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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 181 records · Page 10
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NASA NTRS1988

A multiple-vortex-ring model of the DFW microburst

A multiple-vortex-ring model of the winds associated with a microburst is verified by matching the model-generated winds to those encountered at the Dallas-Ft. Worth (DFW) microburst. The basis of the model consists of time-invariant vortex ring filaments embedded in irrotational flow. Each ring's viscous core is modeled by distributing the vorticity over a small distance (relative to the ring diameter) radially from the filaments. Parameters such as the size and strength of the vortex rings are identified using a modified Newton-Raphson technique. The parameters identified from the analysis of the DFW microburst encounter indicate a large ring with a radius of 8500 ft and a smaller ring with a radius of 1700 ft.

Schultz, Thomas A.↗
NASA NTRS1989

Voyager radio occultation by Uranus' rings. I - Observational results

The January 24, 1986 radio occultation of Voyager 2 by the Uranian rings yielded fine-resolution, 50- and 200-m optical depth profiles at a 3.6 cm wavelength for the nine pre-Voyager rings; these are found to be opaque, reaching optical depths as great as 6-8 in rings gamma and epsilon, which are the only two rings possessing extremely sharp inner and outer edges. Rings 6, 5, 4, and delta share a morphology characterized by a sharp outer-edge transition and a quasi-exponential inner edge one. In ring eta, this behavior is reversed, and ring beta exhibits both diffuse edges and the smallest of the observed opacities.

Gresh, Donna L.↗
NASA NTRS1989

Simulations of light scattering in planetary rings

The current status of efforts to model light scattering in the Uranian and Saturnian rings is briefly reviewed, with an emphasis on the treatment of thin, vertically warped, or clumpy rings. Consideration is given to observations of an opposition surge in the rings (indicating that they are physically thick) and possible reasons for the failure of classical models to predict the forward-scattering characteristics of the B and inner A rings of Saturn, the quadrupole brightness variations in the A ring, the increase in reflectivity with optical depth in the A and B rings, the increase in brightness with solar and observer angle in the B ring, and systematic brightness variations in images of the inner Cassini division. It is pointed out that some of these discrepancies are eliminated by the use of a ray-tracing simulation code which determines the singly scattered light from a layer one or many particles thick.

Dones, L.↗
NASA NTRS1991

Dynamics and structure of planetary rings

Recent research efforts were directed towards sharpening the understanding of kinematical and dynamical properties of the Uranian rings, with the combination of Earth-based and Voyager observations, and in obtaining and interpreting new observations of the Saturn system from the remarkable stellar occultation of 3 Jul. 1989. Some of the highlights studied include: (1) a detailed comparison of structure and dynamics of the Uranus rings from joint analysis of high quality Earth-based data and the complete set of Voyager occultation measurements; (2) a comprehensive search for weak normal modes excited in the Uranian rings, analogous to the m = 2 and m = 0 normal modes previously identified for the delta and gamma rings; (3) an ongoing search for faint rings and ring arcs of Uranus, using both Voyager images of the rings and Earth-based and spacecraft stellar occultation data; (4) a comparison of upper stratospheric temperatures of Uranus inferred from Voyager ultraviolet occultations with results of ground-based occultation observations; and (5) observations of the 3 Jul. 1989 Saturn occultation of 28 Sgr.

French, R. G.↗
NASA NTRS1993

Seeing ghosts - Photometry of Saturn's G Ring

Saturn's faint and narrow G Ring is only visible to the eye in two Voyager images, each taken at a rather high solar phase angle of about 160 deg. In this paper we introduce a new photometric technique for averaging across multiple Voyager images, and use it to detect the G Ring at several additional viewing geometries. The resultant phase curve suggests that the G Ring is composed of dust particles obeying a very steep power-law size distribution. The dust is generally smaller than that seen in other rings, ranging down to 0.03 micron. The G Ring occupies the region between orbital radii 166,000 and 173,000 km, and has a peak somewhat closer to the inner edge. Based on these limits, we demonstrate that Voyager 2 passed through and directly sampled this ring during its 1981 encounter with Saturn. Combined analysis of additional data sets suggests that a population of larger bodies is also present in the G Ring; these bodies occupy a narrower band near the observed peak and are likely the source for the visible dust. Based on some preliminary dynamical models, we propose that these larger bodies represent leftover debris from the collisional breakup of a small moon in Saturn's distant past.

Showalter, Mark R.↗
NASA NTRS1993

Morphology of large impact craters and basins on Venus: Implications for ring formation

A nearly complete examination of the Magellan radar data for the Venusian surface reveals 72 unequivocal peak-ring craters and 4 larger structures that we interpret to be multiringed. This report updates our earlier studies and that of the Magellan team. The general morphology of peak-ring craters, decreasing ring diameter ratio trends with increasing crater diameter, and the general size-morphology progression from complex central-peak crater to peak-ring crater on Venus and the terrestrial planets suggest similar processes of peak-ring formation. Observations are consistent with a model of dynamic collapse, downward and outward, of an unstable central peak to form a ring. We interpret the four larger ringed structures (Klenova, Lise Meitner, Mead, and Isabella) to be morphologically similar to the Orientale Basin on the Moon, and thus, true multiringed basins.

Alexopoulos, Jim S.↗
NASA NTRS1993

Mass of Saturn's A ring

The mass of Saturn's A ring is reestimated using the behavior of spiral density waves embedded in the ring. The Voyager photopolarimeter (PPS) observed the star delta-Scorpii as it was occulted by Saturn's rings during the Voyager 2 flyby of Saturn in 1981 producing a radial profile of the rings. We examined forty spiral density waves in the Voyager PPS data of the A ring including 10 weaker waves that have not been previously analyzed by means of an autoregressive power spectral technique called Burg. The strengths of this new method for ring studies are that weaker, less extended waves are easily detected and characterized. This method is also the first one which does not require precise knowledge of the resonance location and phase of the wave in order to calculate the surface mass density. Uncertainties of up to 3 km are present in the currently available radial scales for Saturn's rings.

Horn, L. J.↗
NASA NTRS1993

Effect of temperature and O-ring gland finish on sealing ability of Viton V747-75

As a part of the redesign project of the Space Shuttle solid rocket motor (SRM) following the Challenger accident, the field joint was redesigned to minimize the relative joint motion caused by internal motor pressurization during ignition. The O-ring seals and glands for the field joint were designed both to accommodate structural deflections and to promote pressure-assisted sealing. Tests were conducted in various face seal fixtures to evaluate the ability of Viton V747-75 O-rings to seal for a range of temperatures and surface finishes of the redesigned O-ring gland. The effect of surface finish on the sealing performance and wear characteristics of the O-rings was evaluated during simulated launch conditions that included low-frequency vibrations, gap openings, and rapid pressurizations. The effect of contamination on the sealing performance was also investigated. The O-rings sealed throughout the 75 deg F leak check test and for the seal tests from 50 deg F to 120 deg F for the range of surface finishes investigated. Although abrasions were found in the O-rings from pressurization against the rougher finishes, these abrasions were not detrimental to sealing. Below 50 deg F, Viton V747-75 O-rings were insufficiently resilient to track the test gap opening.

Lach, Cynthia L.↗
NASA NTRS1993

Multi-band reflector antenna with double-ring element frequency selective subreflector

Frequency selective subreflectors (FSS) are often employed in the reflector antenna system of a communication satellite or a deep space exploration vehicle for multi-frequency operations. In the past, FSS's have been designed for diplexing two frequency bands. For example, the Voyager FSS was designed to diplex S and X bands and the TDRSS FSS was designed to diplex S and Ku bands. Recently, NASA's CASSINI project requires an FSS to multiplex four frequency (S/X/Ku/Ka) bands. Theoretical analysis and experimental verifications are presented for a multi-band flat pannel FSS with double-ring elements. Both the exact formulation and the thin-ring approximation are described for analyzing and designing this multi-ring patch element FSS. It is found that the thin-ring approximation fails to predict the electrically wide ring element FSS's performance. A single screen double-ring element FSS is demonstrated for the tri-band system that reflects the X-band signal while transmitting through the S- and Ku-band signals. In addition, a double screen FSS with non-similar double-ring elements is presented for the Cassini's four-band system which reflects the X- and Ka-band signals while passing the S- and Ku-band signals. To accurately predict the FSS effects on a dual reflector antenna's radiation pattern, the FSS subreflector's transmitted/reflected field variation as functions of the polarization and incident angles with respect to the local coordinates was taken into account. An FSS transmission/reflection coefficient table is computed for TE and TM polarizations at various incident angles based on the planar FSS model. Next, the hybrid Geometric Optics (GO) and Physical Optics (PO) technique is implemented with linearly interpolating the FSS table to efficiently determine the FSS effects in a dual reflector antenna.

Wu, Te-Kao↗
NASA NTRS1992

The IRAF Fabry-Perot analysis package: Ring fitting

As introduced at ADASSI, a Fabry-Perot analysis package for IRAF is currently under development as a joint effort of ourselves and Frank Valdes of the IRAF group. Although additional portions of the package were also implemented, we report primarily on the development of a robust ring fitting task, useful for fitting the calibration rings obtained in Fabry-Perot observations. The general equation of an ellipse is fit to the shape of the rings, providing information on ring center, ellipticity, and position angle. Such parameters provide valuable information on the wavelength response of the etalon and the geometric stability of the system. Appropriate statistical weighting is applied to the pixels to account for increasing numbers with radius, the Lorentzian cross-section, and uneven illumination. The major problems of incomplete, non-uniform, and multiple rings are addressed with the final task capable of fitting rings regardless of center, cross-section, or completion. The task requires only minimal user intervention, allowing large numbers of rings to be fit in an extremely automated manner.

Shopbell, P. L.↗
NASA NTRS1995

The detection of molecular gas in the ring galaxy Arp 143

We have used the NRAO 12 m telescope to map the inner 10 kpc of NGC 2445, the ring galaxy in Arp 143, in CO-12(J = 1-0). Emission is peaked near the ring galaxy nucleus, but we find evidence for an additional asymmetric and extended CO component. This extended CO distribution is consistent with an approximately 8 kpc diameter crescent-shaped ring of molecular gas, similar to the one seen in H I, accounting for approximately half of the total CO flux. Assuming this distribution, we derive a total H2 mass for NGC 2445 of 0.4-2.4 x 10(exp 10) solar mass, depending on whether a Galactic or low-metallicity Large Magellanic Cloud (LMC) conversion factor is used, and an H2/H I mass ratio between 0.9 and 5. The ring is experiencing low rates of massive star formation despite very high gas column densities. We find that the gas surface density exceeds the critical threshold for star formation throughout the ring, even without a possible contribution from a significant molecular component. The absence of vigorous star formation is most simply understood in terms of its youth (approximately 30 Myr): massive stars have not had time to form in large numbers. Our results support the interpretation that NGC 2445 is a nascent ring galaxy, seen prior to its ring starburst phase.

Higdon, James L.↗
NASA NTRS1993

Satellite dynamics about a planet with a narrow ring

The dynamics of a satellite attracted by a planet with a ring are investigated. A single model for the gravitational potential of a ring is used which extracts the fundamental gravitational effects on a satellite. Analytical and qualitative results are presented in cases where the satellites orbit is in the plane of the ring and where it is out of the plane of the ring. As final results we present equations expressing the perturbation of a body from two body motion when the ring is narrow. These effects include the advance of the argument of the periapsis and the precession of the plane of the orbit (when the satellite orbit is not contained in the ring plane). Additionally, inequalities on the angular momentum of the satellite (for the planar case) are found which guarantee that the orbits are bounded and do not intersect the ring.

Scheeres, D. J.↗
NASA NTRS1999

Experiments on Diffusion Flame Structure of a Laminar Vortex Ring

The study of flame-vortex interactions provides one of the means to better understand turbulent combustion, and allows for canonical configurations that contain the fundamental elements found in turbulent flames, These include concentrated vorticity, entrainment and mixing, strain and nonequilibrium phenomena, diffusion and differential diffusion, partial premixing and diluent effects, and heat release effects. In flame- vortex configurations, these fundamental elements can be studied under more controlled conditions than is possible in direct investigations of turbulent flames. Since the paper of Marble, the problem of the flame-vortex interaction has received considerable attention theoretically, numerically and experimentally. Several configurations exist for study of the premixed flame/vortex ring interaction but more limited results have been obtained to date for the diffusion flame/vortex ring case. The setup of Chen and Dahm, which is conceptually similar to that of Karagozian and Manda and Karagozian, Suganuma and Strom where the ring is composed of fuel and air and combustion begins during the ring formation process, is used in the current study. However, it is essential to conduct the experiments in microgravity to remove the asymmetries caused by buoyancy and thus obtain highly symmetric and repeatable interactions. In previous studies it was found that the flame structure of the vortex ring was similar to that obtained analytically by Karagozian and Manda. Dilution of propane with nitrogen led mainly to a reduction in flame luminosities, flame burnout times were affected by both fuel volumes and amount of dilution, and a simple model of the burnout times was developed. In this paper, a discussion on reacting ring displacement and flame burnout time will be given, and the flame structures of vortex rings containing ethane and air will be compared to those of propane reacting in air.

Chen, Shin-Juh↗
NASA NTRS2001

Orbits of Saturn's Inner Moons and Other Observations Connected within the 1995-1996 Saturnian Ring Plane Crossing

We reduced and analyzed radio images of Saturn taken in 1990-1995 at 0.35, 2.0, 3.6 and 6.1 cm wavelength. The latitudinal brightness distribution varies substantially over time. Detailed modeling of these data shows that NH4SH condenses at the thermochemical equilibrium temperature of 235.5 K only at Northern mid-latitudes. Over most of the planet, condensation does not occur until the gas cools to 190 +/- 5 K. Supersaturation may also cause the dark equatorial region seen in 1995 at 6.1 cm. Observations of the rings show that the West (dusk) ansa is brighter than the East (dawn) ansa by factors up to 2. The magnitude of the asymmetry increases with increasing wavelength and with decreasing distance to the planet. This East-West asymmetry may be due to multiple scattering in gravitational wakes. We improved the Monte Carlo scattering code used to model the microwave emission from Saturn's rings at multiple wavelengths. The basic code outlined in Dunn, Molnar, and Fix describes the rings as a uniform, infinite layer of ice spheres with a finite vertical extent. The particle sizes follow a power law distribution and scatter light according to a linear combination of Mie and isotropic phase functions. These parameters may be adjusted to reflect the physical properties in the scattering properties of the ring particles. This is used to construct a model which can be directly compared to Very Large Array data. The code has been extended by including anisotropic particle distributions, primarily in the form of the wake structures that are likely to exist in the enhancements. The vertical extent of the wakes equals that of the ring layer. Between the wakes are regions of diminished density so that the optical depth integrated over many wakes matches that of the Voyager occultation experiments. This allows us to model the brightness temperature as a function of azimuth and look for asymmetric scattering behavior in the rings. By changing the width and relative density of the wakes, we can alter the nature of the gross scattering properties of the rings to match what is seen in our data.

Lissauer, Jack J.↗
NASA NTRS2002

Dynamics and Structure of Planetary Rings

We developed a novel technique to determine the macroscopic particle size distribution of Saturn's rings by exploiting diffraction effects during stellar occultations. This was a major undertaking, and resulted in two publications now in press. In the latter paper, we derived power-law particle size distributions for each of Saturn's main ring regions, using observations of the 3 July 1989 stellar occultation of 28 Sgr from Palomar, McDonald, and Lick Observatories. We used the Voyager PPS delta Sco optical depth profile to estimate and then remove the directly transmitted signal from the 28 Sgr observations, leaving high SNR scattered light profiles at wavelengths of 3.9, 2.1, and 0.9 micrometers. The angular distribution of this diffracted signal depends on the ring particle size distribution: the sharpness of the forward lobe is set by the largest particles, while the overall breadth and amplitude of the scattered signal reflects the abundance of smaller, cm-sized particles. We developed both a simple one-dimensional scattering model and a more realistic 2-D model. We assumed for simplicity a single power law particle size distribution for each major ring region, and determined the index q and lower and upper size cutoffs a(sub min) and a(sub max) that provide the best match to all three data sets in each region. Our results in the A and C Rings are fairly consistent with values of q and a(sub max) derived from Voyager radio occultation (RSS) measurements. We extended their results by determining lower limits to the particle size distributions and by probing the B Ring. This technique is applicable to imaging observations of the rings during the Cassini mission.

French, Richard G.↗
NASA NTRS2002

Androgynous, Reconfigurable Closed Loop Feedback Controlled Low Impact Docking System With Load Sensing Electromagnetic Capture Ring

The present invention relates to a fully androgynous, reconfigurable closed loop feedback controlled low impact docking system with load sensing electromagnetic capture ring. The docking system of the present invention preferably comprises two Docking- assemblies, each docking assembly comprising a load sensing ring having an outer face, one of more electromagnets, one or more load cells coupled to said load sensing ring. The docking assembly further comprises a plurality of actuator arms coupled to said load sensing ring and capable of dynamically adjusting the orientation of said load sensing ring and a reconfigurable closed loop control system capable of analyzing signals originating from said plurality of load cells and of outputting real time control for each of the actuators. The docking assembly of the present invention incorporates an active load sensing system to automatically dynamically adjust the load sensing ring during capture instead of requiring significant force to push and realign the ring.

Lewis, James L.↗
NASA NTRS2005

Discovery of Oxygen Kalpha X-ray Emission from the Rings of Saturn

Using the Advanced CCD Imaging Spectrometer (ACIS), the Chandra X-ray Observatory (CXO) observed the Saturnian system for one rotation of the planet (approx.37 ks) on 20 January, 2004, and again on 26-27 January, 2004. In this letter we report the detection of X-ray emission from the rings of Saturn. The X-ray spectrum from the rings is dominated by emission in a narrow (approx.130 eV wide) energy band centered on the atomic oxygen Ka fluorescence line at 0.53 keV. The X-ray power emitted from the rings in the 0.49-0.62 keV band is about one-third of that emitted from Saturn disk in the photon energy range 0.24-2.0 keV. Our analysis also finds a clear detection of X-ray emission from the rings in the 0.49-0.62 keV band in an earlier (14-15 April, 2003) Chandra ACIS observation of Saturn. Fluorescent scattering of solar X-rays from oxygen atoms in the H20 icy ring material is the likely source mechanism for ring X-rays, consistent with the scenario of solar photo-production of a tenuous ring oxygen atmosphere and ionosphere recently discovered by Cassini.

Bhardwaj, Anil↗
NASA NTRS2005

Irregular Wavelike Structure in Saturn's Rings

We have searched Saturn's A, B, and C rings for irregular wavelike structure using Voyager Photopolarimeter (PPS), Ultraviolet Spectrometer (UVS), and Radio Science (RSS) occultation datasets, as well as ring reflectivity profiles derived from Voyager images. A maximum entropy technique for conducting spectral analysis was used to estimate wave frequency power in relation to radial location for each dataset. Using this method we have found irregular structure in the PPS and UVS inner B Ring occultation datasets previously identified in Voyager imaging data. Both finer structure, with a wavelength of around 20 km, and large structure with wavelengths of 200 to 1000 km, are visible in the occultation data and appear similar to that seen in the imaging data. After removing ringlets from the C-Ring data, we have identified what appears to be a 1000-km wave sustained throughout the ring. The large dominant wavelength appears in all datasets; however, tests are currently being conducted in an attempt to verify its existence. Irregular structure with a wavelength of approximately 20 km has been observed in the C Ring reflectivity profiles, but not within the occultation datasets. This leads us to doubt it is caused by ring surface mass density fluctuations detectable by the occultation experiments.

Pollard, Benjamin J.↗