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Saleh, Joseph H.

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Author: Dubos, Gregory F. ×

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    Spacecraft Technology Portfolio: Probabilistic Modeling and Implications for Responsiveness and Schedule Slippage
    (Georgia Institute of Technology, 2010-04) Dubos, Gregory F. ; Saleh, Joseph H.
    Addressing the challenges of Responsive Space and mitigating the risk of schedule slippage in space programs require a thorough understanding of the various factors driving the development schedule of a space system. The present work contributes theoretical and practical results in this direction. A spacecraft is here conceived of as a technology portfolio. The characteristics of this portfolio are defined as its size (e.g., number of instruments), the technology maturity of each instrument and the resulting Technology Readiness Level (TRL) heterogeneity, and their effects on the delivery schedule of a spacecraft are investigated. Following a brief overview of the concept of R&D portfolio and its relevance to spacecraft design, a probabilistic model of the Time-to-Delivery of a spacecraft is formulated, which includes the development, Integration and Testing, and Shipping phases. The Mean-Time-To-Delivery (MTTD) of the spacecraft is quantified based on the portfolio characteristics, and it is shown that the Mean-Time-To-Delivery (MTTD) of the spacecraft and its schedule risk are significantly impacted by decreasing TRL and increasing portfolio size. Finally, the utility implications of varying the portfolio characteristics are investigated, and "portfolio maps" are provided as guides to help system designers identify appropriate portfolio characteristics when operating in a calendar-based design environment (which is the paradigm shift that space responsiveness introduces).
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    Statistical Reliability Analysis of Satellites by Mass Category: Does Spacecraft Size Matter?
    (Georgia Institute of Technology, 2009-10) Dubos, Gregory F. ; Castet, Jean-Francois ; Saleh, Joseph H.
    Reliability has long been recognized as a critical attribute for space systems, and potential causes of on-orbit failures are carefully sought for identification and elimination through various types of testing prior to launch. From a statistical or actuarial perspective, several parameters of the spacecraft, such as mission type, orbit, or spacecraft complexity, can potentially affect the probability of failure of satellites. In this paper, we explore the correlation between satellite mass, considered here as a proxy for size, and satellite reliability, and we investigate whether different classes of satellite, defined in terms of mass, exhibit different reliability profiles. To this end, we first conduct nonparametric analysis of satellite reliability based on a sample of 1,444 satellites. The satellites are organized in three main categories defined by satellite mass (Small – Medium – Large). Three nonparametric reliability curves are thus derived. We then provide parametric fits of the reliability curves to facilitate the identification of failure trends. We proceed to the comparative analysis of failure profiles over time and clearly identify different reliability behaviors for the various satellite mass categories. Finally, we discuss possible structural and causal reasons for these trends and failure differences, in particular with respect to design, testing and procurement.
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    Technology Readiness Level, Schedule Risk and Slippage in Spacecraft Design: Data Analysis and Modeling
    (Georgia Institute of Technology, 2007-09) Dubos, Gregory F. ; Saleh, Joseph H. ; Braun, Robert D.
    Schedule slippage plagues the space industry, and is antinomic with the recent emphasis on space responsiveness. The Government Accountability Office has repeatedly noted the difficulties encountered by the Department of Defense in keeping its acquisition of space systems on schedule, and identified the low Technology Readiness Level (TRL) of the system/payload under development as a principal culprit driving schedule risk and slippage. In this paper, we analyze based on data from past space programs the relationship between technology uncertainty and schedule risk in the acquisition of space systems, and propose an analytical framework to identify appropriate schedule margins for mitigating the risk of schedule slippage. We also introduce the TRL-schedule-risk curves to help program managers make riskinformed decisions regarding the appropriate schedule margins for a given program, or the appropriate TRL to consider should the program’s schedule be exogenously and rigidly constrained. We recommend based on our findings, that the industry adopts and develops schedule risk curves (instead of single schedule point estimates), 2) that these schedule risk curves be made available to policy- and decision-makers in acquisition programs; and 3) that adequate schedule margins be defined according to an agreed upon and acceptable schedule risk level.