# Closing Force Evaluation of a Sample Return Capsule for a Phobos Sample Return Mission
## 3 Design and Manufacturing of the Testing Devices  
To define the optimal design, to characterize the engineering parameters and to gather data, a device representative of the vault and SC was designed and manufactured for this test. This partial breadboard (dummy) was used to measure the closing force and, after that, the results of the tests were used to optimize the design. The testing device is presented in Figure 9.  
As it was previously mentioned, the Vault (Figure 5) was designed with closed grooves, however, to prevent the destruction of the sealing elements during the dismounting operation, the vault dummy was designed and manufactured with open grooves. The closing of the Vault is made by pushing the SC. The two sealing elements oppose by creating a resistant force during the Vault's closing when they get in contact with the SC surfaces. At the end of the stroke, the seals are in contact with the cylindrical surfaces of the SC. Due to the axial symmetry of the Vault and SC, the opposing force is radial. The closing stroke of the Vault is approximately 8 mm. \begin{table}
\begin{tabular}{c c c} \hline
**Seal Type** & **Shape** & **Characteristics** \\ \hline Type I & & **Jacket Material:** Turcon T01 (a high-grade virgin PTFE) \\  & & **Spring:** slant coil type, stainless steel \\  & & 302 material and approximately 1 N radial \\  & & \\  & & **Jacket Material:** Fluorology A28 \\  & & **Spring:** U shape, nickel alloy material and \\  & &  approximately 1.35 N radial load per each \\  & &  mm of the circumference \\ \end{tabular}
\end{table} 
Table 2: Sealing elements main characteristics. Figure 9: Cross-section through the testing device. The pushing force exerted on the SC has two components:  
* Forces necessary to enlarge the jackets and springs of the sealing elements;
* Forces to overcome the friction between the SC and sealing elements jackets on the entire length of the closing stroke. Two partial breadboards (dummies) for the SC were designed, manufactured and used to measure the closing force. The difference between them is represented by different geometrical shapes in the areas that ensure the enlargement of the sealing elements, namely conical (CO) and toroidal shape (TO). The difference is depicted in Figure 10a,b and the manufactured parts (grade 5 Titanium alloy) are presented in Figure 10c. Hereinafter, the vault dummy will be called cylinder and the SC dummy will be called piston. The surface finish is an important feature when it comes to ensuring the Vault's tightness. The sealing elements manufacturers recommended a roughness average (Ra) between 0.2 \(\upmu\)m and 0.4 \(\upmu\)m. Following the finishing process, the corresponding values have been obtained, making the test more suggestive. The roughness evaluation was performed with a MahrSurf PS 10 device (Mahr GmbH, Gottingen, Germany), over a distance of 10 mm with a 1 mm/s rate. The sealing elements mounted in the cylinder have smaller inside diameters than those of the piston to achieve the necessary tightening for sealing when the piston is mounted in the cylinder. The evaluation of the necessary closing force was made for a stroke of 8 mm, corresponding to the insertion path of the Sample Container in the Vault during the closing operation, illustrated in Figure 11.  
When introducing the piston inside the cylinder (simulating the vault closing operation), the following phases take place:  
* In the start position the piston does not touch the sealing elements;
* When moving the piston, it touches the sealing elements with the conical and rounded areas (Figure 12);
* When moving the piston up to 6 mm, the opposing forces are those needed to enlarge the sealing elements and the friction forces that occurred on the sealing elements;
* After moving the piston, the sealing elements do not enlarge anymore and, until the end of the stroke, the forces opposing the movement of the piston are only the friction forces on the sealing elements.