leads to uncertainties in materials properties and processes and can contribute to unexpected behavior. Prototypes may be needed to detect some of these uncertain events. Regulatory agencies often require safety tests prior to the production and sale of certain products (e.g., automobiles and aircraft). Microprocessors can be completely designed in software using design rules, once the production processes have been verified on test chips that have the required device sizes, materials, and line widths and spacing. Verification of these processes still requires hardware. In software development, prototypes are used to test the new software against customers' expectations. Thus, even if programming aids eliminate bugs, there will still be a need for prototypes.
In some industries, development of prototypes and computer simulations go hand in hand. In aircraft jet engine design, simulations are used to make conceptual, preliminary, and detailed designs of fans, compressors, combustors, and turbines. Each of these components is built in prototype form and tested, as is the final engine. These tests not only determine whether the engine meets its requirements but also provide essential information for updating the