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At least 343 records · Page 19

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NASA NTRS1982

Rapid quenching effects in glassy polymers

Using a specially constructed microbalance for hydrostatic weighing, density changes in PVC thin film due to rapid quenching through the glass transition temperature were observed. The more severe the quench, the greater the free volume content. Isobaric volume recovery of PVC was also studied by volume dilatometry. Both show aging of relaxing molecular rearrangements taking place as a linear function of logarithmic aging time at room temperature. Distribution of retardation times and Primak's distributed activation energy spectra were applied to the volume recovery data. The concomitant changes in mechanical properties of PVC after quenching were monitored by tensile creep stress-stran to failure rates. All reflect the presence of excess free volume content due to rapid quenching.

Mcgarry, F. J.↗

NASA NTRS1982

Photovoltaic array reliability optimization

An overview of the photovoltaic array reliability problem is presented, and a high reliability/minimum cost approach to this problem is presented. Design areas covered are cell failure, interconnect fatigue, and electrical insulation breakdown, and three solution strategies are discussed. The first involves controlling component failures in the solar cell (cell cracking, cell interconnects) and at the module level (must be statistically treated). Second, a fault tolerant circuit is designed which reduces array degradation, improves module yield losses, and controls hot-spot heating. Third, cost optimum module replacement strategies are also effective in reducing array degradation. This can be achieved by minimizing the life-cycle energy cost of the photovoltaic system. The integration of these solutions is aimed at reducing the 0.01% failure rate.

Ross, R. G., Jr.↗

NASA NTRS1981

Defect design of insulation systems for photovoltaic modules

A defect-design approach to sizing electrical insulation systems for terrestrial photovoltaic modules is presented. It consists of gathering voltage-breakdown statistics on various thicknesses of candidate insulation films where, for a designated voltage, module failure probabilities for enumerated thickness and number-of-layer film combinations are calculated. Cost analysis then selects the most economical insulation system. A manufacturing yield problem is solved to exemplify the technique. Results for unaged Mylar suggest using fewer layers of thicker films. Defect design incorporates effects of flaws in optimal insulation system selection, and obviates choosing a tolerable failure rate, since the optimization process accomplishes that. Exposure to weathering and voltage stress reduces the voltage-withstanding capability of module insulation films. Defect design, applied to aged polyester films, promises to yield reliable, cost-optimal insulation systems.

Mon, G. R.↗

NASA NTRS1984

Reliability analysis of an ultra-reliable fault tolerant control system

This report analyzes the reliability of NASA's Ultra-reliable Fault Tolerant Control System (UFTCS) architecture as it is currently envisioned for helicopter control. The analysis is extended to air transport and spacecraft control using the same computational and voter modules applied within the UFTCS architecture. The system reliability is calculated for several points in the helicopter, air transport, and space flight missions when there are initially 4, 5, and 6 operating channels. Sensitivity analyses are used to explore the effects of sensor failure rates and different system configurations at the 10 hour point of the helicopter mission. These analyses show that the primary limitation to system reliability is the number of flux windings on each flux summer (4 are assumed for the baseline case). Tables of system reliability at the 10 hour point are provided to allow designers to choose a configuration to meet specified reliability goals.

Curry, R. E.↗

NASA NTRS1984

A Manual Control Test for the Detection and Deterrence of Impaired Drivers

A brief manual control test and a decision strategy were developed, laboratory tested, and field validated which provide a means for detecting human operator impairment from alcohol or other drugs. The test requires the operator to stabilize progressively unstable controlled element dynamics. Control theory and experimental data verify that the human operator's control ability on this task is constrained by basic cybernetic characteristics, and that task performance is reliably affected by impairment effects on these characteristics. Assessment of human operator control ability is determined by a statistically based decision strategy. The operator is allowed several chances to exceed a preset pass criterion. Procedures are described for setting the pass criterion based on individual ability and a desired unimpaired failure rate. These procedures were field tested with apparatus installed in automobiles that were designed to discourage drunk drivers from operating their vehicles. This test program demonstrated that the control task and detection strategy could be applied in a practical setting to screen human operators for impairment in their basic cybernetic skills.

Stein, A. C.↗

NASA NTRS1985

Estimate of the probability of a lightning strike to the Galileo probe

Lightning strikes to aerospace vehicles occur mainly in or near clouds. As the Galileo entry probe will pass most of its operational life in the clouds of Jupiter, which is known to have lightning activity, the present study is concerned with the risk of a lightning strike to the probe. A strike to the probe could cause physical damage to the structure and/or damage to the electronic equipment aboard the probe. It is thought to be possible, for instance, that the instrument failures which occurred on all four Pioneer Venus entry probes at an altitude of 12 km were due to an external electric discharge. The probability of a lightning strike to the Galileo probe is evaluated. It is found that the estimate of a strike to the probe is only 0.001, which is about the same as the expected failure rate due to other design factors. In the case of entry probes to cloud-covered planets, a consideration of measures for protecting the vehicle and its payload from lightning appears to be appropriate.

Borucki, W. J.↗

NASA NTRS1986

Reliability models applicable to space telescope solar array assembly system

A complex system may consist of a number of subsystems with several components in series, parallel, or combination of both series and parallel. In order to predict how well the system will perform, it is necessary to know the reliabilities of the subsystems and the reliability of the whole system. The objective of the present study is to develop mathematical models of the reliability which are applicable to complex systems. The models are determined by assuming k failures out of n components in a subsystem. By taking k = 1 and k = n, these models reduce to parallel and series models; hence, the models can be specialized to parallel, series combination systems. The models are developed by assuming the failure rates of the components as functions of time and as such, can be applied to processes with or without aging effects. The reliability models are further specialized to Space Telescope Solar Arrray (STSA) System. The STSA consists of 20 identical solar panel assemblies (SPA's). The reliabilities of the SPA's are determined by the reliabilities of solar cell strings, interconnects, and diodes. The estimates of the reliability of the system for one to five years are calculated by using the reliability estimates of solar cells and interconnects given n ESA documents. Aging effects in relation to breaks in interconnects are discussed.

Patil, S. A.↗

NASA NTRS1986

Slip ring experience in long duration space applications

Ball Aerospace experience with slip rings in space extends back to 1962. Over 40 multi-ring assemblies have been flown and continuous operating lifetimes greater than 8 years at up to 60 rpm have been demonstrated. Slip rings provide multi-channel transfer of electrical power and signals in assemblies that are small in size and weight, and low in cost. By use of multiple brushes and sufficient copper within the assembly, power transfer efficiency better than 99.95 percent for high voltage circuits can be achieved. A low slip ring failure rate based on actual space operation totalling billions of ring revolutions has been established. Well qualified suppliers who have been making slip rings for space use for over 25 years are available. It is hoped that the suspected problem in SEASAT will not be allowed to prejudice space system designer against these very useful mechanisms.

Phinney, Damon D.↗

NASA NTRS1987

Electronics reliability and measurement technology

A summary is presented of the Electronics Reliability and Measurement Technology Workshop. The meeting examined the U.S. electronics industry with particular focus on reliability and state-of-the-art technology. A general consensus of the approximately 75 attendees was that "the U.S. electronics industries are facing a crisis that may threaten their existence". The workshop had specific objectives to discuss mechanisms to improve areas such as reliability, yield, and performance while reducing failure rates, delivery times, and cost. The findings of the workshop addressed various aspects of the industry from wafers to parts to assemblies. Key problem areas that were singled out for attention are identified, and action items necessary to accomplish their resolution are recommended.

Heyman, Joseph S.↗

NASA NTRS1986

A stress index model for ascending balloons

Attention is given to the development on the part of NASA of a simplified stress 'index' model to establish the relative stress magnitudes along a balloon's gore position as a function of altitude. Application of this model to several hundred balloon flights showed a good correlation between balloon failure rate and stress 'index' level. This model can be used during the balloon design process to lower the levels of stress in the balloon. By increasing the wall thickness of the balloon, adding caps, lengthening caps, or using external caps, lower stress can be accomplished. As a result, in January 1985, the NASA Balloon Program established a stress index specification to limit the design and flight stresses for NASA balloons.

Smith, I. S.↗

NASA NTRS1987

Measurement-based analysis of error latency

This paper demonstrates a practical methodology for the study of error latency under a real workload. The method is illustrated with sampled data on the physical memory activity, gathered by hardware instrumentation on a VAX 11/780 during the normal workload cycle of the installation. These data are used to simulate fault occurrence and to reconstruct the error discovery process in the system. The technique provides a means to study the system under different workloads and for multiple days. An approach to determine the percentage of undiscovered errors is also developed and a verification of the entire methodology is performed. This study finds that the mean error latency, in the memory containing the operating system, varies by a factor of 10 to 1 (in hours) between the low and high workloads. It is found that of all errors occurring within a day, 70 percent are detected in the same day, 82 percent within the following day, and 91 percent within the third day. The increase in failure rate due to latency is not so much a function of remaining errors but is dependent on whether or not there is a latent error.

Chillarege, Ram↗

NASA NTRS1988

Integrated avionics reliability

The integrated avionics reliability task is an effort to build credible reliability and/or performability models for multisensor integrated navigation and flight control. The research was initiated by the reliability analysis of a multisensor navigation system consisting of the Global Positioning System (GPS), the Long Range Navigation system (Loran C), and an inertial measurement unit (IMU). Markov reliability models were developed based on system failure rates and mission time.

Alikiotis, Dimitri↗

NASA NTRS1989

Markov reliability models for digital flight control systems

The reliability of digital flight control systems can often be accurately predicted using Markov chain models. The cost of numerical solution depends on a model's size and stiffness. Acyclic Markov models, a useful special case, are particularly amenable to efficient numerical solution. Even in the general case, instantaneous coverage approximation allows the reduction of some cyclic models to more readily solvable acyclic models. After considering the solution of single-phase models, the discussion is extended to phased-mission models. Phased-mission reliability models are classified based on the state restoration behavior that occurs between mission phases. As an economical approach for the solution of such models, the mean failure rate solution method is introduced. A numerical example is used to show the influence of fault-model parameters and interphase behavior on system unreliability.

Mcgough, John↗

NASA NTRS1989

Hubble Space Telescope maintenance and refurbishment planning analysis

This paper presents an updated analytical approach toward the identification of Hubble Space Telescope system failures and downstates, maintenance requirements, and overall support to maintenance mission planning. Different sparing options of Orbital Replacement Units are evaluated, and the optimum spares complement that satisfies the expected servicing requirements is identified. Specific Space Telescope Reliability and maintenance Simulation Computer Program (SPATEL) updates and refinements are reported, and input data updates relevant to failure rates, downstate ground rules, and maintenance policy are addressed. A summary of the latest SPATEL outputs is provided along with maintenance analysis results.

Pizzano, F.↗

NASA NTRS1990

Factors influencing manual ability to recover from an anomalous thruster input during a simulated docking maneuver

An experiment was performed in the Space Station Proximity Operations Simulator. Five test subjects were instructed to perform twenty remote docking maneuvers of an orbital maneuvering vehicle (OMV) to the Space Station in which they were located. The OMV started from an initial range of 304.8 m on the Space Station's negative velocity vector. Time and fuel to return to a nominal trajectory, total mission duration, total fuel consumption, failure rate, and time histories of commanded burns were recorded for each trial. Analyses of the results added support to the hypothesis that slow approach velocities are not inherently safer than their more rapid counterparts. Naive subjects were capable of docking successfully at velocities faster than those prescribed by the '0.1 pct Rule' even when a simulated faulty thruster disturbed the nominal trajectory.

Brody, Adam R.↗

NASA NTRS1990

The cleanroom case study in the Software Engineering Laboratory: Project description and early analysis

This case study analyzes the application of the cleanroom software development methodology to the development of production software at the NASA/Goddard Space Flight Center. The cleanroom methodology emphasizes human discipline in program verification to produce reliable software products that are right the first time. Preliminary analysis of the cleanroom case study shows that the method can be applied successfully in the FDD environment and may increase staff productivity and product quality. Compared to typical Software Engineering Laboratory (SEL) activities, there is evidence of lower failure rates, a more complete and consistent set of inline code documentation, a different distribution of phase effort activity, and a different growth profile in terms of lines of code developed. The major goals of the study were to: (1) assess the process used in the SEL cleanroom model with respect to team structure, team activities, and effort distribution; (2) analyze the products of the SEL cleanroom model and determine the impact on measures of interest, including reliability, productivity, overall life-cycle cost, and software quality; and (3) analyze the residual products in the application of the SEL cleanroom model, such as fault distribution, error characteristics, system growth, and computer usage.

Green, Scott↗

NASA NTRS1991

Test chips and ASIC qualification

A test chip set being developed to aid in the qualification of spaceborne Application Specific Integrated Circuits (ASICs) is described. The chip set consists of a process monitor for process parameter verification, a fault chip for yield analysis, a reliability chip for ASIC failure rate analysis, and total ionizing dose and single event upset chips for radiation effect analysis. The test structures contained in these chips are discussed along with representative test results.

Buehler, M. G.↗

NASA NTRS1991

Error latency measurements in symbolic architectures

Error latency, the time that elapses between the occurrence of an error and its detection, has a significant effect on reliability. In computer systems, failure rates can be elevated during a burst of system activity due to increased detection of latent errors. A hybrid monitoring environment is developed to measure the error latency distribution of errors occurring in main memory. The objective of this study is to develop a methodology for gauging the dependability of individual data categories within a real-time application. The hybrid monitoring technique is novel in that it selects and categorizes a specific subset of the available blocks of memory to monitor. The precise times of reads and writes are collected, so no actual faults need be injected. Unlike previous monitoring studies that rely on a periodic sampling approach or on statistical approximation, this new approach permits continuous monitoring of referencing activity and precise measurement of error latency.

Young, L. T.↗