The Extravehicular Mobility Unit: case study in requirements evolution

Requirements are rarely static, and are ever more likely to evolve as the development time of a system stretches out and its service life increases. In this paper, we discuss the evolution of requirements for the U.S. spacesuit, the Extravehicular Mobility Unit (EMU), as a case study in the need for flexibility in system design. We explore one fundamental environmental change, using the Shuttle EMU aboard the International Space Station (ISS), and the resulting EMU requirement and design changes. The EMU, like most complex systems, faces considerable uncertainty during its service life. Changes in the technical, political, or economic environment cause changes in requirements, which in turn necessitate design modifications or upgrades. We make the case that flexibility is a key attribute that needs to be embedded in the design of long-lived systems to enable them to efficiently meet the inevitability of changing requirements after they have been fielded.