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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 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Collisional dynamics of perturbed particle disks in the solar system

Investigations of the collisional evolution of particulate disks subject to the gravitational perturbation of a more massive particle orbiting within the disk are underway. Both numerical N-body simulations using a novel collision algorithm and analytical kinetic theory are being employed to extend our understanding of perturbed disks in planetary rings and during the formation of the solar system. Particular problems proposed for investigation are: (1) The development and testing of general criteria for a small moonlet to clear a gap and produce observable morphological features in planetary rings; (2) The development of detailed models of collisional damping of the wavy edges observed on the Encke division of Saturn's A ring; and (3) The determination of the extent of runaway growth of the few largest planetesimals during the early stages of planetary accretion.

Roberts, W. W.↗
NASA NTRS1987

Timescales for planetary accretion and the structure of the protoplanetary disk

Models of planetary accretion which assume the mass of condensable matter in the protoplanetary disk was equal to that present in the planets today predict accretion timescales for the outer planets approximately or less than 10 to the 8th years. Such timescales are inconsistent with observations of star forming regions, which suggest that most of the gas in disks around one solar mass is removed in a few x 10 to the 6th years. A unified scenario was outlined for solar system formation consistent with astrophysical constraints. Jupiter's core could have grown by runaway accretion of planetesimals to a mass sufficient to initiate rapid accretion of gas in times of order of 500,000 to 5,000,000 years, provided the surface density of solids in its accretion zone was at least 5 to 10 times greater than that required by minimum mass models of the protoplanetary disk. The inner planets and the asteroids can be accounted for in this picture if the surface density of the solar nebula was relatively uniform out to Jupiter's orbit. The formation of such a protoplanetary disk requires significant transport of mass and angular momentum, and is consistent with viscous accretion disk models of the solar nebula.

Lissauer, Jack J.↗
NASA NTRS1987

A scaling law for accretion zone sizes

Current theories of runaway planetary accretion require small random velocities of the accreted particles. Two body gravitational accretion cross sections which ignore tidal perturbations of the Sun are not valid for the slow encounters which occur at low relative velocities. Wetherill and Cox have studied accretion cross sections for rocky protoplanets orbiting at 1 AU. Using analytic methods based on Hill's lunar theory, one can scale these results for protoplanets that occupy the same fraction of their Hill sphere as does a rocky body at 1 AU. Generalization to bodies of different sizes is achieved here by numerical integrations of the three-body problem. Starting at initial positions far from the accreting body, test particles are allowed to encounter the body once, and the cross section is computed. A power law is found relating the cross section to the radius of the accreting body (of fixed mass).

Greenzweig, Yuval↗
NASA NTRS1987

AKM capture device

In an effort to recover the Westar and Palapa satellites and the considerable investment each represented, NASA and Hughes undertook the Satellite Retrieval Mission. The mechanism used to capture each of the errant satellites was the AKM (Apogee Kick Motor) Capture Device (ACD), also referred to as the Stinger. The ACD had three interface requirements: interface with the Manned Maneuvering Unit (MMU) for transportation to and stabilization of the spacecrafts; interface with each satellite for retrieval; and finally, interface with the Shuttle's Remote Manipulator System (RMS or robot arm) for satellite transport back to the Orbiter's payload bay. The majority of the design requirements were associated with the capture and release of the satellites. In addition to these unique requirements, the general EVA, RMS grapple, and RMS manipulation requirements applied. These requirements included thermal, glare, snag, RMS runaway and crewman safety considerations.

Harwell, William D.↗
NASA NTRS1985

Application of radiative boundary conditions to nonhydrostatic primitive equation models

A procedure is presented for applying a radiative boundary condition to the lateral boundary of a limited domain model. The model considered, the Terminal Area Simulation System (TASS), has nonhydrostatic compressible and unsteady governing equations. The primary applications to date have been studies of downbursts, wind shear, thunderstorms and mass fires. A time-splitting integration is used with TASS, with the acoustic terms being integrated separately with a smaller time step. A first-order closure approximation is employed to ensure subgrid turbulence closure. TASS applications for two-dimensional axisymmetric and three-dimensional simulations are described. The radiative boundary conditions were applied in the two-dimensional case to the temperature, pressure and non-normal velocity components. Solutions in both cases avoided runaway circulations and experienced minimal domain-wide mass loss. Also, distorsion was minimized at the boundaries.

Proctor, F. H.↗
NASA NTRS1987

On the theory of type I X-ray bursts - The energetics of bursts and the nuclear fuel reservoir in the envelope

A comparison is made between the observed properties of type I X-ray bursts from 4U/MXB 1636-53 and those of models of thermonuclear flashes on accreting neutron stars. Possible explanations of variations in the burst recurrence properties without an apparent correlation with accretion rate are discussed. The strongest bursts with photospheric expansion are described well by current spherically symmetric models. The observed weak bursts suggest a mechanism that can promote the inward propagation and heating of accreted fuel so as to give rise to thermonuclear runaways much earlier than expected from spherical models. A sufficient amount of fuel to feed the weak bursts can be stored in the outer layers for neutron stars with radii greater than 8 km. The proposed mechanism and fuel storage can resolve the longstanding problem of bursts in rapid succession with intervals shorter than about 10 minutes. The proposed model explains the variation of burst peak fluxes observed from 4U/MSB 1636-53.

Fujimoto, Masayuki Y.↗
NASA NTRS1988

Interactions of planetesimals with protoplanetary atmospheres

The 'core instability' mechanism presently postulated for the interaction of planetesimals with the envelopes of growing giant planets involves core growth through the accretion of solid bodies in the solar nebula; the core thereby becomes more effective in the gravitational attraction of gas from the surrounding nebula into an envelope, until a runaway accretion stage is reached. Two-body trajectory curves of planetesimals through this envelope are computed which demonstrate the depth of penetration and the cross-sections for capture of icy and rocky planetesimals.

Podolak, Morris↗
NASA NTRS1988

Accretion flows onto supermassive black holes

The radiative and hydrodynamic properties of an angular momentum-dominated accretion flow onto a supermassive black hole depend largely on the ratio of the accretion rate to the Eddington accretion rate. High values of this ratio favor optically thick flows which produce largely thermal radiation, while optically thin 'two-temperature' flows may be present in systems with small values of this ratio. Observations of some AGN suggest that thermal and nonthermal sources of radiation may be of comparable importance in the 'central engine'. Consideration is given to the possibilities for coexistence of different modes of accretion in a single flow. One intriguing possibility is that runaway pair production may cause an optically thick 'accretion annulus' to form at the center of a two-temperature inflow.

Begelman, Mitchell C.↗
NASA NTRS1988

Energy dependent pitch angle distributions of auroral primary electrons

Double-layer/parallel-electric field accelerations and the subsequent electron-beam plasma interactions involving Cerenkov and anomalous cyclotron resonances are considered. It is found that these phenomena yield pitch angle distributions as noted from rocket and satellite studies. Although the electron acceleration by weak parallel electric fields forming a runaway electron tail is limited to a critical parallel energy determined by the anomalous cyclotron resonance, such a limitation does not occur with acceleration by a localized parallel electric field such as that in a double layer.

Singh, N.↗
NASA NTRS1988

Cosmological quantum chromodynamics, neutron diffusion, and the production of primordial heavy elements

A simple one-dimensional model is used to describe the evolution of neutron density before and during nucleosynthesis in a high-entropy bubble left over from the cosmic quark-hadron phase transition. It is shown why cosmic nucleosynthesis in such a neutron-rich environment produces a surfeit of elements heavier than lithium. Analytical and numerical techniques are used to estimate the abundances of carbon, nitrogen, and heavier elements up to Ne-22. A high-density neutron-rich region produces enough primordial N-14 to be observed in stellar atmospheres. It shown that very heavy elements may be created in a cosmological r-process; the neutron exposure in the neutron-rich regions is large enough for the Ne-22 to trigger a catastrophic r-process runaway in which the quantity of heavy elements doubles in much less than an expansion time due to fission cycling. A primordial abundance of r-process elements is predicted to appear as an excess of rare earth elements in extremely metal-poor stars.

Applegate, J. H.↗
NASA NTRS1989

On the effects of Cosmions upon the structure and evolution of very low mass stars

A number of recent studies have suggested that cosmions, or WIMPS, may play an important role in the energetics of the solar interior; in particular, it has been argued that these hypothetical particles may transport sufficient energy within the nuclear-burning solar core so as to depress the solar core temperature to the point of resolving the solar neutrino problem. Solutions to the solar neutrino problem have proven themselves to be quite nonunique, so that it is of some interest whether the cosmion solution can be tested in some independent manner. It is argued that if cosmions solve the solar neutrino problem, then they must also play an important role in the evolution of low mass main sequence stars; and, second, that if they do so, then a simple (long mean free path) model for the interaction of cosmions with baryons leads to changes in the structure of the nuclear-burning core which may be in principal observable. Such changes include suppression of a fully-convective core in very low mass main sequence stars; and a possible thermal runaway in the core of the nuclear burning region. Some of these changes may be directly observable, and hence may provide independent constraints on the properties of the cosmions required to solve the solar neutrino problem, perhaps even ruling them out.

Deluca, E. E.↗
NASA NTRS1987

A mathematical model of a lithium/thionyl chloride primary cell

A 1-D mathematical model for the lithium/thionyl chloride primary cell was developed to investigate methods of improving its performance and safety. The model includes many of the components of a typical lithium/thionyl chloride cell such as the porous lithium chloride film which forms on the lithium anode surface. The governing equations are formulated from fundamental conservation laws using porous electrode theory and concentrated solution theory. The model is used to predict 1-D, time dependent profiles of concentration, porosity, current, and potential as well as cell temperature and voltage. When a certain discharge rate is required, the model can be used to determine the design criteria and operating variables which yield high cell capacities. Model predictions can be used to establish operational and design limits within which the thermal runaway problem, inherent in these cells, can be avoided.

Evans, T. I.↗
NASA NTRS1988

Low temperature safety of lithium-thionyl chloride cells

The use of lithium thionyl chloride cells for low-temperature applications is presently restricted because of their unsafe behavior. An attempt is made in the present investigation to identify the safe/unsafe low temperature operating conditions and to understand the low temperature cell chemistry responsible for the unsafe behavior. Cells subjected to extended reversal at low rate and -40 C were found to explode upon warm-up. Lithium was found to deposit on the carbon cathodes during reversal. Warming up to room temperature may be accelerating the lithium corrosion in the electrolyte. This may be one of the reasons for the cell thermal runaway.

Subbarao, S.↗
NASA NTRS1989

A comparative study of the early terrestrial atmospheres

The CO2 abundance required for maintenance of the climatic conditions speculated to have existed 4 billion years ago on the earth, Mars, and Venus is presently sought by means of a one-dimensional radiative-convective atmospheric model. It is ascertained on this basis that the terrestrial and Martian atmospheres were highly stable, and unlikely to proceed to a moist or runaway greenhouse state even if CO2 partial pressure were equivalent to 100 bars on earth. The proximity of Venus to the sun rendered its atmosphere highly susceptible to rapid water vapor photodissociation and hydrogen escape.

Durham, Richard↗
NASA NTRS1989

X-ray emission from O-type stars - Parameters which affect it

As a result of previous analyses of the X-ray fluxes of O-type stars, it has been established that, surprisingly, there is no correlation between X-ray flux and such basic parameters as M, R, v(rot), T(eff), mass accretion rate v(infinity), L(w) = mass accretion rate v(infinity), and wind energy. In this paper, other factors influencing the X-ray flux of the hottest stars are sought. As a result of the present analysis, it is found that stars embedded in dense clouds are often X-ray brighter, runaway stars are fainter in X-rays, massive binaries emit more X-rays than single stars, and radio-bright stars are also more active in X-rays. All these correlations lead to the conclusion that the stellar environment and the interaction of the stellar wind with circumstellar matter (possible remnants of protostellar clouds) play a dominant role in the production of the X-ray flux.

Chlebowski, T.↗
NASA NTRS1989

A thermal analysis of a spirally wound battery using a simple mathematical model

A two-dimensional thermal model for spirally wound batteries has been developed. The governing equation of the model is the energy balance. Convective and insulated boundary conditions are used, and the equations are solved using a finite element code called TOPAZ2D. The finite element mesh is generated using a preprocessor to TOPAZ2D called MAZE. The model is used to estimate temperature profiles within a spirally wound D-size cell. The model is applied to the lithium/thionyl chloride cell because of the thermal management problems that this cell exhibits. Simplified one-dimensional models are presented that can be used to predict best and worst temperature profiles. The two-dimensional model is used to predict the regions of maximum temperature within the spirally wound cell. Normal discharge as well as thermal runaway conditions are investigated.

Evans, T. I.↗
NASA NTRS1989

Particle-in-cell simulations of stochastic electron acceleration

The results of a series of particle-in-cell simulations of stochastic wave-particle interaction are presented. The threshold for stochasticity was confirmed. The simulations demonstrate that in a strong magnetic field plasma waves with quiver velocities much less than the speed of light but above a certain threshold can stochastically accelerate electrons to energies far greater than 1 MeV. Moreover, self-consistency effects drive return currents and produce energetic runaway electrons that violate an invariant of motion.

Akimoto, K.↗
NASA NTRS1990

Simulations and observations of cloudtop processes

Turbulent entrainment at zero mean shear stratified interfaces has been studied extensively in the laboratory and theoretically for the classical situation in which density is a passive tracer of the mixing and the turbulent motions producing the entrainment are directed toward the interface. It is the purpose of the numerical simulations and data analysis to investigate these processes and, specifically, to focus on the following questions: (1) Can local cooling below cloudtop play an important role in setting up convective circulations within the cloud, and bringing about entrainment; (2) Can Cloudtop Entrainment Instability (CEI) alone lead to runaway entrainment under geophysically realistic conditions; and (3) What are the important mechanisms of entrainment at cloudtop under zero or low mean shear conditions.

Siems, S. T.↗