R&R: Development Issues and Solutions  The design of the R&R started by using the linear actuator in its originally designed configuration given our  budgetary and time constraints. However, it became apparent that the linear actuator needed some  modifications to meet our mission’s application.  One of the most critical changes was the SMA material. Initial requirements listed a thermal environment  that may have exceeded the activation temperature of commercially available SMAs. This would cause a  premature activation and release of the SAs, which could cause the CubeSat to jam inside the deployer.  Fortunately, an SMA wire with a higher phase transition temperature had already been developed by Glenn.  This wire was incorporated into the design and alleviated the thermal concerns. However, this wire had  never been tested or used in other flight applications and therefore, the ALBus mission was the first  opportunity to use it.  Another change to the existing SMA linear actuator was the location of the pin-puller attachment point  (output stage). The existing actuator caused the pin-puller to be off-center from the four guide rails. This  was due to packaging the actuator in order to physically fit inside the CubeSat chassis. To install the linear  actuator so the pin-puller would be centered, it would have to be offset so much that it would protrude  outside of the walls of the chassis. The off-center pin-puller was clearly not ideal, however, that  configuration was attempted nonetheless to identify potential issues and solutions. A plastic 3D-printed  prototype was created and the concept worked. A metal prototype followed and was also shown to work,  however, it was prone to occasional binding from the off-center pin-puller. Since the R&R is a missioncritical mechanism, this anomaly was unacceptable. The linear actuator was then modified to move the pinpuller attachment point to allow the actuator to be mounted in the R&R and align the pin-puller on center.  This substantially reduced the observed binding, but did not remove it completely.  The R&R was still prone to binding occasionally when the release plate tilted. This led to the next change,  which added guide bushings made out of a polyimide plastic. Prior to the bushings being added to the  design, the thin 3.17-mm-thick aluminum release plate only had holes drilled in it so it could travel on steel  guide rods. NASA-STD-5017 [8] provides guidance and recommends a length ratio of 2:1 for plain bearings  that are used in this guide rod type configuration. For the ALBus, this is a physical impossibility as the  bushing and resulting guide rails would become too long and take too much space inside the CubeSat. If  this ratio cannot be met, as it is in this case, the specification recommends taking into account several  potential binding-causing issues. It also suggests to perform an analysis based on report work by J.R.  Schroeder [11]. This analysis was performed and a compromise between overall length of the guide rails  and bushings to the physical packaging limitations of the CubeSat was made. As a result, the bushings  were made as long as possible toward the base plate and extended toward the releasing direction. This  change significantly reduced the observed binding, but did not prevent all of the binding occurrences.  The final change made to remove binding, also learned from the EDU, was to free up the guide rods. When  the guide rods were fully installed by tightening the installation nut, the R&R was still prone to binding.  When the nuts were loosened to allow the rails some freedom to move, the mechanisms released smoothly  and consistently. Therefore, the final design allowed the guide rod to float. This was designed into the flight  mechanism such that the guide rod diameter to the through hole in the base plate has a diametral clearance  gap of 0.229 to 0.381 mm (0.009 to 0.015 in). When the installation nut was installed, it was preloaded onto  a collar of the guide rod that extended past the base plate. As a result, the nut did not lock out the guide  rod to the base plate, allowing each of the guide rods to float a small amount. All of these design  modifications allowed the mechanism to consistently function during the development tests.  Hinge: Early Concepts  There was one previous concept for the hinge. This design consisted of solar cells on both sides of the SA,  which complicated the assembly and design. This concept used one large blade-like hinge knuckle with  smaller hinge brackets. However, three superelastic springs were needed to provide the power from both  sides of the SA. The concept had solar cells installed onto PCBs that sandwiched the hinge and SMA  springs. The difficulty with this design was getting the electrical power from the solar cells to the SMAs and  NASA/CP—2018-219887 382