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# Model Building Philosophy

The power of a full directed acyclic graph structure for representing the organization of the database gives a designer a great deal of flexibility in structuring a model. In order to prevent chaos, most designers at BRL choose to design the overall structure of their model in a top-down manner, selecting meaningful names for the major structures and sub-structures within the model. Actual construction of the details of the model generally proceeds in a bottom-up manner, where each sub-system is fabricated from component primitives.

The first sub-systems to be constructed are the chassis and skin of the vehicle, after which a set of analyses are run to validate the geometry, checking for unintentional gaps in the skin or for solids which overlap. The second stage of model construction is to build the features of the main compartments of the vehicle. If necessary for the analysis codes that will be used, the different types of air compartments within the model also need to be described. The final stage of model construction is to build the internal objects to the desired level of detail. This might include modeling engines, transmissions, radios, people, seats, etc. In this stage of modeling, the experienced designer will draw heavily on the parts-bin of model components and on pieces extracted from earlier models, modifying those existing structures to meet his particular requirements.

Throughout the model building process it is important for the model builder to choose part names carefully, as the MGED database currently has a global name space, with individual node names limited to 16 characters. In addition, BRL has defined conventions for naming the elements in the top three levels of database structure, allowing people to easily navigate within models prepared at different times by different designers. This naming convention facilitates the integration of design changes into existing models.

Next: THE BASIC EDITING Up: THE COMBINATORIAL GEOMETRY Previous: Directed Acyclic Graph