Design Solution Definition Adapted for EE400D from the NASA Systems Engineering Handbook by Alia Bonetti 1.1 Design Solution Definition Process The Design Solution Definition Process takes the high level customer requirements and the PBS and translates them into a design solution. The goal is to create a set of alternative solutions from the PBS and the technical requirements. Once a substantial set of alternative design solutions is obtained, they must be analyzed to determine the ideal solution. This analysis can be completed via trade studies, experiments, and/or simulations. The final design solution definition will then be utilized to create end product specifications and for development of product verification. 1.2 Inputs Technical requirements (level 2) based on customer needs are necessary inputs to the design solution definition process. A design solution is not realizable if it still does not meet the defined customer requests (level 1). Decomposition models including WBS, PBS, and project schedule are DESIGN SOLUTION DEFINITION 1
also inputs into the process. These models aide in breaking down requirements into specific products and responsibilities, so they are needed to derive design solutions. 1.3 Outputs Specifications and plans to be used for product realization are the outputs of the design solution process. These include system specifications, external interface specifications, end-product specifications, subsystem specifications, product verification and validation plans, and operating procedures. The means to achieve these outputs are explained in the rest of this document. 1.4 Methods in Determining Design Solutions The realization of a system over its life cycle involves a succession of decisions among alternative courses of action. NASA Systems Engineering Handbook In order to eventually decide upon the best design solution, alternative solutions must also be defined. The systems engineer can only make an educated choice for one solution or another if the alternatives are all well-defined and understood. As the design solution process is carried out, requirements must always be cross-referenced with customer expectations. This is the root of the recursive and iterative nature of the process, which is driven by those customer expectations. The ConOps should also be considered during this time. Also, take into consideration the available technology and the past results of technologies used. At this point, all project groups should have thoroughly researched past projects, but it may be useful to review this information to identify potential drawbacks of particular technologies based on the lessons learned data available. The design must reflect realistic technology that is not only available but feasible given the major project constraints. In the lifetime of a project s development, there comes a point when the particulars of the project become clearer but also more rigid. The role of the systems engineer is to ensure that the most cost- DESIGN SOLUTION DEFINITION 2
effective system determined by the design solution process is carefully analyzed, with alternatives defined and considered before the hard to undo decisions are made. In some cases, the designers can become attached to the designs that they have made when approaching the process from the bottom-up. By the time the systems engineer catches up via the top-down approach, it may be difficult to persuade the designer away from their attachment. Therefore, it is critical that the systems engineer retain objectivity by viewing the project as an outsider. This trend can also be seen in technology selection. Often times, a particular technology is chosen because it was implemented in the past, also known as heritage equipment. Do not choose technology solely based on the fact that it was used in past iterations. Keep in mind that as a systems engineer, full integration of the project is essential. Throughout the design process, systems engineers must act as the voice of reason and account for all interface requirements. This includes asking questions such as Did you include the cabling? 1.5 Analyzing Design Solution Alternatives The most straightforward way to analyze design solution alternatives is through trade studies. The purpose of the trade study process is to ensure that the system architecture and design decisions move toward the best solution that can be achieved with the available resources. Guiding Steps for a Trade Study Analysis: 1. Create alternative solutions to meet the functional needs of the project from system architecture to system designs. 2. Evaluate alternatives based on the mission objective and cost budget. Models, both in simulation and mathematical, can be utilized at this stage. 3. Rank all of the alternative solutions based on an agreed criteria. 4. Remove the alternatives that are least feasible and then continue through this process. DESIGN SOLUTION DEFINITION 3
When choosing the best design solution, many factors must be taken into consideration including subjective factors that could not be quantified, estimates of how well the design meets quantitative requirements, available technology, effectiveness, cost, schedule, risk, etc. Systems engineering includes weighing the quantifiable and unquantifiable factors against each other. Once a design solution has been determined, it is critical to baseline the design. This ensures that everyone on the team is focusing on the same goal. Prior to a determined design solution, members of the project may struggle to design their portion, because essential decisions cannot be made without explicit design direction. Once the design has been baselined, system requirements, specifications, and configurations can be updated, created, or completed. 1.6 Identifying Intangibles Most documents will refer to this as reliability. The reliability of a project is contingent on its ability to continue to operate normally under normal conditions. To ensure this, the risks associated with the project must be analyzed. Event sequence diagrams can be used to exhibit not only the events but also the responses to abnormal operating conditions. Possible failures should be analyzed from the bottom-up and the top-down. In bottom-up analysis, the causes and strategies to control the DESIGN SOLUTION DEFINITION 4
effects should be identified. In top-down analysis, the trickle-down effect of the failure must be identified. Reliability depends on the probability of mission success, were probability is the likelihood of the occurrence of an event. These estimates can be based on historical data. There will be uncertainty surrounding such estimates, but the uncertainty should decrease as the data and overall system understanding improve. Prototypes can mitigate intangible discovers before implementation is done on the final model. DESIGN SOLUTION DEFINITION 5