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At least 1,081 records · Page 60

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NASA NTRS1986

Adhesive joint evaluation by ultrasonic interface and lamb waves

Some results on the application of interface and Lamb waves for the study of curing of thin adhesive layers were summarized. In the case of thick substrates (thickness much more than the wave length) the interface waves can be used. In this case the experimental data can be inverted and the shear modulus of the adhesive film may be explicitly found based on the measured interface wave velocity. It is shown that interface waves can be used for the study of curing of structural adhesives as a function of different temperatures and other experimental conditions. The kinetics of curing was studied. In the case of thin substrates the wave phenomena are much more complicated. It is shown that for successful measurements proper selection of experimental conditions is very important. This can be done based on theoretical estimations. For correctly selected experimental conditions the Lamb waves may be a sensitive probe of adhesive bond quality and may be used or cure monitoring.

Rokhlin, S. I.↗

NASA NTRS1986

System impacts of solar dynamic and growth power systems on space station

Concepts for the 1990's space station envision an initial operational capability with electrical power output requirements of approximately 75 kW and growth power requirements in the range of 300 kW over a period of a few years. Photovoltaic and solar dynamic power generation techniques are contenders for supplying this power to the space station. A study was performed to identify growth power subsystem impacts on other space station subsystems. Subsystem interactions that might suggest early design changes for the space station were emphasized. Quantitative analyses of the effects of power subsystem mass and projected area on space station controllability and reboost requirements were conducted for a range of growth station configurations. Impacts on space station structural dynamics as a function of power subsystem growth were also considered.

Farmer, J. T.↗

NASA NTRS1986

Optical models of the molecular atmosphere

The use of optical and laser methods for performing atmospheric investigations has stimulated the development of the optical models of the atmosphere. The principles of constructing the optical models of molecular atmosphere for radiation with different spectral composition (wideband, narrowband, and monochromatic) are considered in the case of linear and nonlinear absorptions. The example of the development of a system which provides for the modeling of the processes of optical-wave energy transfer in the atmosphere is presented. Its physical foundations, structure, programming software, and functioning were considered.

Zuev, V. E.↗

NASA NTRS1986

Simulations of collisionless shocks

A problem of critical importance to space and astrophysics is the existence and properties of high-Mach-number shocks. In this letter the results of simulations of perpendicular shocks with Alfven Mach number 22 are presented. It is shown that the shock structure is a sensitive function of resistivity, becoming turbulent when the resistivity is too low. The problem of electron heating, and the extension of the results to higher Mach numbers are discussed.

Quest, K. B.↗

NASA NTRS1988

Testing expert systems

Software quality is of primary concern in all large-scale expert system development efforts. Building appropriate validation and test tools for ensuring software reliability of expert systems is therefore required. The Expert Systems Validation Associate (EVA) is a validation system under development at the Lockheed Artificial Intelligence Center. EVA provides a wide range of validation and test tools to check correctness, consistency, and completeness of an expert system. Testing a major function of EVA. It means executing an expert system with test cases with the intent of finding errors. In this paper, we describe many different types of testing such as function-based testing, structure-based testing, and data-based testing. We describe how appropriate test cases may be selected in order to perform good and thorough testing of an expert system.

Chang, C. L.↗

NASA NTRS1988

The determination of the propagation constant for the traveling wave in an infinite ground plane

The propagation constant for the traveling wave in a trough in an infinite ground plane is examined. The null-field integral is used to determine the EM field in the trough structure, and pulse basis functions give the distribution of the aperture fields. From this, the propagation constant is solved for, using the Newton-Raphson iterative scheme. Various sizes of geometries are examined. The far-field patterns are calculated and compared with other solutions, thereby validating the integral formulation which subsequently provided the propagation constant. Measurements of two trough geometries are performed to validate the theoretical results.

Shamansky, Harry T.↗

NASA NTRS1989

NASA'S controls-structures interaction program

A NASA program is about to start which has the objective to advance Controls-Structures Interaction (CSI) technology to a point where it can be used in spacecraft design for future missions. Because of the close interrelationships between the structure, the control hardware, and the analysis/design, a highly interdisciplinary activity is defined in which structures, dynamics, controls, computer and electronics engineers work together on a daily basis and are co-located to a large extent. Methods will be developed which allow the controls and structures analysis and design functions to use the same mathematical models. Hardware tests and applications are emphasized and will require development of concepts and test methods to carry out. Because of a variety of mission application problem classes, several time-phased, focus ground test articles are planned. They will be located at the Langley Researdh Center (LaRC), the Marshall Space Flight Center (MSFC) and at the Jet Propulsion Laboratory (JPL). It is anticipated that the ground tests will be subject to gravity and other environmental effects to the extent that orbital flights tests will be needed for verification of some technology items. The need for orbital flight experiments will be quantified based on ground test results and mission needs. Candidate on-orbit experiments will be defined and preliminary design/definition and cost studies will be carried out for one or more high-priority experiments.

Hanks, Brantley R.↗

NASA NTRS1991

A superconducting large-angle magnetic suspension

The component technologies were developed required for an advanced control moment gyro (CMG) type of slewing actuator for large payloads. The key component of the CMG is a large-angle magnetic suspension (LAMS). The LAMS combines the functions of the gimbal structure, torque motors, and rotor bearings of a CMG. The LAMS uses a single superconducting source coil and an array of cryoresistive control coils to produce a specific output torque more than an order of magnitude greater than conventional devices. The designed and tested LAMS system is based around an available superconducting solenoid, an array of twelve room-temperature normal control coils, and a multi-input, multi-output control system. The control laws were demonstrated for stabilizing and controlling the LAMS system.

Downer, James↗

NASA NTRS1989

Detection and avoidance of errors in computer software

The acceptance test errors of a computer software project to determine if the errors could be detected or avoided in earlier phases of development. GROAGSS (Gamma Ray Observatory Attitude Ground Support System) was selected as the software project to be examined. The development of the software followed the standard Flight Dynamics Software Development methods. GROAGSS was developed between August 1985 and April 1989. The project is approximately 250,000 lines of code of which approximately 43,000 lines are reused from previous projects. GROAGSS had a total of 1715 Change Report Forms (CRFs) submitted during the entire development and testing. These changes contained 936 errors. Of these 936 errors, 374 were found during the acceptance testing. These acceptance test errors were first categorized into methods of avoidance including: more clearly written requirements; detail review; code reading; structural unit testing; and functional system integration testing. The errors were later broken down in terms of effort to detect and correct, class of error, and probability that the prescribed detection method would be successful. These determinations were based on Software Engineering Laboratory (SEL) documents and interviews with the project programmers. A summary of the results of the categorizations is presented. The number of programming errors at the beginning of acceptance testing can be significantly reduced. The results of the existing development methodology are examined for ways of improvements. A basis is provided for the definition is a new development/testing paradigm. Monitoring of the new scheme will objectively determine its effectiveness on avoiding and detecting errors.

Kinsler, Les↗

NASA NTRS1991

An empirical comparison of software fault tolerance and fault elimination

Reliability is an important concern in the development of software for modern systems. Some researchers have hypothesized that particular fault-handling approaches or techniques are so effective that other approaches or techniques are superfluous. The authors have performed a study that compares two major approaches to the improvement of software, software fault elimination and software fault tolerance, by examination of the fault detection obtained by five techniques: run-time assertions, multi-version voting, functional testing augmented by structural testing, code reading by stepwise abstraction, and static data-flow analysis. This study has focused on characterizing the sets of faults detected by the techniques and on characterizing the relationships between these sets of faults. The results of the study show that none of the techniques studied is necessarily redundant to any combination of the others. Further results reveal strengths and weakness in the fault detection by the techniques studied and suggest directions for future research.

Shimeall, Timothy J.↗

NASA NTRS1991

Optical interferometer testbed

Viewgraphs on optical interferometer testbed presented at the MIT Space Research Engineering Center 3rd Annual Symposium are included. Topics covered include: space-based optical interferometer; optical metrology; sensors and actuators; real time control hardware; controlled structures technology (CST) design methodology; identification for MIMO control; FEM/ID correlation for the naked truss; disturbance modeling; disturbance source implementation; structure design: passive damping; low authority control; active isolation of lightweight mirrors on flexible structures; open loop transfer function of mirror; and global/high authority control.

Blackwood, Gary H.↗

NASA NTRS1993

New materials: Fountainhead for new technologies and new science

The role of materials as the benchmark technologies which give epochs of human history their names continues into the present. The discovery of new materials has nearly always been the source of new materials science, and frequently of new technologies. This paper analyzes the actual processes by which new materials are synthesized, i.e. whether driven by serendipitous observations, new knowledge is pulled by the market, or integrated into a technological thrust. This analysis focuses on modern ceramic materials discoveries, since World War 2 and uses 45 years experience in materials synthesis in the author's own laboratory as case studies. A dozen different families of materials or processes are involved: hydrothermal reactions; sol-gel processing; clays and zeolites; electroceramics; zero expansion ceramics; diamond films; and radioactive waste host phases. Nanocomposite concepts introduced by the author a decade ago offer an entire, large, new class of materials which will dominate synthesis for the next period. The future of materials research for the next 25 years cannot be extrapolated from the past 25 years. We are near the asymptote for materials utilization in most metals. Likewise we are approaching saturation in improvement of many useful properties. Justifying much further 'basic' R/D for incremental improvement in civilian-oriented industries will not be easy. In materials synthesis, the near-term future is sure to emphasize not new phases, but tailored micro- and nanocomposites for chemical, electrical, optical, and magnetic uses. Unexpected new discoveries such as the Lanxide process may offer rarer chances for step function advances. The new structure of knowledge management will rely less on local research than on integration of worldwide inputs. Better scientific and technological opportunities will lie in designing knowledge intensive materials to meet the new environmental and conservation goals, and the human needs of the very large numbers at the bottom of the socio-economic structures of the world.

Rustum, Roy↗

NASA NTRS1992

The stability functions in the bulk similarity formulation for the unstable boundary layer

An update is presented for the function C in the heat transfer equation and for the function Bw in the momentum transfer equation of the bulk similarity approach for the atmospheric boundary layer (ABL). Motives for this update are recent developments in the formulation of Monin-Obukhov functions for the surface layer, and the availability of the new data set of FIFE-89, the 1989 phase of the First ISLSCP Field Experiment, which took place over the same hilly prairie terrain in northeastern Kansas as the 1987 phase, i.e., FIFE-87. Functional forms developed in earlier studies are considered. In addition, a new form is derived based on a simple dual structure of the ABL. The functions are calibrated with the data set obtained during FIFE-87; the results are then verified with the independent data set acquired during FIFE-89.

Sugita, Michiaki↗

NASA NTRS1991

Robust controller design for structures with displacement sensors

The passivity theorem may be used to design robust controllers for structures with positive transfer functions. The authors obtain a similar result for systems with displacement sensors. In order to illustrate the ideas discussed, a specific second-order dynamic system is considered.

Morris, K. A.↗

NASA NTRS1992

Incorporation of spatial information in Bayesian image reconstruction - The maximum residual likelihood criterion

We have developed a new figure of merit, a 'maximum-residual-likelihood' (MRL) statistic, for the goodness of fit for Bayesian image restoration which explicitly incorporates spatial information. The MRL constraint provides a natural means of incorporating the prior knowledge that the residuals contain no spatial structure through the autocorrelation function of the residuals. We demonstrate that this statistic follows a Chi-square distribution and that forcing this statistic to have its most probable value leads to a restored image whose residuals are consistent with the noise model. Our numerical experiments suggest that image restoration using the MRL statistic alone is numerically robust and produces results which are independent of the initial guess for the restored image. However, we caution that using the MRL statistic without an image prior can result in overresolution in low SNR portions of the image.

Pina, R. K.↗

NASA NTRS1992

Low frequency astronomy from lunar orbit

As part of preparation for establishing a permanently manned lunar outpost, a plan is proposed whereby relatively simple phased arrays of dipoles will be placed on individual lunar satellites to perform interferometry between the spacecraft. The dipole arrays will be deployed as inflatable structures. The experiment will function at 13 and 26 MHz. For a scenario in which two spacecraft are in perpendicular lunar polar orbits at altitudes of 100 km, the useful physical baselines will range from about 1 to about 1500 km. With a mission lifetime of one year, the two-element interferometer will produce synthetic aperture imaging with a resolution of about 2 arcsec.

Basart, John P.↗

NASA NTRS1956

Compressive crippling of structural sections

A method is presented for calculating crippling stresses of structural sections as a function of material properties and the proportions of the section. The presence of formed or anisotropic material is accounted for by the use of an effective stress-strain curve. The method of analysis applies to many sections for which a procedure for calculating crippling was not previously available.

Anderson, Melvin S↗

NASA NTRS1988

The effects of area postrema lesions and selective vagotomy on motion-induced conditioned taste aversion

Conditioned taste aversion (CTA) is one of several behaviors which was suggested as a putative measure of motion sickness in rats. A review is made of studies which used surgical disruption of area postrema or the vagus nerve to investigate whether CTA and vomiting induced by motion may depend on common neural pathways or structures. When the chemoreceptive function of the area postrema (AP) is destroyed by complete ablation, rats develop CTA and cats and monkeys develop CTA and vomit. Thus the AP is not crucially involved in either CTA or vomiting induced by motion. However, after complete denervation of the stomach or after labyrinthectomy rats do not develop CTA when motion is used as the unconditioned stimulus. Studies of brainstem projections of the vagus nerve, the area postrema, the periaqueductal grey, and the vestibular system are used as the basis for speculation about regions which could mediate both motion-induced vomiting and behavioral food aversion.

conditioned taste aversion↗