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NASA NTRS2002

Risk-Based Probabilistic Approach to Aeropropulsion System Assessment

In an era of shrinking development budgets and resources, where there is also an emphasis on reducing the product development cycle, the role of system assessment, performed in the early stages of an engine development program, becomes very critical to the successful development of new aeropropulsion systems. A reliable system assessment not only helps to identify the best propulsion system concept among several candidates, it can also identify which technologies are worth pursuing. This is particularly important for advanced aeropropulsion technology development programs, which require an enormous amount of resources. In the current practice of deterministic, or point-design, approaches, the uncertainties of design variables are either unaccounted for or accounted for by safety factors. This could often result in an assessment with unknown and unquantifiable reliability. Consequently, it would fail to provide additional insight into the risks associated with the new technologies, which are often needed by decision makers to determine the feasibility and return-on-investment of a new aircraft engine. In this work, an alternative approach based on the probabilistic method was described for a comprehensive assessment of an aeropropulsion system. The statistical approach quantifies the design uncertainties inherent in a new aeropropulsion system and their influences on engine performance. Because of this, it enhances the reliability of a system assessment. A technical assessment of a wave-rotor-enhanced gas turbine engine was performed to demonstrate the methodology. The assessment used probability distributions to account for the uncertainties that occur in component efficiencies and flows and in mechanical design variables. The approach taken in this effort was to integrate the thermodynamic cycle analysis embedded in the computer code NEPP (NASA Engine Performance Program) and the engine weight analysis embedded in the computer code WATE (Weight Analysis of Turbine Engines) with the fast probability integration technique (FPI). FPI was developed by Southwest Research Institute under contract with the NASA Glenn Research Center. The results were plotted in the form of cumulative distribution functions and sensitivity analyses and were compared with results from the traditional deterministic approach. The comparison showed that the probabilistic approach provides a more realistic and systematic way to assess an aeropropulsion system. The current work addressed the application of the probabilistic approach to assess specific fuel consumption, engine thrust, and weight. Similarly, the approach can be used to assess other aspects of aeropropulsion system performance, such as cost, acoustic noise, and emissions. Additional information is included in the original extended abstract.

Tong, Michael T.↗

NASA NTRS1981

Helicopter propulsion system reliability and engine monitoring assessments

The major short life, unreliable, and high maintenance engine and power components and subsystems in current civil helicopters were identified. Categories included both reciprocating and turbine engines, single and multiple engine configurations, single and tandem trotor vehicles, and light, medium, and heavy helicopters. The major focus was on the following parameters: accident rate data; maintenance rate data; and direct operator input.

Murphy, J. A.↗

NASA NTRS2011

Shuttle Risk Progression: Use of the Shuttle Probabilistic Risk Assessment (PRA) to Show Reliability Growth

It is important to the Space Shuttle Program (SSP), as well as future manned spaceflight programs, to understand the early mission risk and progression of risk as the program gains insights into the integrated vehicle through flight. The risk progression is important to the SSP as part of the documentation of lessons learned. The risk progression is important to future programs to understand reliability growth and the first flight risk. This analysis uses the knowledge gained from 30 years of operational flights and the current Shuttle PRA to calculate the risk of Loss of Crew and Vehicle (LOCV) at significant milestones beginning with the first flight. Key flights were evaluated based upon historical events and significant re-designs. The results indicated that the Shuttle risk tends to follow a step function as opposed to following a traditional reliability growth pattern where risk exponentially improves with each flight. In addition, it shows that risk can increase due to trading safety margin for increased performance or due to external events. Due to the risk drivers not being addressed, the risk did not improve appreciably during the first 25 flights. It was only after significant events occurred such as Challenger and Columbia, where the risk drivers were apparent, that risk was significantly improved. In addition, this paper will show that the SSP has reduced the risk of LOCV by almost an order of magnitude. It is easy to look back afte r 30 years and point to risks that are now obvious, however; the key is to use this knowledge to benefit other programs which are in their infancy stages. One lesson learned from the SSP is understanding risk drivers are essential in order to considerably reduce risk. This will enable the new program to focus time and resources on identifying and reducing the significant risks. A comprehensive PRA, similar to that of the Shuttle PRA, is an effective tool quantifying risk drivers if support from all of the stakeholders is given.

Hamlin, Teri L.↗

NASA NTRS1994

Probabilistic assessment of space nuclear propulsion system nozzle

In assessing the reliability of a space nuclear propulsion system (SNPS) nozzle, uncertainties associated with the following design parameters were considered: geometry, boundary conditions, material behavior, and thermal and pressure loads. A preliminary assessment of the reliability was performed using NESSUS (Numerical Evaluation of Stochastic Structures Under Stress), a finite-element computer code developed at the NASA Lewis Research Center. The sensitivity of the nozzle reliability to the uncertainties in the random variables was quantified. With respect to the effective stress, preliminary results showed that the nozzle spatial geometry uncertainties have the most significant effect at low probabilities whereas the inner wall temperature has the most significant effect at higher probabilities.

Shah, Ashwin R.↗

NASA NTRS2022

NASA Physics of Failure (PoF) for Reliability

An item’s reliability or longevity is dependent not only on its design but also on how it is used, manufactured, tested, and the stresses it has or will experience. Stresses include operational and environmental exposures to thermal, voltage, current, age/exposure, mechanical, and radiation mechanisms. Therefore, in reliability analysis, it is important to consider the contributions of all of these factors when predicting the failure rates of components. Historically, there has been a reliance on handbook data (e.g., MIL-HDBK-217), but experience has shown that these values and distributions are not representative of actual performance (1,2). Therefore, to make more credible reliability and risk assessments for its missions, NASA must transition to estimating likelihoods of failure based on an item’s reliability/longevity factors (or the physical susceptibilities and strengths impacting the design’s performance) has or will experience, whenever possible. To facilitate this transition a “Handbook on Methodology for Physics of Failure Based Reliability Assessments” has been developed by NASA to assist in applying physics experiences or experiment physics for empirical analysis and conceptualized physics exposures or theoretical physics for deterministic analysis, to develop and aggregate realistic likelihoods of failure leading to more credible forecasts of item performance and longevity. In addition, since it is NASA’s intention that this document continues to evolve based on community lessons learned and the introduction of new assessment methodologies, NASA is encouraging and appreciates the contributions of current and future authors to maintain and enhance this handbook and its supporting case studies.

Physics of Failure↗

NASA NTRS2001

Probabilistic Risk-Based Approach to Aeropropulsion System Assessment Developed

Tong, Michael T.↗

NASA NTRS2014

Comparison of Knee and Ankle Dynamometry between NASA's X1 Exoskeleton and Biodex System 4

Pre- and post-flight dynamometry is performed on International Space Station crewmembers to characterize microgravity-induced strength changes. Strength is not assessed in flight due to hardware limitations and there is poor understanding of the time course of in-flight changes. PURPOSE: To assess the reliability of a prototype dynamometer, the X1 Exoskeleton (EXO) and its agreement with a Biodex System 4 (BIO). METHODS: Eight subjects (4 M/4 F) completed 2 counterbalanced testing sessions of knee extension/flexion (KE/KF), 1 with BIO and 1 with EXO, with repeated measures within each session in normal gravity. Test-retest reliability (test 1 and 2) and device agreement (BIO vs. EXO) were evaluated. Later, to assess device agreement for ankle plantarflexion (PF), 10 subjects (4 M/6 F) completed 3 test conditions (BIO, EXO, and BIOEXO); BIOEXO was a hybrid condition comprised of the Biodex dynamometer motor and the X1 footplate and ankle frame. Ankle comparisons were: BIO vs. BIOEXO (footplate differences), BIOEXO vs. EXO (motor differences), and BIO vs. EXO (all differences). Reliability for KE/KF was determined by intraclass correlation (ICC). Device agreement was assessed with: 1) repeated measures ANOVA, 2) a measure of concordance (rho), and 3) average difference. RESULTS: ICCs for KE/KF were 0.99 for BIO and 0.96 to 0.99 for EXO. Agreement was high for KE (concordance: 0.86 to 0.95; average differences: -7 to +9 Nm) and low to moderate for KF (concordance: 0.64 to 0.78; average differences: -4 to -29 Nm, P<0.05). BIO vs. BIOEXO PF concordance ranged from 0.89 to 0.92 and mean differences ranged from -9 to +3 Nm (BIO < BIOEXO). BIOEXO vs. EXO PF concordance ranged from 0.73 to 0.80 while mean differences were -18 to -36 Nm (BIOEXO < EXO, P<0.05). PF concordance for BIO vs. EXO was slightly lower (0.61 to 0.84) and mean differences were greater (-27 to -33 Nm; BIO < EXO, P<0.05). CONCLUSION: BIO and EXO were similarly reliable for KE and KF. KE measures produced high agreement between devices; KF did not. For ankle PF, torque differences due to the two footplates were small. However, the X1 motor reports greater torques than the Biodex motor during PF. This first prototype provides proof of concept for a reliable, robotic-based exoskeleton to perform portable dynamometry for large muscle groups of the lower body.

English, K. L.↗

NASA NTRS2016

Comparative Analysis of NOAA REFM and SNB3GEO Tools for the Forecast of the Fluxes of High-Energy Electrons at GEO

Reliable forecasts of relativistic electrons at geostationary orbit (GEO) are important for the mitigation of their hazardous effects on spacecraft at GEO. For a number of years the Space Weather Prediction Center at NOAA has provided advanced online forecasts of the fluence of electrons with energy >2 MeV at GEO using the Relativistic Electron Forecast Model (REFM). The REFM forecasts are based on real-time solar wind speed observations at L1. The high reliability of this forecasting tool serves as a benchmark for the assessment of other forecasting tools. Since 2012 the Sheffield SNB3GEO model has been operating online, providing a 24 h ahead forecast of the same fluxes. In addition to solar wind speed, the SNB3GEO forecasts use solar wind density and interplanetary magnetic field B(sub z) observations at L1. The period of joint operation of both of these forecasts has been used to compare their accuracy. Daily averaged measurements of electron fluxes by GOES 13 have been used to estimate the prediction efficiency of both forecasting tools. To assess the reliability of both models to forecast infrequent events of very high fluxes, the Heidke skill score was employed. The results obtained indicate that SNB3GEO provides a more accurate 1 day ahead forecast when compared to REFM. It is shown that the correction methodology utilized by REFM potentially can improve the SNB3GEO forecast.

Balikhin, M. A.↗

NASA NTRS2009

Designing a Better Spacecraft: Assessing Flight Operability of Human Rated Spacecraft

The design of a human rated spacecraft is a complex and costly process requiring the integrated assessment of many individual criteria. Historically, it has been difficult to include in that integrated assessment the design s full impact on the flight operations community and its costs. The unique "operability requirements" have not been well understood, nor has there been a well-defined set of criteria for assessing operability. As a result, flight operations organizations and program managers are often faced with difficult and costly operations phase implementations. In response, the Mission Operations Directorate at NASA s Lyndon B. Johnson Space Center has established a formal technique to evaluate and communicate the operational characteristics of spacecraft system designs for the Constellation Program. This process is not intended to replace or replicate other critical assessments such as risk, reliability and safety assessments. Instead, this new technique adds to the assessment toolset a means to address the concerns and potential cost drivers that are unique to the operational phase of a program and the flight operations community. This paper describes the implementation and application of this "Spacecraft Flight Operability Assessment Scale" in supporting vehicle design efforts. The six key factors of flight operability are defined, with guiding principles and goals stated for each factor. A standardized rating technique provides feedback that is useful to both the operations and program management communities. Sample assessments of legacy spacecraft, including the Space Shuttle and International Space Station systems, are provided to provide real world examples of this technique s application.

Crocker, Alan R.↗

NASA NTRS1965

Procedure for obtaining fluid amplifier reliability data

Data collecting procedures for assessing fluid amplifier reliability and failure mechanisms

Hahn, G. J.↗

NASA NTRS1979

The 747 primary flight control systems reliability and maintenance study

The major operational characteristics of the 747 Primary Flight Control Systems (PFCS) are described. Results of reliability analysis for separate control functions are presented. The analysis makes use of a NASA computer program which calculates reliability of redundant systems. Costs for maintaining the 747 PFCS in airline service are assessed. The reliabilities and cost will provide a baseline for use in trade studies of future flight control system design.

Source record↗

NASA NTRS1992

Applying reliability models to the maintenance of Space Shuttle software

Software reliability models provide the software manager with a powerful tool for predicting, controlling, and assessing the reliability of software during maintenance. We show how a reliability model can be effectively employed for reliability prediction and the development of maintenance strategies using the Space Shuttle Primary Avionics Software Subsystem as an example.

Schneidewind, Norman F.↗

NASA NTRS2009

Johnson Space Center's Risk and Reliability Analysis Group 2008 Annual Report

The Johnson Space Center (JSC) Safety & Mission Assurance (S&MA) Directorate s Risk and Reliability Analysis Group provides both mathematical and engineering analysis expertise in the areas of Probabilistic Risk Assessment (PRA), Reliability and Maintainability (R&M) analysis, and data collection and analysis. The fundamental goal of this group is to provide National Aeronautics and Space Administration (NASA) decisionmakers with the necessary information to make informed decisions when evaluating personnel, flight hardware, and public safety concerns associated with current operating systems as well as with any future systems. The Analysis Group includes a staff of statistical and reliability experts with valuable backgrounds in the statistical, reliability, and engineering fields. This group includes JSC S&MA Analysis Branch personnel as well as S&MA support services contractors, such as Science Applications International Corporation (SAIC) and SoHaR. The Analysis Group s experience base includes nuclear power (both commercial and navy), manufacturing, Department of Defense, chemical, and shipping industries, as well as significant aerospace experience specifically in the Shuttle, International Space Station (ISS), and Constellation Programs. The Analysis Group partners with project and program offices, other NASA centers, NASA contractors, and universities to provide additional resources or information to the group when performing various analysis tasks. The JSC S&MA Analysis Group is recognized as a leader in risk and reliability analysis within the NASA community. Therefore, the Analysis Group is in high demand to help the Space Shuttle Program (SSP) continue to fly safely, assist in designing the next generation spacecraft for the Constellation Program (CxP), and promote advanced analytical techniques. The Analysis Section s tasks include teaching classes and instituting personnel qualification processes to enhance the professional abilities of our analysts as well as performing major probabilistic assessments used to support flight rationale and help establish program requirements. During 2008, the Analysis Group performed more than 70 assessments. Although all these assessments were important, some were instrumental in the decisionmaking processes for the Shuttle and Constellation Programs. Two of the more significant tasks were the Space Transportation System (STS)-122 Low Level Cutoff PRA for the SSP and the Orion Pad Abort One (PA-1) PRA for the CxP. These two activities, along with the numerous other tasks the Analysis Group performed in 2008, are summarized in this report. This report also highlights several ongoing and upcoming efforts to provide crucial statistical and probabilistic assessments, such as the Extravehicular Activity (EVA) PRA for the Hubble Space Telescope service mission and the first fully integrated PRAs for the CxP's Lunar Sortie and ISS missions.

Valentine, Mark↗

NASA NTRS1992

Rocket engine propulsion 'system reliability'

A system reliability approach is discussed which is based on a quantitative assessment of both the engine system and hardware reliability during the design process. The approach makes it possible to evaluate design trades as the design matures. It is concluded that the system reliability assessment approach offers the following benefits. The uncertainty of each design variable is explicitly considered in the analysis process. The most significant design variables are ranked in order of their effect on reliability. Design trades can be assessed for reliability impact. It is concluded that the approach facilitates communication between disciplines and thus aids in concurrent engineering.

O'Hara, K. J.↗

NASA NTRS1975

Comparison of 70 deg tilt, LBNP, and passive standing as measures of orthostatic tolerance

The present study was performed to assess the reliability of lower body negative pressure (LBNP) as a test of orthostatic tolerance. The need for this assessment arose from the prior observation in this laboratory that some subjects show wide day-to-day variation in heart rate responses to LBNP. The extent of these variations was so great as to raise a serious question as to the value of LBNP as a measure of study-induced alterations (e.g., those produced by bedrest or weightlessness) in orthostatic tolerance. Five healthy volunteers were subjected to a series of tests, consisting of 70 deg tilt, LBNP, and passive standing, on three occasions preceding and three occasions following a 2-week period of bedrest. Study results show that it is possible to subdivide the volunteers into subgroups which show either great or little day-to-day variability in any of the three tests. All three tests revealed bedrest-induced alterations in orthostatic tolerance quite adequately. Of the three tests studied, LBNP most frequently resulted in the largest test-induced heart rate alterations, followed by quiet standing and, finally 70 deg tilt.

Hyatt, K. H.↗

NASA NTRS1987

Absolute magnitude estimation and relative judgement approaches to subjective workload assessment

Two rating scale techniques employing an absolute magnitude estimation method, were compared to a relative judgment method for assessing subjective workload. One of the absolute estimation techniques used was an unidimensional overall workload scale and the other was the multidimensional NASA-Task Load Index technique. Thomas Saaty's Analytic Hierarchy Process was the unidimensional relative judgment method used. These techniques were used to assess the subjective workload of various single- and dual-tracking conditions. The validity of the techniques was defined as their ability to detect the same phenomena observed in the tracking performance. Reliability was assessed by calculating test-retest correlations. Within the context of the experiment, the Saaty Analytic Hierarchy Process was found to be superior in validity and reliability. These findings suggest that the relative judgment method would be an effective addition to the currently available subjective workload assessment techniques.

Vidulich, Michael A.↗

NASA NTRS2009

Solid Rocket Booster Large Main and Drogue Parachute Reliability Analysis

The parachutes on the Space Transportation System (STS) Solid Rocket Booster (SRB) are the means for decelerating the SRB and allowing it to impact the water at a nominal vertical velocity of 75 feet per second. Each SRB has one pilot, one drogue, and three main parachutes. About four minutes after SRB separation, the SRB nose cap is jettisoned, deploying the pilot parachute. The pilot chute then deploys the drogue parachute. The drogue chute provides initial deceleration and proper SRB orientation prior to frustum separation. At frustum separation, the drogue pulls the frustum from the SRB and allows the main parachutes that are mounted in the frustum to unpack and inflate. These chutes are retrieved, inspected, cleaned, repaired as needed, and returned to the flight inventory and reused. Over the course of the Shuttle Program, several improvements have been introduced to the SRB main parachutes. A major change was the replacement of the small (115 ft. diameter) main parachutes with the larger (136 ft. diameter) main parachutes. Other modifications were made to the main parachutes, main parachute support structure, and SRB frustum to eliminate failure mechanisms, improve damage tolerance, and improve deployment and inflation characteristics. This reliability analysis is limited to the examination of the SRB Large Main Parachute (LMP) and drogue parachute failure history to assess the reliability of these chutes. From the inventory analysis, 68 Large Main Parachutes were used in 651 deployments, and 7 chute failures occurred in the 651 deployments. Logistic regression was used to analyze the LMP failure history, and it showed that reliability growth has occurred over the period of use resulting in a current chute reliability of R = .9983. This result was then used to determine the reliability of the 3 LMPs on the SRB, when all must function. There are 29 drogue parachutes that were used in 244 deployments, and no in-flight failures have occurred. Since there are no observed drogue chute failures, Jeffreys Prior was used to calculate a reliability of R =.998. Based on these results, it is concluded that the LMP and drogue parachutes on the Shuttle SRB are suited to their mission and changes made over their life have improved the reliability of the parachute.

Clifford, Courtenay B.↗

NASA NTRS2001

Reliability of Fault Tolerant Control Systems

This paper reports Part I of a two part effort, that is intended to delineate the relationship between reliability and fault tolerant control in a quantitative manner. Reliability analysis of fault-tolerant control systems is performed using Markov models. Reliability properties, peculiar to fault-tolerant control systems are emphasized. As a consequence, coverage of failures through redundancy management can be severely limited. It is shown that in the early life of a syi1ein composed of highly reliable subsystems, the reliability of the overall system is affine with respect to coverage, and inadequate coverage induces dominant single point failures. The utility of some existing software tools for assessing the reliability of fault tolerant control systems is also discussed. Coverage modeling is attempted in Part II in a way that captures its dependence on the control performance and on the diagnostic resolution.

Wu, N. Eva↗