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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Development of far infrared detection techniques

This grant supports the development of a variety of advanced far infrared detection techniques that will be used in future NASA missions such as the Space Infrared Telescope Facility (SIRTF). These studies span the wavelength region of 30-200 microns and include development of focal planes and electronics that would utilize them. Efforts reported here represent collaborations among the University of Arizona's Steward Observatory; Lawrence Berkeley Laboratories (LBL); and the University of California at Berkeley. The overall goal of this program is to demonstrate extremely high performance detectors for low background applications between 30-200 microns. For the 40-120 micron region, the program is developing a 32x32 filled detector array. Previous work has demonstrated the required performance with a Z-Plane array architecture; we are now upgrading construction facilities and techniques to increase yields and reliability. We have completed the initial tradeoff analysis for the interconnects between the detectors and and readouts. We found satisfactory performance for both Flex-Cable and Tape Automated Bonding (TAB) devices, but analysis showed that an all-sapphire device would not meet our requirements. In addition, the effort continued to develop readouts that can operate close to the detector element temperature; success would substantially improve the manufacturability of the arrays. For the 100-200 micron range, previous work has demonstrated good performance with individual detector elements of stressed Ge:Ga; current efforts are to increase the quantum efficiency of these devices. Work continues to discover how to construct an alternate type of long wave detector, Ge:B Blocked Impurity Band devices. Following descoping of SIRTF, we are closing out the bolometer and refrigerator development. We documented the optical designs and approaches developed previously to meet the specific requirements of these detector types in terms of modulation of the signals for good photometric behavior within the constraints of a compact and self-contained cryogenic instrument. Further study was initiated for the Band 3 optical train, where we have concerns about complexity and the manufacturability of one component.

Rieke, George↗
NASA NTRS1996

Silicon Carbide High-Temperature Power Rectifiers Fabricated and Characterized

The High Temperature Integrated Electronics and Sensors (HTIES) team at the NASA Lewis Research Center is developing silicon carbide (SiC) for use in harsh conditions where silicon, the semiconductor used in nearly all of today's electronics, cannot function. Silicon carbide's demonstrated ability to function under extreme high-temperature, high power, and/or high-radiation conditions will enable significant improvements to a far ranging variety of applications and systems. These improvements range from improved high-voltage switching for energy savings in public electric power distribution and electric vehicles, to more powerful microwave electronics for radar and cellular communications, to sensors and controls for cleaner-burning, more fuel-efficient jet aircraft and automobile engines. In the case of jet engines, uncooled operation of 300 to 600 C SiC power actuator electronics mounted in key high-temperature areas would greatly enhance system performance and reliability. Because silicon cannot function at these elevated temperatures, the semiconductor device circuit components must be made of SiC. Lewis' HTIES group recently fabricated and characterized high-temperature SiC rectifier diodes whose record-breaking characteristics represent significant progress toward the realization of advanced high-temperature actuator control circuits. The first figure illustrates the 600 C probe-testing of a Lewis SiC pn-junction rectifier diode sitting on top of a glowing red-hot heating element. The second figure shows the current-versus voltage rectifying characteristics recorded at 600 C. At this high temperature, the diodes were able to "turn-on" to conduct 4 A of current when forward biased, and yet block the flow of current ($quot;turn-off") when reverse biases as high as 150 V were applied. This device represents a new record for semiconductor device operation, in that no previous semiconductor electronic device has ever simultaneously demonstrated 600 C functionality, and 4-A turn-on and 150-V rectification. The high operating current was achieved despite severe device size limitations imposed by present-day SiC wafer defect densities. Further substantial increases in device performance can be expected when SiC wafer defect densities decrease as SiC wafer production technology matures.

Source record↗
NASA NTRS2015

Dynamic Simulation Probabalistic Risk Assessment Model for an Enceladus Sample Return Mission

Enceladus, a moon of Saturn, has geyser-like jets that spray plumes of material into orbit. These jets could enable a free-flying spacecraft to collect samples and return them to Earth for study to determine if they contain the building blocks of life. The Office of Planetary Protection at NASA requires containment of any unsterilized samples and prohibits destructive impact of the spacecraft upon return to Earth, with a sample release probability of less than 1 in 1,000,000 as a recommended goal. This paper describes a probabilistic risk assessment model that uses dynamic simulation techniques to capture the physics-based, time- and state-dependent interactions between the sample return system and the environment, which drive the risk of sample release. The dynamic approach uses a Monte Carlo-style simulation to integrate the many phases and sources of risk for a sample return mission. The model is used to assess the achievability of the planetary protection reliability goal. This is accomplished by performing sensitivity studies assessing the impact of modeling assumptions to identify where uncertainties drive the risk. These results, in turn, are used to examine the feasibility of meeting key design and performance parameters that are needed to achieve the reliability goal for a given architecture with existing technologies.

Dynamic Simulation↗
NASA NTRS2006

Vapor-Compression Heat Pumps for Operation Aboard Spacecraft

Vapor-compression heat pumps (including both refrigerators and heat pumps) of a proposed type would be capable of operating in microgravity and would be safe to use in enclosed environments like those of spacecraft. The designs of these pumps would incorporate modifications of, and additions to, vapor-compression cycles of heat pumps now used in normal Earth gravitation, in order to ensure efficiency and reliability during all phases of operation, including startup, shutdown, nominal continuous operation, and peak operation. Features of such a design might include any or all of the following: (1) Configuring the compressor, condenser, evaporator, valves, capillary tubes (if any), and controls to function in microgravitation; (2) Selection of a working fluid that satisfies thermodynamic requirements and is safe to use in a closed crew compartment; (3) Incorporation of a solenoid valve and/or a check valve to prevent influx of liquid to the compressor upon startup (such influx could damage the compressor); (4) Use of a diode heat pipe between the cold volume and the evaporator to limit the influx of liquid to the compressor upon startup; and (5) Use of a heated block to vaporize any liquid that arrives at the compressor inlet.

Ruemmele, Warren↗
NASA NTRS1985

Ordering actions for visibility

The notion of 'atomic actions' has been considered in recent work on data integrity and reliability. It has been found that the standard database operations of 'read' and 'write' carry with them severe performance limitations. For this reason, systems are now being designed in which actions operate on 'objects' through operations with more-or-less arbitrary semantics. An object (i.e., an instance of an abstract data type) comprises data, a set of operations (procedures) to manipulate the data, and a set of invariants. An 'action' is a unit of work. It appears to be primitive to its surrounding environment, and 'atomic' to other actions. Attention is given to the conventional model of nested actions, ordering requirements, the maximum possible visibility (full visibility) for items which must be controlled by ordering constraints, item management paradigms, and requirements for blocking mechanisms which provide the required visibility.

Mckendry, M. S.↗
NASA NTRS2004

Ejecta Generation and Redistribution on 433 Eros: Modeling Ejecta Launch Conditions

The NEAR-Shoemaker mission to asteroid 433 Eros presents an unprecedented opportunity to gain fundamental new knowledge about the processes governing regolith formation and redistribution on small bodies. NEAR-Shoemaker’s high-resolution imaging of the surface of Eros makes the asteroid a valuable and heretofore unparalleled laboratory for the detailed study of impact ejecta reaccretion and regolith redistribution on low-gravity (of order 10 -3 g) objects. Regolith is produced on asteroids by impact cratering, and the existence of regolith on the smallest solar system bodies supports the view that some of the ejecta from impact events on such objects may be retained. Impact craters and retained ejecta on low-gravity objects like Eros represent valuable natural laboratories for evaluating various models of impact cratering processes, since they may present crater structures or ejecta features that either do not form or are hidden on higher-gravity bodies like the Moon. Further, quantifying the extent to which impact processes generate and redistribute regoliths on small body surfaces (excavation depths, retained fraction, turnover timescales, etc.) is pivotal to the issue of how to relate meteoritical samples to their asteroidal parent bodies when surficial processes ( i.e., “space weathering”) may disguise or cover up underlying material and confound the ability of remote sensing techniques to provide reliable mineralogical assays of the parent objects. The rich variety of data on Eros’ regolith properties and distribution returned by NEAR-Shoemaker now require detailed analysis in order to take full advantage of the clues these observations offer for elucidating details of the impact cratering process on small bodies. Complicating simple interpretations of crater and ejecta morphology are dynamical effects on ejecta emplacement resulting from Eros’ irregular shape, rapid (5.27 hr) rotation, and low gravity. Figure 1 shows the very different ejecta deposit morphology that can result if the effects of rotation alone are neglected. Considering the additional complicating factors of Eros’ irregular shape and complex gravitational field, simple calculations of the extent and thickness of ejecta blankets and the spatial distribution of ejecta blocks from basic crater scaling laws or numerical hydrocodes alone do not suffice. In order to fully interpret the suite of NEAR-Shoemaker observations of regolith features across the surface of Eros and to evaluate various impact models for specific craters on the asteroid, detailed dynamical modeling of the deposition of crater ejecta from those craters is required . Here, I describe some modifications and improvements to the dynamical model being used for these studies.

D D Durda↗
NASA NTRS1995

Airborne spectrograph for the thermal IR: Broadband Array Spectrograph System

Spectroscopic studies in the 'fingerprint' region of the thermal IR from 3 to 14 microns of celestial dust components and the overall energy distribution of the sources are best served by moderate spectral resolution (R = lambda/Delta lambda approximately 30 to 200), high sensitivity observations. Spectral purity and the reproducibility of the spectral shape are critical as well, when using the spectral shape to assign temperatures to dust grains or to gas clouds based on the wavelength and shape of molecular bands. These sensor attributes are also important to the use of wavelengths and ratios of solid state features to derive compositions of dust grains in celestial sources. The advent of high quality linear arrays of blocked impurity band (BIB) detectors of Si:As permitted the development of a state-of-the-art, patented, cooled prism spectrograph. Developed at The Aerospace Corporation largely with in-house funds, the Broadband Array Spectrograph System (BASS) has been used for a variety of remote sensing applications, but especially for IR astronomical studies on the Kuiper Airborne Observatory and at the NASA Infrared Telescope Facility (IRTF). The attributes of the spectrograph, specifically having the pupil imaged onto the 2 linear 58 element detector arrays so that the effects of guiding errors are minimized, being able to maximally exploit the limited observing time by acquiring all 116 spectral channels simultaneously, and having all spectral channels imaged through the same aperture so that spectral mapping is readily and reliably accomplished, afford the scientist with a unique opportunity to conduct both surveys of examples of many different types of sources as well as in-depth studies of a given class of object by thoroughly sampling the class members. This duality was demonstrated with the BASS through a combination of KAO flights where spectral maps were obtained as part of in-depth studies of specific source regions (such as Orion and W3) and flights where up to 21 legs/flight were used to survey many different sources of different types.

Russell, Ray W.↗
NASA NTRS2000

Life and Reliability Characteristics of TurboBrayton Coolers

Wear and internal contaminants are two of the primary factors that influence reliable, long-life operation of turbo-Brayton cryocoolers. This paper describes tests that have been conducted and methods that have been developed for turbo-Brayton components and systems to assure reliable operation. The turbomachines used in these coolers employ self-acting gas bearings to support the miniature high-speed shafts, thus providing vibration-free operation. Because the bearings are self-acting, rubbing contact occurs during initial start-up and shutdown of the machines. Bearings and shafts are designed to endure multiple stop/start cycles without producing particles or surface features that would impair the proper operation of the machines. Test results are presented for a variety of turbomachines used in these systems. The tests document extended operating life and start/stop cycling behavior for machines over a range of time and temperature scales. Contaminants such as moisture and other residual gas impurities can be a source of degraded operation if they freeze out in sufficient quantities to block flow passages or if they mechanically affect the operation of the machines. A post-fabrication bakeout procedure has been successfully used to reduce residual internal contamination to acceptable levels in a closed cycle system. The process was developed during space qualification tests on the NICMOS cryocooler. Moisture levels were sampled over a six-month time interval confirming the effectiveness of the technique. A description of the bakeout procedure is presented.

Breedlove, Jeff J.↗
NASA NTRS1978

On the tones and pressure oscillations induced by flow over rectangular cavities

Experimental measurements of the frequencies of discrete tones induced by flow over rectangular cavities were carried out over a range of low subsonic Mach numbers to provide a reliable data base for (aircraft wheel well) cavity noise consideration. A mathematical model of the cavity tones and pressure oscillation phenomenon based on the coupling between shear layer instabilities and acoustic feedback is developed to help in understanding the tone generation mechanism. Good agreement is found between discrete tone frequencies predicted by the model and experimental measurements over a wide range of Mach numbers. Evidence of tones generated by the cavity normal mode resonance mechanism at very low subsonic Mach numbers is also presented.

Tam, C. K. W.↗
NASA NTRS1992

Sudbury Breccia and suevite as glacial indicators transported 800 km to Kentland Astrobleme, Indiana

A glacial erratic whose place of origin is known by direct comparison with bedrock is known as an indicator. In 1971, while visiting the known astrobleme at Kentland, Indiana, Peredery recognized and sampled in the overlying glacial drift deposits a distinctive boulder of Sudbury suevite (black member, Onaping Formation) that normally occurs within the Sudbury Basin as an impact fall-back or wash-in deposit. The rock was sampled (but later mislaid) from a farmer's cairn next to a cleared field. Informal reports of this discovery prompted the other authors to recently reconnoiter the Kentland locality in an attempt to relocate the original boulder. Several breccia blocks were sampled but laboratory examination proved most of these probably to be diamictites from the Precambrian Gowganda Formation, which outcrops extensively in the southern Ontario. However, one sample was confirmed as typical Sudbury Breccia, which outcrops in the country rock surrounding the Sudbury Basin. Thus two glacial indicators were transported by Pleistocene continental glaciers about 820 km over a tightly proscribed path and, curiously, from one astrobleme to another. Brecciated boulders in the Illinois/Indiana till plain are usually ascribed to the Gowganda or Mississagi formations in Ontario. But impact-generated rocks need not be confused. The carbonaceous matrix of the suevite, for example, was sufficiently distinctive to assign it to the upper portion of the black Onaping. The unique and restricted source area of these indicators provide an accurate and reliable control for estimating Pleistocene ice movement.

Mchone, John F.↗
NASA NTRS1992

Open systems storage platforms

The building blocks for an open storage system includes a system platform, a selection of storage devices and interfaces, system software, and storage applications CONVEX storage systems are based on the DS Series Data Server systems. These systems are a variant of the C3200 supercomputer with expanded I/O capabilities. These systems support a variety of medium and high speed interfaces to networks and peripherals. System software is provided in the form of ConvexOS, a POSIX compliant derivative of 4.3BSD UNIX. Storage applications include products such as UNITREE and EMASS. With the DS Series of storage systems, Convex has developed a set of products which provide open system solutions for storage management applications. The systems are highly modular, assembled from off the shelf components with industry standard interfaces. The C Series system architecture provides a stable base, with the performance and reliability of a general purpose platform. This combination of a proven system architecture with a variety of choices in peripherals and application software allows wide flexibility in configurations, and delivers the benefits of open systems to the mass storage world.

Collins, Kirby↗
NASA NTRS1988

User's guide to the Fault Inferring Nonlinear Detection System (FINDS) computer program

Described are the operation and internal structure of the computer program FINDS (Fault Inferring Nonlinear Detection System). The FINDS algorithm is designed to provide reliable estimates for aircraft position, velocity, attitude, and horizontal winds to be used for guidance and control laws in the presence of possible failures in the avionics sensors. The FINDS algorithm was developed with the use of a digital simulation of a commercial transport aircraft and tested with flight recorded data. The algorithm was then modified to meet the size constraints and real-time execution requirements on a flight computer. For the real-time operation, a multi-rate implementation of the FINDS algorithm has been partitioned to execute on a dual parallel processor configuration: one based on the translational dynamics and the other on the rotational kinematics. The report presents an overview of the FINDS algorithm, the implemented equations, the flow charts for the key subprograms, the input and output files, program variable indexing convention, subprogram descriptions, and the common block descriptions used in the program.

Caglayan, A. K.↗
NASA NTRS2004

High-Temperature Probe Station Developed to Characterize Microwave Devices Through 500 C

A photograph and a block diagram of the high-temperature probe station are shown. The system consists of the ceramic heater mounted on a NASA shuttle tile insulator, a direct current power supply, a personal-computer-based data acquisition and temperature controller, microwave probes, a microscope, and a network analyzer. The ability to perform microwave tests at high temperatures is becoming necessary. There is now a need for sensors and communication circuits that can operate at 500 C and above for aircraft engine development and monitoring during flight. To address this need, researchers have fabricated devices using wide bandgap semiconductors such as SiC with targeted operating temperatures of 500 to 600 C. However, the microwave properties of these devices often change drastically with temperature, so any designs that are intended to be used in such an environment must be characterized at high temperatures. For some reliability, lifetime, and direct-current testing, the device under test can be packaged and characterized in an oven. However, for RF and microwave measurements, it is usually not possible to establish a calibrated reference plane at the device terminals within a package. In addition, the characteristics of the package would vary over a 500 C temperature range, and this would have to be accounted for when the data were analyzed. A high temperature probe station allows circuits and devices to be characterized through on wafer measurements across a broad temperature range with known reference plane. The conventional, commercially available thermal wafer-probe stations that are used to evaluate microwave devices across a controlled temperature range have a typical upper limit of 200 C. Standalone thermal heating chucks are available with an extended upper temperature range of 300 to 400 C. To effectively characterize devices at temperatures up to and surpassing 500 C, Glenn researchers developed a custom probe station. In the past, custom probe stations have been developed to test devices under other extreme environments, such as cryogenic temperatures as low as 37 K. Similarly, this custom probe station was specifically modified for high-temperature use. It allows devices to be measured quickly and flexibly, without the use of wire bonds and test fixtures. The probe station is shown making scattering parameter measurements from 1 to 50 GHz with a Hewlett-Packard 8510C Network Analyzer. There is a half-wafer of silicon directly on top of the heater to provide a uniform heated platform for our sample. A quarter wafer of silicon carbide forms the substrate for our test circuit of several transmission lines.

Downey, Alan N.↗
NASA NTRS1993

Micromechanical simulation of damage progression in carbon phenolic composites

Carbon/phenolic composites are used extensively as ablative insulating materials in the nozzle region of solid rocket motors. The current solid rocket motor (RSRM) on the space shuttle is fabricated from woven rayon cloth which is carbonized and then impregnated with the phenolic resin. These plies are layed up in the desired configuration and cured to form the finished part. During firing, the surface of the carbon/phenolic insulation is exposed to 5000 F gases from the rocket exhaust. The resin pyrolizes and the material chars to a depth which progresses with time. The rate of charring and erosion are generally predictable, and the insulation depth is designed to allow adequate safety margins over the firing time of the motor. However, anomalies in the properties and response of the carbon/phenolic materials can lead to severe material damage which may decrease safety margins to unacceptable levels. Three macro damage modes which were observed in fired nozzles are: ply lift, 'wedge out', and pocketing erosion. Ply lift occurs in materials with plies oriented nearly parallel to the surface. The damage occurs in a region below the charred material where material temperatures are relatively low - about 500 F. Wedge out occurs at the intersection of nozzle components whose plies are oriented at about 45 deg. The corner of the block of material breaks off along a ply interface. Pocketing erosion occurs in material with plies oriented normal to the surface. Thermal expansion is restrained in two directions resulting in large tensile strains and material failure normal to the surface. When a large section of material is removed as a result of damage, the insulation thickness is reduced which may lead to failure of the nozzle due to excessive heating of critical components. If these damage events cannot be prevented with certainty, the designer must increase the thickness of the insulator thus adding to both weight and cost. One of the difficulties in developing a full understanding of these macro damage mechanisms is that the loading environment and the material response to that environment are extremely complex. These types of damage are usually only observed in actual motor firings. Therefore, it is difficult and expensive to evaluate the reliability of new materials. Standard material tests which measure mechanical and thermal properties of test specimens can only provide a partial picture of how the material will respond in the service environment. The development of the ANALOG test procedure which can combine high heating rates and mechanical loads on a specimen will improve the understanding of the interactive effects of the various loads on the system. But a mechanistic model of material response which can account for the heterogeneity of the material, the progression of various micromechanical damage mechanisms, and the interaction of mechanical and thermal stresses on the material is required to accurately correlate material tests with response to service environments. A model based on fundamental damage mechanisms which is calibrated and verified under a variety of loading conditions will provide a general tool for predicting the response of rocket nozzles. The development of a micromechanical simulation technique was initiated and demonstrated to be effective for studying across-ply tensile failure of carbon/phenolic composites.

Slattery, Kerry T.↗
NASA NTRS1994

Unique passive diagnostic for slapper detonators

The objective of this study was to find a material and configuration that could reliably detect the proper functioning of a slapper (non-explosive) detonator. Because of the small size of the slapper geometry (on the order of a 15 mils), most diagnostic techniques are not suitable. This program has the additional requirements that the device would be used on centrifuge so that it could not use any electrical power or output signals. This required that the diagnostic be completely passive. The paper describes the three facets of the development effort: complete characterization of the slapper using VISAR measurements, selection of the diagnostic material and configuration, and testing of the prototype designs. The VISAR testing required that use of a special optical probe to allow the laser light to reach both bridges of the dual-slapper detonator. Results are given in the form of flyer velocity as a function of the initiating charge voltage level. The selected diagnostic design functions in a manner similar to a dent block except that the impact of the Kapton disk from a properly-functioning slapper causes a fracture pattern. A quick visual inspection is all that is needed to determine if the flyer velocity exceeded the threshold value. Sub-threshold velocities produce a substantially different appearance.

Brigham, William P.↗
NASA NTRS2023

Effect of Loading Rate on the Open Hole Compression Strength of Quasi-Isotropic Carbon/Epoxy Laminates

This study was undertaken to examine how well the 0.05 inch/min loading rate typically used to develop allowables for open hole compression strength of carbon/epoxy laminates represents structures that may experience a much lower loading rate (such as civil engineering structures) or a much higher loading rate (such as a rocket interstage at liftoff). The Composite Materials Handbook-17 (CMH-17) suggests developing strength allowables at the coupon level (lowest level of the “building block” approach) and using a rigorous statistical analysis to arrive at a strength allowable upon which the structure is to be further designed at higher levels of the building block pyramid. But do these coupon level tests truly represent meaningful stresses at which the composite in the final structure can withstand considering the effects of loading rate? Open hole compression (OHC) strength was chosen for this study since most structures are designed to some damage tolerance criteria and OHC strength testing is rather simple to perform. A quasi-isotropic lay-up was chosen since this is a very common lay-up sequence. The results show that for structures that experience a very small loading rate (0.0005 in/min) the allowables developed at the standard 0.05 in/min loading rate are higher than what the composite can withstand at this lower loading rate. Conversely, at a high loading rate of 10 in/min the strength values found at the standard test rate of 0.05 in/min are slightly lower and not “in family” using the statistics suggested in CMH-17. These results question if standardized test rates are reliable, at least to the fidelity of being “in family” per CMH-17. Some creep rupture tests were also performed to demonstrate that OHC strength values could be lower than the slowest test speed used in this study (0.0005 in/min).

open hole compression strength↗
NASA NTRS1999

A Single-Lap Joint Adhesive Bonding Optimization Method Using Gradient and Genetic Algorithms

A natural process for any engineer, scientist, educator, etc. is to seek the most efficient method for accomplishing a given task. In the case of structural design, an area that has a significant impact on the structural efficiency is joint design. Unless the structure is machined from a solid block of material, the individual components which compose the overall structure must be joined together. The method for joining a structure varies depending on the applied loads, material, assembly and disassembly requirements, service life, environment, etc. Using both metallic and fiber reinforced plastic materials limits the user to two methods or a combination of these methods for joining the components into one structure. The first is mechanical fastening and the second is adhesive bonding. Mechanical fastening is by far the most popular joining technique; however, in terms of structural efficiency, adhesive bonding provides a superior joint since the load is distributed uniformly across the joint. The purpose of this paper is to develop a method for optimizing single-lap joint adhesive bonded structures using both gradient and genetic algorithms and comparing the solution process for each method. The goal of the single-lap joint optimization is to find the most efficient structure that meets the imposed requirements while still remaining as lightweight, economical, and reliable as possible. For the single-lap joint, an optimum joint is determined by minimizing the weight of the overall joint based on constraints from adhesive strengths as well as empirically derived rules. The analytical solution of the sin-le-lap joint is determined using the classical Goland-Reissner technique for case 2 type adhesive joints. Joint weight minimization is achieved using a commercially available routine, Design Optimization Tool (DOT), for the gradient solution while an author developed method is used for the genetic algorithm solution. Results illustrate the critical design variables as a function of adhesive properties and convergences of different joints based on the two optimization methods.

Smeltzer, Stanley S., III↗
NASA NTRSDate not supplied

The System Complexity Metric (SCM) Predicts System Costs and Failure Rates

A complex system has many parts and interactions and so is difficult to understand. Systems with higher complexity generally have higher costs and failure rates. A System Complexity Metric (SCM) is defined to be the sum of the number of nodes, N, in the system block diagram plus the number of one-way interactions, I, between the nodes. SCM = N + I. SCMs are easily determined by direct inspection of high level block diagrams of life support systems. System cost was found to be directly proportional to SCM. The system MTBF (Mean Time Before Failure) is the inverse of the system failure rate. MTBF = 1/f. The system MTBF was found to be proportional to SCM^(-2.2) for estimated preflight MTBFs. As is typical for systems that are not extensively tested and redesigned to eliminate unexpected failure modes, the life support flight failure rates were about ten times higher than the preflight estimates and the MTBFs one-tenth the preflight estimates. The system MTBF was found to be proportional to SCM^(-2.6) for observed flight MTBFs.

System compleity↗