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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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All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Metallurgical characterization of the fracture of several high strength aluminum alloys

The fracture behavior for structural aluminum alloys (2024, 6061, 7075, and 7178) was examined in selected heat treatments. The investigation included tensile, shear, and precracked notch-bend specimens fractured at ambient temperature under monotonic loading. Specimens were obtained from thin sheets and thick plates and were tested in longitudinal and transverse orientations at different strain rates. Microstructures of alloys were examined using the optical microscope and the scanning electron microscope with associated energy dispersive X ray chemical analysis. Several different types of second phase particles, some not reported by other investigators, were identified in the alloys. Fracture morphology was related to microstructural variables, test variables, and type of commercial product. Specimen orientation examined in the present investigation had little effect on fracture morphology. Test strain rate changes resulted in some change in shear fracture morphology, but not in fracture morphology of tensile specimens.

Bhandarkar, M. D.↗
NASA NTRS1983

Embedded cavity drag in steady and unsteady flows

The numerical solution of the laminar boundary-layer flow over an embedded cavity is studied. The purpose is to examine the relevant drag characteristics of laminar cavity flow. The solution field is obtained in terms of velocity and vorticity variables, with the stream function and pressure derivable from the directly computed variables. An analysis and comparison is made among four square cavities, ranging in size from 0.25 to 1.00 boundary-layer thicknesses deep. The dominant flow features are examined in the vicinity of the cavity by means of the stream function and iso-vorticity contours. The dominant physics in the overall drag characteristics of the flow is examined by an analysis of the pressure and wall shear stress distributions in the cavity, and upstream and downstream of the cavity. Pressure forces and frictional forces in, and in the vicinity of, the cavity are determined. Stress relaxation distances, both upstream and downstream of the cavity, are calculated and analyzed. The flow dynamics of the boundary-layer flow over an embedded cavity is summarized. Finally, the relevance of the results to the control of flow separation in such flows is discussed.

Gatski, T. B.↗
NASA NTRS1983

Stiffness Properties for Dynamic Modeling of Composite Graphite-Epoxy Cylindrical Orthotropic Shells

Traditional composite lamination theory was used to predict composite graphite-epoxy laminate stiffnesses for comparison to quasi-experimental stiffnesses developed from cylindrical bottle pressure testing. Stiffness sensitivities were examined for variations in constituent materials and geometric properties. The material component interactions examined were the fiber longitudinal and transverse Young's modulus, the fiber shear modulus and primary Poisson's ratio, the resin Young's modulus, shear modulus, and Poisson's ratio. The geometric variation of the helical winding angle was also examined. Two computer programs were written to generate the data used to demonstrate the stiffness variations.

Tolbert, R. N.↗
NASA NTRS1984

Fast algorithms for combustion kinetics calculations: A comparison

To identify the fastest algorithm currently available for the numerical integration of chemical kinetic rate equations, several algorithms were examined. Findings to date are summarized. The algorithms examined include two general-purpose codes EPISODE and LSODE and three special-purpose (for chemical kinetic calculations) codes CHEMEQ, CRK1D, and GCKP84. In addition, an explicit Runge-Kutta-Merson differential equation solver (IMSL Routine DASCRU) is used to illustrate the problems associated with integrating chemical kinetic rate equations by a classical method. Algorithms were applied to two test problems drawn from combustion kinetics. These problems included all three combustion regimes: induction, heat release and equilibration. Variations of the temperature and species mole fraction are given with time for test problems 1 and 2, respectively. Both test problems were integrated over a time interval of 1 ms in order to obtain near-equilibration of all species and temperature. Of the codes examined in this study, only CREK1D and GCDP84 were written explicitly for integrating exothermic, non-isothermal combustion rate equations. These therefore have built-in procedures for calculating the temperature.

Radhakrishnan, K.↗
NASA NTRS1984

Two probable optical flashes from gamma-ray bursters

Two images on archival photographic plates which are most likely records of optical flashes from gamma-ray bursters (GRBs) were examined. One of these images appears on a 1901 plate in the field of the 5 Nov. 1979 GRB, while the other is in the field of the 13 Jan. 1979 GRB on a plate exposed in 1944. The 1901 optical transient image is circular in shape, while all normal star images are trailed by 8 in. No optical transients are found in a control region which is 34.3 times larger than the GRB error regions examined. Independent limits on the optical flash rate from the sky yield a probability of less than 0.0001 that any one of the optical transients is due to a background flash. A total exposure of 2.7 years was examined for GRB flashes at known GRB locations on the Harvard plates and a total of three GRB flashes were seen, that the average recurrence time scale for optical flashes is roughly one year. The optical fluence of these optical flashes was measured. For the three currently known GRB optical flashes, the ratio of gamma-ray fluence (from a modern burst) to the optical fluence (from a archival burst) were measured to be 800, 900, and 900.

Schaefer, B. E.↗
NASA NTRS1984

Mechanical behavior of carbon-carbon composites

A general background, test plan, and some results of preliminary examinations of a carbon-carbon composite material are presented with emphasis on mechanical testing and inspection techniques. Experience with testing and evaluation was gained through tests of a low modulus carbon-carbon material, K-Karb C. The properties examined are the density - 1.55 g/cc; four point flexure strength in the warp - 137 MPa (19,800 psi) and the fill - 95.1 MPa (13,800 psi,) directions; and the warp interlaminar shear strength - 14.5 MPa (2100 psi). Radiographic evaluation revealed thickness variations and the thinner areas of the composite were scrapped. The ultrasonic C-scan showed attenuation variations, but these did not correspond to any of the physical and mechanical properties measured. Based on these initial tests and a survey of the literature, a plan has been devised to examine the effect of stress on the oxidation behavior, and the strength degradation of coated carbon-carbon composites. This plan will focus on static fatigue tests in the four point flexure mode in an elevated temperature, oxidizing environment.

Rozak, G. A.↗
NASA NTRS1983

Pulse asymptotics of three-dimensional baroclinic waves

The asymptotic development at large time of waves arising from localized disturbances in a baroclinic flow is examined. Vertical structures unlike those associated with the more commonly examined temporal normal modes are found both for the pulse confined to a channel as previously examined, and for the unconfined pulse on an infinite beta-plane. These structures and their implied transports are compared to observations in the regions of storm tracks. It is also found that the meridional extent of the asymptotic solution becomes large compared to observed cyclone wavetrains, emphasizing the importance of flow inhomogeneity and sphericity effects in determining the latitudinal structure of eddies.

Farrell, B. F.↗
NASA NTRS1984

Embedded cavity drag in steady and unsteady flows

The numerical solution of the laminar boundary-layer flow over an embedded cavity is studied. The purpose of the study is to examine the relevant drag characteristics of laminar cavity flow. The solution field is obtained in terms of velocity and vorticity variables, with the stream function and pressure derivable from the directly computed variables. An analysis and comparison is made among four square cavities, ranging in size from 0.25 to 1.00 boundary-layer thicknesses deep. The dominant flow features are examined in the vicinity of the cavity by means of the stream function and iso-vorticity contours. The dominant physics in the overall drag characteristics of the flow is examined by an analysis of the pressure and wall shear stress distributions in the cavity, and upstream and downstream of the cavity. Pressure forces and frictional forces in, and in the vicinity of, the cavity are determined. Stress relaxation distances, both upstream and downstream of the cavity, are calculated and analyzed. The flow dynamics of the boundary-layer flow over an embedded cavity is summarized. Finally, the relevance of the present results to the control of flow separation in such flows is discussed.

Gatski, T. B.↗
NASA NTRS1983

A comparative study on synthetic aperture radar and optical imagery of ocean waves

The ocean imaging capability of synthetic aperture radar (SAR) is examined in terms of an intercomparison between simultaneous, spatially collocated images of a coastal ocean scene, as recorded by aerial photography and aircraft-borne SAR. Based on a detailed study of these images, this paper examines various local features of oceanographic interest as they are imaged by the two methods. This provides information not available in the usual comparisons of normalized SAR and single point buoy spectra. In addition, aspects of the intensity wavenumber spectra of the two images are examined and discussed. An interesting and surprising conclusion arising from the comparison study is the ability of the SAR to image accurately non-uniform (possibly nonlinear) wave features which are imaged by the aerial photography. This result should have important implications on the development of SAR imaging models and the interpretation of the effect of ocean surface motion on the SAR images.

Banner, M. L.↗
NASA NTRS1983

Application of the Parker flash method to high temperature thermoelectrics

Mathematical aspects of thermal diffusivity measurements are reviewed and applied to develop an experimental setup at JPL. Certain approximations commonly appearing in the literature are examined in some detail and rejected based upon sensitive slope variations having significant impact on the results. The final mathematical formulation used was one developed by Parker et al. (1961), which evaluated the diffusivity in a way that could be simply evaluated by examining an oscilloscope trace. A computer algorithm was written to examine solution sensitivity to the number of terms taken in the infinite series. The mathematical technique is being used at JPL to evaluate diffusivities of graphite, Boron Carbon and Lanthanum Sulphur compounds in the temperature range of 300 K to 1200 K.

Noon, E. L.↗
NASA NTRS1984

Subcritical crack growth and other time- and environment-dependent behavior in crustal rocks

Stable crack growth strongly influences both the fracture strength of brittle rocks and some of the phenomena precursory to catastrophic failure. Quantification of the time and environment dependence of fracture propagation is attempted with the use of a fracture mechanics technique. Some of the difficulties encountered when applying techniques originally developed for simple synthetic materials to complex materials like rocks are examined. A picture of subcritical fracture propagation is developed that embraces the essential ingredients of the microstructure, a microcrack process zone, and the different roles that the environment plays. To do this, the results of (1) fracture mechanics experiments on five rock types, (2) optical and scanning electron microscopy, (3) studies of microstructural aspects of fracture in ceramics, and (4) exploratory tests examining the time-dependent response of rock to the application of water are examined.

Swanson, P. L.↗
NASA NTRS1984

The effect of plasma on solar cell array arc characteristics

The influence from the ambient plasma on the arc characteristics of a negatively biased solar cell array was investigated. The arc characteristics examined were the peak current during an arc, the decay time as the arc terminates, and the charge lost during the arc. These arc characteristics were examined in a nitrogen plasma with charge densities ranging from 15,000 to 45,000 cu cm. Background gas pressures ranged from 8x1,000,000 to 6x100,000 torr. Over these ranges of parameters no significant effect on the arc characteristics were seen. Arc characteristics were also examined for three gas species: helium, nitrogen and argon. The helium arcs have higher peak currents and shorter decay times than nitrogen and argon arcs. There are slight differences in the arc characteristics between nitrogen and argon. These differences may be caused by the differences in mass of the respective species. Also, evidence is presented for an electron emission mechanism appearing as a precursor to solar array arcs. Occassionally the plasma generator could be turned off, and currents could still be detected in the vacuum system. When these currents are presented, arcs may occur.

Snyder, D. B.↗
NASA NTRS1985

Optimal symmetric flight studies

Several topics in optimal symmetric flight of airbreathing vehicles are examined. In one study, an approximation scheme designed for onboard real-time energy management of climb-dash is developed and calculations for a high-performance aircraft presented. In another, a vehicle model intermediate in complexity between energy and point-mass models is explored and some quirks in optimal flight characteristics peculiar to the model uncovered. In yet another study, energy-modelling procedures are re-examined with a view to stretching the range of validity of zeroth-order approximation by special choice of state variables. In a final study, time-fuel tradeoffs in cruise-dash are examined for the consequences of nonconvexities appearing in the classical steady cruise-dash model. Two appendices provide retrospective looks at two early publications on energy modelling and related optimal control theory.

Weston, A. R.↗
NASA NTRS1985

Vibration characteristics of the HPOTP (High Pressure Oxygen Turbopump) of the SSME (Space Shuttle Main Engine)

A review is presented of various rotordynamic problems which have been encountered and eliminated in developing the current flight engines, and continuing subsynchronous problems which are being encountered in developing a 109% power level engine. The basic model for the High Pressure Oxygen Turbopump (HPOTP) of the SSME including the structural dynamic model for the rotor and housing and component models for the liquid and gas seals, turbine-clearance excitation forces, and impeller-diffuser forces are discussed. Results from a linear model are used to examine the synchronous response and stability characteristics of the HPOTP, examining bearing load and stability problems associated with the second critical speed. Various seal modifications are examined and shown to have favorable consequences with respect to bearing reactions and stability.

Childs, D. W.↗
NASA NTRS1985

Dynamic performance of several large antenna concepts

Four antenna concepts the box truss, tetrahedral truss, wrap-radial rib, and hoop and column antenna were examined for the dynamic performance when subjected to an operational environment. Space applications for the concepts are numerous; however, the land mobile satellite system (LMSS) was chosen as a baseline study and its operational constraints are applied to each concept. The dynamic response of each concept is examined for structural displacement and structural damping effects. The necessity of a control system for vibrational displacement reduction is examined along with a comparison of the relative merits of each antenna concept.

Andersen, G. C.↗
NASA NTRS1985

Particle acceleration due to shocks in the interplanetary field: High time resolution data and simulation results

Data were examined from two experiments aboard the Explorer 50 (IMP 8) spacecraft. The Johns Hopkins University/Applied Lab Charged Particle Measurement Experiment (CPME) provides 10.12 second resolution ion and electron count rates as well as 5.5 minute or longer averages of the same, with data sampled in the ecliptic plane. The high time resolution of the data allows for an explicit, point by point, merging of the magnetic field and particle data and thus a close examination of the pre- and post-shock conditions and particle fluxes associated with large angle oblique shocks in the interplanetary field. A computer simulation has been developed wherein sample particle trajectories, taken from observed fluxes, are allowed to interact with a planar shock either forward or backward in time. One event, the 1974 Day 312 shock, is examined in detail.

Kessel, R. L.↗
NASA NTRS1984

Two probable optical flashes from gamma-ray bursters

Two images on archival photographic plates which are most likely records of optical flashes from gamma-ray bursters (GRBs) were examined. One of these images appears on a 1901 plate in the field of the Nov. 5, 1979 GRB, while the other is in the field of the Jan. 13, 1979 GRB on a plate exposed in 1944. The 1901 optical transient image is circular in shape, while all normal star images are trailed by 8 in. No optical transients are found in a control region which is 34.3 times larger than the GRB error regions examined. Independent limits on the optical flash rate from the sky yield a probability of less than 0.0001 that any one of the optical transients is due to a background flash. A total exposure of 2.7 years was examined for GRB flashes at known GRB locations on the Harvard plates and a total of three GRB flashes were seen, that the average recurrence time scale for optical flashes is roughly one year. The optical fluence of these optical flashes was measured. For the three currently known GRB optical flashes, the ratio of gamma-ray fluence (from a modern burst) to the optical fluence (from a archival burst) were measured to be 800, 900, and 900.

Schaefer, B. E.↗
NASA NTRS1985

The effect of plasma on solar cell array arc characteristics

The influence from the ambient plasma on the arc characteristics of a negatively biased solar cell array was investigated. The arc characteristics examined were the peak current during an arc, the decay time as the arc terminates, and the charge lost during the arc. These arc characteristics were examined in a nitrogen plasma with charge densities ranging from 15,000 to 45,000 cu cm. Background gas pressures ranged from 8x1,000,000 to 6x100,000 torr. Over these ranges of parameters no significant effect on the arc characteristics were seen. Arc characteristics were also examined for three gas species: helium, nitrogen and argon. The helium arcs have higher peak currents and shorter decay times than nitrogen and argon arcs. There are slight differences in the arc characteristics between nitrogen and argon. These differences may be caused by the differences in mass of the respective species. Also, evidence is presented for an electron emission mechanism appearing as a precursor to solar array arcs. Occasionally the plasma generator could be turned off, and currents could still be detected in the vacuum system. When these currents are presented, arcs may occur.

Snyder, D. B.↗