Systems Engineering got started after World War II as an organized method of developing large, complex "systems". A system means a project built up out of other complex engineered parts, often "systems" in their own right. Thus an aircraft system engineer need to know what performance numbers (thrust, weight, fuel consumption) can be expected from an engine in order to properly design the aircraft. Optimizing the design of the system may not (probably will not) mean optimizing the designs of the constituent parts (sub-systems). Nancy Leveson in has an excellent introduction to Systems Engineering in Chapter 7 of Safeware. Also see the Wikipedia article on Systems Engineering.
The Systems Engineering process generally takes several years and goes through several phases as details are added to the design of the system. Each phase requires the agreement of all parties concerned: the system engineers, the engineers of the sub-systems and components, and of course the customer - the organization paying the bill for the project.
The process consists of identifying key performance variables of the system (being able to carry a specified amount of weight a specified distance at a specified speed, using a specified amount of fuel, for example), then identifying how much each sub-system may contribute to the goals. The goals typically interact, meaning improving one may hurt another (increasing fuel load will increase range but hurt payload). Systems Engineering will have to establish budgets for factors like weight and reliability which are then shared out to the sub-systems.
Typical milestones in the development process are:
A review of the conceptual design - often done at the proposal phase, where a contractor is selected. This is sometimes called the
An early review of the initial design done at a "Preliminary Design Review" (PDR) where the initial system design is reviewed and the sub-system designs are started.
A Critical Design Review (CDR) is at the end of the design phase and is intended to show how well the resulting design meets the project goals.
Along with the development of the main performance goals of the system, the systems engineering team has to follow the other factors including: Reliability, Maintainability, Human Factors, System Safety, and Electromagnetic Compatibility. These are sometimes called the "ilities". The specification of the overall system will also have requirements for performance in each of these fields and they often trade off against the main performance design parameters.
Phase A: Concept and Technology Development (i.e., define the project and identify and initiate necessary technology)
Phase B: Preliminary Design and Technology Completion (i.e., establish a preliminary design and develop necessary technology)
Phase C: Final Design and Fabrication (i.e., complete the system design and build/code the components)
Phase D: System Assembly, Integration and Test, Launch (i.e., integrate components, and verify the system, prepare for operations, and launch)
Phase E: Operations and Sustainment (i.e., operate and maintain the system)
Phase F: Closeout (i.e., disposal of systems and analysis of data)
System Safety has work elements to perform at each phase starting with a Hazard List (simply a list of hazards which can be expected with the type of system being developed) at the beginning, to high-level Hazard Analysis and then more detailed Hazard Analysis as the design progresses.