Systems Engineering

Although we don't want to prematurely select a design solution, we do have to generate alternate concepts for how the system will function. At the level of the system as a whole it is difficult to define requirements and measures without an idea of how the system will work. Concept identification involves identifying alternate approaches to how it will operate, and synthesizing one or more system concepts for each potential approach. System level concepts give the general approach to design and function, without specifying exact values of parameters or what components will be used. For the Apollo program, for example, there was a choice of "Direct to the Moon" vs "Lunar Orbit Rendezvous" as mission concepts, with the latter being the one actually chosen. System concepts may include major variables such as type of propulsion (i.e. chemical or nuclear), service life (one or multiple missions), and supply concept (i.e. closed or open loop life support). At the least it covers what main tasks the system will perform, and how it will be operated and maintained. Once the system concepts are established, the process of analysis, optimization, and selection can begin to find the best version of each competing concept, and then compare and select among the concepts to carry to the next stage of development.

At lower levels of the design process, this step is repeated when there are multiple possible approaches. An example would be thermal protection for re-entry. An ablative heat shield burns off some material each time, so checking thickness and periodic replacement would be part of the necessary operations. A metallic heat shield might not burn off, but suffer cracking from heating and cooling cycles, and require different types of inspection. So when listing design alternatives, it is not just ablative vs metallic that is important, but also how that choice affects the total flow of operations in the system.