# Dynamics Analysis of Space Netted Pocket System Capturing Non-Cooperative Target
## 4 Dynamic Simulation and Results
### Simulation Results and Discussion  
In this subsection, the dynamics simulation results for three different capturing strategies with different closing times of the netted pocket system are investigated. Here, the closing time is 10 s, 7.5 s and 5 s for each case, respectively. Then, after capturing the target, the service spacecraft starts to tow the target at t = 10 s. In the simulation, the initial positions of the spacecraft and the target are (0, \(-\)1 m, 0) and (0, 7.5 m, 0), respectively. For the first case, the closing time is 10 s. At the initial state (t = 0 s), the distance L between the ends of adjacent inflation rods is 5.78 m. Then, L shortens from 5.78 m to 2.1 m. At 10 s, L decreases to the minimum value of 2.1 m, and the capturing phase ends. After the capturing phase, the service spacecraft tows the target. The capturing rule of the distance L is shown in Figure 3, in which the distance L and velocity of adjacent inflation are presented. The dynamics simulation of capturing process is performed and the simulation results are presented in Figure 4. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Mass (kg)** & \multicolumn{3}{c}{**Moment of Inertia (kg m\({}^{2}\))**} \\ \cline{2-5}  & **Ixx** & **Iyy** & **Izz** \\ \hline Spacecraft & 1210 & 4282 & 12,736 & 14,498 \\ Target & 200 & 1210 & 1210 & 1210 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Parameters of the spacecraft and target. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Diameter (m)** & **Density (kg/m\({}^{3}\))** & **Poisson Ratio** & **Modulus of Elasticity** \\ \hline Nets & 0.004 & 1430 & 0.3 & 12 \\ Inflatable rods & 0.01 & 164 & 0.3 & 0.75 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of the netted pocket system. The simulation results for the first case are presented in Figures 5 and 6. Figure 5 presents the collision forces on the captured target. Figure 6 presents the positions of the target and the service spacecraft, respectively. Figure 5 shows that the collision force in the Y direction is larger than the collision forces in X and Z directions. The maximum collision force occurs before the end of the closing phase. After the closing phase is completed, the service spacecraft tows the target. In the towing phase, the collision force on the target is decreased obviously. Both the collision forces in the X and Z directions appear after the end of the closing phase. Figure 4: Simulation results of capturing process (case 1). Figure 3: Capturing process for first case (case 1). Figure 5: Collision forces on the captured target (case 1). Figure 6: Position of the target and service spacecraft for case 1: (**a,b**) Position of the target; (**c,d**) Position of the service spacecraft. In (**a,c**), blue curve shows capturing phase and red curve shows towing phase. Comparing with the collision forces in Figure 5 and the positions of the target and spacecraft in Figure 6, it can be found that the target collides with the rope net in the closing phase, and is subjected to a negative collision force in the Y direction, resulting in a decrease in the distance between the target and the service spacecraft. Subsequently, the target collides with the rope net near the service spacecraft, resulting a positive collision force in the Y direction, which leads to the distance between the target and the spacecraft increasing. The target contacts and collides with the rope net at the closing point again, resulting a negative collision force in the Y direction, and the closing capture process is completed. After the closing process is completed, the service spacecraft rows the target. From Figure 6, it clearly shows the service spacecraft and target motion together in the Y direction in the towing phase, indicating that the target is captured successfully. In the towing phase, the target and the spacecraft have continuous contact and there will be smaller collision forces generated in the Y direction.