# Closing Force Evaluation of a Sample Return Capsule for a Phobos Sample Return Mission
## 1 Introduction
In the first Hayabusa mission, the sample containment system was featured with two Viton O-rings, which were replaced in the second mission with a metallic seal due to the fluorocarbon material of the sealing elements which can interact with organic molecules (either terrestrial or from the sample) [17]. Although the Hayabusa concept (Figure 2a,b) uses the same actuation means as in the case of the concept developed in this paper, it shall be stated that the seal proposed by JAXA requires a pretension force because the sealing  
Figure 1: Sample Return Missions chronology in history [13] (image credit: NASA, JAXA, CNSA). elements are mounted axially. Moreover, the Hayabusa concept is based on a touch-and-go mission architecture, while the proposed sealing and closing system are designed for a sampling-by-landing mission. The closing force evaluation is one of the key aspects of the mission proposed under the MREP-2 programme. The current architecture presumes using a robotic arm with limited capacities in terms of force (maximum 40 N of pushing force), having a sampling tool at the end which is used to collect the regolith in a Sample Container (SC), as seen in Figure 3a. After the regolith sampling, the vault is closed with the SC by pushing it with the necessary force using a closing mechanism, illustrated in Figure 3b. Considering the requirements for a Phobos Sample Return Mission, a special closing mechanism with mechanical actuators was designed under a project coordinated by ESA. The actuators are helical compression springs, and their characteristics and specifications were established based on the experimental data and motorization factors given by the ECSS-E-ST-33-01C standard [19]. The closing mechanism, presented in Figure 4, is physically separated from the SC after the closing operation, remaining on Phobos along with the Robotic Arm). Figure 3: Sample return configuration: (**a**) after regolith sampling in the SC; (**b**) prior to performing the closing operation (images courtesy: Airbus Defence and Space UK). Figure 2: Sample Container concepts developed for the Hayabusa missions: (**a**) Hayabusa 1, foreseen with two Viton O-rings, (**b**) Hayabusa 2, foreseen with a metallic gasket [18]. A set of requirements were used as input data for the development of the sealing and closing system. Several ones were considered as driving requirements when it comes to evaluating the necessary closing force to achieve the sealing and closing of the sample container vault, such as the ones presented in Table 1. It is to be noted that, during the mission, the vault shall withstand operational and non-operational temperatures, however, the vault closing operations are performed when the vault is in the range of the defined operational temperature. Consequently, to determine the required closing force through tests, the operational temperature domain was considered, altogether with \(+/-5\)\({}^{\circ}\)C margins according to ECSS-E-ST-33-01C. The critical review of the technical requirements imposed by ESA consists in identifying their effects over the design, with respect to the optimal dimensioning of the components and the entire system. The first step was to determine the most suitable sealing technologies for this mission. This was achieved by performing a comprehensive trade-off analysis [20] which considered a large number of factors and aspects relevant to the mission. The results of this trade-off concluded that the best sealing technology is the use of radial seals with a composite jacket and metallic spring energizer. To achieve redundancy, the vault design featured two sealing elements with different diameters.