Systems engineering (or systems design engineering) as a field originated around the time of World War II. Large engineering projects, such as the development of a new airliner or warship, can often be subdivided and managed through the execution of a series of smaller, interconnected engineering projects. This approach to engineering is inherently complex, since the behavior of and interaction between such "subsystems" is not always apparent. Characterizing such complex systems is the domain of systems engineering.
Admiral Grace Hopper has been quoted as saying "Life was simple before World War II. After that, we had systems."
The first significant systems engineering was performed for telephone systems. All the different parts of the phone system have to interoperate reliably. An excellent overview of the interfaces and logic, with some history, is "Digital Telephony" by John C. Bellamy. For operational telephony terms, see Newton's Telecom Dictionary, for example.
In recent times, industry in general has begun to accept that the engineering of systems, both large and small, can lead to unpredictable behavior and the emergence of unforeseen system characteristics. Decisions made at the beginning of a project whose consequences are not clearly understood can have enormous implications later in the life of a system, and it is the task of the modern systems engineer to explore these issues and make critical decisions. There is no method which guarantees that decisions made today will still be valid when a system goes into service years or decades after it is first conceived but there are techniques to support the process of systems engineering. Examples include the use of soft systems methodology, Jay Wright Forrester's System dynamics method and the Unified Modelling Language (UML), each of which are currently being explored, evaluated and developed to support the engineering decision making process.
Systems engineering often involves the modelling or simulation of some aspects of the proposed system in order to validate assumptions or explore theories. For example, highly complex systems such as aircraft are usually modeled and simulated before flight. In this way the initial aeroelastic engineering and control equations can be drafted initially and improved before the physical system is constructed. Since aircraft are often very expensive, this reduces the expense and difficulty of debugging the controls and reduces the risk of crashing real aircraft. Careful initial testing and flight envelope expansion are typically still required to reach acceptable levels of safety and performance in advanced aircraft.
System engineers perform testing and validation when a system has to have predictable behavior. For example medical machinery such as heart and lung machines usually consist of several parts, engineered by different companies. Testing and validation assures that normal operation and possible failures of each part will not harm the patient. Other applications are communications systems, or bankingsoftware, where failures can cause loss of property or liability. Test plans can often be adjusted to save significant amounts of money, by testing partial systems, or including special features in a system to aid testing.
The techniques of safety engineering can be applied by everyday people to planning complex events to assure that the systems cannot cause harm. Most of safety engineering is just a way of making plans that cope with failures.
Usually a failure in safety-certified systems is acceptable if less than one life per 30 years of operation (10^9 hours) is lost to mechanical failure. Most Western nuclear reactors, medical equipment and commercial aircraft are certified to this level. This level is accepted not because loss of life is acceptable, but rather because a design near this level usually has significant mechanical redundancy, and the failures will be gradual enough that repairs can be scheduled before significant loss of life can occur.
