Systems Engineering

Complex systems evolve through a Life Cycle much the way living things do, from conception to disposal. The life cycle is divided into a number of stages where different tasks are performed (Figure 1.5-3). The design stages (the first three boxes in the figure) can be organized in different ways depending on the nature of the system. These include linear, parallel, spiral, or closed loop sequences, or some mixture of these. The illustration shows a typical linear sequence. A spiral process repeats stages in increasing detail, while a closed loop repeats at the same level of detail. Beyond the design stages, the process is more typically linear from production, through test, installation, operation, and retirement.

Life cycle stages are used for two important reasons. First, the design process should consider all the later stages, so that the best total solution is found, rather than optimizing for just one part of a system's life. Second, breaking down a system by time is another way to simplify the design work, along with breaking it down by subsystems and components. The stages are further broken down into internal tasks which have inputs and outputs that connect them, and have decision points for when it is time to proceed to the next stage.

A life cycle is a time oriented view of an entire system. Other views of the same system include functional diagrams, which show what tasks it performs and their inputs and outputs, and a work breakdown, which tabulates the elements and sub-elements which make up the system. Which view of the system is used depends on the design task at hand, though all the views need to be kept current or the design process can become disjointed.