Methods to expand the usefulness of the design spiral have been developed. The factor of time was added to the model [Andrews (1981)]. The essential concept remains the same, but the visual representation moved into three dimensions, with the added third dimension representing time. Figure 2 is the resulting cone shaped model. The design progresses through time by “cork-screwing” down the cone following a helical path. A cross section of the cone, essentially a spiral, represents a snapshot of the design process at a given instance. Design convergence is achieved at the cone’s apex.