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At least 847 records · Page 47

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NASA NTRS1992

Postexplosion hydrodynamics of SN 1987A

A comprehensive study is presented of the postexplosion hydrodynamics of the ejecta of SN 1987A encompassing the Rayleigh-Taylor instabilities at the shell interfaces and the dynamical effects of the energy input due to the radioactive decay of Ni-56 and Co-56. The dependence of the hydrodynamical behavior on progenitor structure, initial perturbations, explosion energy, and dimensionality of the numerical simulation is investigated in order to explain the observed high-velocity wings of the iron line of SN 1987A. It is shown that none of these factors leads to the required high-velocity tail of iron. When nickel is artificially premixed in the initial model, it is found that this early mixing has to affect the inner 1.5 solar mass of the ejecta to reproduce the high-velocity wings at a time of 90 d. It is argued that the high-entropy bubble associated with the delayed mechanism is responsible for this premixing by driving convective motions early in the explosion.

Herant, Marc↗

NASA NTRS1992

Textural constraints on effusive silicic volcanism - Beyond the permeable foam model

The paper reports textural observations and presents isotopic evidence from active and recent silicic lava flows which show that at least some vesiculation occurs during surface advance of extrusions, after magma has reached the earth's surface. This view is in contrast to the widely promoted 'permeable foam' model, which states that all volatiles escape during ascent of the magma, and that all dense glassy material in lava flows forms from the collapse of pumiceous lava, i.e., that silicic lavas emerge as highly inflated foam flows. The permeable foam model also implies the unlikely requirement that explosive-to-effusive transitions be associated with an increase in the eruption rate. A more comprehensive model for the emplacement of silicic extrusions that allows for early gas loss during ascent, as well as late-stage vesiculation, is presented. The way in which the redistribution of volatiles during surface flow can increase explosive hazards from silicic lavas days, weeks, or months after the lava emerges from the event is discussed.

Fink, Jonathan H.↗

NASA NTRS1992

On the nonlinear three dimensional instability of Stokes layers and other shear layers to pairs of oblique waves

The nonlinear evolution of a pair of initially oblique waves in a high Reynolds Number Stokes layer is studied. Attention is focused on times when disturbances of amplitude epsilon have O(epsilon(exp 1/3)R) growth rates, where R is the Reynolds number. The development of a pair of oblique waves is then controlled by nonlinear critical-layer effects. Viscous effects are included by studying the distinguished scaling epsilon = O(R(exp -1)). This leads to a complicated modification of the kernel function in the integro-differential amplitude equation. When viscosity is not too large, solutions to the amplitude equation develop a finite-time singularity, indicating that an explosive growth can be introduced by nonlinear effects; we suggest that such explosive growth can lead to the bursts observed in experiments. Increasing the importance of viscosity generally delays the occurrence of the finite-time singularity, and sufficiently large viscosity may lead to the disturbance decaying exponentially. For the special case when the streamwise and spanwise wavenumbers are equal, the solution can evolve into a periodic oscillation. A link between the unsteady critical-layer approach to high-Reynolds-number flow instability, and the wave vortex approach is identified.

Wu, Xuesong↗

NASA NTRS1993

Proposed system safety design and test requirements for the microlaser ordnance system

Safety for pyrotechnic ignition systems is becoming a major concern for the military. In the past twenty years, stray electromagnetic fields have steadily increased during peacetime training missions and have dramatically increased during battlefield missions. Almost all of the ordnance systems in use today depend on an electrical bridgewire for ignition. Unfortunately, the bridgewire is the cause of the majority of failure modes. The common failure modes include the following: broken bridgewires; transient RF power, which induces bridgewire heating; and cold temperatures, which contracts the explosive mix away from the bridgewire. Finding solutions for these failure modes is driving the costs of pyrotechnic systems up. For example, analyses are performed to verify that the system in the environment will not see more energy than 20 dB below the 'No-fire' level. Range surveys are performed to determine the operational, storage, and transportation RF environments. Cryogenic tests are performed to verify the bridgewire to mix interface. System requirements call for 'last minute installation,' 'continuity checks after installation,' and rotating safety devices to 'interrupt the explosive train.' As an alternative, MDESC has developed a new approach based upon our enabling laser diode technology. We believe that Microlaser initiated ordnance offers a unique solution to the bridgewire safety concerns. For this presentation, we will address, from a system safety viewpoint, the safety design and the test requirements for a Microlaser ordnance system. We will also review how this system could be compliant to MIL-STD-1576 and DOD-83578A and the additional necessary requirements.

Stoltz, Barb A.↗

NASA NTRS1993

The ring around SN1987A

Stars in the 9-40 solar mass range play a prominent role in the hydrodynamical and chemical evolution of galaxies. Their stellar winds and supernova explosions are believed to create the hot component of the interstellar medium (ISM). In some galactic disks, the kiloparsec sized super bubbles formed around clusters of massive stars may blow out of the disk plane and release hot, metal enriched gas into the galaxy's halo. Additionally, the expanding shock front of a super bubble in the disk may trigger additional star formation. Furthermore, similar processes probably drive the galactic winds associated with star burst nuclei that enrich the intracluster and intergalactic mediums. Nonetheless, the explosion of a blue super giant in the Large Magellanic Cloud (LMC), SN1987A, illuminated the incompleteness of our understanding of massive stars. Evolutionary models of massive stars do not synthesize the observed super giant populations in either the Milky Way or LMC. Our modeling of the formation of SN1987A's ring will improve our knowledge of both the post-main-sequence evolution of massive stars and their coupling to the ISM in galaxies.

Martin, Crystal L.↗

NASA NTRS1993

COBE looks back to the Big Bang

An overview is presented of NASA-Goddard's Cosmic Background Explorer (COBE), the first NASA satellite designed to observe the primeval explosion of the universe. The spacecraft carries three extremely sensitive IR and microwave instruments designed to measure the faint residual radiation from the Big Bang and to search for the formation of the first galaxies. COBE's far IR absolute spectrophotometer has shown that the Big Bang radiation has a blackbody spectrum, proving that there was no large energy release after the explosion.

Mather, John C.↗

NASA NTRS1993

Hard X-ray and gamma-rays from supernovae

The hard electromagnetic radiation produced by and accompanying the explosion of supernovae is discussed. This radiation may have three origins: (i) the eruption of the shock wave from the surface of the star; (ii) radioactive decay of unstable isotopes produced during the explosion; and (iii) gamma-rays produced by neutrino annihilation following accretion-induced collapse. Though the latter has been proposed as a mechanism for making cosmological gamma-ray bursts, it is shown that this is unlikely.

Woosley, S. E.↗

NASA NTRS1993

Gamma-ray bursts from planetesimal accretion

Planetesimals that form following the explosion of supernovae provide a reservoir of material that can be accreted and generate high energy transient emission long after the neutron star has ceased being a pulsar. Blue supergiants, which would have been the predominant progenitor of supernovae during galaxy formation, may be particularly efficient in producing such systems. Owing to their birth at an early epoch and the kick velocity given the neutron star during the explosion, these sources would lie in a very extended halo. In this picture, some or most gamma-ray bursters would be 10 exp 10 year old relics of galaxy formation. At a minimum a very thick disk population would be expected.

Woosley, S. E.↗

NASA NTRS1932

The Effect of Clearance Distribution on the Performance of a Compression-ignition Engine with a Precombustion Chamber

The clearance distribution in a precombustion chamber cylinder head was varied so that for a constant compression ratio of 13.5 the spherical auxiliary chambers contained 20, 35, 50, and 70 per cent of the total clearance volume. Each chamber was connected to the cylinder by a single circular passage, flared at both ends, and of a cross-sectional area proportional to the chamber volume, thereby giving the same calculated air-flow velocity through each passage. Results of engine-performance tests are presented with variations of power, fuel consumption, explosion pressure, rate of pressure rise, ignition lag, heat loss to the cooling water, and motoring characteristics. For good performance the minimum auxiliary chamber volume, with the cylinder head design used, was 35 per cent of the total clearance volume; for larger volumes the performance improves but slightly. With the auxiliary chamber that contained 35 percent of the clearance volume there were obtained the lowest explosion pressures, medium rates of pressure rise, and slightly less than the maximum power. For all clearance distributions an increase in engine speed decreased the ignition lag in seconds and increased the rate of pressure rise.

Moore, C. S.↗

NASA NTRS1929

Impact waves and detonation. Part I

Among the numerous thermodynamic and kinetic problems that have arisen in the application of the gaseous explosive reaction as a source of power in the internal combustion engine, the problem of the mode or way by which the transformation proceeds and the rate at which the heat energy is delivered to the working fluid became very early in the engine's development a problem of prime importance. The work of Becker here given is a notable extension of earlier investigations, because it covers the entire range of the explosive reaction in gases - normal detonation and burning.

DETONATION↗

NASA NTRS1932

Combustion of Gaseous Mixtures

This report not only presents matters of practical importance in the classification of engine fuels, for which other means have proved inadequate, but also makes a few suggestions. It confirms the results of Withrow and Boyd which localize the explosive wave in the last portions of the mixture burned. This being the case, it may be assumed that the greater the normal combustion, the less the energy developed in the explosive form. In order to combat the detonation, it is therefore necessary to try to render the normal combustion swift and complete, as produced in carbureted mixtures containing benzene (benzol), in which the flame propagation, beginning at the spark, yields a progressive and pronounced darkening on the photographic film.

Duchene, R↗

NASA NTRS1993

Collisions of small Spacewatch asteroids with the Earth

The discovery, with the Spacewatch Telescope, of 8 Earth-approaching objects smaller than 100 m in diameter has been reported. I have calculated the probability and velocity of collision with Earth for each of these objects. Using a code we have successfully employed to model the Tunguska and Revelstoke atmospheric explosions, I have simulated the entry of these objects into Earth's atmosphere, assuming iron, stony, carbonaceous, and cometary compositions. The smallest of these objects, with tens to hundreds of kilotons of kinetic energy, pose a substantial threat at the surface only if they are iron objects. An object is taken to 'pose a substantial threat' if it either craters the ground, or explodes in the atmosphere with sufficient energy at low enough altitude to create an overpressure at the surface capable of felling trees or destroying buildings. Larger objects, with energies greater than about a megaton, devastate the surface regardless of whether they are of iron, stony, or carbonaceous composition. Iron objects crater the ground, whereas stony and carbonaceous objects explode in the atmosphere low enough to fell trees and damage buildings over thousands of square kilometers. Spacewatch objects, if presumed to be of carbonaceous composition, are as dangerous as their stony counterparts, as the former objects' lower yield strengths (and hence, higher explosion altitudes) are roughly compensated by their larger masses (as derived from their lower albedos for a given observed magnitude). Although comets are intrinsically less dangerous than asteroids, the 90 m diameter Spacewatch objects would devastate hundreds of square kilometers at the surface, even if cometary.

Chyba, C. F.↗

NASA NTRS1993

Macroscopic electric charge separation during hypervelocity impacts: Potential implications for planetary paleomagnetism

The production of transient magnetic fields by hypervelocity meteoroid impact has been proposed to possibly explain the presence of paleomagnetic fields in certain lunar samples as well as across broader areas of the lunar surface. In an effort to understand the lunar magnetic record, continued experiments at the NASA Ames Vertical Gun Range allow characterizing magnetic fields produced by the 5 km/s impacts of 0.32-0.64 cm projectiles over a broad range of impact angles and projectile/target compositions. From such studies, another phenomenon has emerged, macroscopic electric charge separation, that may have importance for the magnetic state of solid-body surfaces. This phenomenon was observed during explosive cratering experiments, but the magnetic consequences of macroscopic electric charge separation (as opposed to plasma production) during explosion and impact cratering have not, to our knowledge, been explored before now. It is straightforward to show that magnetic field production due to this process may scale as a weakly increasing function of impactor kinetic energy, although more work is needed to precisely assess the scaling dependence. The original intent of our experiments was to assess the character of purely electrostatic signals for comparison with inferred electrostatic noise signals acquired by shielded magnetic sensors buried within particulate dolomite targets. The results demonstrated that electrostatic noise does affect the magnetic sensors but only at relatively short distances (less than 4 cm) from the impact point (our magnetic studies are generally performed at distances greater than approximately 5.5 cm). However, to assess models for magnetic field generation during impact, measurements are needed of the magnetic field as close to the impact point as possible; hence, work with an improved magnetic sensor design is in progress. In this paper, we focus on electric charge separation during hypervelocity impacts as a potential transient magnetic field production mechanism in its own right.

Crawford, D. A.↗

NASA NTRS1991

The fragmentation of the Nimbus 6 rocket body

On 1 May 1991, the Nimbus 6 second stage Delta Rocket Body experienced a major breakup at an altitude of approximately 1,100 km. There were numerous pieces left in long-lived orbits, adding to the long-term hazard in this orbital regime already present from previous Delta Rocket Body explosions. The assessed cause of the event is an accidental explosion of the Delta second stage by documented processes experienced by other similar Delta second stages. Various aspects of the event are discussed.

Nauer, David J.↗

NASA NTRS1993

Pressure versus drag effects on crater size

The topic of atmospheric effects on crater formation is very complex because it includes not only pressure effects on excavation, but also drag effects on ejecta placement. Experiments have to be designed very carefully to allow isolation of the two phenomena. Historically, numerous investigators have shown an influence of atmospheric pressure. However, none have identified the scaling that correctly isolates pressure from drag effects. On-going work in explosive cratering has produced scaling paradigms for deeply buried explosive charges where drag effects are negligible. Here it was found that increased pressure caused significant induced strength effects that impeded crater excavation. The effect is more pronounced with increasing burial depth and less pronounced with increased yield.

Schmidt, R. M.↗

NASA NTRS1993

Demonstrating damage tolerance of composite airframes

Commercial transport aircraft operating in the United States are certified by the Federal Aviation Authority to be damage tolerant. On 28 April 1988, Aloha Airlines Flight 243, a Boeing 727-200 airplane, suffered an explosive decompression of the fuselage but landed safely. This event provides very strong justification for the damage tolerant design criteria. The likely cause of the explosive decompression was the linkup of numerous small fatigue cracks that initiated at adjacent fastener holes in the lap splice joint at the side of the body. Actually, the design should have limited the damage size to less than two frame spacings (about 40 inches), but this type of 'multi-site damage' was not originally taken into account. This cracking pattern developed only in the high-time airplanes (many flights). After discovery in the fleet, a stringent inspection program using eddy current techniques was inaugurated to discover these cracks before they linked up. Because of concerns about safety and the maintenance burden, the lap-splice joints of these high-time airplanes are being modified to remove cracks and prevent new cracking; newer designs account for 'multi-site damage'.

Poe, Clarence C., Jr.↗

NASA NTRS1993

Orbital debris and near-Earth environmental management: A chronology

This chronology covers the 32-year history of orbital debris and near-Earth environmental concerns. It tracks near-Earth environmental hazard creation, research, observation, experimentation, management, mitigation, protection, and policy-making, with emphasis on the orbital debris problem. Included are the Project West Ford experiments; Soviet ASAT tests and U.S. Delta upper stage explosions; the Ariane V16 explosion, U.N. treaties pertinent to near-Earth environmental problems, the PARCS tests; space nuclear power issues, the SPS/orbital debris link; Space Shuttle and space station orbital debris issues; the Solwind ASAT test; milestones in theory and modeling the Cosmos 954, Salyut 7, and Skylab reentries; the orbital debris/meteoroid research link; detection system development; orbital debris shielding development; popular culture and orbital debris; Solar Max results; LDEF results; orbital debris issues peculiar to geosynchronous orbit, including reboost policies and the stable plane; seminal papers, reports, and studies; the increasing effects of space activities on astronomy; and growing international awareness of the near-Earth environment.

Portree, David S. F.↗

NASA NTRS1994

Supernovae and mass extinctions

Shklovsky and others have suggested that some of the major extinctions in the geological record might have been triggered by explosions of nearby supernovae. The frequency of such extinction events will depend on the galactic supernova frequency and on the distance up to which a supernova explosion will produce lethal effects upon terrestrial life. In the present note it will be assumed that a killer supernova has to occur so close to Earth that it will be embedded in a young, active, supernova remnant. Such young remnants typically have radii approximately less than 3 pc (1 x 10(exp 19) cm). Larger (more pessimistic?) killer radii have been adopted by Ruderman, Romig, and by Ellis and Schramm. From observations of historical supernovae, van den Bergh finds that core-collapse (types Ib and II) supernovae occur within 4 kpc of the Sun at a rate of 0.2 plus or minus 0.1 per century. Adopting a layer thickness of 0.3 kpc for the galacitc disk, this corresponds to a rate of approximately 1.3 x 10(exp -4) supernovae pc(exp -3) g.y.(exp -1). Including supernovae of type Ia will increase the total supernovae rate to approximately 1.5 x 10(exp -4) supernovae pc(exp -3) g.y.(exp -1). For a lethal radius of R pc the rate of killer events will therefore be 1.7 (R/3)(exp 3) x 10(exp -2) supernovae per g.y. However, a frequency of a few extinctions per g.y. is required to account for the extinctions observed during the phanerozoic. With R (extinction) approximately 3 pc, the galactic supernova frequency is therefore too low by 2 orders of magnitude to account for the major extinctions in the geological record.

Vandenbergh, S.↗