# Reliability Assessment of Space Station Based on Multi-Layer and Multi-Type Risks
## 5 Reliability Model of Space Station Configuration  
The space station configuration mission profile can be divided into three phases: key technology verification (KTV), main module launch (MML), and assembly construction (AC), respectively. The configuration mission can be divided into four final states: mission success (MS), LOM, LOC, and LOP. The whole mission is functioning only if all the submissions in different phases are functioning. The KTV, MML, and AC phases failure will lead to LOM, LOP, and the worst final state, LOC, respectively.  
The configuration reliability model can be built using QRAS, as shown in Figures 4-7. Figure 4 is the event model, which includes the initial event mission beginning, the pivotal events KTV, MML, and AC. It has four final states as mentioned above. Figures 5-7 are the fault tree models. Figure 5 represents the failure of the KTV phase, which includes five high load risks (RP11, RP12, RP13, RP14, and RP15). Figure 6 represents the failure of the MML phase, and includes four high load risks (RP21, RP22, RP23, and RP24). Figure 7 represents the failure of the AC phase, and includes six high load risks (RP31, RP32, RP33, RP34, RP35, and RP36). All the high load risks are connected to each pivotal event through logic gates because of their high propagation probabilities.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **Risk Name** & **Risk Layer** & **Risk Type** & **Symbol** \\ \hline
1 & Lack of adequate verification of space & Between- & & \\  & docking technology & mission & & \\
2 & Insufficient verification of large assembly & Between- & & \\  & control technology & mission & & \\
3 & Incomplete coverage of critical & Between- & & \\  & measurement control segment & system & & \\
4 & Insufficient continuous launch support & Between- & & \\  & & system & & \\
5 & Power supply interruption & system & & \\
6 & Main module failed to enter the & Between- & & \\  & scheduled orbit & mission & & \\
7 & Main module out of control & Between- & & \\  & & mission & & \\
8 & Rocket thrust deficiency & Between- & & \\  & & system & & \\
9 & Inadequate measurement and control & Between- & & \\  & accuracy & system & & \\
10 & Insufficient on-orbit material support & Between- & & \\
11 & Main module structure damage & Between- & & \\  & Lack of emergency life-saving training in & Between- & & \\  & the crew & system & & \\
13 & Cargo mission failed & Between- & & \\
14 & Cargo ship failure & Between- & & \\
15 & Main module docking interface & Inner- & & \\  & damaged & system & & \\ \hline \hline \end{tabular}
\end{table}
Table 5: High load risks of the space station configuration mission.  
probabilities should be used synthetically. Generally, \(p_{oi}\leq 10^{-6}\), \(p_{pi}\leq 10^{-8}\), and \(p_{oi}\geq p_{pi}\). The assessment probability of the \(i\)th high load risk \(p_{ai}\) can be defined by Equation (3)  
\[p_{ai}=\sqrt{p_{oi}p_{pi}},1\geq i\geq 15 \tag{3}\]  
where \(P_{oi}\) is the \(i\)th high load risk's occurrence probability, \(P_{pi}\) is the high load risk's propagation probability, and \(p_{ai}\) is the \(i\)th high load risk's assessment probability. Because \(p_{pi}\leq p_{ai}\leq p_{oi}\), so \(p_{ai}\) can better describe the risk feature. The value of \(p_{ai}\). is shown in Table 6.