us the NIRSpec instrument.   The actuator’s operation in an envir onment close to contamination crit ical optical equipment rules out  lubrication systems with particulate or molecula r contamination but requires solid lubrication. 

Initial Design of First Actuator and Lessons Learned   The first RMA actuator consisted of a material mix of six different sorts of steel with an estimated  coefficient of thermal expansion (C TE) at 30K varying between 7.5 x 10 -6 1/K (ball bearings) and 15 x 10-6  1/K (gearbox housing). This design was based on t he experiences in gearbox development for usual  applications – every single material has its specific strength playing on the individual function. But this  cryogenic application cannot be called usual and different strategies have to be pursued. 

Figure 2: Material concept of first actuator   The shrinking at cryo is likely to have caused incr eased friction and therefore affected the life-time  significantly. The lubricant used, Dicronite, showed si gnificant signs of abrasion after life test (about  330,000 motor revolutions) resulting in metal-to-metal  contact as well as a major increase in necessary  current; according to ECSS, a clear failure of life test.  NASA/CP-2010-216272