Reliability tool for initial quantified functional risk and hazard analysis

2015 Annual Reliability and Maintainability Symposium (RAMS)

Mathias

2015

This study examines three different methodologies for producing loss-of-mission (LOM) and loss-of-crew (LOC) risks estimates for probabilistic risk assessments (PRA) of crewed spacecraft. The three bottom-up, component-based PRA approaches examined are a traditional static fault tree, a dynamic Monte Carlo simulation, and a fault tree hybrid that incorporates some dynamic elements. These approaches were used to model the reaction control system thruster pod of a generic crewed spacecraft and mission, and a comparative analysis of the methods is presented.The methodologies are assessed in terms of the process of modeling a system, the actionable information produced for the design team, and the overall fidelity of the quantitative risk evaluation generated. The system modeling process is compared in terms of the effort required to generate the initial model, update the model in response to design changes, and support mass-versus-risk trade studies. The results are compared by examining the top-level LOM/LOC estimates and the relative risk driver rankings at the failure mode level. The fidelity of each modeling methodology is discussed in terms of its capability to handle real-world system dynamics such as cold-sparing, changes in mission operations due to loss of redundancy, and common cause failure modes.The paper also discusses the applicability of each methodology to different phases of system development and shows that a single methodology may not be suitable for all of the many purposes of a spacecraft PRA. The fault tree hybrid approach is shown to be best suited to the needs of early assessments during conceptual design phases. As the design begins to mature, the level of detail represented in the risk model must go beyond redundancy and nominal mission operations to include dynamic, time-and state-dependent system responses as well as diverse system capabilities. This is best accomplished using the dynamic simulation approach, since these phenomena are not easily captured by static methods. Ultimately, once the design has been finalized and the goal of the PRA is to provide design validation and requirement verification, more traditional, static fault tree approaches may become as appropriate as the simulation method.

Section: Rapid Fault Tree Hybrid Approachmentioning

confidence: 99%

Comparative analysis of static and dynamic probabilistic risk assessment

2015 Annual Reliability and Maintainability Symposium (RAMS)

Mathias

2015

“…The risk of failure stemming from the unreliability of subsystem components was estimated by assuming an exponential failure model for each MEL component, utilizing surrogate failure rates from the Space Shuttle (SS) PRA 3.0 [7], the International Space Station (ISS) Modeling and Analysis Data Set, and the Valador Failure Data Handbook [8]. The subsystem reliability model [10] took into account component redundancy and assumed a different "baseline" level of redundancy for each element.…”

Section: Element Leo Loiter Risk Assessmentmentioning

confidence: 99%

Near-Earth phase risk comparison of human Mars campaign architectures

Manning

Nejad

2013 Proceedings Annual Reliability and Maintainability Symposium (RAMS)

2013

A risk analysis of the launch, orbital assembly, and Earthdeparture phases of human Mars exploration campaign architectures was completed as an extension of a probabilistic risk assessment (PRA) originally carried out under the NASA Constellation Program Ares V Project [1]. The objective of the updated analysis was to study the sensitivity of loss-ofcampaign risk to such architectural factors as composition of the propellant delivery portion of the launch vehicle fleet (Ares V heavy-lift launch vehicle vs. smaller/cheaper commercial launchers) and the degree of launcher or Mars-bound spacecraft element sparing. Both a static PRA analysis and a dynamic, event-based Monte Carlo simulation were developed and used to evaluate the probability of loss of campaign under different sparing options. Results showed that with no sparing, loss-of-campaign risk is strongly driven by launcher count and on-orbit loiter duration, favoring an all-Ares V launch approach. Further, the reliability of the all-Ares V architecture showed significant improvement with the addition of a single spare launcher/payload. Among architectures utilizing a mix of Ares V and commercial launchers, those that minimized the on-orbit loiter duration of Mars-bound elements were found to exceed the reliability of no spare all-Ares V campaign if unlimited commercial vehicle sparing was assumed.

“…To resolve this issue and make comparison of the other aspects of the NSWC approach more meaningful, the RBD that was used for the LRO PRA was also used for the NSWC approach.) The innovative approach used failure rates from a variety of sources, tailored risk exposure to expected mission operations, and precisely applied redundancy at the failure mode level based upon previous studies [5,10]. Additionally, the innovative approach was the only method considered here that included common-cause failure modes.…”

Section: Comparative Study Overviewmentioning

confidence: 99%

“…Reviewed in this paper are descriptions, applications and analysis of the PRA results of the approach outlined in the Naval Surface Warfare Center (NSWC) 2007 Handbook; an innovative approach utilizing previously developed PRA tools that have been successfully applied on previous projects; the actual PRA approach that was implemented by the LRO project team; and a top-down methodology [3,10,5,4,5].…”

Section: Introductionmentioning

confidence: 99%

An exploration of PRA methodology used in spacecraft design

2013 Proceedings Annual Reliability and Maintainability Symposium (RAMS)

Nejad

2013

In order to achieve an optimal design of a complex space system that meets all constraints, the requirements placed upon the performance, mass, cost, and risk of the system must be considered, understood and traded against each other during the conceptual design of the system to avoid costly redesigns or project cancellation later in the development process [1]. A design process that follows this tenet of riskinformed design will need detailed insight into the relative risks facing the system, as well as quantitative estimates that can be produced through probabilistic risk assessment (PRA), in order to evaluate design decisions based upon the impact to all requirements on a co-equal basis [2].In this study, four types of methodologies used to produce risk estimates for spacecraft and satellites are examined. These include two traditional PRA methodologies [3,4], an innovative approach [5], and a top-down approach [6], all of which are explored by using the propulsion subsystem of the Lunar Reconnaissance Orbiter (LRO) as a comparative basis for the methodologies considered [7]. Similarities, differences, benefits, and drawbacks of various bottom-up, componentbased PRA approaches and the top-down approach are elucidated in terms of the process of modeling a system, the actionable information produced for the design team, and the overall quantitative risk evaluation of the system as compared to similar heritage space systems.Results of the various PRA methodologies are examined at the level of component failure rates, single-component failure probabilities, single-function failure probabilities where redundancy exists in the design, as well as the subsystem failure probability for the nominal LRO mission.Ultimately, all of the bottom-up, component-based PRA methods capture only the risk of a mature system and miss the risk contribution of design defects, which have been shown to be key drivers of reliability in single-use developmental systems [8,9]. Therefore, further steps must be taken to incorporate this contribution in future PRA methodologies.