# Dynamics Analysis of Space Netted Pocket System Capturing Non-Cooperative Target
## 1 Introduction  
The Earth's orbital environment is a limited resource [1]. With the increase in human space activities, the number of satellites has risen sharply. Space debris resulting from spacecraft disintegration or failure has become an increasing threat to space activities [2, 3]. To deal with the threat of space debris to space activities, many debris-capturing methods have been proposed. Generally, contact capturing methods include net capturing [4], harpoon capturing [5], gripper capturing [6] and link space manipulators [7]. The net capturing methods are lightweight, low cost, and more flexible for capturing missions than other capture methods [8]. Furthermore, because of the flexible capturing range and the larger permission for the shape and size of space targets, net capturing technologies are a more promising solution for space debris and have been extensively studied. The space net system can throw a flexible net to wrap around space debris and then drag it to the expected orbit. It usually deploys to a predetermined shape and captures the space debris. Accordingly, the whole capturing task includes three phases, namely the launch phase, deployment phase and capture phase of the space net. Several dynamics and simulations studies have been carried out on these phases [9, 10]. However, the space fly net cannot maintain its configuration for a long time in the deployment phase [11, 12], which reduces the success rate of capture [13]. There is a proposed solution to maintain the configuration of the net by maneuvering satellites at the corner of the space net using the maneuverable tethered space net robot (TSNR) [14, 15]. However, the drawbacks of the maneuverable TSNR are the high production cost and difficulty of control. Moreover, the space inflatable net (SIN) can also help to maintain the configuration of the net [16]. TheSIN consists of inflatable rods and rope nets, which are supported by inflatable rods to unfold and form a large capture net. The stiffness of the inflatable rods can maintain the net configuration to improves the capture success rate. The SIN inherits the advantages of the space fly net, and it also has a more stable configuration and controllable attitude [17]. Due to the high nonlinear dynamics of the space inflatable net capture system, it is important and difficult to derive a precise dynamic model. Generally, the methods successfully applied to flexible rope dynamic modeling are mainly divided into two kinds: mass-spring models [18; 19] and absolute nodal coordinate formulation (ANCF) models [20; 21]. [PERSON] et al. [22] proposed the centralized mass method to simplify the modeling of complex flexible net structures, and the dynamics mechanism of the space net was studied. In [23], the mass-spring model was used to establish a dynamic model of a square fly net, including the bending stiffness of the rope. [PERSON] et al. [24; 25] performed a parabolic flight experiment to validate the simulation results of the mass-spring model and the ANCF model. After that, to analyze the influence of the flexibility modeling on the net simulation, the dynamics models of a space net based on the ANCF and the mass-spring models was presented, respectively. [PERSON] et al. [26] studied a capture device which can throw a sticky rope which winds around and captures the target based on the ANCF model. [PERSON] et al. [27] presented a dynamics simulation method of capturing a space debris cloud, in which the gradient-deficient beam element of the ANCF is employed to discretize threads which are woven into the net. The normal contact force between the net and the debris cloud and among debris particles is computed by using the penalty method. [PERSON] et al. [28] designed a new net projectile mechanical structure to capture the target and simulated the projection process to make references on the projectile strategy of the space net.