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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Analytic theory for the selection of a two-dimensional needle crystal at arbitrary Peclet number

An accurate analytic theory is presented for the velocity selection of a two-dimensional needle crystal for arbitrary Peclet number for small values of the surface tension parameter. The velocity selection is caused by the effect of transcendentally small terms which are determined by analytic continuation to the complex plane and analysis of nonlinear equations. The work supports the general conclusion of previous small Peclet number analytical results of other investigators, though there are some discrepancies in details. It also addresses questions raised on the validity of selection theory owing to assumptions made on shape corrections at large distances from the tip.

Tanveer, S.↗
NASA NTRS1991

Guidance, navigation, and control subsystem equipment selection algorithm using expert system methods

Enhanced engineering tools can be obtained through the integration of expert system methodologies and existing design software. The application of these methodologies to the spacecraft design and cost model (SDCM) software provides an improved technique for the selection of hardware for unmanned spacecraft subsystem design. The knowledge engineering system (KES) expert system development tool was used to implement a smarter equipment section algorithm than that which is currently achievable through the use of a standard data base system. The guidance, navigation, and control subsystems of the SDCM software was chosen as the initial subsystem for implementation. The portions of the SDCM code which compute the selection criteria and constraints remain intact, and the expert system equipment selection algorithm is embedded within this existing code. The architecture of this new methodology is described and its implementation is reported. The project background and a brief overview of the expert system is described, and once the details of the design are characterized, an example of its implementation is demonstrated.

Allen, Cheryl L.↗
NASA NTRS1991

UBVRI photometry of stars in the fields of X-ray selected BL Lacertae objects

UBVRI photometry is presented of stars in the fields of 38 X-ray selected BL Lacertae objects, two X-ray selected AGNs, and one X-ray binary system (possibly a cataclysmic variable). These stars are suitable for use as photometric comparison stars for optical monitoring programs involving these X-ray selected objects. A total of 166 observations of 87 stars were made on seven photometric nights. At least two stars were observed in every field. UBVRI measurements of three stars were made in the fields of MS 0950.9 + 4929, MS 1552.1 + 2020, and H 2154 - 304. Four stars were measured in the field of H 1426 + 428. Finding charts for all of these objects are included.

Smith, Paul S.↗
NASA NTRS1990

Actuator selection for large space structures

The paper discusses the process of selecting the actuator locations and the determination of the required number of actuators for large space structures. The selection is based on the definitions of the degree of controllability, the independence of actuators, and the effectiveness of the individual actuators. An algorithm is developed that can be used for the selection of the essential number of actuators and for finding some defects of the system, such as the insuffiency of the available actuator locations for effective control of the whole system or a too crowded frequency distribution. The efficiency of the algorithm was demonstrated by an application to the Space Station.

Reddy, A. S. S. R.↗
NASA NTRS1992

Selective Emitters

This invention relates to a small particle selective emitter for converting thermal energy into narrow band radiation with high efficiency. The small particle selective emitter is used in combination with a photovoltaic array to provide a thermal to electrical energy conversion device. An energy conversion apparatus of this type is called a thermo-photovoltaic device. In the first embodiment, small diameter particles of a rare earth oxide are suspended in an inert gas enclosed between concentric cylinders. The rare earth oxides are used because they have the desired property of large emittance in a narrow wavelength band and small emittance outside the band. However, it should be emphasized that it is the smallness of the particles that enhances the radiation property. The small particle selective emitter is surrounded by a photovoltaic array. In an alternate embodiment, the small particle gas mixture is circulated through a thermal energy source. This thermal energy source can be a nuclear reactor, solar receiver, or combustor of a fossil fuel.

Chubb, Donald L.↗
NASA NTRS1992

Screening programme to select a resin for Gravity Probe-B composites

The Gravity Probe-B (GP-B) program undertook a screening program to select a possible replacement resin for the E-787 resin currently used in composite neck tubes and support struts. The goal was to find a resin with good cryogenic and structural properties, low-helium permeation and an easily repeatable fabrication process. Cycom 92, SCI REZ 081 and RS-3 were selected for comparison with E-787. Identical composite tubes made from each resin and gamma-alumina fiber (85 percent Al2O3, 15 percent SiO2) were evaluated for cryogenic and structural performance and for processability. Cryogenic performance was evaluated by measuring low-temperature permeation and leaks to determine cryogenic strain behavior. Structural performance was evaluated by comparing the resin-dominated shear strength of the composites. Processability was evaluated from fabrication comments and GP-B's own experience. SCI REZ 081 was selected as the best overall resin with superior strength and cryogenic performance and consistent processability.

Will, E. T.↗
NASA NTRS1992

Spacecraft flight control system design selection process for a geostationary communication satellite

The Earth's first artificial satellite, Sputnik 1, slowly tumbled in orbit. The first U.S. satellite, Explorer 1, also tumbled out of control. Now, as we launch the Mars observer and the Cassini spacecraft, stability and control have become higher priorities. The flight control system design selection process is reviewed using as an example a geostationary communication satellite which is to have a life expectancy of 10 to 14 years. Disturbance torques including aerodynamic, magnetic, gravity gradient, solar, micrometeorite, debris, collision, and internal torques are assessed to quantify the disturbance environment so that the required compensating torque can be determined. Then control torque options, including passive versus active, momentum control, bias momentum, spin stabilization, dual spin, gravity gradient, magnetic, reaction wheels, control moment gyros, nutation dampers, inertia augmentation techniques, three-axis control, reactions control system (RCS), and RCS sizing, are considered. A flight control system design is then selected and preliminary stability criteria are met by the control gains selection.

Barret, C.↗
NASA NTRS1992

Lunar Observer Laser Altimeter observations for lunar base site selection

One of the critical datasets for optimal selection of future lunar landing sites is local- to regional-scale topography. Lunar base site selection will require such data for both engineering and scientific operations purposes. The Lunar Geoscience Orbiter or Lunar Observer is the ideal precursory science mission from which to obtain this required information. We suggest that a simple laser altimeter instrument could be employed to measure local-scale slopes, heights, and depths of lunar surface features important to lunar base planning and design. For this reason, we have designed and are currently constructing a breadboard of a Lunar Observer Laser Altimeter (LOLA) instrument capable of acquiring contiguous-footprint topographic profiles with both 30-m and 300-m along-track resolution. This instrument meets all the severe weight, power, size, and data rate limitations imposed by Observer-class spacecraft. In addition, LOLA would be capable of measuring the within-footprint vertical roughness of the lunar surface, and the 1.06-micron relative surface reflectivity at normal incidence. We have used airborne laser altimeter data for a few representative lunar analog landforms to simulate and analyze LOLA performance in a 100-km lunar orbit. We demonstrate that this system in its highest resolution mode (30-m diameter footprints) would quantify the topography of all but the very smallest lunar landforms. At its global mapping resolution (300-m diameter footprints), LOLA would establish the topographic context for lunar landing site selection by providing the basis for constructing a 1-2 km spatial resolution global, geodetic topographic grid that would contain a high density of observations (e.g., approximately 1000 observations per each 1 deg by 1 deg cell at the lunar equator). The high spatial and vertical resolution measurements made with a LOLA-class instrument on a precursory Lunar Observer would be highly synergistic with high-resolution imaging datasets, and will allow for direct quantification of critical slopes, heights, and depths of features visible in images of potential lunar base sites.

Garvin, James B.↗
NASA NTRS1993

Selection of experimental modal data sets for damage detection via model update

When using a finite element model update algorithm for detecting damage in structures, it is important that the experimental modal data sets used in the update be selected in a coherent manner. In the case of a structure with extremely localized modal behavior, it is necessary to use both low and high frequency modes, but many of the modes in between may be excluded. In this paper, we examine two different mode selection strategies based on modal strain energy, and compare their success to the choice of an equal number of modes based merely on lowest frequency. Additionally, some parameters are introduced to enable a quantitative assessment of the success of our damage detection algorithm when using the various set selection criteria.

Doebling, S. W.↗
NASA NTRS1989

Optimal selection of space transportation fleet to meet multi-mission space program needs

A space program that spans several decades will be comprised of a collection of missions such as low earth orbital space station, a polar platform, geosynchronous space station, lunar base, Mars astronaut mission, and Mars base. The optimal selection of a fleet of several recoverable and expendable launch vehicles, upper stages, and interplanetary spacecraft necessary to logistically establish and support these space missions can be examined by means of a linear integer programming optimization model. Such a selection must be made because the economies of scale which comes from producing large quantities of a few standard vehicle types, rather than many, will be needed to provide learning curve effects to reduce the overall cost of space transportation if these future missions are to be affordable. Optimization model inputs come from data and from vehicle designs. Each launch vehicle currently in existence has a launch history, giving rise to statistical estimates of launch reliability. For future, not-yet-developed launch vehicles, theoretical reliabilities corresponding to the maturity of the launch vehicles' technology and the degree of design redundancy must be estimated. Also, each such launch vehicle has a certain historical or estimated development cost, tooling cost, and a variable cost. The cost of a launch used in this paper includes the variable cost plus an amortized portion of the fixed and development costs. The integer linear programming model will have several constraint equations based on assumptions of mission mass requirements, volume requirements, and number of astronauts needed. The model will minimize launch vehicle logistic support cost and will select the most desirable launch vehicle fleet.

Morgenthaler, George W.↗
NASA NTRS1993

Design and pilot evaluation of the RAH-66 Comanche selectable control modes

The RAH-66 Comanche helicopter has been designed to possess superior handling qualities over a wide range of flight conditions. The control laws have been tailored to satisfy the requirements of ADS-33C and the Weapon System Specification (WSS). This paper addresses the design of the Comanche Selectable Mode control laws (Velocity Stabilization/Hover Hold and Altitude Hold), which provide the additional stabilization and control augmentation needed when flying in a Degraded Visual Environment (DVE). An overview of the RAH-66 control laws is presented, including a detailed description of the Selectable Modes design. The primary focus of this paper is the results of piloted evaluation of these control laws in the Boeing motionbase simulator. These tests substantiate the detailed design of the Comanche Selectable Mode control laws. All tested DVE tasks (ADS-33C, sections 4.4 and 4.5) were rated Level 1. Other evaluation tasks confirmed the mission suitability of the control system. These control laws are ready for formal ADS-33C compliance testing in the Sikorsky Full Mission Simulator (FMS).

Phillip J Gold↗
NASA NTRS1994

Selection process for trade study: Reusable Hydrogen Composite Tank System (RHCTS)

This document describes the selection process that will be used to identify the most suitable structural configuration option for an SSTO winged vehicle capable of delivering 25,000 lbs to a 220 nm circular orbit at 51.6 degree inclination. The most suitable RHCTS is within this configuration and will be the prototype design for subsequent design and analysis and the basis for the design and fabrication of a scale test article to be subjected to life cycle testing. The selection process for this TA 1 trade study is the same as that for the TA 2 trade study. As the trade study progresses additional insight may result in modifications to the selection criteria within in this process. Such modifications will result in an update of this document as appropriate.

Greenberg, H. S.↗
NASA NTRS1992

Validation of astronaut psychological select-in criteria

An optional astronaut selection strategy would select-in individuals on the basis of personality attributes associated with superior performance. Method: A test battery, the Astronaut Personal Characteristics Inventory (ASTROPCI) was developed which assesses positive and negative components of achievement, motivation, and interpersonal orientations and skills. The battery was administered to one hundred three astronaut candidates and sixty-six current U.S. Shuttle astronauts. To determine performance, a series of conceptual areas related to space flight performance were defined. Astronauts rated their peers on each of these dimensions. Ratings were obtained on all eighty-four current astronauts (excluding those selected in 1990). In addition to peer ratings, supervisor assessments of the same dimensions were obtained for each astronaut. Results: Cluster and factor analysis techniques were employed to isolate subgroups of astronauts. Those astronauts with both high achievement needs and interpersonal skills were most often rated among the top five by their peers and least often rated among the lowest five. A number of scales discriminated between astronauts rated high and low on one or more performance dimensions. Conclusions: The results parallel findings from the personality assessment of individuals in other demanding professions, including aircraft pilots and research scientists, suggesting that personality factors are significant determinants to performance in the space environment.

Rose, R. M.↗
NASA NTRS1994

On the development of an expert system for wheelchair selection

The presentation of wheelchairs for the Multiple Sclerosis (MS) patients involves the examination of a number of complicated factors including ambulation status, length of diagnosis, and funding sources, to name a few. Consequently, only a few experts exist in this area. To aid medical therapists with the wheelchair selection decision, a prototype medical expert system (ES) was developed. This paper describes and discusses the steps of designing and developing the system, the experiences of the authors, and the lessons learned from working on this project. Wheelchair Advisor, programmed in CLIPS, serves as diagnosis, classification, prescription, and training tool in the MS field. Interviews, insurance letters, forms, and prototyping were used to gain knowledge regarding the wheelchair selection problem. Among the lessons learned are that evolutionary prototyping is superior to the conventional system development life-cycle (SDLC), the wheelchair selection is a good candidate for ES applications, and that ES can be applied to other similar medical subdomains.

Madey, Gregory R.↗
NASA NTRS1994

A CLIPS-based expert system for the evaluation and selection of robots

This paper describes the development of a prototype expert system for intelligent selection of robots for manufacturing operations. The paper first develops a comprehensive, three-stage process to model the robot selection problem. The decisions involved in this model easily lend themselves to an expert system application. A rule-based system, based on the selection model, is developed using the CLIPS expert system shell. Data about actual robots is used to test the performance of the prototype system. Further extensions to the rule-based system for data handling and interfacing capabilities are suggested.

Nour, Mohamed A.↗
NASA NTRS1994

Rare earth garnet selective emitter

Thin film Ho-YAG and Er-YAG emitters with a platinum substrate exhibit high spectral emittance in the emission band (epsilon(sub lambda) approximately equal to 0.74, ((4)l(sub 15/2)) - ( (4)l(sub13/2)), for Er-YAG and epsilon(sub lambda) approximately equal to 0.65, ((5)l(sub 7))-((5)l(sub 8)) for Ho-YAG) at excellent candidates for high efficiency selective emitters in the thermophotovoltaics (TPV) systems operating at moderate temperatures (1200-1500K). Spectral emittance measurements of the thin films were made (1.2 less than lambda less than 3.0 microns) and compared to the theoretical emittances calculated using measured values of the spectral extinction coefficient. In this paper we present the results for a new class of rare earth ion selective emitters. These emitters are thin sections (less than 1 mm) of yttrium aluminum garnet (YAG) single crystal with a rare earth substitutional impurity. This paper presents normal spectral emittance, epsilon(sub lambda), measurements of holmium (Ho), and erbium (Er) doped YAG thin film selective emitters at 1500 K, and compares those results with the theoretical spectral emittance.

Lowe, Roland A.↗
NASA NTRS1993

Optimal band selection for dimensionality reduction of hyperspectral imagery

Hyperspectral images have many bands requiring significant computational power for machine interpretation. During image pre-processing, regions of interest that warrant full examination need to be identified quickly. One technique for speeding up the processing is to use only a small subset of bands to determine the 'interesting' regions. The problem addressed here is how to determine the fewest bands required to achieve a specified performance goal for pixel classification. The band selection problem has been addressed previously Chen et al., Ghassemian et al., Henderson et al., and Kim et al.. Some popular techniques for reducing the dimensionality of a feature space, such as principal components analysis, reduce dimensionality by computing new features that are linear combinations of the original features. However, such approaches require measuring and processing all the available bands before the dimensionality is reduced. Our approach, adapted from previous multidimensional signal analysis research, is simpler and achieves dimensionality reduction by selecting bands. Feature selection algorithms are used to determine which combination of bands has the lowest probability of pixel misclassification. Two elements required by this approach are a choice of objective function and a choice of search strategy.

Stearns, Stephen D.↗
NASA NTRS1994

The optical polarization properties of X-ray-selected BL Lacertae objects

We discuss the optical polarization properties of X-ray-selected BL Lacertae objects (XSBLs) as determined from 3 yr of monitoring the polarization of 37 BL Lac objects and candidates. The observed objects include a complete X-ray flux limited sample drawn from the Einstein Extended Medium Sensitivity Survey (EMSS). We find that the majority of the XSBLs classified solely on the appearance of their optical spectra are true members of the class of BL Lacertae objects since they possess intrinsically polarized and variable continua. The duty cycle of polarized emission (fraction of time spent with the degree of polarization greater than 4%) from XSBLs is 44%. The maximum observed percent polarizations are of order 10%, considerably below the maximum values observed for radio-selected BL Lacertae objects and blazars (30%-40%). While XSBLs have variable polarized emission, the majority (approximately equal to 85%) have preferred polarization position angles on timescales at least as long as 3 yr. This reflects stability in the geometry of the region emitting the linearly polarized optical emission. We describe the spectral dependence of the degree of polarization and discuss some of the possible mechanisms producing the observed characteristics. While dilution of the polarized emission by the host galaxy starlight is certainly present in some objects, we demonstrate that the average polarization properties of XSBLs derived from our observations are not drastically affected by this effect. While the confirmed BL Lac objects are shown to be photometric variables, the objects in our monitored sample did not display the larger than 1 mag variations generally used to characterize the optical variability of radio-selected BL Lacertae objects or blazars in general.

Jannuzi, Buell T.↗