View source for System Life Cycle Models

The life cycle model is one of the key concepts of systems engineering ([[Acronyms|SE]]).  A [[Life Cycle (glossary)|life cycle (glossary)]] for a [[System (glossary)|system (glossary)]] generally consists of a series of [[Stage (glossary)|stages (glossary)]] regulated by a set of management decisions confirming that the system is mature enough to leave one stage and enter another. 

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==Topics==
The topics contained within this knowledge area include:
*[[System Life Cycle Process Drivers and Choices]]
*[[System Life Cycle Process Models: Vee]]
*[[System Life Cycle Process Models: Iterative]]
*[[Lean Engineering]]
*[[Integration of Process and Product Models]]

==A Generic System Life Cycle Model==
A [[Life Cycle Model (glossary)|life cycle model]] for a system identifies the major [[Stage (glossary)|stages]] that the system goes through, from its inception to its retirement.  The stages are culminated by [[Decision Gate (glossary)|decision gates]], at which the key stakeholders decide whether to proceed into the next stage, remain in the current stage, or terminate or re-scope related projects.   Its inception begins with a set of [[Stakeholder (glossary)|stakeholders]] agreeing to the need for a system and exploring whether a new system can be developed whose [[Life Cycle (glossary)|life cycle]] benefits are worth the investments in [[Life Cycle Cost (LCC) (glossary)|life cycle costs]].

As most organizations adapt to the need for “Competing on Internet Time” (Cusumano and Yoffee 1998), the need for rapid adaptation to unforeseen changes has caused many organizations to emphasize [[Evolutionary (glossary)|evolutionary]] development with emergent requirements as compared with traditional development to a fixed set of requirements. Yet, there are still significant areas in which development driven by fixed requirements is appropriate.

Thus, there is no single “one-size-fits-all” system life cycle model that can provide specific guidance for all project situations.  Figure 1, adapted from (Lawson 2010), provides a generic life cycle model that will then be used to describe the most common versions of prespecified, evolutionary, sequential, opportunistic, and concurrent life cycle processes.
















     Figure 1.  A Generic Life Cycle Model (adapted from Lawson 2010)

The initial dotted line in Figure 1 recognizes that some conceptual effort is generally needed before committing resources to element and system prototyping or development, but that often concurrent conceptual and development activity will take place.  Depending on the system’s scale, complexity, and criticality, stage gates may occur at the beginning, end, and/or points in the middle of the dotted line.  Although it is not shown in Figure 1, some SE conceptual effort will continue through the life cycle to address needed changes or emerging opportunities.  Once a first-article system of interest is developed and assured of readiness for use, the system may proceed with Production of further instances, will provide support for users during its Utilization.  Generally, but not necessarily, the aftermarket support function will continue after production is complete, and many users will continue to use the system even after its support function is discontinued.  Almost always the system will undergo Retirement and disposal, even though counter examples exist (for instance some very ancient irrigation systems are still in use). 

During a system life cycle various views of the evolving system are generated that reflect, for example, capabilities, requirements, abstract functions and/or objects, concrete elements to be integrated, production parts lists and services provided.

Figure 1 shows just the single-step approach for proceeding through the stages of a system’s life cycle.  There are also several incremental and evolutionary approaches for sequencing the stages.  Next are examples of how a system-of-interest’s value propositions may lead to different sequencing approaches, and a discussion of how aspects such as complexity, dynamism, new technologies such as 3-dimensional printing, and non-physical building materials (e.g., software) can introduce variations on the overall generic life-cycle theme.   The subsequent article on [[Incremental (glossary)|Incremental]] and Evolutionary Development will summarize the primary approaches, including their main strengths and weaknesses, along with criteria for choosing the best-fit approach.

==References== 


===Works Cited===
Lawson, H. 2010. ''A Journey Through the Systems Landscape.'' London, UK: College Publications.

===Primary References===
Blanchard, B. S., and W. J. Fabrycky. 2011. [[Systems Engineering and Analysis]], 5th ed. Prentice-Hall International series in Industrial and Systems Engineering. Englewood Cliffs, NJ, USA: Prentice-Hall.

Lawson, H. 2010. ''[[A Journey Through the Systems Landscape]].''  London, UK:  College Publications.

===Additional References===
The following three books are not referenced in the SEBOK text, nor are they systems engineering "texts"; however, they contain important systems engineering lessons, and readers of this SEBOK are encouraged to read them.

<blockquote>Kinder, G. 1998. ''Ship of Gold in the Deep Blue Sea.'' New York, NY, USA: Grove Press.</blockquote>

This is an excellent book that follows an idea from inception to its ultimately successful conclusion. Although systems engineering is not discussed, it is clearly illustrated in the whole process from early project definition to alternate concept development to phased exploration and “thought experiments” to addressing challenges along the way. It also shows the problem of not anticipating critical problems outside the usual project and engineering scope. It took about five years to locate and recover the 24 tons of gold bars and coins from the sunken ship in the 2,500-meter-deep sea, but it took ten years to win the legal battle with the lawyers representing insurance companies who claimed ownership based on 130-year-old policies they issued to the gold owners in 1857. 

<blockquote>McCullough, D. 1977. ''The Path Between the Seas: The Creation of the Panama Canal (1870 – 1914).'' New York, NY, USA: Simon & Schuster.</blockquote>

Although “systems engineering” is not mentioned, this book highlights many systems engineering issues and illustrates the need for SE as a discipline. The book also illustrates the danger of applying a previously successful concept (the sea level canal used in Suez a decade earlier) in a similar but different situation. Ferdinand de Lesseps led both the Suez and Panama projects. It illustrates the danger of not having a fact-based project cycle and meaningful decision gates throughout the project cycle. It also highlights the danger of providing project status without visibility, since after five years into the ten-year project investors were told the project was more than 50 percent complete when in fact only 10 percent of the work was complete. The second round of development under Stevens in 1904 focused on “moving dirt” rather than digging a canal, a systems engineering concept key to the completion of the canal. The Path Between the Seas won the National Book Award for history (1978), the Francis Parkman Prize (1978), the Samuel Eliot Morison Award (1978), and the Cornelius Ryan Award (1977).

<blockquote>Shackleton, Sir E.H. 2008. (Originally published in by William Heinemann, London, 1919). ''South: The Last Antarctic Expedition of Shackleton and the Endurance.'' Guilford, CT, USA: Lyons Press.</blockquote>

This is the amazing story of the last Antarctic expedition of Shackleton and the ''Endurance'' in 1914 to 1917. The systems engineering lesson is the continuous, daily risk assessment by the captain, expedition leader, and crew as they lay trapped in the arctic ice for 18 months. All 28 crew members survived.
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