# A Reliability Engineering Approach for Managing Risks in CubeSats
## 4 Results
### Step 3: Criticality Analysis on SPEC (Example for STR)  
Detailed examples of how the criticality analysis has been conducted during the SPEC preliminary design phase is now provided in order to clarify the applicability of Step 3 (cf. Section 3.3). Considering the STR subsystem, a logical loop is presented in Figure 6, in order to briefly show all the analyzed items: purple, grey and light-blue colors enlighten the causal relationships among failure modes, effects, subsystems and specific fails. By means of colored areas, it is possible to track back and relate a fail in any subsystem to all the possible effects of one or more failure modes. For example, if the payload extending mechanism results to be blocked, its cause may be correlated to any of the 1 to 5 effects of each failure mode of the STR subsystem. It implies (e.g.,) that a structural deformation may have compromised a mechanical interface of the payload subsystem, or vibrations may have caused a break-up in the wirings as well (notice that they are not the only possible causes). By means of other information, it would be possible to distinguish which is the actual cause of the specific fail. In case of the wirings, either power source or data lines of the payload might be compromised. Notice that, in Figure 6, the number identifying the effect (B) follows a numerical order referred to each failure mode (A), so that different items may have the same number (i.e., 7, _radiation shield damage_, _short circuit_, _damage to launcher and other payload_). When the same item has more than one number (_break wiring_), it means that in the complete list it is referred to two different cases (i.e., 5, _break wirings and compromising interfaces_ and 6, _break wirings and compromising subsystems_). \begin{table}
\begin{tabular}{c c} \hline \hline
**CODE** & **STR-1.1** \\ \hline “SUBSYSTEM FAILING” & STR \\ “FAILURE MODE” & 1 \\ EFFECT & 1 \\ AFFECTED Subsystem & SYS \\ \hline \hline \end{tabular} The first effect of the first failure mode of the structural subsystem (affects the entire system).  
\begin{tabular}{c c} \hline \hline
**CODE** & **ADC-1.1-ADC.5** \\ \hline “SUBSYSTEM FAILING” & ADC \\ “FAILURE MODE” & 1 \\ EFFECT & 1 \\ AFFECTED Subsystem & ADC \\ AFFECTED Unit & 5 \\ \hline \hline \end{tabular} The fifth unit of the ADC subsystem is affected by the first effect caused by the first failure mode of the ADC subsystem itself.  
\begin{tabular}{c} **Specific Fail**: ST mirror misalignment \\ \hline \hline \end{tabular}
\end{table} 
Table 13: Traceability of Failures in FMECA Analysis. _Vibration_ is considered as the failure mode to provide pragmatical example for the evaluation of the criticality indexes (i.e., O, S, D). Three examples have been considered as possible effects of _vibration_: one refers to an _unseparated payload adapter_ case (i.e., STR-3.2), and two of them concern a _compromised subsystem_ effect (i.e., \"Shield damaged\"STR-3.3-THM-1, and _extending mechanism block_ STR-3.3-PLD-6). Starting from the calculation of O, a failure probability (cf. Table 2) and a conditioned probability index (cf. Table 3) have to be assigned. In case STR is undergoing vibrations, the failure mode probability is set to the maximum level 5/5 in all the three cases (cf. Table 2, \"the likelihood of the failure mode is estimated as extremely high\"). Still for the O index, the conditioned probability shall be assigned at both subsystem level (dealing with the likelihood of the effect on the subsystem to arise by means of the undergoing failure mode) and part level (dealing with the likelihood of the specific fail to arise by means of the undergoing effect). According to Table 3, each case requires a conditioned probability equal to 4/5, i.e., \"the effect is the most probable consequence of failure mode under exam\". Similarly, following Table 3 applied in case of conditioned probability analysis at part level, for STR-3.2 the assigned value is 3/5; for STR-3.3-THM-1 is 2/5, and for STR-3.3-PLD-6 is 3/5.