## 4 Discussion  
This paper has presented situations from an ongoing development initiative that demonstrates how the satellite business is becoming a very innovative and dynamic business environment. This requires designing highly flexible architectures to cope with such dynamicity. However, the flexibility enabled in the architecture needs to be quantified and traded off with other attributes of a design (such as performances and qualification ability) in order to make robust decisions [22]. Previous studies (e.g., [56]) have discussed the flexibility implications of space propulsion architectures by varying design parameters (e.g., required/targeted _Isp_ vs. Mass). Compared to these previous studies, this paper focused more on the difficulty of including uncertainty in market behavior during architecture trade studies--which have a profound impact on which architecture is preferred [22; 23; 25]). To support these decisions, this paper has adapted and refined existing techniques (e.g., surplus value, discrete-event simulation, tradespace exploration) in a methodology (Figure 3). The use of the methodology is illustrated through an industrial case related to the study of xenon vs. krypton architectures for megaconstellation businesses. The purpose of the proposed case study is to demonstrate the methodology in an industrially relevant set-up, rather than a detailed comparison between alternative propellant systems. For the industrial partners, the use of the methodology has contributed to quantify the 'thought process' of satellite and propulsion engineers into value-based models [57]. This has been considered beneficial in order to promote monetary value assessment during the component and sub-system design and development activities. One program manager reported: _\"we often do these calculations at the beginning of a project or program, but not when we are doing design work at sub-system or component level\"_. This was considered very important for engineering teams to be able to present and communicate the impact of alternative technological concepts to program managers and decision-makers. Therefore, the objective of this paper is to provide insights into how to apply the proposed methodology in other case studies, for example multimode space propulsion [56]. Regarding the results of the case study, some aspects require further attention and future work. The analysis of the xenon vs. krypton choice presented in this paper has a layer missing, which is the storage density. Xenon stores are about three times denser than those of krypton, so for a given mission/deltaV requirement, a krypton tank would be almost three times as big as the xenon tank. This can become a problem for \"small spacecraft\" constellations where the propellant system is the major volume driver (conversely, less of a problem for bigger satellites). Therefore, there actually exists a scenario that, despite xenon costing six times more than krypton, \"smaller\" xenon-fueled spacecraft would enable more satellites per launch and thus save the total amount of needed launches. The authors reserve the analysis of the impact of satellite size at system level (considering the different tank volumes required by xenon and krypton) for future publications. The reason for this is that the contributions of the paper (described in the paragraph above) are mainly on the real-life applications of value-based methodologies, rather than a detailed analysis of the xenon vs. krypton choice. To quantify flexibility and qualification ability, this paper has adapted and refined existing techniques (e.g., surplus value, discrete-event simulation, tradespace exploration). While such refinements can be considered incremental from a methodological standpoint, the real contributions of the paper are twofold. From a research perspective, this paper has contributed by applying the surplus value equation--as formulated originally in VDD [19]--in a long industrial project with many iterations and partners involved.