# Closing Force Evaluation of a Sample Return Capsule for a Phobos Sample Return Mission
## 5 Closing Force Measurement
### Piston-Conical Shape  
Following the initial empty tests, Type I and Type II seals were tested at the worst-case scenario (\(-65\)\({}^{\circ}\)C), ambient temperature (\(+20\)\({}^{\circ}\)C) and the highest possible temperature (\(+70\)\({}^{\circ}\)C). Figure 14 illustrates the testing set-up for both extreme temperatures and the thermocouple positioning. For each type of test/configuration, several determinations at 1-min intervals were made successively. The forces obtained decreased after each test because, when introducing the piston in the cylinder, the sealing elements are enlarged. After pulling out the piston, the sealing elements are free, but they do not come back immediately to the initial shape. Because of that, when performing the next closing, their enlargement is smaller and, thus, the necessary force is smaller. For the piston with conical shape (CO) tests were performed firstly and for each sealing element along with tests with both sealing elements mounted at the same time. In the following paragraphs, the term \"new seal\" will be used every time a sealing element is tested for the first time. Maximum obtained closing forces for each Type I and Type II seals at environment temperature (\(+20\)\({}^{\circ}\)C) are marked in Figure 15. \begin{table}
\begin{tabular}{c l} \hline \hline
**Phase** & **Description** \\ \hline \multirow{4}{*}{1} & Device set-up on Instron tensile testing machine (an “empty test” was performed to determine the friction force without the sealing elements). To be sure that there is no friction force between the testing device metallic components, an “empty test” was performed before and after the testing campaign. During the “empty test” the testing device was not equipped with the sealing elements. \\  & Installing and testing the large seal (L), dismounting the large seal and mounting the small seal (S). \\  & Install and test both sealing elements (large and small). \\  & Perform an “empty test”–to see if the testing device suffers any deformation which can lead to additional friction forces. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Testing phases for sealing force evaluation. From the results presented in Figure 15, the maximum measured values for the friction force at environmental temperature were extracted, respectively 608 N for Type I seal and 379 N for Type II seal, as shown in Figure 16.  
Considering the graph plotted in Figure 16, it can be seen that the Type I seals that were individually tested (closing force evaluation for one seal), have a different behaviour than the new seals. The maximum force for Type I-b new seals (CO) configuration is around 608 N and the maximum force for Type I-a (CO) (individually tested) configuration is around 470 N. The Type II seals have almost the same behaviour and the maximum force is around 379 N. The force curves do not have the same starting point, where Type I-b new seals (CO) start at 1.7 mm, Type I-a (CO) start at 2.3 mm and Type II-a (CO) start at 3.1 mm. This is because there are some gaps between the piston/vault mock-up and the testing machine gripping devices. This aspect can be observed for all the graphs which are presented in this paper, but these gaps do not affect the maximum closing force determination. Figure 14: Closing force evaluation for (**a**) \(-\)60 \({}^{\circ}\)C and (**b**) +70 \({}^{\circ}\)C using a thermocouple and the (**c**) test set-up with (**d**) a detailed view of the testing device. Figure 13: Empty tests results at: (**a**) the start of the testing campaign, (**b**) the end of the testing campaign) and (**c**) “empty test” set-up. To analyse the temperature effect over the closing force, it was decided to perform a new test at \(-20\)\({}^{\circ}\)C. After this test, it was identified that the force for Type I seals was \(1330\) N, while for Type II was \(799\) N (Figure 17). Figure 16: Closing force curves obtained at environmental temperature.