Search NASA⌕ Search

SEARCH · Search NASA

Results for “Re-Evaluation”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Cometary atmospheres: Modeling the spatial distribution of observed neutral radicals

Progress on modeling the spatial distributions of cometary radicals is described. The Monte Carlo particle-trajectory model was generalized to include the full time dependencies of initial comet expansion velocities, nucleus vaporization rates, photochemical lifetimes and photon emission rates which enter the problem through the comet's changing heliocentric distance and velocity. The effect of multiple collisions in the transition zone from collisional coupling to true free flow were also included. Currently available observations of the spatial distributions of the neutral radicals, as well as the latest available photochemical data were re-evaluated. Preliminary exploratory model results testing the effects of various processes on observable spatial distributions are also discussed.

Combi, M. R.↗
NASA NTRS1985

Performance degradation of helicopter rotor in forward flight due to ice

This study addresses the analytical assessment of the degradation in the forward flight performance of the front rotor Boeing Vertol CH47D helicopter in a rime ice natural icing encounter. The front rotor disk was divided into 24 15-deg sections and the local Mach number and angle of attack were evaluated as a function of azimuthal and radial location for a specified flight condition. Profile drag increments were then calculated as a function of azimuthal and radial position for different times of exposure to icing, and the rotor performance was re-evaluated including these drag increments. The results of the analytical prediction method, such as horsepower required to maintain a specific flight condition, as a function of icing time have been generated. The method to illustrate the value of such an approach in assessing performance changes experienced by a helicopter rotor as a result of rime ice accretion is described.

Korkan, K. D.↗
NASA NTRS1985

Fundamental Studies of Droplet Combustion at Reduced Gravity

Reduced gravity is used to carry out scientific investigations of droplet combustion. In earlier work a preliminary conceptual design had been developed for droplet burning experiments for Spacelab. Refinements of the earlier work with special consideration given to possible experiments for Mid-Deck Modules of the Space Shuttle is discussed. A re-evaluation of suitable experiments on droplet combustion is done to ascertain whether influences of reduced buoyancy on time-dependent processes of heat and mass transfer in the gas or in the liquid on extinction processes or on disruptive burning phenomena are best suited for further investigation. Components of the experimental apparatus, which include a droplet dispensing system, a droplet positioning system, a droplet ignition system and provision for recording, primarily photographically, the combustion of the individual droplet in a chamber having a controlled gas atmosphere, are studied to determine optimal approaches to the experimental design. Methods for data reduction and interpretation are being made more specific, particularly in the context of an objective to calculate overall gas-phase chemical-kinetic parameters for the combustion from observations of extinction conditions in different atmospheres.

Williams, F. A.↗
NASA NTRS1985

An overview of integrated flywheel technology for aerospace application

Space missions ranging from small scientific satellites to large manned spacecraft have, for many years, utilized systems of spinning flywheels to maintain vehicle attitude. These systems have included momentum and reaction wheels as well as control moment gyros. Extension of that technology to satisfy the additional tasks associated with energy storage has also been pursued. The combining of control and energy storage features into one system has been examined by NASA for space applications and demonstrated in the laboratory. The impact of technology advances in such areas as composite material rotors, magnetic suspensions, motor/generators, and electronics have prompted a re-evaluation of the viability of the flywheel storage system concept for aerospace applications. This paper summarizes the results of this re-examination and identifies shortfalls in the various technology areas.

Keckler, C. R.↗
NASA NTRS1986

Characterization of elastomeric materials with application to design

Redesign of the Space Shuttle Solid Booster has necessitated re-evaluation of the material used in the field joint O-ring seals. The viscoelastic characteristics of five candidate materials were determined. The five materials are: two fluorocarbon compounds, two nitrile compounds, and a silicon compound. The materials were tested in a uniaxial compression test to determine the characteristic relaxation functions. These tests were performed at five different temperatures. A master material curve was developed for each material from the experimental data. The results are compared to tensile relaxation tests. Application of these results to the design analysis is discussed in detail.

Bower, Mark V.↗
NASA NTRS1986

Manned Mars mission radiation environment and radiobiology

Potential radiation hazards to crew members on manned Mars missions are discussed. It deals briefly with radiation sources and environments likely to be encountered during various phases of such missions, providing quantitative estimates of these environments. Also provided are quantitative data and discussions on the implications of such radiation on the human body. Various sorts of protective measures are suggested. Recent re-evaluation of allowable dose limits by the National Council of Radiation Protection is discussed, and potential implications from such activity are assessed.

Nachtwey, D. S.↗
NASA NTRS1987

On a criterion for vortex breakdown

A criterion for the onset of vortex breakdown is proposed. Based upon previous experimental, computational, and theoretical studies, an appropriately defined local Rossby number is used to delineate the region where breakdown occurs. In addition, new numerical results are presented which further validate this criterion. A number of previous theoretical studies concentrating on inviscid standing-wave analyses for trailing wing-tip vortices are reviewed and reinterpreted in terms of the Rossby number criterion. Consistent with previous studies, the physical basis for the onset of breakdown is identified as the ability of the flow to sustain such waves. Previous computational results are reviewed and re-evaluated in terms of the proposed breakdown criterion. As a result, the cause of breakdown occurring near the inflow computational boundary, common to several numerical studies, is identified. Finally, previous experimental studies of vortex breakdown for both leading edge and trailing wing-tip vortices are reviewed and quantified in terms of the Rossby number criterion.

Spall, R. E.↗
NASA NTRS1987

A criterion for vortex breakdown

A criterion for the onset of vortex breakdown is proposed. Based upon previous experimental, computational, and theoretical studies, an appropriately defined local Rossby number is used to delineate the region where breakdown occurs. In addition, new numerical results are presented which further validate this criterion. A number of previous theoretical studies concentrating on inviscid standing-wave analyses for trailing wing-tip vortices are reviewed and reinterpreted in terms of the Rossby number criterion. Consistent with previous studies, the physical basis for the onset of breakdown is identified as the ability of the flow to sustain such waves. Previous computational results are reviewed and re-evaluated in terms of the proposed breakdown criterion. As a result, the cause of breakdown occurring near the inflow computational boundary, common to several numerical studies, is identified. Finally, previous experimental studies of vortex breakdown for both leading edge and trailing wing-tip vortices are reviewed and quantified in terms of the Rossby number criterion.

Spall, R. E.↗
NASA NTRS1988

Galileo Earth-Venus trajectory correction maneuver design

As a result of delays in the Space Shuttle program and a re-evaluation of safety concerns which resulted in cancellation of the Shuttle Centaur Upper Stage program, the Galileo mission to Jupiter has undergone a substantial redesign effort. Use of the Inertial Upper Stage (IUS) for injection has necessitated a complicated Venus-Earth-Earth gravity assist trajectory in order to reach Jupiter. One flyby of Venus and two flybys of Earth are required to compensate for the lower injection energy available from the IUS. As the spacecraft was not originally designed for the environment within 1 AU of the Sun, modifications to the spacecraft and operating procedures have been developed. This paper describes some of these changes and how they have affected the design and implementation of trajectory correction maneuvers on the Earth-Venus leg of the mission. A strategy of biasing the IUS target at injection to control the statistical velocity distribution of a subsequent trajectory correction maneuver is discussed.

Wilson, M. G.↗
NASA NTRS1984

USL/DBMS NASA/PC R and D project system design standards

A set of system design standards intended to assure the completeness and quality of designs developed for PC research and development projects is established. The standards presented address the areas of problem definition, initial design plan, design specification, and re-evaluation.

Dominick, Wayne D.↗
NASA NTRS1988

Geological remote sensing signatures of terrestrial impact craters

Geological remote sensing techniques can be used to investigate structural, depositional, and shock metamorphic effects associated with hypervelocity impact structures, some of which may be linked to global Earth system catastrophies. Although detailed laboratory and field investigations are necessary to establish conclusive evidence of an impact origin for suspected crater landforms, the synoptic perspective provided by various remote sensing systems can often serve as a pathfinder to key deposits which can then be targetted for intensive field study. In addition, remote sensing imagery can be used as a tool in the search for impact and other catastrophic explosion landforms on the basis of localized disruption and anomaly patterns. In order to reconstruct original dimensions of large, complex impact features in isolated, inaccessible regions, remote sensing imagery can be used to make preliminary estimates in the absence of field geophysical surveys. The experienced gained from two decades of planetary remote sensing of impact craters on the terrestrial planets, as well as the techniques developed for recognizing stages of degradation and initial crater morphology, can now be applied to the problem of discovering and studying eroded impact landforms on Earth. Preliminary results of remote sensing analyses of a set of terrestrial impact features in various states of degradation, geologic settings, and for a broad range of diameters and hence energies of formation are summarized. The intention is to develop a database of remote sensing signatures for catastrophic impact landforms which can then be used in EOS-era global surveys as the basis for locating the possibly hundreds of missing impact structures. In addition, refinement of initial dimensions of extremely recent structures such as Zhamanshin and Bosumtwi is an important objective in order to permit re-evaluation of global Earth system responses associated with these types of events.

Garvin, J. B.↗
NASA NTRS1989

Recent results in the NASA research balloon program

The NASA Balloon Program has progressed from a total hiatus in the fall of 1985 to an unprecedented flight success rate in the fall of 1988. Using heavy-lift balloons being regularly supplied by two manufacturers, the program has provided a timely response for investigations of Supernova 1987A from Australia, low energy cosmic ray investigations from Canada during periods of near-solar-minimum, and routine domestic turnaround flights for a variety of investigations. Recent re-evaluation of balloon flight-safety have resulted in severe constraints on flights launched from the Palestine, Texas facility. The future program must rely heavily on the use of remote launch sites to meet the growing requirements for more frequent and longer duration flights being planned for the next 3 - 5 years.

Jones, W. Vernon↗
NASA NTRS1989

Viscoelastic properties of elastomeric materials for O-ring applications

Redesign of the Space Shuttle Solid Rocket Booster necessitated re-evaluation of the material used in the field joint O-ring seals. This research project was established to determine the viscoelastic characteristics of five candidate materials. The five materials are: two fluorocarbon compounds, two nitrile compounds, and a silicon compound. The materials were tested in a uniaxial compression test to determine the characteristic relaxation functions. These tests were performed at five different temperatures. A master material curve was developed for each material from the experimental data. The results of this study are compared to tensile relaxation tests. Application of these results to the design analysis is discussed in detail.

Bower, Mark V.↗
NASA NTRS1990

Evaluation of finite-element models and stress-intensity factors for surface cracks emanating from stress concentrations

A re-evaluation of the 3-D finite-element models and methods used to analyze surface crack at stress concentrations is presented. Previous finite-element models used by Raju and Newman for surface and corner cracks at holes were shown to have ill-shaped elements at the intersection of the hole and crack boundaries. Improved models, without these ill-shaped elements, were developed for a surface crack at a circular hole and at a semi-circular edge notch. Stress-intensity factors were calculated by both the nodal-force and virtual-crack-closure methods. Comparisons made between the previously developed stress-intensity factor equations and the results from the improved models agreed well except for configurations with large notch-radii-to-plate-thickness ratios. Stress-intensity factors for a semi-elliptical surface crack located at the center of a semi-circular edge notch in a plate subjected to remote tensile loadings were calculated using the improved models.

Tan, P. W.↗
NASA NTRS1991

Velocity measurements in a turbulent trailing vortex and their application to BWI noise prediction

The objectives were to observe the turbulence structure and spectral characteristics of the trailing vortex shed by a rectangular NACA 0012 wing over a range of conditions and to incorporate these observations into the blade-wake interaction (BWI) noise-prediction method of Glegg (1989). The following sections are presented: (1) measurements performed during the first year of this two year investigation; (2) presentation and discussion of a representative sample of the results; (3) implications for the BWI noise prediction method; and (4) re-evaluation of work planned for the second year.

Devenport, William J.↗
NASA NTRS1991

Mars geologic mapping program: Review and highlights

The Mars Geologic Mapping (MGM) Program was introduced by NASA in 1987 as a new initiative in the Planetary Geology and Geophysics (PGG) Program. The overall purpose of the program is to support research on topical science problems that address specific questions. Among the objectives of the project are: (1) to produce highly detailed geologic maps that will greatly increase the knowledge of the materials and processes that have contributed to the evolutionary history of Mars; (2) to define areas of special interest for possible future investigation by planned missions (Mars Observer, Mars Sample Return); and (3) to maintain the interest of the planetary community in the development of new concepts and the re-evaluation of Martian geology as new data in usable form become available. Some interesting highlights of the geologic mapping indicate that multiple flood episodes occurred at different times during the Hesperian Period in both Kasei and Maja Valles. Studies of small channels in the Memnonia, Mangala, and Tharsis regions show that fluvial events appear to have occurred during the Amazonian Period at equatorial latitudes. Flood waters occurred during the Amazonian Period at equatorial latitudes. Flood waters from Mangala Valles may have seeped into surficial materials with the subsequent development of numerous sapping channels and debris flows; this suggests that the ancient highland terrain consists of relatively unconsolidated materials. Multiple layers were observed for the first time in the ridged plains lava flows covering large areas of Lunae Planum; some wrinkle ridges in this area are associated with grabens and collapse volcanic units at Hadriaca and Tyrrhena Paterae indicates that the units may have been emplaced by gravity-driven pyroclastic flows. Unlike the north polar layered deposits, those in the south polar region show no angular unconformities or evidence of faulting and folding. Water ice in the south polar layered deposits may be protected from solar heating and sublimation by a weathering rind or lag deposit on the surface.

Scott, David H.↗
NASA NTRS1990

Hubble Space Telescope: Faint object spectrograph instrument handbook. Version 1.1

The Faint Object Spectrograph (FOS) has undergone substantial rework since the 1985 FOS Instrument Handbook was published, and we are now more knowledgeable regarding the spacecraft and instrument operations requirements and constraints. The formal system for observation specification has also evolved considerably, as the GTO programs were defined in detail. This supplement to the FOS Instrument Handbook addresses the important aspects of these changes, to facilitate proper selection and specification of FOS observing programs. Since the Handbook was published, the FOS red detector has been replaced twice, first with the best available spare in 1985 (which proved to have a poor, and steadily degrading red response), and later with a newly developed Digicon, which exhibits a high, stable efficiency and a dark-count rate less than half that of its predecessors. Also, the FOS optical train was realigned in 1987-88 to eliminate considerable beam-vignetting losses, and the collimators were both removed and recoated for greater reflectivity. Following the optics and detector rework, the FOS was carefully recalibrated (although only ambient measurements were possible, so the far-UV characteristics could not be re-evaluated directly). The resulting efficiency curves, including improved estimates of the telescope throughput, are shown. A number of changes in the observing-mode specifications and addition of several optional parameters resulted as the Proposal Instructions were honed during the last year. Target-brightness limitations, which have only recently been formulated carefully, are described. Although these restrictions are very conservative, it is imperative that the detector safety be guarded closely, especially during the initial stages of flight operations. Restrictions on the use of the internal calibration lamps and aperture-illumination sources (TA LEDs), also resulting from detector safety considerations, are outlined. Finally, many changes have been made to the instructions for target acquisition specification.

Ford, Holland C.↗