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At least 1,495 records · Page 83
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

The importance of improved facular observations in understanding solar constant variations

A new study of solar irradiance modeling has been undertaken to improve the previous modeling efforts and perhaps to resolve the energy-balance question. In the present study, the daily sunspot and facular areas (using plages as a proxy measure of faculae) have been utilized, as well as a plage intensity index to examine brightness variations. It is noted that the reported plage areas changed by a factor of 2 near the end of 1979. Although this can be partially modeled because a commensurate change in plage brightness occurs, it leads to the conclusion that facular areas and brightness uncertainties prevent a definitive answer to the energy-balance question with this technique.

Schatten, K. H.↗
NASA NTRS1986

Strength of graphite/epoxy bolted wing-skin splice specimens subjected to outdoor exposure under constant load and yearly fatigue loading

The results of an experimental study to provide long-term durability data on detailed full-scale graphite/epoxy wing-skin joint designs under environmental exposure and cyclic loading associated with commercial transport aircraft are reported. The specimens consisted of a single-row bolt configuration fabricated from T300/5208 and a double-row bolt configuration fabricated from T300/5209. The unpainted specimens were exposed to the outdoor environment under a sustained tensile load, and at yearly intervals, they were subjected to fatigue loading. Experimental results showed a slight reduction in residual tensile strength for both graphite/epoxy joints under the exposure times and fatigue loadings reported. A 7.5-percent decrease in residual strength was observed for the T300/5208 single-row joint after 5 years exposure and two lifetimes of fatigue loading. A 5.3-percent decrease in residual strength was observed for the T300/5209 double-row joint after 7 years exposure and 2.8 lifetimes of fatigue loading. The 5208 epoxy material was more susceptible to degradation by ultraviolet radiation than the 5209 epoxy material.

Wichorek, G. R.↗
NASA NTRS1985

Magnetic optical activity in intense laser fields. I - Self-rotation and Verdet constant

The paper is concerned with the Faraday rotation and circular dichroism of dilute gases in longitudinal magnetic fields subject to strong radiation fields. In particular, magnetic fields are considered which produce Zeeman shifts much smaller than inverse collision times but which may be greater or smaller then the multipole relaxation times. Doppler broadening is considered, and explicit expressions are given for dipole transitions in a two-level system with arbitrary angular momentum for strong and weak magnetic fields. The same system is then studied as a function of the external magnetic-field strength with a fixed laser frequency. It is shown that this experimental arrangement, referred to as forward scattering, has certain advantages that are not present in the more usual magnetic rotation spectroscopy.

Giraud-Cotton, S.↗
NASA NTRS1986

Channel specific rate constants for reactions of O(1D) with HCl and HBr

The absolute rate coefficients and product yields for reactions of O(1D) with HCl(1) and HBr(2) at 287 K are presently determined by means of the time-resolved resonance fluorescence detection of O(3P) and H(2S) in conjunction with pulsed laser photolysis of O3/HX/He mixtures. Total rate coefficients for O(1D) removal are found to be, in units of 10 to the -10th cu cm/molecule per sec, k(1) = 1.50 + or - 0.18 and k(2) 1.48 + or - 0.16; the absolute accuracy of these rate coefficients is estimated to be + or - 20 percent.

Wine, P. H.↗
NASA NTRS1985

A constant altitude flight survey method for mapping atmospheric ambient pressures and systematic radar errors

The flight test technique described uses controlled survey runs to determine horizontal atmospheric pressure variations and systematic altitude errors that result from space positioning measurements. The survey data can be used not only for improved air data calibrations, but also for studies of atmospheric structure and space positioning accuracy performance. The examples presented cover a wide range of radar tracking conditions for both subsonic and supersonic flight to an altitude of 42,000 ft.

Larson, T. J.↗
NASA NTRS1986

A comparison of flexural properties of aramid reinforced pultrusions having constant fiber volume and varied matrices, pretreatments, and postcures

Aramid reinforcement composite materials of equal fiber volume having varied polymer thermoset matrices have been pultruded and flexurally tested to failure. The objective was to improve flexural properties of aramid reinforced pultrusions. Pultrusions of both sized and unsized aramid fiber with four different resin systems were compared to determine the effects of sizing compounds and post-thermal treatments on the flexural strength as an indication of fiber wettability and fiber-to-resin interface bonding. Improvements in flexural strength as the result of pretreatments with sizing solutions used in this study were marginal. The most significant improvements in flexural properties were the results of postcuring. Overall improvements ranged from a low 39,647 Psi(273 Mpa) to a high of 80,390 Psi(554 Mpa), or 103 percent. The fact that post-thermal treatments improved the flexural properties of the four pultrusions indicates that a full cure did not occur in either resin system during the pultrusion process. The increased flexural strengths of the polyester and vinyl ester pultrusions were the most surprising. Based on data presented, the most promising resin system of the four examined (VE 8300, Aropol 7430, Epon 9302, and Epon 9310) for Kevlar reinforced pultrusion is Epon 9310 epoxy.

Wilson, Maywood L.↗
NASA NTRS1987

Elastic interaction of a crack with a microcrack array. I - Formulation of the problem and general form of the solution. II - Elastic solution for two crack configurations (piecewise constant and linear approximations)

The elastic interactions of a two-dimensional configuration consisting of a crack with an array of microcracks located near the tip are studied. The general form of the solution is based on the potential representations and approximations of tractions on the microcracks by polynomials. In the second part, the technique is applied to two simple two-dimensional configurations involving one and two microcracks. The problems of stress shielding and stress amplification (the reduction or increase of the effective stress intensity factor due to the presence of microcracks) are discussed, and the refinements introduced by higher order polynomial approximations are illustrated.

Chudnovsky, A.↗
NASA NTRS1988

Variable-Speed, Constant-Frequency Generation Of Power

Feedback of stator power and reactive volt-amperes determines rotor excitation. New method involves control circuit separating rotor excitation into generation of slip frequency and control of amplitude and phase. In control circuit, speed determines slip frequency, while stator power and reactive volt-amperes determine amplitude and phase of rotor current.

Brady, Frank J.↗
NASA NTRS1987

Upper limit on the rate constant for isotope exchange between molecular oxygen and ozone at 298 K

The gas phase bimolecular isotope exchange reaction between molecular oxygen and ozone has been investigated directly for the first time. Its rate coefficient is found to be less than 2 x 10 to the -25th cu cm/sec at 298 K, over six orders of magnitude below recent estimates. Much faster exchange was observed over condensed ozone at 77 K, suggesting isotopic scrambling is catalyzed under these conditions. The low rate coefficient implies that homogeneous exchange between ground state oxygen and ozone molecules cannot play a significant role in heavy ozone chemistry.

Anderson, S. M.↗
NASA NTRS1988

An investigation of the optical constants and band gap of chromium disilicide

Optical properties of polycrystalline thin films of CrSi2 grown by the diffusion couple method on silicon substrates were investigated. An analysis of the energy dependence of the absorption coefficient indicates that the material is an indirect forbidden gap semiconductor with a band-gap value of slightly less than 0.35 eV. This result was confirmed by measurements of the temperature dependence of the intrinsic conductivity. The value of the bandgap corresponds well to an important window of transparency in the earth's atmosphere (3-5 microns), which makes the material of potential interest for IR detector applications.

Bost, M. C.↗
NASA NTRS1988

A complete dc characterization of a constant-frequency, clamped-mode, series-resonant converter

The dc behavior of a clamped-mode series-resonant converter is characterized systematically. Given a circuit operating condition, the converter's mode of operation is determined and various circuit parameters are calculated, such as average inductor current (load current), rms inductor current, peak capacitor voltage, rms switch currents, average diode currents, switch turn-on currents, and switch turn-off currents. Regions of operation are defined, and various circuit characteristics are derived to facilitate the converter design.

Tsai, Fu-Sheng↗
NASA NTRS1988

The single crystal elastic constants of hexagonal SiC to 1000 C

The relationships between the sound velocities in the cubic and hexagonal crystal structures and the tensor transformations for the two structures are applied to determine the elastic stiffnesses for the hexagonal structures of SiC to 1000 C. These results are then applied to calculate the polycrystalline elastic moduli, E and G, and their temperature variations. The calculated values for E and G at 20 C are 420 and 180 GPa; for (dE/dT) and (dG/dT), the values are -0.020 and -0.007 GPa/C, respectively.These agree well with published experimental values for E and G of dense polycrystalline alpha silicon carbides.

Li, Z.↗