# Closing Force Evaluation of a Sample Return Capsule for a Phobos Sample Return Mission
## 5 Closing Force Measurement
### Mechanical Press Motorization  
Using the worst-case scenario for Type II seals with toroidal shape for the piston (Figure 24) and taking into account the ECSS-E-ST-33-01C standard [19], the next development step consisted in performing the mechanical press motorization. The actuation force necessary to close the vault is supplied by the Mechanical Press which shall be equipped with several helical compression springs defined after the motorization calculus. For the mechanical press motorization calculus, it was considered the worst-case scenario of the closing force operation which is the one performed at minimum operational temperature (\(-40\)\({}^{\circ}\)C).  
In Figure 26 the measured values of the closing force are shown and they were obtained following the testing campaign (\(-60\)\({}^{\circ}\)C, environmental temperature and \(+70\)\({}^{\circ}\)C) for the toroidal shape piston and using Type II seals.  
Interpolating the results of the test obtained at \(-40\)\({}^{\circ}\)C, two force values were determined, at 3 mm displacement (Fs1 = 748 N), respectively 8 mm displacement (end of the stroke, Fs2 = 552 N). Considering the specifications mentioned in the ECSS-E-ST-33-01C standard [19] for motorization and actuation factors (sections 4.7.5.3.1e and 4.7.5.3.2e), the minimum actuation forces which shall be ensured by the MP were obtained. The results regarding the minimum actuation forces are summarized in Table 5.  
Figure 25: Type I and Type II seals closing forces for the piston with toroidal shape as a function of the temperature.  
The driving force applied by the MP springs will linearly decrease from the initial value to a final value at the end of the closing stroke. The driving force values must be above the reference values F1 and F2 set above. Based on the obtained F1 and F2 forces (Table 5), the helical compression springs of the MP were dimensioned. Considering this, the MP dimensioning was made, respectively how the spring force varies during the 8 mm stroke, which must be ensured by the closing mechanism so that the vault will be in a closed position. The force delivered by the MP during the closing stroke is given in Figure 27 and it can be observed that the driving force curve is above the reference value set according to the testing campaign results and ECSS standards guideline.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Displacement [mm]**} & **Closing Force Obtained** & **Closing Force Considering** & **Observations** \\  & **from Test [N]** & **ECSS-E-ST-33-01C Specifications [N]** & **-** \\ \hline
3 & Fs1 = 748 & F1 = 0.8 \(\times\) 3 \(\times\) Fs1 = 1795.2 & Motorization factor: \\
8 & Fs2 = 552 & F2 = 0.8 3 \(\times\) Fs2 = 1324.8 & 2 \(\times\) 1.5 = 3 \\  & & & Actuation factor: 0.8 \\ \hline \hline \end{tabular} Where, Fs1, Fs2-closing forces obtained from tests; 0.8-actuation factor; 3-motorization factor.  
\end{table}
Table 5: Mechanical press motorization calculus considering forces at \(-\)40 \({}^{\circ}\)C.  
Figure 27: Mechanical Press delivered force during the Vault’s closing.  
Taking into consideration all this information, the mechanical press was designed and presented in Figure 28, with 6 compression springs (spring codification 76/1/3, Hennlich supplier) for the following reasons:  
* The available space for mounting the compression springs is small;
* To achieve a uniform distribution of the force on the vault circumference;
* The requirement to obtain a high closing force (according to the test results presented above);
* The necessity to ensure an 8 mm stroke to perform the vault closing operation.