The hybrid simulation (sometimes referred to as Engineering of Simulation and testing) tries to optimally integrate the procedures and points of view of numerical analysis and applied experimentation as a necessary condition to make virtual prototypes a reliable tool in the taking of engineering decisions.

In the initial phases of the project, simulation is used more qualitatively than quantitatively, as a tool to aid the thinking process, specifying the conceptual design and the architecture of the new development. The agility of the evaluations takes priority over the exactness of them always and only when the results are qualitatively correct and allow us to take adequate decisions.

In the detail development phase, simulation and testing is used as a tool to verify the fulfilment of requirements or technical specifications where the optimal combination is that which minimizes time and total cost. Physical testing should be started when the cost of continuing with simulation increases the total cost; equally the tests should end when they can not be justified in terms of time, cost and associated risk. This way with good acquired knowledge management, shown by valid simulation procedures, we can find ourselves in the situation that practically all the development process can be done by virtual simulation and that physical testing can be used as a simple validation of success. Various real cases undertaken by the technical team of AST Ingeniería S.L. illustrate this situation.

Finally it seems clear that the development and diffusion of simulation can not replace the need for physical testing but it is modifying the role of experimentation, relating to the decrease in the number of tests necessary forcing the appearance of new forms of experimentation aimed at the acquisition of information for the mathematical models used.