## 3 Model-Based System Engineering for Preliminary Design
### Brief Overview of Existing MBSE Methods and Techniques
#### 3.1.2 The V-Model  
The systems engineering \"V\" model, a.k.a., the V-model, has been widely used in the aerospace and defense industries [97]. It is called V-model because the activities are presented by using a \"V\" letter metaphor. The left part of the \"V\" is for the specification activities that go from high level specifications and designs to more detailed and granular ones; there are the implementation tasks (represented by the foundation of the \"V\" letter base); and finally there are the testing activities represented by the right part of the \"V\" letter that go from more granular tests (e.g., unit level tests) to high level ones (e.g., acceptance tests). The V-model is a systems development process that emphasizes an upfront specification and design stage, similar to the NASA systems engineering process. It is often associated with the validation-and-verification approach, as it structures the development process in a way that allows for continuous validation and verification at each stage [97; 98; 99]. However, this association is sometimes debated in the literature, and care should be taken to distinguish between the general V-model and specific implementations that emphasize validation and verification activities. The V-Model inherently supports vertical and horizontal traceability, which are crucial to ensure alignment and consistency throughout the development process. Vertical traceability ensures that each phase, from initial requirements to detailed design and testing, is aligned with the mission objectives, providing a clear lineage of requirements. Horizontal traceability ensures consistency across various components and subsystems at each development stage, maintaining coherence in design and implementation. In our application to the \"ColStar mission, vertical traceability was achieved by maintaining comprehensive documentation that mapped each specification and design decision back to the initial mission requirements. This was further supported by regular reviews and validation steps in each phase, ensuring that the design remained aligned with the mission objectives. Horizontal \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Phase** & **Purpose** & **Typical Outcome** \\ \hline Pre-Phase A. Concept Studies & To produce a broad spectrum of ideas and alternatives for missions from which new programs/projects can be selected. Determine feasibility of desired system, develop mission concepts, draft system-level requirements, assess performance, cost, and schedule feasibility; identify potential technology needs, and scope. & Feasible system concepts in the form of simulations, analysis, study reports, models, and mock-ups \\ \hline Phase A. Concept and Technology Development & To determine the feasibility and desirability of a suggested new system and establish an initial baseline compatibility with NASA’s strategic plans. Develop final mission concept, system-level requirements, needed system technology developments, and program/project technical management plans. & System concept definition in the form of formation of simulations, analysis, engineering models and mock-ups, and trade study definition \\ \hline Phase B. Preliminary Design and Technology Completion & To define the project in enough detail to establish an initial baseline capable of meeting mission needs. Develop system structure end product (and enabling product) requirements and generate a preliminary design for each system structure end product. & End products in the form of mock-ups, trade study results, specification and interface documents, and prototypes \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Design-related phases of the NASA Systems Engineering Project Life Cycle [96]. traceability was facilitated through integrated subsystem design strategies and weekly cross-functional meetings, which ensured that interdependencies between subsystems were consistently addressed and managed.