SEARCH · Engineering Papers

Results for “sublimation”

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.

SourceResearch connectionFrom yearThrough year

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

At least 613 records · Page 34

All sources NASA NTRS DOE OSTI

NASA NTRS1981

Nature and origin of organic molecules in comets

The organic molecules sublimating from the cometary nucleus either represent the 'frost' of interstellar molecules that condensed onto those interstellar grains that have later accreted into comets, or they represent the 'snows' that condensed onto silicate grains during the cooling phase of the presolar nebula. Even if one does not accept the speculations of Hoyle and Wickramashinghe (1977), the fact that prebiotic chemistry could have been brought from the interstellar space or from the solar nebula to the Earth by comets is an intriguing possibility that cannot be ruled out at this stage.

Delsemme, A. H.↗

NASA NTRS1982

The path and surviving tail of a comet that fell into the sun

A satisfactory orbital solution for Comet Howard-Koomen-Michels 1979 XI is found on the assumption that the comet's line of apsides coincided with that of the Kreutz sungrazing comet group. The derived perihelion distance then shows that this is the first known case of a comet falling into the sun. A dust tail that survived the comet is studied as a particle flow phenomenon controlled by no force other than solar gravity and solar radiation pressure. The tail's outline is interpreted in terms of an onset of dust production, a peak repulsive force on the particles, and a circumsolar dustfree zone due to particle sublimation. It is shown that the surviving debris consisted mostly of absorbing, submicron size particles in hyperbolic trajectories convex to the sun and curving toward the earth. The tail width may be a product of the interaction of charged dust in the tail with a complicated structure of the coronal magnetic field.

Sekanina, Z.↗

NASA NTRS1982

Wind-tunnel investigation of a full-scale canard-configured general aviation aircraft

As part of a broad research program to provide a data base on advanced airplane configurations, a wind-tunnel investigation was conducted in the Langley 30-by 60-Foot Wind Tunnel to determine the aerodynamic characteristics of an advanced canard-configured general aviation airplane. The investigation included measurements of forces and moments of the complete configuration, isolated canard loads, and pressure distributions on the wing, winglet, and canard. Flow visualization was obtained by using surface tufts to determine regions of flow separation and by using a chemical sublimation technique to determine boundary-layer transition locations. Additionally, other tests were conducted to determine simulated rain effects on boundary layer transition. Investigation of configuration effects included variations of canard locations, canard airfoil section, winglet size, and use of a leading-edge droop on the out-board section of the wing.

Yip, L. P.↗

NASA NTRS1982

Laboratory studies of charged particle erosion of SO2 ice and applications to the frosts of Io

The removal and/or redistribution of SO2 frosts on the surface of the first Galilean satellite, Io, can occur through the erosion of these frosts by the magnetosphere particle environment of the satellite. The energy, species, and temperature dependence of the erosion rates of SO2 ice films by charged particles have been studied in laboratory experiments. Rutherford backscattering and thin film techniques are used in the experiments. The ice temperature is varied between about 10 K and the sublimation temperature. The erosion rates are found to have a temperature-independent and a temperature-dependent regime and to be much greater, for 10-2000 keV ions, than those predicted by the usual sputtering process. The laboratory results are used together with measured magnetosphere particle fluxes in the vicinity of Io to estimate the erosion rates of SO2 ice films from the satellite and implications therefrom on an SO2 atmosphere on Io.

Lanzerotti, L. J.↗

NASA NTRS1982

Spatially resolved methane band photometry of Saturn. I - Absolute reflectivity and center-to-limb variations in the 6190-, 7250-, and 8900-A bands

Spatially resolved measurements of Saturn's absolute reflectivity in methane bands at 6190, 7250, and 8900 A and in nearby continuum regions are presented. Images were obtained through narrow-band interference filters with a 500 x 500-pixel charge-coupled device. Band/continuum ratios were measured to high accuracy by referencing to the ring brightness in each image. Several data processing techniques enhanced the quality of the observations. These are the use of the ring symmetry to find center position and orientation, accurate subtraction of ring light, and constrained image deconvolution. Uncertainty in the continuum absolute reflectivity is within 10%. Uncertainties in band/continuum ratios are from one to several percent. The Equatorial Zone was much brighter than any other latitude in the strong 8900 band image. Northern midlatitudes were brighter than southern midlatitudes. The latter observation indicates fewer high-altitude aerosols in the south, a possible result of atmospheric dynamics or seasonal sublimation of NH3 crystals. The data are tabulated and presented in a form suitable for quantitative scattering model analyses.

West, R. A.↗

NASA NTRS1983

Cooling by Para-to-Ortho-Hydrogen Conversion

Catalyst speeds conversion, increasing capacity of solid hydrogen cooling system. In radial-flow catalytic converter, para-hydrogen is converted to equilibrium mixture of para-hydrogen and ortho-hydrogen as it passes through porous cylinder of catalyst. Addition of catalyst increases capacity of hydrogen sublimation cooling systems for radiation detectors.

Sherman, A.↗

NASA NTRS1982

Chemical composition of cometary nuclei

Observational evidence pertaining to the origin and composition of cometary material is reviewed. Arguments favoring the undifferentiated character of the icy conglomerate are summarized. Theoretical descriptions of the sublimation of a cometary nucleus and the velocity field of the expanding gas are presented and compared with observations. The nature of cometary dust and the atomic abundances of H, C, N, O, and S in the volatile fraction are examined, and data on the dust and volatile fractions are combined to derive elemental abundances. It is shown that O, N, and S in cometary nuclei appear to have essentially cosmic abundances but that both H and C are drastically depleted with respect to the cosmic abundances. The apparent depletion of C by a factor of more than three is discussed. It is suggested that the missing carbon might be hidden in the dust fraction in the form of heavy organic molecules or might have remained in either the primeval solar nebula or interstellar space.

Delsemme, A. H.↗

NASA NTRS1982

Characterization of the outgassing of spacecraft materials

Equations which describe material outgassing in spaceflight conditions are integrated over temperatures up to 125 C and with outgassing kinetic energies up to 40 kcal/mole. The outgassing rate is noted to be formulated as an expression of sublimation and evaporation, and can be a function of the materials of the outgassing process. The predicted mass loss rates for various materials are compared with mass loss rates from materials with known activation energies, e.g., silicone and solar panel samples, held at different temperatures and for varying time lengths. The comparisons are made for first order and more complex outgassing kinetics. The technique is concluded to be useful for a quick characterization of candidate materials for spaceflight considerations when contamination is a significant factor.

Scialdone, J. J.↗

NASA NTRS1982

Remote comets and related bodies - VJHK colorimetry and surface materials

VJHK colors for a number of asteroids and eight comets at various solar distances and levels of activity were obtained, and the observations are interpreted in terms of a two-component mixing model in which outer solar system interplanetary bodies are viewed as mixtures of ice and dark carbonaceous-type (RD and C) dirt. It is inferred that the observed comets have comae, and perhaps surfaces, of dirty ice or ice dirt grains colored by an RD-dirt component. This inference is supported by systematics of an 'alpha index' based on VJHK colors and empirically correlated with albedo and ice/dirt ratio. Among comets the alpha index correlates with solar distance in a way that suggests comets emit dirty ice grains which are stable at large solar distance but from which the ice component sublimes and leaves dirt grains at small solar distance.

Hartmann, W. K.↗

NASA NTRS1982

Computer modeling of thermoelectric generator performance

Features of the DEGRA 2 computer code for simulating the operations of a spacecraft thermoelectric generator are described. The code models the physical processes occurring during operation. Input variables include the thermoelectric couple geometry and composition, the thermoelectric materials' properties, interfaces and insulation in the thermopile, the heat source characteristics, mission trajectory, and generator electrical requirements. Time steps can be specified and sublimation of the leg and hot shoe is accounted for, as are shorts between legs. Calculations are performed for conduction, Peltier, Thomson, and Joule heating, the cold junction can be adjusted for solar radition, and the legs of the thermoelectric couple are segmented to enhance the approximation accuracy. A trial run covering 18 couple modules yielded data with 0.3% accuracy with regard to test data. The model has been successful with selenide materials, SiGe, and SiN4, with output of all critical operational variables.

Chmielewski, A. B.↗

NASA NTRS1983

On the nature of the interaction of the Jovian magnetosphere with the icy Galilean satellites

The effects of plasma and micrometeoroid bombardment of each satellite are considered in the case of interactions between the Jovian magnetosphere and Europa, Ganymede, and Callisto, in an attempt to understand the influences of these exogenic processes on the surface properties of each of the three moons. Sublimated and sputtered H2O results in a net O2 atmosphere on each of the three moons, and the effectiveness of such an atmosphere in preventing the Jovian plasma from reaching each satellite surface is considered for a variety of magnetospheric conditions. Also studied are the effects of possible satellite magnetic field magnitude and orientation, in order to estimate satellite magnetosphere properties, and the orbits of charged micrometeroids in the Jupiter magnetosphere, in order to determine the relative flux of these particles over the satellite surfaces.

Wolff, R. S.↗

NASA NTRS1981

Natural laminar flow data from full-scale flight and wind-tunnel experiments

Experimental results obtained at NASA Langley during studies of natural laminar flow (NLF) over commercially produced aircraft surfaces are reported. The general aviation aircraft examined were light aircraft, yet displayed NLF extents close to the maximum available and equivalent to high performance business aircraft flying envelopes. Sublimating chemicals and acoustic detection techniques were employed to measure the boundary layer transition. Theoretical predictions of boundary layer stability were found to match well with the experimental data, with consideration given to both swept wings and the amplitudes of allowable waves on the airfoil surfaces. The presence of the NLF on the airfoil surfaces confirmed the benefits available from use of composite materials for airfoil surfaces.

Holmes, B. J.↗

NASA NTRS1982

Cryogenic particle collection on a cometary mission

The present investigation is concerned with research related to the design of a method for capturing effectively particulate matter and volatiles from the tail of a comet for the purpose of chemical analysis. It is envisaged to conduct a space mission with a means of propulsion which would make it possible to maneuver the space probe into the coma of the comet. The collecting medium has to be chemically inert and easily distinguishable from the particles being collected. It is proposed to employ a film of solid xenon at a temperature of 65 K as the collecting medium. Sublimation of this film and the resultant pressure will be used to concentrate the particles on a small area suitable for analysis. Attention is given to the laboratory apparatus, the experimental work, the experimental results, and the cryogenic requirements for a space mission.

Szara, R. J.↗

NASA NTRS1983

Cometary impacts with the sun - Physical and dynamical considerations

Michels et al. (1982) observed the apparent impacting of the sun by a comet, and Sekanina (1982) showed that the comet, 1979XI, was probably a member of the Kreutz group of sun-grazing comets, although its perihelion of 0.35 solar radii was much smaller than the 1.2-1.9 solar radii common for this comet class. The perihelion change cannot be explained by planetary, stellar, or nongravitational perturbations. The most plausible explanation is collision with another body, probably a comet, at large heliocentric distance. The probability of such an event is, however, extremely small. The sublimation of the comet's nucleus before impact is discussed, and it is suggested that its ultimate destruction probably resulted from the shock of entry into the denser regions of the solar atmosphere, just above the photosphere.

Weissman, P. R.↗

NASA NTRS1984

Investigation of the thermophysical properties of high-melting materials with the aid of a complex of instruments

The evaporation rate, vapor pressure, heats of evaporation reaction (sublimation, dissociation), enthalpy, electrical resistance, heat capacity, emissivity, and heat conductivity of various carbides, borides, sulfides, nitrides, selenides, and phosphides were investigated. A set of high temperature high vacuum devices, calorimeters (designed for operation at 400 to 1300 K and from 1200 K), and mass spectrometers, most of which were specially developed for these studies, is described.

Bolgar, A. S.↗

NASA NTRS1984

Cometary nucleus and active regions

On the basis of the icy conglomerate model of cometary nuclei, various observations demonstrate the spotted nature of many or most nuclei, i.e., regions of unusual activity, either high or low. Rotation periods, spin axes and even precession of the axes are determined. The observational evidence for variations in activity over the surfaces of cometary nuclei are listed and discussed. On June 11 the comet IRAS-ARAKI-ALCOCK approached the Earth to a distance of 0.031 AU, the nearest since C/Lexell, 1770 I, providing a unique opportunity for near-nucleus observations. Preliminary analysis of these images establishes the spin axis of the nucleus, with an oblioquity to the orbit plane of approximately 50 deg, and a lag angle of sublimation approximately 35 deg from the solar meridian on the nucleus. Asymmetries of the inner coma suggests a crazy-quilt distribution of ices with differing volatility over the surface of the nucleus. The observations of Comet P/Homes 1892 III, exhibiting two 8-10 magnitude bursts, are carefully analyzed. The grazing encounter produced, besides the first great burst, an active area on the nucleus, which was rotating retrograde with a period of 16.3hr and inclination nearly 180 deg. After the first burst the total magnitude fell less than two magnitudes from November 7 to November 30 (barely naked eye) while the nuclear region remained diffuse or complex, rarely if ever showing a stellar appearance. The fading was much more rapid after the second burst. The grazing encounter distributed a volume of large chunks in the neighborhood of the nucleus, maintaining activity for weeks.

Whipple, F. L.↗

NASA NTRS1984

Recent comet impacts on the moon: The evidence from spectral reflectance studies

The results of analyses of near infrared reflectance spectra are inconsistent with numerous previous interpretations of the Reiner Gamma Formation. These include: (1) nue ardente or volcanic ash deposits, (2) volcanically derived sublimates, (3) high albedo volcanic deposits, and (4) highlands debris emplaced as impact ejecta. These results, strongly suggest that the selective preservation of high albedo features (formed by secondaries) by a local magnetic field enhancement is not a viable hypothesis. The results are generally consistent with, but place constraints on, the cometary impact hypothesis of Schultz and co-workers. While the presence of a magnetized component was not detected in either the bright or dark portions of the Reiner Gamma Formation, this material may be present in amounts under the current detection limits.

Hawke, B. R.↗

NASA NTRS1984

Wind Erosion and Dune Formation on High Frozen Bluffs

Frost penetration increases upslope on barren, windswept bluffs in cold environments. Along the south shore of Lake Superior, near the brow of 100 m high bluffs it typically exceeds 5 m. Frost increases the shear strength of damp sand to a level comparable to that of concrete, making winter slopes highly stable despite undercutting by waves and ground-water sapping along the footslope. Sublimation of interparticle ice in the slope face increases with wind speed and lower vapor pressures. The cold and dry winter winds of Lake Superior ablate these slopes through loss of binding ice. Wind erosion rates, based on measurements of sand accumulation on the forest floor downwind of the brow, show most airborne sand falls out within several meters of the brow, forming a berm 1 to 3 m high after many years. The spatial pattern of sand deposition, however, varies considerably over distances of several hundred meters along the top bluffs in response to frost conditions and the build-up of gravel lag on the slope face, sand exposure from mass movements, and local aerodynamics of the crest slope. The formation of perched sand dunes in the Great Lakes region is clearly related to wind erosion of sand from high bluffs in winter. Broadly similar processes may operate on Mars.

Marsh, W. M.↗