For the design and engineering of large systems, testing scale models is often an integral part of the process. For many systems, this is a relatively straightforward process. For example, early in the design process of new ships, aircraft, spacecraft, and vehicles, progressively larger models from about 1/20th to 1/4th are often employed. Even for such a narrow range of scaling ratios, some physical parameters change significantly, which is why there is often a progression of scale model testing. For example, a physical structure that is 1/20th scale in linear dimension is only 1/8000 or 1.25 × 10-4 times the mass of the full-sized structure. Such scale models are nevertheless extremely useful in engineering endeavors where building multiple full-scale prototypes is overly expensive and impractical, as there is a well-developed methodology for the testing of scale models that yields valuable information used in design modifications before the next-larger model is built.