As products became more complex, conceptualizing shape and fit from a two-dimensional drawing became increasingly difficult. As a pre-production version or prototype of the product was constructed, the consequences of inaccurate conceptualization evidenced itself in structural components that would not properly mate, hydraulic lines that did not connect as planned, and so forth. To solve this dilemma, engineers began using mockups of critical sections of the product, sometimes of the entire product. Constructed of inexpensive materials, mockups are non-functioning, dimensionally accurate, and usually full-scale models of the product. Mockups allow the fit and mating of components to be checked before constructing any functional hardware. Although being supplanted by computer-aided design and virtual reality, mockups are still useful tools due to their simplicity and relatively low cost 11.
11 Very sophisticated mockups have been constructed. For example, an expensive, full-scale, left half (bisected down the longitudinal axis) of the B-1A bomber was built by Rockwell. The wing was sweepable. Normally, mockups are relatively simple and inexpensive.