The second idea of our agile approach for complex systems engineering consists in iteratively defining the product by organizing its development into successive collaborative design loops of increasing length and granularity where the product relevancy, consistency and feasibility is assessed at more and more precise levels of analysis (see next figure). Each loop shall in particular cover and study several possible product variants at different levels of grain along their full development cycle, starting from top-level architecture up to manufactured part design and definition of manufacturing and maintenance concepts. The design choices shall be gradually frozen and deepened until achieving the full complete specification of the product and of its associated manufacturing and maintenance processes. All system stakeholders – in particular manufacturing & maintenance actors – shall be involved in these design loops in order to capture as early as possible their constraints. Such a development model makes it possible to freeze quickly structuring choices related to system global performance, system architecture, external or internal interfaces, critical parts or technologies and development, manufacturing & maintenance processes, ensuring the technical and industrial feasibility of the target system and providing a good control of risks at any time.