# Design and Analysis of a Novel Floating Docking Mechanism for On-Orbit Refueling
###### Abstract  
The docking mechanism is a key component for on-orbit refueling technology. In this paper, the design and analysis of a novel floating docking mechanism for on-orbit berthing-based refueling is presented. Compared with traditional berthing and docking, the berthing here is high in success rate and low in impact, which is accomplished by stretching out a docking subassembly instead of pulling back the client spacecraft. However, the berthing also has two problems: initial deviations between two spacecraft and an additional force generated by a hard alloy refueling pipe, which both seriously affect the docking operation. Thus, the docking mechanism is designed to have alignment abilities and decrease the additional force as much as possible. Based on the principles above, we introduced spring pins and a helical refueling pipe to design a light, compact, and simple docking mechanism. To further reduce the additional force, we proposed an elliptical-helical pipe and analyzed its mechanical properties. Finally, simulations and experiments were conducted to validate the proposed mechanism. The results show that the proposed mechanism with an elliptical-helical pipe has a high tolerance for linear and angular misalignment and superior dynamic performance during docking. diating docking mechanism; dynamic analysis; elliptical-helical pipe; on-orbit refueling; berthing +
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Michigan Aerospace Autonomous Microsatellite Docking System II [23], a probe-drogue docking mechanism that can achieve soft docking, was proposed. Then, the Assist project presented a type of probe-drogue docking mechanism [15] that used a pantograph to capture and pull back the target spacecraft in the mid-2010s. Recently, [PERSON] et al. presented two probe-drogue docking mechanisms: (1) Autonomous Rendezvous Control and Docking Experiment [24] proposed in 2013, which uses an electromagnet to achieve soft docking and release and uses three locking solenoids to lock the probe-drogue; and (2) a miniature docking mechanism for CubeSats [25] proposed in 2020, which uses photoelectric sensors to judge contact and a rotating tip to lock. As a classical docking configuration, the probe-drogue configuration has been used to this day due to its simple structure. The main methods for on-orbit joining operations are docking and berthing [13; 21]--e.g., Automated Transfer Vehicle and Progress join the ISS by docking--and other refueling/refurbishing automatic modules of the ISS adopt berthing. For docking, two spacecraft dock in a floating state and do not need robotic manipulators to fasten each other. The requirements of the spacecraft navigation system and the impact force of docking are all very high. Moreover, docking has a high failure risk due to floating collisions and bouncing off [25]. For berthing, the client spacecraft berths next to the service spacecraft by robotic manipulators, and the two spacecraft are fastened. Then, the client spacecraft is pulled back to join the service spacecraft or the service spacecraft stretches a docking subassembly to dock with the client spacecraft. The requirements of the robotic manipulators' flexibility and control error are all very high. However, berthing does not exhibit failure risk due to collision and bouncing off, and the impact force is lower than docking due to the low joining velocity [26]. Moreover, compared with the two types of berthing, the impact force of stretching is much lower than that of pulling back due to the lower moving mass. Thus, berthing and joining by stretching a docking subassembly to dock has better application prospects for on-orbit joining operations. The steps are shown in Figure 1 and described as follows:  
1. Approach--the service spacecraft moves toward the client spacecraft using a navigation system until the distance between them is below the maximum capture distance. 2.