The Spiral Model as a Tool for Evolutionary Acquisition

Since its original publication [1], the spiral development model diagrammed in Figure 1 has been used successfully in many defense and commercial projects. To extend this base of success, the Department of Defense (DoD) has recently rewritten the defense acquisition regulations to incorporate “evolutionary acquisition,” an acquisition strategy designed to mesh well with spiral development. In particular, DoD Instruction 5000.2 subdivides acquisition [2]: “There are two ... approaches, evolutionary and single step to full capability. An evolutionary approach is preferred. ... [In this] approach, the ultimate capability delivered to the user is divided into two or more blocks, with increasing increments of capability.” (p. 20) Here, a block corresponds to a single product release. The text goes on to specify the use of spiral development within blocks: “For both the evolutionary and singlestep approaches, software development shall follow an iterative spiral development process in which continually expanding software versions are based on learning from earlier development.” (p. 20) Given this reliance on the spiral development model, an in-depth definition is appropriate. Two recent workshops provided one. The University of Southern California (USC) Center for Software Engineering and the Carnegie Mellon University Software Engineering Institute held two workshops last year to study spiral development and identify a set of critical success factors and recommended approaches. Their results appear in two reports, [3, 4] and are available on the workshop Web site www.sei.emu.edu/cbs/spiral2000 The first author’s presentations at these workshops defined spiral development and are followed below. The definition was first converted to a report [5], where details, suggestions, and further references can be found. Additionally, a follow-on article appearing in a later CR O S S TA L K issue, will address the relationships among spiral development, evolutionary acquisition, and the Integrated Capability Maturity Model.