478  Despite variability in interference fits from device to device, actuation data are tightly grouped and  can be used, when factored, for timed separation commands when the bus is at room  temperature in vacuum.   The tell-tale function of the actuators was determined to be reliable though the vibration testing  and subsequent thermal cycling testing they underwent.     Tell-tale reliability is not affected by vibration or thermal cycling.   Operating condition effects:  o The effect of supplied current on total energy and “on-time” is significant.  o The effect of initial temperature on actuation times is also significant.    o Since the “on-time” can vary wildly with initial temperature and supplied current, these  devices should not be used open loop to avoid damaging the heaters . We recommend  either: implement tell-tale reading circuits, as APL’s CubeSat did, or implement temperature sensors with each actuator and shut the devices off when the devices reach the maximum allowable temperature, or code in a two dimensional look-up table in the flight software, which sets “on-time” as a function of battery state of charge (resulting in current delivered to the device) and initial temperature. 

Lessons Learned 

 Don’t skimp on Ground Support Equipment (GSE) and test equipment. Test GSE are just as  important as the flight hardware:  o It would have been immediately obvious that the first (and second) generation devices  were incompatible with the CubeSat solar array deployment motion if we had tested with a flight-like wing mechanism from the beginning. Representing boundary conditions correctly would have saved one design iteration.  o Pay attention to data acquisition. We lost some data due to shortcuts in setting up the  data acquisition system. We also would have been able to capture more data (such as temperature vs time curves) with a more advanced data acquisition system.  o We could have saved money and time by testing more than one device at a time with  more elaborate GSE  o Enlist subject matter experts initially. We could have eliminated one design revision if we  had known of electrical parts de-rating guidelines and selected the right resistors in the beginning.   Incremental development is cost effective. By making really cheap parts, testing a little and  learning a lot, fabricating a slightly more elaborate second generation device, testing a little and learning a lot more, we were still able to develop and test the flight device in a short, inexpensive effort.   Manufacturing methods are critical to the success of a mechanism design; it wouldn’t have cost  much more to fabricate the first generation parts on the newer generation CNC lathe, rather than the manually operated lathe.  

Conclusion 

A compact device for releasing deployable structures has been developed exploiting differential coefficients of thermal expansion of two dissimilar materials. The resultant device is economical, miniature, simple in concept and execution, uses little power, produces negligible shock, is easily re-settable, and is able to restrain a relatively large load, considering its size. Through a total of 37 test actuations, the third generation design demonstrated a robust device that performed reliably in a flight-like environment. This device is applicable to small deployable devices on instruments, CubeSats and Nanosatellites, and has the capability to be used as an initiation device for mechanisms used on conventional spacecraft and larger instruments. Future work will focus on alternate materials, such as invar, which has a very low CTE, and manufacturing processes, as well as integration into larger load capacity mechanisms.