Non-traditional design criteria such as flexibility, robustness, survivability and others (collectively referred to as the "ilities") are increasingly recognized as critical system properties for the success of aerospace programs. While most decision makers would agree that the ilities are important system properties, they are neither well-defined nor easily evaluated in isolation. While some evaluation methodologies do exist (e.g., real options for flexibility), there is a need for a holistic

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... ramework for describing systems with these properties. This paper will explore the use of these "ilities" properties as selection criteria in tradespace studies. A framework is described that can be used to incorporate ilities into conceptual design and tradespace studies in a systematic way. The framework is based on describing ilities in terms of changes in three dimensions: changes in the context, changes in the needs, and changes in the systems itself. The ilities are then interpreted as methods of navigating changes in this space. Quantitative analysis can be accomplished through Epoch/Era Analysis linking multiple discrete tradespace studies into a coherent timeline. An extended example is presented to demonstrate an analysis and visualization of the survivability of a space tug system to LEO orbital debris over a ten year lifetime. The results indicate that passive shielding for satellites in LEO provides only marginal benefit, at best, given the debris distribution. The smaller satellite designs in the tradespace display an unfavorable tradeoff of cost versus survivability. The example case study is a simple application of the framework, and points out the potential challenges of displaying multidimensional, non-aggregate data in "ilities" dynamic tradespace studies. The framework and case study suggest that using Epoch/Era Analysis, "ilities" will emerge as properties describing how systems change across epochs, and can be prescribed as visual and quantitative strategies that can be traded in terms of benefit and cost over time. American Institute of Aeronautics and Astronautics * This implicitly excludes some ilities, such as manufacturability, that affect the design of the system but not its operation.