## 4 Analysis of the Factors Affecting on the Gathering Process and Self-Preservation of the STACER
### Effect of Layered Friction Coefficient on the Self-Preservation Performance of STACER  
From the analysis above, the radius of the elastic spring reel is selected to be 8.7 mm. During the launch process of satellites, the spring reel may be affected by the extreme conditions, such as high temperature and microgravity, which may change the friction conditions between the layers of the spring reel. Therefore, it is necessary to investigate the effects of friction coefficient on the self-preservation of the STACER. From the experiment [28], the friction coefficient is around 0.1 at room temperature on the earth. The friction coefficient was also determined by the experimental validation. The friction coefficient was about 0.1 in the test. For the simulation, the friction coefficients was set to 0.009, 0.025, 0.05, 0.1, and 0.3 in case a sudden change happens on the friction coefficient of a STACER. Taking the gathered state of the spring reel as the initial state, the effect of friction coefficient between the layers of the spring reel on the self-preservation performance of the reel was studied. The simulation results are shown in Figure 8. The spring reel shows an unstable state only when the friction coefficient is 0.009, as shown in Figure 8a. The STACER is completely gathered with a stable state and the maximum stress value is still under the required stress condition under all other friction coefficient conditions, as shown in Figure 8b. Figure 8: The effect of friction coefficients on the top morphology of the spring reel when (**a**) \(\upmu=0.009\); (**b**) \(\upmu=0.025,0.05,0.1\), and 0.3. Figure 7: Gathering process with the top rod radius of 9.3 mm when the gathering length is (**a**) 230 mm; (**b**) 770 mm; (**c**) 890 mm; or (**d**) 1050 mm. When the friction coefficient is 0.009, the evolution of top morphology is shown in Figure 9a-e. At the beginning, the spring reel is completely gathered and the bulge height of the spring reel is 0. From 0.24 s, two layers begin to exceed the top of the storage barrel and keep rises until the maximum height is reached at 0.80 s with a height of 20.2 mm. Then the bulge height is slightly decreased and when at 1.0 s, three layers exceed the barrel top and the bulge height is 18.1 mm. The effect of friction coefficients on the force at the top rod is further analyzed, as shown in Figure 10a. The force on the top rod fluctuated between 17.1348 and 39.6567 N with a small changes when the friction coefficient was 0.009. As shown in Figure 10b, the force on the top rod reaches a peak with the maximum value around 115 N when the friction coefficient increases from 0.009. The reason is that when the friction coefficient changes, the spring reel reaches its balance state, and the relative movement between the layers occurs. Then the friction force is generated which leads to a force fluctuation at the top rod. It can be concluded that the changes in friction coefficient show limited effect on the stability and self-preservation performance when the friction coefficient is between 0.009 and 0.3. Figure 10: Force diagram of top rod with (**a**) a friction coefficient of 0.009 and (**b**) different friction coefficients. Figure 9: Evolution of top morphology of spring reel when the friction coefficient is 0.009 at (**a**) 0 s; (**b**) 0.24 s; (**c**) 0.52 s; (**d**) 0.80 s and (**e**) 1.0 s.  
However, when the friction coefficient is equal to or less than 0.009, the friction force between the spring reel layers cannot resist the elastic force from the spring reel. Some layers move and exceed the top of storage barrel. The initial state of the spring reel with a stable state is destroyed. Therefore, consideration should be taken when a spring reel is applied with a friction coefficient below 0.009 between layers. Therefore, when the STACER works, the spring reel deploys from the storage barrel, and the friction coefficient will affect the deploying process.