Docking Mechanism Evolution  Initial Prototype Docking Mechanism Beginning in 1999, SRC undertook a design effort to develop autonomous soft docking. The project  began with a Phase I SBIR grant from AFRL, who provided bounding conditions to the problem that included an estimated spacecraft size and mass (a cylinder, 50-cm OD x 130-cm L, 50 kg) and a requirement to dock two small microsat spacecraft and transfer fluid and/or electrical data between them. Yet, definition of the various needs that might arise when servicing obsolete or exhausted spacecraft had to be formulated. A key challenge became one of both establishing concept requirements as well as designing to address those requirements.  Evaluation of existing designs revealed a myriad of related technology. Most available technology related  to extra-vehicular activity (EVA) interfaces, robotic boom end-effector configurations, or impact docking  mechanisms. Much of this technology, by nature, required manned intervention. Most of these systems  were primarily intended for large spacecraft and impact docking applications, and were not well suited for the more precise requirements of autonomous soft docking and alignment of fluid and electrical  couplings. A critical component in identifying design requirements involved defining the eventual use and  application of the intended mechanical docking system. Further research did not yield any definitive  mission scenarios, but did present general ideas regarding potential applications. Two mission scenarios were conceived:  Client (on-orbit spacecraft needing service) / microsat (spacecraft servicing the client):  A microsat  could be launched on an as-needed basis to directly service an orbiting client. This would provide very specific refurbishment based on the individual needs of the client. Response time would include arranging a specific launch for the re-supply microsat.  Various client satellites and a microsat base station (both on-orbit):  The client calls on the microsat  base as needed. This could provide faster service, but would be limited to common, more generic  types of refurbishment or repair.  In either of these scenarios, the tender would need to approach the client, match velocity and orientation,  establish initial mechanical contact, pull together to rigidize, establish electrical contact, and finally, the pair must fly together as a single unit while docked. Prototype Trade Studies In parallel with organizing general mission requirements, the design team looked at many different  methods of joining, grappling, and aligning. Four techniques that seemed reasonable with respect to a  plausible flight control approach were traded in more detail:  1. “Harpoon”  2. A telescoping probe 3. Impact docking with a large conical guide 4. “Claw-type” linkages interfacing with a trefoil  The harpoon configuration, in which a probe is launched at a target, latches on, and is reeled back into  guide features, seemed unpredictable and presented complications concerning the alignment of fluid and electrical couplings. A telescoping probe, where a telescoping pole extends to the mating spacecraft, engages a target feature, and retracts to join the two, turned into a complicated multi-mechanism apparatus not well suited for the direction and vision taking form. Impact docking was eliminated as an option because soft docking seemed safer for the two spacecraft and better suited to precisely align fluid and electrical couplings and prevent potential damage of components.  After extensive evaluation, SRC selected a three arm grapple design (Figure 1). The design would  consist of two main subassemblies: an active mechanism and a passive structure. The active mechanism  would consist of a motor driven lead screw that would actuate three individual linkages. The linkages would engage the passive structure, whose geometry would allow it to be constrained by the linkages.  Further retraction of the linkages would seat the passive structure into a three point kinematic mount,  208