Fractionated Space Systems, as exemplified by Defense Advanced Research Projects Agency's Future Fast, Flexible, Fractionated, Free-Flying Spacecraft united by Information eXchange (DARPA’s System F6), represent real challenges to software cost estimation. The concept is a traditional 'monolithic' spacecraft is replaced by a cluster of wirelessly interconnected spacecraft modules to create a virtual satellite, delivering capability which is at least equivalent to the monolithic spacecraft. Concurrently, they significantly enhance flexibility and system robustness, and reduce risk throughout the mission life and the spacecraft development cycle. Such systems present real challenges to software cost estimation which arise from both the concept of a Directed System of Systems (DSOS) and the reduced risk which is primarily achievable only through the application of an Incremental Commitment Model (ICM). This paper will briefly introduce a next-generation synthesis of the spiral model and other leading process models into the ICM, which is being piloted or considered for adoption in some parts of Department of Defense (DoD). The ICM emphasizes architecting systems (or DSOSs) to encapsulate subsystems (or systems) undergoing the most rapid change; and architecting the incremental development process by having agile systems engineers handling longer-range change traffic to rebaseline the plans for future increments while largely plan-driven teams develop and continuously verify and validate (V&V) the current increment, as is usually required for safe or secure software.
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