To maximize the profitability of modern satellites, designers must invest their spacecraft with both capability and reliability. On-board, autonomous software holds the promise of greatly enhancing spacecraft abilities, yet software glitches have been directly to blame for recent, highly publicized failures. This paper presents the flight-software development framework created for the Generalized FLight Operations Processing Simulator (GFLOPS) testbed. Based upon a robust, commercial, real-time operating system, the methodology applies principles of object-oriented design in order to separate subsystem software functions into protected, quasi-independent modules. The flight software modules are coupled to simulation modules, which provide high fidelity, real-time, representations of system hardware and dynamics. The GFLOPS approach provides tools and a methodology suitable for rapid flight software development. Since the basic implementation of the framework does not rely on any advanced techniques such as autonomy, it can be used in both conservative and aggressive engineering programs. Focus applications include an MIT-designed formation flying experiment (SPHERES) and a U.S. Air Forcefunded distributed satellite mission (TechSat 21).