Hugh McManus

A framework for understanding uncertainty and its mitigation and exploitation in complex systems

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Thermally induced damage in composite laminates : Predictive methodology and experimental investigation

Abstract A general analysis method is presented to predict matrix cracks in a composite laminate, together with the resulting degradations of laminate properties, as functions of temperature or thermal cycles. All plies except the surface plies are considered. A shear-lag solution for the stresses in the vicinity of cracks and a fracture mechanics crack formation criterion are used to predict cracks. Damage is modeled incrementally, which allows the inclusion of the effects of temperature-dependent material properties and softening of the laminate due to previous cracking. The analysis is incorporated into an easy-to-use computer program. Experimentally, crack densities are measured in a variety of laminates exposed to decreasing temperatures. Crack densities are measured at the edges of specimens by microscopic inspection, and throughout the specimen volumes by X-radiography and sanding down of the edges. In specimens with thick ply groups (several plies of the same angle stacked together), cracks behave ‘classically’, running the width of the specimens. In specimens with single ply groups, smaller, discontinuous cracks developed. Correlation between the analytical results and the edge crack densities were reasonable for those specimens which behaved classically. Crack densities measured at specimen edges do not agree with internal crack densities (or analyses) in specimens with non-classical cracks. A free-edge stress analysis clarifies the reasons for these discrepancies.

New Methods for Rapid Architecture Selection and Conceptual Design

New methods for rapid front-end development of complex systems are introduced. New tradespace exploration techniques, advances in integrated concurrent engineering, and application of risk analysis methods early in the design process allow rapid progress from poorly defined user needs to fairly detailed conceptual designs. An overview is provided of the methods. A process is described that allows thousands of system architecture alternatives to be quickly and quantitatively assessed vs user needs. The result is an understanding of the tradespace, including its key constraints and sensitivities, as well as an optimum architecture. This architecture is used to specify needs for space vehicles, which are designed using integrated concurrent engineering techniques. Research in risk and uncertainty, policy impacts, and information technology methods allows quantitative consideration of these factors, resulting in designs that are robust to uncertainties and policy impacts and potentially more versatile and flexible. Eight systems designed to date using the method are briefly reviewed. Key literature and a number of companion papers that go into depth on various aspects of the method are cited.

A Framework for Incorporating "ilities" in Tradespace Studies

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 framework 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.

Responsive systems comparison method: Dynamic insights into designing a satellite radar system

Often shifts in context, such as changes in budgets, administrations, and warfighter needs, occur more frequently than high-cost space-based system development timelines. In order to ensure the successful development and operation of such systems, designers must balance between anticipating future needs and meeting current constraints and expectations. This paper describes the application of Multi-Epoch Analysis on a previously introduced satellite radar system program case study, quantitatively analyzing the impact of changing contexts and preferences on “best” system designs for the program. Each epoch characterizes a fixed set of context parameters, such as available technology, infrastructure, environment, and mission priorities. For each epoch, several thousand design alternatives are parametrically assessed in terms of their ability to meet imaging, tracking, and programmatic expectations using Multi-Attribute Tradespace Exploration. While insights on tradeoffs are discovered within a particular epoch, further dynamic insights become apparent when comparing tradespaces across multiple epochs. The Multi-Epoch Analysis reveals three key insights: 1) the ability to quantitatively investigate the impact of “requirements” across many systems and contexts, 2) the ability to quantitatively identify value “robust” systems, including both passively robust and changeable systems, and 3) the ability to quantitatively identify key system tradeoffs and compromises across stakeholders and missions.

Prediction of thermal cycling induced matrix cracking

Thermal fatigue has been observed to cause matrix cracking in laminated composite materials. A method is presented to predict transverse matrix cracks in a composite laminate subjected to cyclic thermal load. Shear lag stress approximations and a simple energy-based failure criteria are used to predict crack density as a function of temperature. Prediction of crack density as a function of thermal cycling is accomplished by assuming that fatigue degrades the materials inherent resistance to cracking. The method is implemented as a computer program. Simple experiments provide data on progressive cracking of a laminate with decreasing temperature, and on cracking induced by thermal cycling. Correlation of the analytical predictions to the data is very good. A parametric study using the analytical method is presented which provides insight into material behavior under cyclical thermal loads.

Responsive Systems Comparison Method: Case Study in Assessing Future Designs in the Presence of Change

In this short paper, the Responsive Systems Comparison (RSC) method is introduced. RSC is a structured method for collecting information and conducting analysis to characterize a wide variety of possible futures in order to enable the comparison of the performance of proposed systems in those futures. A case study uses the RSC to analyze a satellite radar system. The needs and expectations of a user community for such a system, the context it will operate in, and its technical basis are determined both at the present time, and with possible changes over the next 15 years. This information is used to set up an analysis that should be able to highlight systems that will deliver value under a wide variety of future situations. The case study illustrates the practicality of the method, and provides lessons for improvement and implementation.

UNDERSTANDING THE ORBITAL TRANSFER VEHICLE TRADE SPACE

This study uses new methods to explore the theoretical performance of over a hundred possible orbital transfer vehicle designs. The designs have varying propulsion types, fuel mass fractions, and grappling/observation equipment capabilities. Simple sizing rules are used to calculate the performance of the designs and their utility to several types of users. Designs of interest are further explored using Integrated Concurrent Engineering techniques, resulting in complete conceptual designs. The results give an understanding of the trade-space for such vehicles, including sensitivities to both design variables and assumed user needs. This clarifies some of the challenges involved such as physical constraints and sensitivities to uncertain user preferences. Several potentially viable designs are identified including an electric-propulsion high delta-V vehicle dubbed the Electric Cruiser, and a class of lower delta-V vehicles dubbed Tenders which are studied in a companion paper. NOMENCLATURE

Mechanical Properties of Degraded PMR-15 Resin

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, February 1999.

Creating advanced architectures for space systems: Emergent lessons from new processes

Techniques are considered for developing clean sheet designs of advanced architectures for space systems. The terrestrial observer swarm A, an ionospheric mapping system using a swarm of small satellites, is used as an example. A process for exploring open-ended trade spaces is briefly described, and its application to the system followed. The utilities and costs of 1380 possible architectures are calculated. The trade space is explored; the two primary missions of the system are found to drive the architecture in opposite directions, suggesting a rethinking of the mission mix. This tension is coupled to the available funding level because larger, more expensive swarms can do both missions. The process is observed to require careful definition of a set of utility metrics to capture user needs, and updating of the design parameter set to be explored, as understanding of the system and its technical challenges emerges. Overall, both the process used and the swarm technology considered are found to be useful for defining and understanding a large set of available architectures.