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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 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Microgravity Environment on the International Space Station

A primary feature of the International Space Station will be its microgravity environment--an environment in which the effects of gravity are drastically reduced. The International Space Station design has been driven by a long-standing, high-level requirement for a microgravity mode of operation. Various types of data are gathered when science experiments are conducted. The acceleration levels experienced during experiment operation should be factored into the analysis of the results of most microgravity experiments. To this end, the Space Acceleration Measurement System records the acceleration levels to support microgravity researchers for nearly three years of International Space Station operations. The Principal Investigator Microgravity Services project assists the experiments principal investigators with their analysis of the acceleration (microgravity) environment. The Principal Investigator Microgravity Services project provides cataloged data, periodic analysis summary reports, specialized reports for experiment teams, and real-time data in a variety of user-defined formats. Characterization of the various microgravity carriers (e.g., Shuttle and International Space Station) is also accomplished for the experiment teams. Presented in this paper will be a short description of how microgravity disturbances may affect some experiment classes, a snapshot of the microgravity environment, and a view into how well the space station is expected to meet the user requirements.

DeLombard, Richard↗
NASA NTRS1982

Ion beamlet vectoring by grid translation

Ion beamlet vectoring is investigated by collecting deflection and divergence angle data for two-grid systems as a function of the relative displacement of these grids. Results show that at large displacements, accelerator grid impingement becomes a limiting factor and this determines the useful range of beamlet deflection. Beamlet deflection was shown to vary linearly with grid offset angle over this range. Values of deflection-to-offset angle ratio and useful range of deflection are presented as functions of grid-hole geometries, perveance levels, and accelerating voltages. It is found that the divergence of the beamlets is unaffected by deflection over the useful range of beamlet deflection. The grids of a typical dished-grid ion thruster are examined to determine where over the grid surface the grid offsets exceed the useful range, which indicates the regions on the surface where high accelerator grid impingment is probably occurring.

Homa, J. M.↗
NASA NTRS1983

Parallel solution of finite element equations

The paper examines several parallel processing solution algorithms for finite element equations arising in linear equilibrium problems. Two basic groups of algorithms, direct and iterative, are investigated with respect to a number of parallel computer architectures and associated selection criteria. The direct algorithms include: LR-Gauss, Crout, Cholesky, Cyclic Reduction and WZ-factorization. The iterative methods examined are: Accelerated Gauss-Seidel, Surrogate Stiffness, Jacobi, Series Expansion, and Energy Monte Carlo. For real-time applications, where the object is to minimize the execution time, Cyclic Reduction appears to be best suited. This assumes a computer with an unlimited number of parallel processors. However, for computers with a limited number of parallel processors that must be used efficiently, both Gauss factorization and Jacobi-like iterative methods rank favorably.

Salama, M.↗
NASA NTRS1979

Accelerated stress testing of terrestrial solar cells

A program to investigate the reliability characteristics of unencapsulated low-cost terrestrial solar cells using accelerated stress testing is described. Reliability (or parametric degradation) factors appropriate to the cell technologies and use conditions were studied and a series of accelerated stress tests was synthesized. An electrical measurement procedure and a data analysis and management system was derived, and stress test fixturing and material flow procedures were set up after consideration was given to the number of cells to be stress tested and measured and the nature of the information to be obtained from the process. Selected results and conclusions are presented.

Prince, J. L.↗
NASA NTRS2005

Comprehensive Validation of an Intermittency Transport Model for Transitional Low-Pressure Turbine Flows

A transport equation for the intermittency factor is employed to predict transitional flows under the effects of pressure gradients, freestream turbulence intensities, Reynolds number variations, flow separation and reattachment. and unsteady wake-blade interactions representing diverse operating conditions encountered in low-pressure turbines. The intermittent behaviour of the transitional flows is taken into account and incorporated into computations by modifying the eddy viscosity, Mu(sub t), with the intermittency factor, gamma. Turbulent quantities are predicted by using Menter's two-equation turbulence model (SST). The onset location of transition is obtained from correlations based on boundary-layer momentum thickness, acceleration parameter, and turbulence intensity. The intermittency factor is obtained from a transport model which can produce both the experimentally observed streamwise variation of intermittency and a realistic profile in the cross stream direction. The intermittency transport model is tested and validated against several well documented low pressure turbine experiments ranging from flat plate cases to unsteady wake-blade interaction experiments. Overall, good agreement between the experimental data and computational results is obtained illustrating the predicting capabilities of the model and the current intermittency transport modelling approach for transitional flow simulations.

Suzen, Y. B.↗
NASA NTRS1973

Estimates of trapped radiation encountered on low-thrust trajectories through the Van Allen belts

Estimates were made of the number of trapped protons and electrons encountered by vehicles on low-thrust trajectories through the Van Allen belts. The estimates serve as a first step in assessing whether these radiations present a problem to on-board sensitive components and payload. The integrated proton spectra and electron spectra are presented for the case of a trajectory described by a vehicle with a constant-thrust acceleration A sub c equal to 0.001 meter/sq sec. This value of acceleration corresponds to a trip time of about 54 days from low earth orbit to synchronous orbit. It is shown that the time spent in the belts and hence the radiation encountered vary nearly inversely with the value of thrust acceleration. Thus, the integrated spectral values presented for the case of A sub c = 0.001 meter/sq sec can be generalized for any other value of thrust acceleration by multiplying them by the factor 0.001/A sub c.

Karp, I. M.↗
NASA NTRS1973

Nonlinear analysis of aortic flow in living dogs.

A nonlinear theory which considered the convective accelerations of blood and the nonlinear elastic behavior and taper angle of the vascular wall was used to study the nature of blood flow in the descending thoracic aorta of living dogs under a wide range of pressures and flows. Velocity profiles, wall friction, and discharge waves were predicted from locally measured input data about the pressure-gradient wave and arterial distention. The results indicated that a major part of the mean pressure gradient was balanced by convective accelerations; the theory, which took this factor into account, predicted the correct velocity distributions and flow waves.

Ling, S. C.↗
NASA NTRS1974

Automatic classification of eutrophication of inland lakes from spacecraft data

The author has identified the following significant results. Spacecraft data and computer techniques can be used to rapidly map and store onto digital tapes watershed land use information. Software is now available by which this land use information can be rapidly and economically extracted from the tapes and related to coliform counts and other lake contaminants (e.g. phosphorus). These tools are basic elements for determining those land use factors and sources of nutrients that accelerate eutrophication in lakes and reservoirs.

Rogers, R. H.↗
NASA NTRS1974

Selection of appropriate test method for simulating steady-state spacecraft launch loads

In ground testing prior to launch the static loads portion of the launch profile may be simulated by either applying static loads directly to the spacecraft through an appropriate loading facility or by subjecting the spacecraft to acceleration loads on a centrifuge. The factors to be considered in selecting one of the two simulation methods are discussed, giving attention to the relative advantages of the two methods in each case.

Shockey, E. F.↗
NASA NTRS1976

A technique for measurement of material damping in metals

The paper outlines the theory, design, and application of an apparatus based on the single beam resonant dwell technique to determine the damping capacity of metallic materials by measuring the response of a structural element to excitation at a modal frequency. In this apparatus, a cantilever beam specimen of a test material is clamped to a bar which is connected at one end to an electromagnetic shaker and at the other to a heavy base. The thickness of the bar at the base end is reduced by two saw cuts to provide a pivot around which the remainder of the bar can rotate when excited by the shaker which is connected to the bar by a rod passing through a hole in the base. The response of the supporting system to shaker excitation is measured with an accelerometer mounted on the bar at the root of the specimen. Specimen response is measured optically with a low-power microscope with a reticle. Specimen loss factor is determined in terms of acceleration at the beam root, beam tip displacement, and the beam natural frequency.

Heine, J. C.↗
NASA NTRS1984

A study of environmental characterization of conventional and advanced aluminum alloys for selection and design. Phase 1: Literature review

A review of the literature is presented with the objectives of identifying relationships between various accelerated stress corrosion testing techniques, and for determining the combination of test methods best suited to selection and design of high strength aluminum alloys. The following areas are reviewed: status of stress-corrosion test standards, the influence of mechanical and environmental factors on stress corrosion testing, correlation of accelerated test data with in-service experience, and procedures used to avoid stress corrosion problems in service. Promising areas for further work are identified.

Sprowls, D. O.↗
NASA NTRS1986

Predicted and measured strain response of rectangular panels due to acoustic loading

The objective of the research described in this paper is to assess the accuracy of classical linear acceleration/strain prediction theory for flat, rectangular panels excited by low level, acoustic plane waves. Predictions for both aluminum and laminated composite panels were based on the Ritz method using an orthotropic, multi-mode analysis with rotational springs on the boundaries. The measured natural frequencies and critical damping ratio of the panels were empirial inputs to the model. The comparison between measured and predicted acceleration was very good. Although a factor of three bias error was found between measured and predicted strains, the model was good for predicting the trends in the spatial variation in strain across the panel.

Roussos, L. A.↗
NASA NTRS1990

Turbulence measurements and noise generation in a transonic cryogenic wind tunnel

A high-frequency combination probe was used to measure dynamic flow quality in the test section of the NASA Langley 0.3-m Transonic Cryogenic Tunnel. The probe measures fluctuating stagnation (total) temperature and pressure, static pressure, and flow angles in two orthogonal planes. Simultaneous measurements of unsteady total temperature and pressure were also made in the settling chamber of the tunnel. The data show that the stagnation temperature fluctuations remain constant, and the stagnation pressure fluctuations increase by a factor of two, as the flow accelerates from the settling chamber to the test section. In the test section, the maximum rms value of the normalized fluctuating velocity is 0.7 percent. Correlation coefficients failed to show vorticity, entropy, or sound as the dominant mode of turbulence in the tunnel.

Ng, W. F.↗
NASA NTRS1991

Transient particle acceleration in strongly magnetized neutron stars

Although several reasonable physical processes have been proposed to account for the initial energy release in transient events on strongly magnetized neutron stars, and although the manifestation of this activity as a burst of X- or gamma-radiation evidently results from the Compton upscattering of soft photons by relativistic electrons, a self-consistent picture for the particle acceleration itself is lacking. Here, a detailed model is developed in which sheared Alfven waves generated by the energized stellar crust (e.g., due to a starquake) accelerate the changes to large Lorentz factors. For canonical stellar parameters and a burst energy of about 10 to the 37th ergs, the maximum attainable Lorentz factor is gamma(max) 50m, where m (of about 2-3/cm) characterizes the shear of the magnetic perturbation. This value of gamma(max) is consistent with the particle energy required to produce the most energetic gamma-rays (of about 10-100 MeV) observed in gamma-ray bursts.

Melia, Fulvio↗
NASA NTRS1995

Noncoeval star formation, starbursts, and the growth of supershells in Ob associations

There is strong evidence that the epoch of massive-star formation in OB associations is spread over intervals of 10-20 Myr as a result of subclustering, continuous or sequential star formation, or delayed collapse of the most massive stars. The growth of supershells is strongly influenced by this star formation history and by mass deposition to the hot bubble interior ('radiative poisoning'). Even in cases of coeval or continuous star formation, the rising energy input from massive stars, as they evlolve off the main sequence, may be observed as a 'starburst' with peak energy deposition 3-10 times that of the late-time rate due to supernovae and an excess of Wolf-Rayet stars (WR/O ratio approximately equal 0.1). The peak in energy deposition results in faster shell growth and velocity evolution than in constant-luminosity models. At finite bubble ages, this mechanism can alter the covering factor of superbubbles in the disk by factors of 2-3 and can accelerate the dynamical effects of galactic 'superwinds.' In contrast, if sufficient matter is entrained in the hot bubble, radiative poisoning will stall its growth.

Shull, J. Michael↗
NASA NTRS1965

The Effect of Yaw Coupling in Turning Maneuvers of Large Transport Aircraft

A study has been made, using a piloted moving simulator, of the effects of the yaw-coupling parameters N(sub p) and N(sub delta(sub a) on the lateral-directional handling qualities of a large transport airplane at landing-approach airspeed. It is shown that the desirable combinations of these parameters tend to be more proverse when compared with values typical of current aircraft. Results of flight tests in a large variable-stability jet transport showed trends which were similar to those of the simulator data. Areas of minor disagreement, which were traced to differences in airplane geometry, indicate that pilot consciousness of side acceleration forces can be an important factor in handling qualities of future long-nosed transport aircraft.

McNeill, Walter E.↗
NASA NTRS2001

Stellar and Binary Evolution in Star Clusters

This paper presents a final report on research activities covered on Stellar and Binary Evolution in Star Clusters. Substantial progress was made in the development and dissemination of the "Starlab" software environment. Significant improvements were made to "kira," an N-body simulation program tailored to the study of dense stellar systems such as star clusters and galactic nuclei. Key advances include (1) the inclusion of stellar and binary evolution in a self-consistent manner, (2) proper treatment of the anisotropic Galactic tidal field, (3) numerous technical enhancements in the treatment of binary dynamics and interactions, and (4) full support for the special-purpose GRAPE-4 hardware, boosting the program's performance by a factor of 10-100 over the accelerated version. The data-reduction and analysis tools in Starlab were also substantially expanded. A Starlab Web site (http://www.sns.ias.edu/-starlab) was created and developed. The site contains detailed information on the structure and function of the various tools that comprise the package, as well as download information, "how to" tips and examples of common operations, demonstration programs, animations, etc. All versions of the software are freely distributed to all interested users, along with detailed installation instructions.

McMillan, Stephen L. W.↗
NASA NTRS1961

An Explicit Linear Filtering Solution for the Optimization of Guidance Systems with Statistical Inputs

The determination of optimum filtering characteristics for guidance system design is generally a tedious process which cannot usually be carried out in general terms. In this report a simple explicit solution is given which is applicable to many different types of problems. It is shown to be applicable to problems which involve optimization of constant-coefficient guidance systems and time-varying homing type systems for several stationary and nonstationary inputs. The solution is also applicable to off-design performance, that is, the evaluation of system performance for inputs for which the system was not specifically optimized. The solution is given in generalized form in terms of the minimum theoretical error, the optimum transfer functions, and the optimum transient response. The effects of input signal, contaminating noise, and limitations on the response are included. From the results given, it is possible in an interception problem, for example, to rapidly assess the effects on minimum theoretical error of such factors as target noise and missile acceleration. It is also possible to answer important questions regarding the effect of type of target maneuver on optimum performance.

Stewart, Elwood C.↗