The spiral model of the software process (see Figure 2) has been evolving for
several years, based on experience with various refinements of the waterfall model as
applied to large government software projects. As will be discussed, the spiral model can
accommodate most previous models as special cases and further provides guidance as to
which combination of previous models best fits a given software situation. Development
of the TRW Software Productivity System (TRW-SPS), described in the next section, is
its most complete application to date.