# Reliability Assessment of Space Station Based on Multi-Layer and Multi-Type Risks
## 3 Risk Definition and Identification of Space Station  
### Risk Definition  
In the space station configuration mission, there are four final states that need to be researched, they are full success, success, partial success, and failure, respectively. In the engineering area, only the first state is the most expected state, and the other three states are undesirable states. The middle two states have no damages to the platform and the crew, and their risks can be defined as mission risks, which can be described by using the terminology loss of mission (LOM). The last state means that the mission failure may cause damage to the platform or the crew, and it should be studied to find the reasons and adopt countermeasures. The risks of the failure state can be defined as safety risks, and described using the terminologies of loss of crew (LOC) and loss of platform (LOP). Therefore, the space station configuration risks include mission risks and safety risks.  
Because of the different compositions and functions implemented by different subsystems (such as the power subsystem, the guidance, navigation, and control subsystems) and different missions (such as manned missions and cargo missions), different layers and different type of risks will affect the space station configuration mission.  
According to the influence domain, configuration risks can be divided into three layers: inner-system risks, between-system risks, and between-mission risks. Their definitions are shown as follows.  
(1) Inner-system risks  
Inner-system risks occur inside one system and affect the system functions. They have the propagation characteristic, and also cause damage to the equipment in the system, but they do not cause damage to the other systems.  
(2) Between-system risks  
Between-system risks occur at the interfaces between the systems and affect single-mission completion. They can cause damage to the other systems in one single flight mission, but not to the whole configuration mission.  
(3) Between-mission risks  
Between-mission risks occur between the adjacent missions. They cause damage to the other flight missions and affect the whole configuration mission.  
According to engineering experience, there are four main types of risks: technology risks, management risks, product risks, and operation risks. Their definitions are shown as follows.  
(1) Technology risks  
Technology risks are caused by immature or invalidated key technologies. They will occur at all stages of the configuration mission with a certain degree of concealment.  
(2) Management risks  
Management risks are caused by unreasonable work plans, inadequate material guarantees, incorrect risk decisions, and insufficient communication and coordination.  
(3) Product risks  
Product risks are caused by product failures occurring in the launching and operation stages, and they have the most propagation characteristics.  
(4) Operation risks  
Operation risks are caused by incorrect operation and handling by the operators in the launch site, flight control center, and on-orbit.  
In summary, the space station configuration risks are shown in Figure 2.