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Results for “Collision Rates”

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 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Low-perigee aerodynamic heating during orbital flight of an atmosphere Explorer

An extensive, low-perigee orbital aerodynamic heating study was undertaken in support of the Atmosphere Explorer-C Temperature Alarm. State of the art of low-density, high-speed flows, some models of the earth's atmosphere, external flow-field definition, thermodynamic and transport properties of atmospheric gases, the accommodation coefficient orbital thermal environment, and correlation of theory and measurements are discussed. Aerodynamic heating rates are determined for eight selected orbits by means of a reduced, analytical model verified by both ground test and flight data. These heating rates are compared with classical free-molecule and first-order collision regime values.

Caruso, P. S., Jr.↗
NASA NTRS1977

The influence of autoionization accompanied by excitation on dielectronic recombination and ionization equilibrium

In the process of dielectronic recombination, the doubly excited state formed by radiationless capture may autoionize preferentially into an excited state of the recombining ion. This additional autoionization process has not been discussed in previous treatments of dielectronic recombination. The dielectronic recombination rates for certain nonhydrogenic Fe ions, although still larger than the direct radiative recombination rates, are found to be substantially reduced by the inclusion of the additional autoionization rate in the branching ratio for the stabilizing radiative transition. Consequently, the temperatures of maximum equilibrium abundance are significantly lower than those predicted by recent calculations. Finally, the radiative energy loss rate coefficients are calculated for radiation processes involving electron Fe-ion collisions in high-temperature plasmas. Electron impact excitation of resonance line radiation is the dominant radiative cooling mechanism in steady-state plasmas at temperatures where ions with bound electrons are abundant. However, it is found that the radiation emitted during dielectronic recombination can be more important than direct recombination radiation and bremsstrahlung.

Jacobs, V. L.↗
NASA NTRS1977

Rotational excitation of linear molecules by collisions with atoms - Comparison of classical and quantum methods

Exact quantum results for the rotational excitation of rigid linear molecules by collisions with atoms are compared with classical trajectory results. The systems studied are CO-He, CS-H2, OCS-H2, HCl-He, and HCl-Ar at collision energies up to 500/cm. Total cross sections and state to state rate constants are compared. The classical results are found to be in good agreement with the quantum results on the average. Differences arising from the existence of purely quantum effects are clearly evident, but consistent and predictable. Two methods of extracting state selective information from moments of the classical distribution are examined and found to be less reliable than the usual histogram method. In conjunction with previous comparisons of classical and quantum results these calculations provide a useful measure of the limitations and reliability of classical trajectories.

Chapman, S.↗
NASA NTRS1984

Perceived threat and avoidance maneuvers in response to cockpit traffic displays

Airline pilots rated their perception of the danger of an air-to-air collision based on cockpit displays of traffic information while they monitored simulated departures. They selected avoidance maneuvers when necessary for separation. Most evasive maneuvers were turns rather than vertical maneuvers. Evasive maneuvers chosen for encounters with lowor moderate-collision danger were generally toward the intruding aircraft. This tendency lessened as the perceived threat level increased. In the highest threat situations, pilots turned toward the intruder only at chance levels. Intruders coming from positions in front of the pilot's ship were more frequently avoided by turns toward than when intruders approached laterally or from behind. Some of the implications of the pilot's turning-toward tendencies are discussed with respect to automatic collision avoidance systems and coordination of avoidance maneuvers of conflicting aircraft.

Smith, J. D.↗
NASA NTRS1987

Theoretical studies of Resonance Enhanced Stimulated Raman Scattering (RESRS) of frequency-doubled Alexandrite laser wavelength in cesium vapor

The solutions for the imaginary susceptibility of the Raman field transition with arbitrary relaxation rates and field strengths are examined for three different sets of relaxation rates. These rates correspond to: (1) Far Infrared (FIR) Raman lasers in the diabatic collision regime without consideration of coupled population decay in a closed system, (2) Raman FIR lasers in the diabatic collision regime with coupled population conserving decay, and (3) IR Raman gain in cesium vapor. The model is further expanded to include Doppler broadening and used to predict the peak gain as a function of detuning for a frequency doubled Alexandrite laser-pumped cesium vapor gain cell.

Lawandy, Nabil M.↗
NASA NTRS1990

Spatial distribution of H2O(+) in Comet P/Halley

Comet Halley spectroscopic and imaging data near the time of the Vega 1 encounter in March 1986 have been analyzed for H2O(+) distribution and near-nucleus ion physics, using a data-reduction procedure encompassing flat fielding, sky removal, continuum subtraction, contamination correction, absolute flux calibration, and conversion to ion-column densities. A 25,000 km-radius collision zone within which collisions dominate the solar wind's sweeping effect is verified. A production rate of H2O(+) ions is calculated on the basis of the average velocity and column density integral across the tail. An analysis of the tailward ion flow indicates constant acceleration beyond 130,000 km from the nucleus.

Disanti, Michael A.↗
NASA NTRS1983

Density drift instabilities and weak collisions

A model is developed which describes the effects of weak collisions on the linear kinetic theory of electrostatic density drift instabilities. A dispersion equation valid at all frequencies and wave numbers is derived using the assumptions of a weak, uniform density gradient; a uniform magnetic field; and the BGK collision operator with a modification of the local approximation. The properties of the universal and collisional density drift instabilities at maximum growth rates are examined in detail. The thresholds of the instabilities are examined for an ionospheric model which includes ion-neutral, electron-neutral, and electron-ion collisions, and are compared with the threshold of the lower hybrid density drift instability. It is concluded that the k to the -5th short wavelength density power spectra observed above 280 km in the PLUMEX experiment are due to the effects of the universal density drift instability.

Gary, S. P.↗
NASA NTRS2016

Comparison of Quantum Mechanical and Empirical Potential Energy Surfaces and Computed Rate Coefficients for N2 Dissociation

Physics-based modeling of hypersonic flows is predicated on the availability of chemical reaction rate coefficients and cross sections for the collisional processes. This approach has been built around the use of quantum mechanical calculations to describe the interaction between the colliding particles. In this approach a potential energy surface (PES) is computed by solving the electronic Schrödinger equation and collision cross sections are determined for that PES using classical, semiclassical or quantum mechanical scattering methods. The rate coefficients are computed by integrating the thermally weighted cross sections. State-to-state rate coefficients are determined by only integrating over a thermal distribution of collisional energies. Finally, thermal rate coefficients are determined by summation of the state-to-state rate coefficients for reactions of molecules in all relevant ro-vibrational energy levels. If the flow is in thermal non-equilibrium, the translational, vibrational and rotational energy modes can be represented in different ways: three unique temperatures can be used to describe the distributions, the populations of individual ro-vibrational energy levels can be determined by solving the Master Equation, or through the use of direct simulation in particle-based Monte Carlo sampling. The PES-to-rate coefficient approach had been proposed and attempted in the early days of digital computing, but it is only in the last 15 years that computer hardware and software have been up to the task of calculating accurate interatomic and intermolecular potentials.

Jaffe, Richard L.↗
NASA NTRS1992

Columbus proto-planetesimal dust aggregation experiment

A microgravity experiment to study the growth of dust particles which has been proposed to be flown on one of the Columbus precursor flights is described. The microgravity environment will allow for low collision velocities (of order of mm/s) of the dust grains and for a large Knudsen number of the embedding gas; conditions expected in the early solar nebula. The outcome of the experiment will yield estimates of the sticking efficiency and the critical velocity for agglomeration. The values of these two parameters will provide substantial improvements in the constraints for models of the formation of planetesimals. In particular, the questions related to growth rate and mode of the aggregation process will be answered. The range of material type, collision velocities, properties of the environment in which growth takes place, and other factors permit a natural extension of this experiment to take advantage of the capabilities of the Space Station Columbus. Other astrophysical applications, such as processes in Saturnian rings, with somewhat different regimes could also be investigated.

Keller, H. U.↗
NASA NTRS2014

Wald Sequential Probability Ratio Test for Space Object Conjunction Assessment

This paper shows how satellite owner/operators may use sequential estimates of collision probability, along with a prior assessment of the base risk of collision, in a compound hypothesis ratio test to inform decisions concerning collision risk mitigation maneuvers. The compound hypothesis test reduces to a simple probability ratio test, which appears to be a novel result. The test satisfies tolerances related to targeted false alarm and missed detection rates. This result is independent of the method one uses to compute the probability density that one integrates to compute collision probability. A well-established test case from the literature shows that this test yields acceptable results within the constraints of a typical operational conjunction assessment decision timeline. Another example illustrates the use of the test in a practical conjunction assessment scenario based on operations of the International Space Station.

Navigation↗
NASA NTRS1990

Accretion rates of protoplanets

Projections are made of the collision/accretion rates of planetesimals in a uniform surface-density disk with a massive protoplanet, whose collision cross-section is enhanced, relative to its geometric cross-section, by a gravitational focusing of the planetesimals' trajectories. A scaling method is presented which generalizes the results obtained to protoplanets of arbitrary mass, radius, and orbital semimajor axis. It is established that, in the case where one protoplanet dominates gravitational scattering in a given protoplanetary disk region, planetesimal inclinations are substantially less rapidly excited than eccentrocities; the skewed velocity dispersion thus generated allows for a more rapid rate of protoplanet growth.

Greenzweig, Yuval↗
NASA NTRS1982

Cratering time scales for the Galilean satellites

An attempt is made to estimate the present cratering rate for each Galilean satellite within the correct order of magnitude and to extend the cratering rates back into the geologic past on the basis of evidence from the earth-moon system. For collisions with long and short period comets, the magnitudes and size distributions of the comet nuclei, the distribution of their perihelion distances, and the completeness of discovery are addressed. The diameters and masses of cometary nuclei are assessed, as are crater diameters and cratering rates. The dynamical relations between long period and short period comets are discussed, and the population of Jupiter-crossing asteroids is assessed. Estimated present cratering rates on the Galilean satellites are compared and variations of cratering rate with time are considered. Finally, the consistency of derived cratering time scales with the cratering record of the icy Galilean satellites is discussed.

Shoemaker, E. M.↗
NASA NTRS1973

Matter-antimatter cosmology

Mechanisms responsible for the orgin and separation of matter and antimatter in the universe are discussed. Particular attention was given to coalesence. This mechanism involves annihilation production of along the boundary of matter-antimatter, high energy photons, electrons, and positrons. These particles together with secondary particles which they put into motion by collisions, carry their momentum to the matter or antimatter fluid. Coalescence and annihilation rates are discussed.

Omnes, R.↗
NASA NTRS1973

Laboratory simulation of graphite formation in stellar envelopes

A technique has been developed which permits laboratory simulation of graphite condensation in the envelopes of cool, late sequence stars. Processes in the stellar envelope were simulated by allowing hydrogen-carbon mixtures of typical circumstellar composition and temperature to expand freely into vacuum, cooling and supersaturating the vapor. Expansion products were determined mass spectrometrically through the use of molecular beam techniques. The results of laboratory expansions were scaled to those occurring near M and N stars by comparing the number of collisional events in each case. Kinetic considerations indicate that the important rate process in rarefied envelopes will be C2 formation; consequently scaling by termolecular collisions was employed. Results of this study imply that envelope expansions near M giants contribute at most .01 of the observed interstellar grain density. In the case of pulsating N stars, on the other hand, the results are not inconsistent with previous predictions of essentially complete carbon condensation.

Dorfeld, W. G.↗
NASA NTRS1977

Solar power satellite. System definition study. Part 1, volume 3: Construction, transportation and cost analyses

Concepts developed for both LEO and GEO construction of photovoltaic and thermal engine satellites are analyzed. Topics discussed include: satellite construction; crew scheduling; crew jobs and organizations; operator productivity rating; constructability rating; transportation systems for cargo launch, refueling operations, personnel transport, and orbit transfer; collision analysis, cost analysis, and radiation evironment and effects.

Source record↗
NASA NTRS1980

A molecular beam investigation of the oxidation of CO on Pt/9/111/x/100//

The CO oxidation on Pt/9(111)x(100)/ was studied by molecular beam relaxation spectroscopy (MBRS). The reaction proceeded via the reaction of adsorbed CO and adsorbed oxygen. No evidence for direct reactive collisions between gaseous CO and adsorbed atomic oxygen was seen. The second order rate constant was measured by linearizing the reaction system to be 10 to the -7th exp/-(9700 kcal/mole)RT/ per sq cm s. The relatively low pre-exponential factor was explained by transition state theory on the basis of a high partition function for adsorbed carbon monoxide obtained previously from studies of CO desorption on this surface.

Fair, J. A.↗