Fayssal M. Safie

Probabilistic modeling of solar power systems

The author presents a probabilistic approach based on Markov chain theory to model stand-alone photovoltaic power systems and predict their long-term service performance. The major advantage of this approach is that it allows designers and developers of these systems to analyze the system performance as well as the battery subsystem performance in the long run and determine the system design requirements that meet a specified service performance level. The methodology presented is illustrated by using data for a radio repeater system for the Boston, Massachusetts, location.<<ETX>>

NASA new approach for evaluating risk reduction due to Space Shuttle upgrades

As part of NASAs intensive effort to incorporate quantitative risk assessment (QRA) tools in the Agencys decision-making process concerning Space Shuttle risk, NASA has developed a powerful risk assessment tool called the quantitative risk assessment system (QRAS). The QRAS is a tool designed to estimate Space Shuttle risk and evaluate Space Shuttle upgrades. This paper presents an overview of the QRAS with focus on its application for evaluating the risk reduction due to proposed Space Shuttle upgrades. The application includes a case study from the Space Shuttle main engine (SSME). The QRAS overview section of the paper includes the QRAS development process, the technical approach to model development, the QRA quantification methods and techniques, and observations concerning the complex modeling involved in QRAS. The application section of the paper describes a practical case study using QRAS models for evaluating critical Space Shuttle Program upgrades, specifically a proposed SSME nozzle upgrade. This paper presents the method for evaluating the proposed upgrade by comparing the current nozzle (old design with well-established probabilistic models) to the channel wall nozzle (new design at the preliminary design level).

Application of Probabilistic Risk Assessment (PRA) During Conceptual Design for the NASA Orbital Space Plane (OSP)

In order to meet the space transportation needs for a new century, America’s National Aeronautics and Space Administration (NASA) has implemented an Integrated Space Transportation Plan to produce safe, economical, and reliable access to space. One near term objective of this initiative is the design and development of a next-generation vehicle and launch system that will transport crew and cargo to and from the International Space Station (ISS), the Orbital Space Plane (OSP). The OSP system is composed of a manned launch vehicle by an existing Evolved Expendable Launch Vehicle (EELV). The OSP will provide emergency crew rescue from the ISS by 2008, and provide crew and limited cargo transfer to and from the ISS by 2012. A key requirement is for the OSP to be safer and more reliable than the Soyuz and Space Shuttle, which currently provide these capabilities.

A risk-assessment methodology for the Space-Shuttle external-tank welds

This paper presents a simulation model designed to evaluate the risk involved in deleting preproof test x-ray inspection of some welds on the Space Shuttle external tank (ET). The simulation model combines engineering and statistical models to evaluate the risk of a weld leak or burst during proof test and flight. Results of the model show that the risk due to the deletion of x-ray inspections is relatively high. This suggests that deleting the x-ray inspection would be a step in the wrong direction since the cost of failure during proof testing could be extremely high. Although this study addresses a weld inspection issue on the ET, the models developed can be applicable to other situations where weld risk assessment is needed.<<ETX>>

A simulation model for risk assessment of turbine wheels

A simulation model has been developed to evaluate the risk of the Space Shuttle auxiliary power unit (APU) turbine wheels for a specific inspection policy. It is an effective tool for risk/reliability evaluation and allows the analyst to study the tradeoffs between wheel reliability, wheel life, inspection interval, and rejection crack size. In the APU application, sensitivity analysis results showed that the wheel life limit has the least effect on wheel reliability when compared to the effect of the inspection interval and the rejection crack size. The simulation model developed represents a flexible tool to predict turbine wheel reliability and study the risk under different inspection policies.<<ETX>>

Reliability and probabilistic risk assessment — How they play together

Since the Space Shuttle Challenger accident in 1986, NASA and aerospace industry has extensively used Probabilistic Risk Assessment (PRA) methods to assess, understand, and communicate the risk of space launch vehicles, especially manned space flight missions. Another area that was given a lot of emphasis at NASA is reliability engineering. Both PRA and reliability are probabilistic in nature; however; the reliability engineering is a broad design discipline that deals with loss of function, while PRA is a system scenario based risk assessment process that deals with Loss of Mission (LOM), Loss of Vehicle (LOV), and Loss of Crew (LOC). This paper discusses the PRA process and the reliability engineering discipline in details. It discusses their differences and similarities and how they are used as complementary analyses to support design and flight decisions. In summary: 1) Reliability Engineering is a discipline that involves the application of engineering principles to the design and processing of products; both hardware and software intended to minimize the loss of functions. 2) PRA at NASA is a process that deals with system risk focusing on understanding the system risk scenarios that could lead to LOM, LOV, and LOC. 3) PRA and reliability engineering are two different areas serving different functions in supporting the design and operation of launch vehicles. However, PRA as a risk assessment, and reliability as a metric could play together in a complementary manner in assessing the risk and reliability of launch vehicles. 4) In general, reliability analyses should be used as a critical data source for PRA.

NASA applications and lessons learned in reliability engineering

Since the Shuttle Challenger accident in 1986, communities across National Aeronautic and Space Administration (NASA) have been developing and extensively using quantitative reliability and risk assessment methods in their decision making process. This paper discusses several reliability engineering applications that NASA has used over the years to support the design, development, and operation of critical space flight hardware. Specifically, the paper discusses several reliability engineering applications used by NASA in areas such as risk management, inspection policies, components upgrades, reliability growth, integrated failure analysis, and physics based probabilistic engineering analysis. In each of these areas, the paper provides a brief discussion of a case study to demonstrate the value added and the criticality of reliability engineering in supporting NASA project and program decisions to fly safely. Examples of these case studies discussed are; reliability based life limit extension of Shuttle Space Main Engine (SSME) hardware, Reliability based inspection policies for Auxiliary Power Unit (APU) turbine disc, probabilistic structural engineering analysis for reliability prediction of the SSME Alternate Turbo-pump Development (ATD), impact of the Space Shuttle External Tank (ET) foam reliability on the Space Shuttle System risk, and reliability based Space Shuttle upgrade for safety. A special attention is given in this paper to the physics based probabilistic engineering analysis applications and their critical role in evaluating the reliability of NASA development hardware including their potential use in a research and technology development environment.

Application of the NASA risk assessment tool to the evaluation of the Space Shuttle external tank re-welding process

The current Space Shuttle external tank design is called the super light weight tank (SLWT). A weight reduction of approximately 30% was achieved relative to the prior design called the light weight tank (LWT). The new NASA risk assessment tool, the quantitative risk assessment system (QRAS), was used to compare the risk of the two designs. The comparison includes consideration of the apparent reduction of the design safety factor for SLWT welds when a weld repair is required. The risk models for the structural failure accident scenario include five initiating events (IEs): (1) liquid oxygen (LO2) tank component failure; (2) liquid hydrogen (LH2) tank component failure; (3) LO2 tank weld failure; (4) LH2 tank weld failure; and (5) intertank failure. Although the risk results for the LH2 and LO2 tank welds for IEs 2 and 4 are higher for the SLWT vs. the LWT, the reverse is true for tank components IEs 1, 3 and 5. The SLWT has a slightly lower risk of structural failure. The impact of this difference is not significant to the total risk when the other six scenarios are also included.

Reliability and Maintainability analysis of a high air pressure compressor facility

This paper discusses a Reliability, Availability, and Maintainability (RAM) independent assessment conducted to support the refurbishment of the Compressor Station at the NASA Langley Research Center (LaRC). The paper discusses the methodologies used by the assessment team to derive the repair by replacement (RR) strategies to improve the reliability and availability of the Compressor Station (Ref.1). This includes a RAPTOR simulation model that was used to generate the statistical data analysis needed to derive a 15-year investment plan to support the refurbishment of the facility. To summarize, study results clearly indicate that the air compressors are well past their design life. The major failures of Compressors indicate that significant latent failure causes are present. Given the occurrence of these high-cost failures following compressor overhauls, future major failures should be anticipated if compressors are not replaced. Given the results from the RR analysis, the study team recommended a compressor replacement strategy. Based on the data analysis, the RR strategy will lead to sustainable operations through significant improvements in reliability, availability, and the probability of meeting the air demand with acceptable investment cost that should translate, in the long run, into major cost savings. For example, the probability of meeting air demand improved from 79.7 percent for the Base Case to 97.3 percent. Expressed in terms of a reduction in the probability of failing to meet demand (1 in 5 days to 1 in 37 days), the improvement is about 700 percent. Similarly, compressor replacement improved the operational availability of the facility from 97.5 percent to 99.8 percent. Expressed in terms of a reduction in system unavailability (1 in 40 to 1 in 500), the improvement is better than 1000 percent (an order of magnitude improvement).

The role of probabilistic design analysis methods in safety and affordability

For the last several years, NASA and its contractors have been working together to build space launch systems to commercialize space. Developing commercial affordable and safe launch systems becomes very important and requires a paradigm shift. This paradigm shift enforces the need for an integrated systems engineering environment where cost, safety, reliability, and performance need to be considered to optimize the launch system design. In such an environment, rule based and deterministic engineering design practices alone may not be sufficient to optimize margins and fault tolerance to reduce cost. As a result, introduction of Probabilistic Design Analysis (PDA) methods to support the current deterministic engineering design practices becomes a necessity to reduce cost without compromising reliability and safety. This paper discusses the importance of PDA methods in NASAs new commercial environment, their applications, and the key role they can play in designing reliable, safe, and affordable launch systems. More specifically, this paper discusses: (1) The involvement of NASA in PDA (2) Why PDA is needed (3) A PDA model structure (4) A PDA example application (5) PDA link to safety and affordability.