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At least 703 records · Page 39

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NASA NTRS1985

Structural error and identifiability

In most identification problems, the possibility of identifying a model from the observation of input and output is approached by first assuming that the model is an exact representation of the process. Since models are only approximations to a real process, this paper presents a concrete way of taking into account the difference in structure between the model and the process it represents. The identifiability problem is reformulated on the basis of equivalence and near-equivalence between model and process. Necessary and sufficient conditions under which the differences between the outputs of the process and a model remains within a desired bound in the presence of modeling error are given. Necessary and sufficient conditions for the identifiability of parameters are presented.

Hadaegh, F. Y.↗

NASA NTRS1985

Sensitivity analysis of low bulk resistivity solar cells

In an effort to identify critical parameters of single-junction solar cells and to determine their relative sensitivity, parametric analyses have been conducted with a computer code SPCOLAY. A receipt for high-efficiency cells emerges from the analyses that requires the primary ingredients of low bulk resistivity (0.1-0.2 ohm-cm), a bulk diffusion length as large as attainable (greater than 100 microns), and a small front surface recombination velocity (about 100 cm/sec). Other parameters appear to play less sensitive roles.

Lin, E. I. H.↗

NASA NTRS1986

The HRS GTO program to study the neutral hydrogen column density and D/H ratio in the local interstellar medium

Theoretical line profiles are presented that demonstrate why 100,000 spectral resolution and high S/N are needed to derive accurate column densities from spectral lines that lie close to the flat part of the curve of growth and may contain multiple velocity components. The aim of the Space Telescope high-resolution-spectrograph observing program is to obtain column densities in the hydrogen and deuterium Lyman alpha lines along a variety of lines of sight within and extending beyond the local cloudlet, in which the sun is located near an edge. The broad chromospheric Lyman alpha emission lines of late-type stars are used as background sources against which to measure the interstellar absorption features. Profiles of interstellar absorption features in Mg II and Fe II lines will be obtained to derive the broadening parameter and/or identify possible multiple velocity components in the lines of sight.

Linsky, J. L.↗

NASA NTRS1987

Fiber composite structural durability and damage tolerance: Simplified predictive methods

Simplified predictive methods and models (theory) to evaluate fiber/polymer-matrix composite material for determining structural durability and damage tolerance are presented and described. This theory includes equations for (1) fatigue and fracture of composites without and with defects, (2) impact resistance and residual strength after impact, (3) thermal fatigue, and (4) combined stress fatigue. Several examples are included to illustrate applications of the theory and to identify significant parameters and sensitivities. Comparisons with limited experimental data are made.

Chamis, Christos C.↗

NASA NTRS1986

A theory for the core flow of leading-edge vortices

Separation-induced leading-edge vortices can dominate the flow about slender wings at moderate to high angles of attack, often with favorable aerodynamic effects. However, at the high angles of attack which are desirable for takeoff and landing as well as subsonic-transonic maneuver the vortices can breakdown or burst in the vicinity of the aircraft causing many adverse effects; these include lift loss, pitchup, and buffet. The flow in the core of leading-edge vortices is generally affiliated with the vortex breakdown phenomenon. A theory is presented for the flow in the core of separation-induced, leading-edge vortices at practical Reynolds numbers. The theory is based on matching inner and outer representations of the vortex. The inner representation models continuously distributed vorticity and includes an asymptotic viscous subcore. The outer representation models concentrated spiral sheets of vorticity and is fully three dimensional. A parameter is identified which closely tracks the vortex breakdown stability boundary for delta, arrow, and diamond wings.

Luckring, James M.↗

NASA NTRS1988

Three parallel computation methods for structural vibration analysis

The Lanczos (1950), multisectioning, and subspace iteration sequential methods for vibration analysis presently used as bases for three parallel algorithms are noted, in the aftermath of three example problems, to maintain reasonable accuracy in the computation of vibration frequencies. Significant computation time reductions are obtained as the number of processors increases. An analysis is made of the performance of each method, in order to characterize relative strengths and weaknesses as well as to identify those parameters that most strongly affect computation efficiency.

Storaasli, Olaf↗

NASA NTRS1988

Protein crystal growth in low gravity

The solubility and growth of the protein canavalin, and the application of the schlieren technique to study fluid flow in protein crystal growth systems were investigated. These studies have resulted in the proposal of a model to describe protein crystal growth and the preliminary plans for a long-term space flight experiment. Canavalin, which may be crystallized from a basic solution by the addition of hydrogen (H+) ions, was shown to have normal solubility characteristics over the range of temperatures (5 to 25 C) and pH (5 to 7.5) studies. The solubility data combined with growth rate data gathered from the seeded growth of canavalin crystals indicated that the growth rate limiting step is a screw dislocation mechanism. A schlieren apparatus was constructed and flow patterns were observed in Rochelle salt (sodium potassium tartrate), lysozyme, and canavalin. The critical parameters were identified as the change in density with concentration (dp/dc) and the change in index of refraction with concentration (dn/dc). Some of these values were measured for the materials listed. The data for lyrozyme showed non-linearities in plots of optical properties and density vs. concentration. In conjunction with with W. A. Tiller, a model based on colloid stability theory was proposed to describe protein crystallization. The model was used to explain observations made by ourselves and others. The results of this research has lead to the development for a preliminary design for a long-term, low-g experiment. The proposed apparatus is univeral and capable of operation under microprocessor control.

Feigelson, Robert S.↗

NASA NTRS1988

A gamma ray observatory ground attitude error analysis study using the generalized calibration system

The Goddard Space Flight Center (GSFC) Flight Dynamics Division (FDD) will be responsible for performing ground attitude determination for Gamma Ray Observatory (GRO) support. The study reported in this paper provides the FDD and the GRO project with ground attitude determination error information and illustrates several uses of the Generalized Calibration System (GCS). GCS, an institutional software tool in the FDD, automates the computation of the expected attitude determination uncertainty that a spacecraft will encounter during its mission. The GRO project is particularly interested in the uncertainty in the attitude determination using Sun sensors and a magnetometer when both star trackers are inoperable. In order to examine the expected attitude errors for GRO, a systematic approach was developed including various parametric studies. The approach identifies pertinent parameters and combines them to form a matrix of test runs in GCS. This matrix formed the basis for this study.

Ketchum, E.↗

NASA NTRS1988

A study of kinematic models for forward calibration of manipulators

The objective of robot calibration is to identify all parameters in a robot model so that the model represents the true robot. Each model has its own set of parameters. The authors prove that regardless of the kinematic model chosen, there are a maximum number of parameters that must be determined, implying that model accuracy cannot be improved by adding parameters. In addition, the authors demonstrate how to model any manipulator so that a minimum size Jacobian is used, thus reducing the computation required for calibration.

Everett, Louis J.↗

NASA NTRS1989

Modeling surface temperature distributions in forest landscapes

A model of the frequency distributions of the spatial variability in surface temperature is presented. Surface temperature data are obtained from two daytime and two nighttime flights of the Thermal IR Multispectral Scanner (TIMS) over forest land in western Oregon in August, 1985. The temperature values are corrected for atmospheric attenuation and thermal radiation emission with the LOWTRAN-6 algorithm. The temperature distributions were modeled with a two-parameter beta probability density distribution and the fit of the model was evaluated by comparison with the TIMS data set. Use of the model's parameters to identify and classify surface types shows good discrimination among various surfaces for the daytime images, with less distinct discrimination for the nighttime images.

Holbo, H. R.↗

NASA NTRS1989

Fiber composite structural durability and damage tolerance - Simplified predictive methods

Chamis, Christos C.↗

NASA NTRS1989

The mean ozone profile and its temperature sensitivity in the upper stratosphere and lower mesosphere - An analysis of LIMS observations

Multiple simultaneous LIMS (Limb Infrared Monitor of the Stratosphere) observations of both O3 and temperature T in the upper stratosphere and lower mesosphere are analyzed to derive O3-T correlations for comparisons with model calculations. The zonally averaged O3 profile is compared with results from a simplified photochemical model that assumes O3 to be in a photochemical steady state. The model profile is systematically lower than the observed profile. Key parameters are identified in which changes can systematically increase the O3 profile in both the stratosphere and the mesosphere. The LIMS-derived values for the sensitivity of O3 to changes in T are compared with equilibrium model calculations which include the T-driven opacity feedback effect on photodissociation rate constants. The theoretical O3 response to temperature perturbations is investigated. It is shown how the O3-T sensitivity coefficient is affected by zonal and vertical advection terms as well as by the photochemical coupling between O3 and T.

Froidevaux, Lucien↗

NASA NTRS1989

Analysis of earth rotation solution from Starlette

Earth rotation parameter (ERP) solutions were derived from the Starlette orbit analysis during the Main MERIT Campaign, using a technique of a consider-covariance analysis to assess the effects of errors on the polar motion solutions. The polar motion solution was then improved through the simultaneous adjustment of some dynamical parameters representing identified dominant perturbing sources (such as the geopotential and ocean-tide coefficients) on the polar motion solutions. Finally, an improved ERP solution was derived using the gravity field model, PTCF1, described by Tapley et al. (1986). The accuracy of the Starlette ERP solution was assessed by a comparison with the LAGEOS-derived ERP solutions.

Schutz, B. E.↗

NASA NTRS1989

Heat sink effects on weld bead: VPPA process

An investigation into the heat sink effects due to weldment irregularities and fixtures used in the variable polarity plasma arc (VPPA) process was conducted. A basic two-dimensional model was created to represent the net heat sink effect of surplus material using Duhamel's theorem to superpose the effects of an infinite number of line heat sinks of variable strength. Parameters were identified that influence the importance of heat sink effects. A characteristic length, proportional to the thermal diffusivity of the weldment material divided by the weld torch travel rate, correlated with heat sinking observations. Four tests were performed on 2219-T87 aluminum plates to which blocks of excess material were mounted in order to demonstrate heat sink effects. Although the basic model overpredicted these effects, it correctly indicated the trends shown in the experimental study and is judged worth further refinement.

Steranka, Paul O., Jr.↗

NASA NTRS1990

Heat sink effects on weld bead: VPPA process

Steranka, Paul O., Jr.↗

NASA NTRS1988

Durability of thermal barrier coatings in a high heat flux environment

Thermal shock is a significant factor in the limited service life of Space Shuttle Main Engine (SSME) high pressure, fuel turbopump (HPFTP) turbine blades. Addition of advanced thermal barrier coatings (TBCs) to the blades could serve to dampen the thermal shock, thereby increasing the life of the blades. However, testing and use of TBCs to date is performed primarily under moderate heat flux conditions which are typical of aircraft turbines. Only limited testing was conducted that addresses high heat flux and severe thermal shock conditions. Therefore, it is not clear if TBCs can survive severe thermal shocks or provide adequate thermal shock protection to the HPFTP turbine blades. The purpose is to experimentally evaluate the potential durability and protective capability of a variety of advanced TBCs in a cyclic thermal shock environment. A secondary goal is to identify significant parameters affecting TBC life during high heat flux testing. Parameters investigated include top coat thickness, bond coat thickness, substrate type, and substrate geometry.

Brindley, William J.↗

NASA NTRS1989

Identification of rotor flapping equation of motion from flight measurements with the RSRA compound helicopter

The application of an integrated rotorcraft identification method to the linear modeling of rotor system dynamics is studied. Two approaches used to describe the rotor flapping parameters in the rotor state dynamic equations and the period coefficients of the blade flapping equation of motion are presented. In the first approach, the parameters are identified in the nonrotating reference frame; in the second, the blade equivalent damping and spring periodic coefficients as well as other periodic coefficients are identified in the rotating reference frame.

Wang, Ji C.↗

NASA NTRS1989

Introduction to the problem

Solid propellant rockets were used extensively in space missions ranging from large boosters to orbit-raising upper stages. The smaller motors find exclusive use in various earth-based applications. The advantage of the solids include simplicity, readiness, volumetric efficiency, and storability. Important recent progress in related fields (combustion, rheology, micro-instrumentation/diagnostics, and chaos theory) can be applied to solid rockets to derive maximum advantage and avoid waste. Main objectives of research in solid propellants include: to identify critical parameters, to establish specification rules, and to develop quantitative criteria.

Ramohalli, Kumar↗