System Engineering Unit-4

Editor's Notes

• #4: For Risk Management details refer to chapter 5 System Engineering by Kosiakoff page numbers 120-128 It should be noted that all new system developments do not have to go through a formal advanced development phase. If all major subsystems are directly derivable from proven predecessor or otherwise mature subsystems, and their characteristics can be reliably predicted, then the system development can proceed on to the engineering design phase. Such is the case with most new model automobiles, in which the great majority of components are directly related to those of previous models. In that case, such critical items as the airbag system or pollution control may be individually built and tested in parallel with the engineering of the new model.
• #19: Assessment of Component Maturity. The determination of whether or not a given component is suffi ciently proven for full - scale engineering can only be made by comparing the component with analogous components that have been successfully engineered and produced. If no proven analogous component is similar to the new one, the comparison may often be made in two parts, functional and physical, by asking the following questions: 1. Are there proven components that have very similar functionality and performance characteristics? Where signifi cant differences exist, are they within the demonstrated performance boundaries of this type of component? 2. Are there existing components whose physical construction uses similar materials and architectures? Are the projected stresses, tolerances, safety, and lifetime characteristics within the demonstrated limits of similar existing components? Risk Analysis. After identifying those system elements that require further development, the next step is to determine the appropriate nature and extent of such development. This is where systems engineering knowledge and judgment are especially important because these decisions involve a careful balance between the cost of a thorough development effort on one hand and the risks inherent in insufficient development and consequent residual uncertainties on the other. Development Planning. It is clear from the above discussion that the planning of the advanced development phase should be based on a component - by - component assessment of the maturity of the proposed system design to defi ne (1) the specifi c character of the unproven design features and (2) the type of analysis, development, and test activities required to resolve the residual issues. Risk Reduction Budget. The result of the above analysis of risks and definition of appropriate risk reduction efforts should be incorporated into a detailed development plan to guide the analysis, development, and testing effort of the advanced development phase. In doing so, an essential step is to revise carefully the relative allocation of effort to the individual components or subsystems that are planned for development. Do the relative allocations correspond to an appropriate balance from the standpoint of a potential gain to investment ratio? Is each allocation adequate to acquire the needed data? Example: Natural Gas - Powered Automobile. The development of an automobile that uses natural gas as a fuel in place of gasoline offers an example of some of the principles discussed above. This development has the dual objective of conforming to future strict auto pollution standards while at the same time preserving all the desirable characteristics of conventional modern automobiles, including affordability. Thus, it seeks to minimize the required changes in standard auto design by limiting them to the fuel system and its immediate interfaces. Other changes to the body, engine, and other components are kept to a minimum.
• #21: Because of the rapid advance of modern technology, a new system that is to be developed to replace an obsolescent current system will inevitably have performance requirements well beyond those of its predecessor. Moreover, in order for the new system to have a long, useful operational life in the face of further projected increases in the capability of competitive or opposing systems, the requirements will specify that its performance more than meets current needs. The increase in system performance frequently requires a signifi cant increase in component complexity, as in many of today ’ s automated computer - based systems. The means for achieving such projected extensions are often not reliably predictable by analytical or simulation methods and have to be determined experimentally.
• #26: Specialized software---In real - time systems, the control of timing can be especially complicated, as when system interrupts occur at unpredictable times and with different priorities for servicing. In distributed software systems, the designer gives up a large degree of control over the location of system data and processing among networked data processors and memories. This makes the course of system operation exceedingly diffi cult to analyze. In graphical user interfaces, the requirements are often incomplete and subject to change. Further, the very flexibility that makes such systems useful is itself an invitation to complexity. Dynamic system Elements---Another form of complexity that usually requires development and testing is inherent in closed - loop dynamic systems such as those that are used for automated controls (e.g., autopilots). While these lend themselves to digital or analog simulation, they often involve coupling and secondary effects (e.g., fl exure of the mounting of an inertial component) that cannot be readily separated from their physical implementation. Thus, the great majority of such system elements must be built and tested to ensure that problems of overall system stability are well in hand.
• #30: • Dynamic Elements. Except for very high frequency dynamic effects, most system dynamics can be simulated with adequate fi delity. The six - degree - of - freedom dynamics of an aircraft or missile can be explored in great detail. • Human – Machine Interfaces. User interfaces are control elements of most complex systems. Their proper design requires the active participation of potential users in the design of this system element. Such participation can best be obtained by providing a simulation of the interface early in the development and by enhancing it as experience accumulates. • Operational Scenarios. Operational systems are usually exposed to a variety of scenarios that impact the system in different ways. A simulation with variable input conditions is valuable in modeling these different effects well before system prototypes or fi eld tests can be conducted. Example: Aircraft Design. Illustrating a use of simulation, assume, as in the example in Chapter 7 , that an aircraft company is considering the development of a new medium - range commercial airplane. The two basic options being considered are to power it with either turbo - prop or jet engines. While the gross characteristics of these options are known, the overall performance of the aircraft with various types and numbers of engines is not suffi ciently well - known to make a choice. It is clearly not practical to build a prototype aircraft to obtain the necessary data. However, in this case, simulation is a practical and appropriate method for this purpose because extensive engineering data on aircraft performance under various conditions are available. Since the primary issue at this stage is the type and number of engines, it is only necessary to have a fi rst - order, two - dimensional (i.e., vertical and longitudinal) model of the aerodynamic and fl ight dynamics of the airplane. The performance of various engines can be represented by expressions of thrust as a function of fuel fl ow, speed, altitude, and so on, known from their measured performance data. From this simple model, basic performance in terms of such variables as take - off distance, climb rate, and maximum cruise speed can be determined for various design parameters such as gross weight, number of engines, and payload. Assuming that this process led to a recommended confi guration, extension of this simple simulation to higher orders of detail could provide the necessary data for advanced analysis. Thus, such simulations can save cost and can build on the experience gained at each stage of effort.
• #35: Unusually High Performance. Most new systems are designed to provide performance well in excess of that of their predecessors. When such systems are at the same time more complex, and also demand greater reliability and operating life, it is almost always necessary to verify the validity of the design approach experimentally. Radars used in air traffi c control systems are examples of complex devices requiring extremely high reliability. These radars are frequently unmanned and must operate without interruption for weeks between maintenance periods. The combination of performance, complexity, and reliability requires special attention to detailed design and extensive validation testing. All key components of these radars require development and testing prior to full - scale engineering. Special Materials and Processes. Advances in technology and new processes and manufacturing techniques continue to produce new materials with remarkable properties. In many instances, it is these new materials and processes that have made possible the advances in component performance Extreme Environmental Conditions. The proper operation of every system component depends on its ability to satisfactorily operate within its environment, including such transport, storage, and other conditions as it may encounter during its life cycle. This includes the usual factors of shock, vibration, extreme temperatures, and humidity, and, in special instances, radiation, vacuum, corrosive fl uids, and other potentially damaging environments. Component Interfaces. Perhaps the most neglected aspect of system design is component interfaces. Since these are seldom identifi ed as critical elements, and since they fall between the domains of individual design specialists, often only systems engineering feels responsible for their adequacy. And the press of more urgent problems frequently crowds out the necessary effort to ensure proper interface management
• #48: The typical contents of a system TEMP are the following: • System Introduction Mission description Operational environment Measures of effectiveness and suitability System description Critical technical parameters • Integrated Test Program Summary Test program schedule Management Participating organizations • Developmental Test and Evaluation Method of approach Confi guration description Test objectives Events and scenarios • Operational Test and Evaluation Purpose Confi guration description Test objectives Events and scenarios • Test and Evaluation Resource Summary Test articles Test sites Test instrumentation Test environment and sites Test support operations Computer simulations and models Special requirements