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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

IUE observations of WX Ceti in outburst

IUE observations of the peculiar dwarf nova WX Ceti roughly two weeks into a rare outburst, the first since 1963, are obtained. The SWP and LWP observations are very similar to other erupting dwarf novae, both in the line and continuum spectrum. The C IV 1549 A doublet displays blueshifted absorption, indicating the presence of a wind which has a terminal velocity of either 3150 km/s or 5450 km/s; the two possible values are due to an uncertain interpretation of features in the blue wing of the line. The line may also show a weak, redshifted emission, completing a P Cygni profile. The derived mass-loss of 0.7-2.2 x 10 to the -11th solar masses/yr is quite typical of dwarf novae in outburst. Thus, despite the unusual photometric properties of this system, long recurrence time, large amplitude outbursts, and long duration eruptions, the IUE observations reveal that the physical characteristics of the outburst are fairly typical of dwarf novae.

Downes, Ronald A.↗
NASA NTRS1990

Relationship between optical and X-ray properties of O-type stars surveyed with the Einstein Observatory

An X-ray luminosity function is derived for a representative volume-limited sample of O-type stars selected from the catalog of Galactic O stars surveyed with the Einstein Observatory. It was found that, for the stars of this sample which is ten times larger than any previously analyzed, the level of X-ray emission is strongly correlated with bolometric luminosity, confirming previous findings of an Lx-L(bol) relationship (e.g., Harnden et al., 1979; Pallavicini et al., 1981). Correlations between the Lx and the mass loss rate with the wind terminal velocity or with the rotation rate were weak. However, there was a strong correlation with wind momentum flux as well as with the wind kinetic energy flux.

Sciortino, S.↗
NASA NTRS1990

A GNM mission and system design proposal

Here, the author takes an advocacy position for the proposed Mars Global Network Mission (GNM); it is not intended to be an objective review, although both pros and cons are presented in summary. The mission consists of launches from earth in the '96, '98, and '01 opportunities on Delta-class launch vehicles (approx. 1000 kg injected to Mars in 8 to 10 ft diameter shroud). The trans Mars boost stage injects a stack of small independent, aeroshelled spacecraft. The stack separates from the boost stage and each rigid (as opposed to deployable) aeroshell flies to Mars on its own, performing midcourse maneuvers as necessary. Each spacecraft flies a unique trajectory which is targeted to achieve approach atmospheric interface at the desired latitude and lighting conditions; arrival times may vary by a month or more. A direct entry is performed, there is no propulsive orbit capture. The aeroshelled rough-landers are targeted to achieve a desired attitude and entry flight path angle, and then follow a passive ballistic trajectory until terminal descent. Based on sensed acceleration (integrated to deduce altitude), the aft aeroshell skirt is jettisoned; a short later a supersonic parachute is deployed. The ballistic coefficient of the parachute is sized to achieve terminal velocity at about 8 km. However the parachute is not deployed until a few Km above the surface to minimize wind-induced drift. The nose cap descent imaging begins, a laser altimeter also measures true altitude. Based on range and range rate to the surface, the parachute is jettisoned and the lander uses descent engines to achieve touchdown velocity. A contact sensor shuts down the motors to avoid cratering, and the lander rough-lands at less than 5 m/sec. The remaining aeroshell and a deployable bladder attenuate landing loads and minimize the possibility of tip over. Science instruments are deployed and activated, and the network is established.

Bailey, Stephen↗
NASA NTRS1991

Circumstellar material around the massive close binary DH Cephei

The expanding atmosphere of the massive close binary DH Cephei is analyzed on the basis of data from ground-based polarimetry and spaceborne UV observations obtained with the IUE satellite. The mass loss from the system is estimated to be about 2 x 10 to the -6th solar mass/yr. This material can be divided into three classes: (1) wind material, which is hot gas driven from the stars via radiation pressure and reaching terminal velocity at a distance of 5a or greater; (2) scattering material, which may be the result of colliding winds or instabilities in the wind flow; and (3) small (0.1 micron) dust grains. The latter may originate in the cold high-density boundary formed as the wind from the binary plows into the ambient interstellar medium or, alternatively, may be the residue of the formation of stellar components.

Corcoran, M. F.↗
NASA NTRS1991

On winds and X-rays of O-type stars

The dependence between stellar wind parameters and their X-ray fluxes is studied. Bolometric corrections are examined as a function of temperature, and a bolometric luminosity scale is proposed for the O-type stars. New mass-loss rates and terminal velocities of 40 well-known, bright single stars or binaries are measured, and the Lyman-alpha profile is modeled for the stars to determine the amount of X-ray-absorbing hydrogen. The mass-loss rates found for the latest O-type stars are significantly smaller than expected from the power-law dependences known for brighter, hot stars. The mass-loss rates for binary system do not differ significantly from those of single stars. A dependence is found between the wind energy of the single stars and the X-ray luminosity, and it is suggested that this dependence is at least partially responsible for the proportionality between L(x) and L(bol). The X-ray fluxes from the close binaries behave in qualitative agreement with predictions from simple theories of colliding stellar winds.

Chlebowski, Tomasz↗
NASA NTRS1991

First results from the Goddard High-Resolution Spectrograph - Spectroscopic determination of stellar parameters of Melnick 42, an O3f star in the Large Magellanic Cloud

GHRS and optical (ESO 3.6 m) observations of the O3f star Melnick 42 in the 30 Doradus complex are reported. A first analysis reveals that with a luminosity of 2.3 million L(solar) and a present mass of 100 M(solar), Melnick 42 is one of the most luminous and massive stars known. An estimate of abundances indicates that iron and oxygen are very likely reduced by a factor of four relative to the sun, whereas carbon is more strongly depleted and nitrogen is approximately solar. The terminal velocity of the stellar wind is 3000 km/s. The mass-loss rate is 4 x 10 to the -6th M(solar)/yr, with a large uncertainty. The excellent quality GHRS spectrum taken in a crowded region of the LMC demonstrates the superiority of the HST for quantitative ultraviolet spectroscopy of hot stars in other galaxies.

Heap, S. R.↗
NASA NTRS1992

Properties of the massive X-ray binary 4U 1700 - 37 = HD 153919

The system parameters for the massive X-ray binary 4U 1700 - 37 are reexamined in the light of modern spectroscopic theory and new UV and X-ray data. The system is composed of HD 153919, an O6f star having a stronger wind, and an accreting neutron star. Revised parameters for the Of component are derived which are in accord with the parameters of single Of stars, which resolves the problem that 'the O star is undermassive by a factor of 2' formulated by Conti (1978). The ratio of the terminal velocity of the wind to the photospheric escape velocity agrees with those of O stars. The discrepancy between the values of the wind acceleration parameter, derived from X-ray and UV data, is resolved. The acceleration of the wind is similar to that of single OB star winds. It is suggested that the supernova explosion producing the neutron star in 4U 1700 - 37 occurred over a million years ago when HD 153919 was still on the main sequence.

Heap, S. R.↗
NASA NTRS1992

Prevention of accretion onto white dwarfs by stellar winds

There is indirect observational evidence that hot white dwarfs may have weak stellar winds. In this paper, the interaction between such a wind and the flow of ISM material in the gravitational field of the white dwarf is investigated with the aim of finding limits on the mass-loss rate and terminal velocity of winds capable of preventing accretion from the ISM. The limiting cases of no relative motion of the star and the ISM and supersonic relative motion of the star through ISM are separately investigated. Each case is treated by generalizing models for the interaction between the solar wind and the local ISM to include the effects of gravity. It is found that, for wind velocities expected for radiatively driven winds, mass-loss rates as low as 10 exp -21 solar mass/yr are sufficient to prevent accretion from the hot phase of the ISM. To prevent accretion during passages through cold clouds, wind mass-loss rates of order 10 exp -18 to 10 exp -17 are required.

Macdonald, James↗
NASA NTRS1992

On the drag of model dendrite fragments at low Reynolds number

An experimental study of low Reynolds number drag on laboratory models of dendrite fragments has been conducted. The terminal velocities of the dendrites undergoing free fall along their axis of symmetry were measured in a large Stokes flow facility. Corrections for wall interference give nearly linear drag vs Reynolds number curves. Corrections for both wall interference and inertia effects show that the dendrite Stokes settling velocities are always less than that of a sphere of equal mass and volume. In the Stokes limit, the settling speed ratio is found to correlate well with the primary dendrite arm aspect ratio and a second dimensionless shape parameter which serves as a measure of the fractal-like nature of the dendrite models. These results can be used to estimate equiaxed grain velocities and distance of travel in metal castings. The drag measurements may be used in numerical codes to calculate the movement of grains in a convecting melt in an effort to determine macrosegregation patterns caused by the sink/float mechanism.

Zakhem, R.↗
NASA NTRS1993

On the drag of model dendrite fragments at low Reynolds number

An experimental study of low Reynolds number drag on laboratory models of dendrite fragments has been conducted. The terminal velocities of the dendrites undergoing free fall along their axis of symmetry were measured in a large Stokes flow facility. Corrections for wall interference give nearly linear drag vs Reynolds number curves. Corrections for both wall interference and inertia effects show that the dendrite Stokes settling velocities are always less than that of a sphere of equal mass and volume. In the Stokes limit, the settling speed ratio is found to correlate well with primary dendrite arm aspect ratio and a second dimensionless shape paremeter which serves as a measure of the fractal-like nature of the dendrite models. These results can be used to estimate equiaxed grain velocities and distance of travel in metal castings. The drag measurements may be used in numerical codes to calculate the movement of grains in a convecting melt in an effort to determine macrosegregation patterns caused by the sink/float mechanism.

Zakhem, R.↗
NASA NTRS1991

New developments in astrodynamics algorithms for autonomous rendezvous

A the core of any autonomous rendezvous guidance system must be two algorithms for solving Lambert's and Kepler's problems, the two fundamental problems in classical astrodynamics. Lambert's problem is to determine the trajectory connecting specified initial and terminal position vectors in a specified transfer time. The solution is the initial and terminal velocity vectors. Kepler's problem is to determine the trajectory that stems from a given initial state (position and velocity). The solution is the state of an earlier or later specified time. To be suitable for flight software, astrodynamics algorithms must be totally reliable, compact, and fast. Although solving Lambert's and Kepler's problems has challenged some of the world's finest minds for over two centuries, only in the last year have algorithms appeared that satisfy all three requirements just stated. This paper presents an evaluation of the most highly regarded Lambert and Kepler algorithms.

Klumpp, Allan R.↗
NASA NTRS1992

Major outburst of periodic Comet Halley at a heliocentric distance of 14 AU

The properties and evolution of the major outburst experienced by Comet Halley at 14 AU during February-March 1991 are studied. It is found that the observed halo is a segment of a conical surface populated by solid particles ejected from a localized, temporarily activated region on the sunlit hemisphere of the spinning nucleus. The total mass of the ejecta was at least 10 exp 12 g and CO was probably the prime driver, accelerating the smallest grains to a terminal velocity of about 45 m/s. The mass loading of the gas flow by particulate matter is enormous, with the mass production rate exceeding the expected production rate of CO by a factor of several tens.

Sekanina, Z.↗
NASA NTRS1992

Deuterium in North Atlantic storm tops

During the ERICA project in 1989, ice crystals were collected from the tops of two winter storms and one broad cirrus cloud. Deuterium concentration in the storm ice samples, together with a model of isotope fractionation, are used to determine the temperature where the ice was formed. Knowledge of the ice formation temperature allows us to determine whether the ice has fallen or been lofted to the altitude of collection. In both storms, the estimated fall distance decreases upward. In the 21 January storm, the fall distance decreases to zero at the cloud top. In the 23 January storm, the fall distance decreases to zero at a point 2 km below the cloud top and appears to become negative above, indicating lofted ice. Cloud particle data from the cloud tops show an ice-to-vapor ratio greater than one and indicate the presence of particles with small terminal velocities; both observations support the idea of ice lofting. The satellite-derived cloud tops lie well below the actual cloud top (e.g., 2.5 km below on 23 January), indicating that the lofted ice in winter storms may not be detectable from space using IR radiance techniques. A comparison of deuterium in cloud-top ice and clear-air vapor suggests that even in winter, when vertical air motions are relatively weak, lofted ice crystals are the dominant source of water vapor in the upper troposphere.

Smith, Ronald B.↗
NASA NTRS1920

Bomb trajectories

The report is a mathematical treatise dealing with the trajectories of bombs of high terminal velocity, dropped from a great altitude.

Wilson, Edwin Bidwell↗
NASA NTRS1993

Presenting the Rain-Sea Interaction Facility

The new Rain-Sea Interaction Facility (RSIF) was established at GSFC/WFF and the first finds are presented. The unique feature of this laboratory is the ability to systematically study microwave scattering from a water surface roughened by artificial rain, for which the droplets are at terminal velocity. The fundamental instruments and systems (e.g., the rain simulator, scatterometers, and surface elevation probes) were installed and evaluated during these first experiments - so the majority of the data were obtained with the rain simulator at 1 m above the water tank. From these initial experiments, three new models were proposed: the square-root function for NCS vs. R, the log Gaussian model for ring-wave elevation frequency spectrum, and the Erland probability density distribution for back scattered power. Rain rate is the main input for these models, although the coefficients may be dependent upon other factors (drop-size distribution, fall velocity, radar configuration, etc.). The facility is functional and we foresee collaborative studies with investigators who are engaged in measuring and modeling rain-sea interaction processes.

Bliven, Larry F.↗
NASA NTRS1994

Rationale for a Mars Pathfinder mission to Chryse Planitia and the Viking 1 lander

Presently the landing site for Mars Pathfinder will be constrained to latitudes between 0 deg and 30 deg N to facilitate communication with earth and to allow the lander and rover solar arrays to generate the maximum possible power. The reference elevation of the site must also be below 0 km so that the descent parachute, a Viking derivative, has sufficient time to open and slow the lander to the correct terminal velocity. Although Mars has as much land surface area as the continental crust of the earth, such engineering constraints immediately limit the number of possible landing sites to only three broad areas: Amazonis, Chryse, and Isidis Planitia. Of these, both Chryse and Isidis Planitia stand out as the sites offering the most information to address several broad scientific topics.

Craddock, Robert A.↗
NASA NTRS1994

A deep atmospheric eclipse of AL Velorum

I have obtained new International Ultraviolet Explorer (IUE) observations in 1992 to search for an accretion column in and to analyze an atmospheric eclipse of AL Vel, a low luminosity zeta Aurigae binary. Rich shell spectra recorded at three phases of atmospheric eclipse give column densities and excitation temperatures for the chromospheric gas. The chromospheric density falls off exponentially with a scale height of 3 solar radius, an order of magnitude greater than expected for support by thermal motions. Excitation temperatures at 2 - 5 scale heights into the chromosphere are approximately 7000 K, with Doppler widths near 17 km/s. Absorption features that could be formed in an accretion column were detected at phases 0.70 and 0.75 in a series of spectra spanning the range 0.63-0.80 in orbital phase. They imply a very slow acceleration of the wind, consistent with results for other zeta Aur binaries, but the wind speed implied is inconsistent with profiles of wind lines. The terminal velocity of this wind, greater than or approximately = 360 km/s, is higher than that in any of the other zeta Aur binaries. Wind and chromospheric features in 1992 were much more prominent than in 1988, which implies that both the wind and chromosphere were much more massive: we seem to have witnessed in 1992 a burst of activity with a mass-loss rate greater than 10(exp -8) solar mass/yr.

Eaton, Joel A.↗
NASA NTRS1994

Modelling the low-latitude boundary layer with reconnection entry

We develop a one-dimensional Low Latitude Boundary Layer (LLBL) model for northward interplanetary magnetic field (IMF). The boundary layer in this model is uniform in the direction normal to the magnetopause, a 'plateau-type' boundary layer. The boundary layer motion is decoupled from the magnetosheath motion and driven by the plasma pressure associated with the incoming solar wind plasma near local noon, which has become entrained on closed field lines as a result of reconnection in the cusp region. Dissipation in the ionosphere at the feet of the boundary layer field lines opposes this motion. There are two physical solutions for the model. In one, the boundary layer reaches a terminal velocity in the tail as the boundary layer plasma effectively joins the solar wind flow. In the other solution, the flow is nearly stopped in the far tail. In combination with other mechanisms, this latter solution may correspond to the case in which the boundary layer plasma participates in magnetospheric convection and returns sunward. The density, velocity, and thickness as functions of distance from local noon are studied, assuming that the magnetopause hasa elliptical shape and the magnetospheric field is dipolar.

Song, P.↗