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At least 775 records · Page 43

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NASA NTRS1980

Meteor Crater: Energy of formation - Implications of centrifuge scaling

Recent work on explosive cratering has demonstrated the utility of performing subscale experiments on a geotechnic centrifuge to develop scaling rules for very large energy events. The present investigation is concerned with an extension of this technique to impact cratering. Experiments have been performed using a projectile gun mounted directly on the centrifuge rotor to launch projectiles into a suitable soil container undergoing centripetal accelerations in excess of 500 G. The pump tube of a two-stage light-gas gun was used to attain impact velocities of approximately 2 km/sec. The results of the experiments indicate that the energy of formation of any large impact crater depends upon the impact velocity. This dependence, shown for the case of Meteor Crater, is consistent with analogous results for the specific energy dependence of explosives and is expected to persist to impact velocities in excess of 25 km/sec.

Schmidt, R. M.↗

NASA NTRS1980

Cratering flow fields - Implications for the excavation and transient expansion stages of crater formation

Theoretical depths, volumes of excavation, hinge radii, ejection angles and transient structural rim uplifts are calculated, for comparison with experimental and field data from impact and explosion craters, by means of a Maxwell Z-model cratering flow field originating at non-zero depths-of-burst. It is found that the formation of a hinge, about which the coherent ejecta flap rotates at the rim of the transient crater, divides material in the flow field into ejected and downward and outward-driven portions. Ejected material originates from an excavation cavity whose geometry is distinct from that of the transient crater. The shallow depths of excavation both observed in impact and explosion craters and predicted by the Z-model flow fields imply that thickness estimates of such lunar geologic units as the maria basalts, which are determined by assuming that excavation depths are similar to (1) final or (2) transient crater depths, must be reduced by factors of two to three respectively. It is concluded that lunar basin excavation cavities exhibit proportional growth, and have maximum depths of excavation near 0.1 the diameter of the basin transient crater.

Croft, S. K.↗

NASA NTRS1980

The equivalent depth of burst for impact cratering

The concept of modeling an impact cratering event with an explosive event with the explosive buried at some equivalent depth of burst (d.o.b.) is discussed. Various and different ways to define this equivalent d.o.b. are identified. Recent experimental results for a dense quartz sand are used to determine the equivalent d.o.b. for various conditions of charge type, event size, and impact conditions. The results show a decrease in equivalent d.o.b. with increasing energy for fixed impact velocity and a decrease in equivalent d.o.b. with increasing velocity for fixed energy. The values for an iron projectile are on the order of 2-3 projectile radii for energy equal to one ton of TNT, decreasing to about 1.5 radii at a megaton of TNT. The dependence on projectile and target mass density matches that included in common jet-penetration formulas for projectile densities greater than target densities and for the higher energies.

Holsapple, K. A.↗

NASA NTRS1982

On the scaling of crater dimensions. II - Impact processes

Holsapple and Schmidt (1980) previously addressed the problem of the scaling of explosive cratering. Their analysis included results which show under which conditions the scaling can be bounded between quarter-root and cube-root rules. The present investigation is an extension of the earlier analysis and approaches the case of impact cratering. More restrictive bounds are found for impact cratering than for the explosive case. These stronger results come from considering the role of the impactor momentum as an independent variable for impact cratering. Attention is given to impact cratering variables, general scaling rules, the bounds on scaling rules, a generalization to more variables, and previous scaling rules and results.

Holsapple, K. A.↗

NASA NTRS1982

Late-stage summit activity of Martian shield volcanoes

The preservation of morphologically fresh lava flows which pre-date the most recent episodes of caldera collapse at the summits of Ascraeus, Arsia and Olympus Montes indicates that explosive eruptions were not associated with this stage of Tharsis shield volcanism. The existence of resurfaced floor segments, complex wrinkle ridges, and lava terraces within the summit craters suggests that lava lakes comprised the dominant form of the intra-caldera activity. Multiple collapse episodes on Ascraeus and Olympus Montes are indicated by the nested summit craters. The most plausible cause of caldera collapse appears to be large-scale sub-terminal effusive activity, which is corroborated by the previously recognized existence of large lava flows on the flanks of these volcanoes. Due to the implied sequence of large-scale explosive (silicic) volcanism followed by effusive (basaltic) activity, it appears highly unlikely that ignimbrites or other forms of pyroclastic flows (previously proposed as possible deposits within the Olympus Mons aureole material) were ever erupted from the Tharsis Montes.

Mouginis-Mark, P. J.↗

NASA NTRS1981

X-ray imaging - Supernova remnants

Consideration is given to imaging observations of supernova remnants (SNRs) obtained during the first year of the Einstein Observatory's operation. Inferences are drawn regarding models for stellar explosions, remnant evolution, neutron star formation and the interstellar medium. Because the X-ray emission traces the expanding shock boundary and dominates the radiative energy losses of an SNR over much of its lifetime, it can provide data on the possible collapsed remnants of the explosion, such as neutron stars and/or black holes. X-ray emission also allows a supernova shock to be used as a probe of interstellar medium structure. The imaging instrument aboard the Einstein satellite has been used to observe over 30 known Galactic remnants, and a similar number of objects in other galaxies, in the 0.15-4.5 keV band.

Helfand, D. J.↗

NASA NTRS1982

Vacillation, sudden warmings and potential enstrophy balance in the stratosphere

The balance of potential enstrophy and its relationship to vacillation cycles and the sudden warming is studied for a beta-channel model of the stratosphere. It is shown that the mean flow cannot be steady in the presence of large-amplitude quasi-geostrophic waves (approximately 1-0.25 geopotential kilometers /gpkm/) when any dissipation is present, and the maximum wave amplitude allowed is approximately 2 gpkm. If wave forcing (transience plus dissipation) is artificially maintained, the mean flow decelerates slowly at first then explosively as the potential vorticity gradient of the basic state is wiped out over the channel. This process is called wave saturation. The initial phase of the explosive deceleration resembles both the observed and modeled mean flow evolution during a sudden stratospheric warming. A simple vacillation model based upon these ideas shows remarkable similarity to the results of Holton and Mass (1976) and Davies (1981).

Schoeberl, M. R.↗

NASA NTRS1982

The formulation of Lamb's Dust Veil Index

A catalog of the major explosive volcanic eruptions since 1500 AD and formulated the Dust Veil Index (DVI) is presented. The DVI quantifies the impact on the Earth's energy balance of changes in atmospheric composition due to explosive volcanic eruptions. The DVI for a particular eruption quantifies the climatic impact of the dust and aerosol injection from the eruption integrated over the years following the event. The formulation of the DVI is described. All references are to Lamb (1970). A distinction is made between the catalog of volcanic activity, and the tabulation of the northern hemisphere DVI apportioned over the years. The DVI data are updated to 1975 for any particular eruption, the catalog gives three DVI values: global, Southern Hemisphere, and Northern Hemisphere. The global DVI given in the catalog is considered. The other two DVIs relate to the impact on the hemispheres considered separately and their estimation involves an additional factor apportioning the dust veil between the hemispheres on the basis of the latitude of injection.

Kelly, P. M.↗

NASA NTRS1982

Magma vesiculation and pyroclastic volcanism on Venus

Theoretical consideration of the magma vesiculation process under observed and inferred venusian surface conditions suggests that vesicles should form in basaltic melts, especially if CO2 is the primary magmatic volatile. However, the high surface atmospheric pressure (about 90 bars) and density on Venus retard bubble coalescence and disruption sufficiently to make explosive volcanism unlikely. The products of explosive volcanism (fire fountains, convecting eruption clouds, pyroclastic flows, and topography-mantling deposits of ash, spatter, and scoria) should be rare on Venus, and effusive eruptions should dominate. The volume fraction of vesicles in basaltic rocks on Venus are predicted to be less than in chemically similar rocks on earth. Detection of pyroclastic landforms or eruption products on Venus would indicate either abnormally high volatile contents of Venus magmas (2.5-4 wt%) or different environmental conditions (e.g., lower atmospheric pressure) in previous geologic history.

Garvin, J. B.↗

NASA NTRS1982

An optimist's guide to supernovae

There are some hints that a satisfactory agreement between observations and models is developing in several areas of supernova research. The light curves and spectra of Type I supernovae look so much like the models that they may well have their origin in the synthesis of Ni-56 in the explosion of a compact star. Type II supernovae are found where massive stars form, have light curves that correspond to 10 to the 51st ergs suddenly deposited inside a red supergiant, and may in fact be the explosions of massive stars. Some young supernova remnants, such as Cas A, show abundance patterns that bear a striking resemblance to those seen deep inside models of massive stars on the verge of destruction. Observations of old supernova remnants provide energy estimate of 10 to the 51st ergs - just the amount needed for the models of the outbursts.

Kirshner, R. P.↗

NASA NTRS1983

A comparison of volcanic eruption processes on earth, moon, Mars, Io and Venus

The physical, chemical, and atmospheric characteristics of the silicate planets and satellites are surveyed in terms of their effects on the volcanic evolution of the surfaces of these bodies. The equations relating the parameters affecting magma ascent through the crust and eruption are analyzed, and three major types of eruption are characterized: effusive, steady explosive, and unsteady explosive. This analytical framework is then used to predict the nature of volcanic activity on each of the planets and satellites, and the predictions are compared with actual observations. Outstanding problems are discussed, with emphasis on the need for a general model of planetary interior dynamics applicable to bodies with varying degrees of interior viscosity, mantle activity, and lithospheric plate tectonics.

Wilson, L.↗

NASA NTRS1982

Einstein observations of supernova remnants

Recent observations of SNRs have produced X-ray images with resolution comparable to that routinely achieved at optical wavelengths. There has also been a great improvement in the quality of X-ray spectra. Since most of the energy radiated by SNRs appears as X-rays, these new data are crucial to studies of SNRs, the interaction of SNRs with the interstellar medium, and the SN explosion itself. Images show a variety of shapes ranging from shell-like remnants to those dominated by the influence of central objects which appear both as point sources and as centers of diffuse activity. Once the temperature and spatial distribution of X-ray emitting material is known, the mass of ejected material and the energy release of the SN explosion can sometimes be calculated. X-ray images and spectra of several remnants are shown, and some quantitative results are given.

Seward, F. D.↗

NASA NTRS1983

The extraordinary extragalactic supernova remnant in NGC 4449. II - X-ray and optical investigations

Einstein Observatory X-ray observations indicate that the SNR in NGC 4449 is the most luminous X-ray, as well as optical and radio, SNR known. New optical spectroscopy, together with the X-ray data, suggest this exceptional luminosity to be due to chemically enriched material at unusually high densities. A model is proposed in which this SNR resulted from the explosion of a massive star in a medium of nearly 25/cu cm density about 100 years ago. The chemistry in question further suggests that the SNR in NGC 4449 resulted from the explosion of a star whose mass was greater than 25 solar masses.

Blair, W. P.↗

NASA NTRS1983

Feasibility demonstration of booster cross-over system for 3 1/2 inch SRB/MLP frangible nut system

Recent testing of the SRB/MLP Frangible Nut System (SOS Part Number 114850-9/Boosters P/N 114848-3) at NASA indicated a need to reduce the function time between boosters (2) within a single frangible nut. These boosters are initiated separately by electrical impulse(s). Coupling the output of each detonator with an explosive cross-over would reduce the function time between boosters (independent of electrical impulse) while providing additional redundancy to the system. The objectives of this program were to: provide an explosive cross-over between boosters, reduce function time between boosters to less than one (1) millisecond within a given nut, reduce cost of boosters, be compatible with the existing frangible nut system, and meet requirements of USBI Spec's (nut 10SPC-0030, booster 10SPC-0031).

Source record↗

NASA NTRS1984

Recent Solar Measurements Results at the Parabolic Dish Test Site

After the Mexican volcanic eruptions of March 28, April 3 and 4, 1982, the question of its effect on insolation levels at the Parabolic Dish Test Site (PDTS) naturally arose. Clearly, the answer to the original question is that the Mexican volcanic explosion had a significant impact on energy and insolation levels at the PDTS and, furthermore, it has been quite long lasting. The first really significant decrease in energy and insolation levels occurred in June 1982 when the energy level decreased by 19.7% while the peak insolation levels went down by 4.0%. June of 1982 was also the first month (of 13 consecutive months) when peak insolation levels did not equal or exceed 1,000 W/sq m. Signs of a recovery from the effects of the volcanic explosion began to appear in May of 1983, when the energy level exceeded that of May 1981 as well as May 1982. It would appear that energy and insolation levels are improving at the PDTS, but have not quite reached normal or pre-volcanic levels. At this time the data would seem to suggest a return to normal energy and insolation levels will occur in the very near future.

Ross, D. L.↗

NASA NTRS1984

Field repair of AH-16 helicopter window cutting assemblies

The U.S. Army uses explosively actuated window cutting assemblies to provide emergency crew ground egress. Gaps between the system's explosive cords and acrylic windows caused a concern about functional reliability for a fleet of several hundred aircraft. A field repair method, using room temperature vulcanizing silicone compound (RTV), was developed and demonstrated to fill gaps as large as 0.250 inch.

Bement, L. J.↗

NASA NTRS1983

Martian rampart crater ejecta - Experiments and analysis of melt-water interaction

The possible effects of explosive water vaporization on ejecta emplacement after impact into a wet target are described. A general model is formulated from analysis of Viking imagery of Mars and experimental vapor explosions as well as consideration of fluidized particulate transport and lobate volcanic deposits. The discussed model contends that as target water content increases, the effects of vapor expansion due to impact increasingly modify the ballistic flow field during crater excavation. This modification results in transport by gravity-driven surface flowage, and is similar to that of atmospheric drag effects on ejecta modelled by Schultz and Gault (1979).

Wohletz, K. H.↗

NASA NTRS1982

Dynamic effects of combustion

The dynamic effects of combustion are due to the evolution of exothermic energy and its deposition in the compressible medium where the process takes place. The paper examines the dynamics of combustion phenomena, including ignition, turbulent flame propagation (inflammation), explosion, and detonation, with emphasis on their exothermic characteristics. Ignition and explosion are treated as problems of nonlinear mechanics, and their dynamic behavior is described in terms of phase space models and cinematographic laser shear interferograms. The results of a numerical random vortex model of turbulent flame propagation are confirmed in a combustion tunnel experiment, where it was observed that a fresh mixture of burnt and unburnt gases can sustain combustion with a relatively small expenditure of overall mass flow, due to the increasing specific volume of burnt gases inside the flame front. An isentropic pressure wave is found to precede the accelerating flame in the process of detonation, and components of this presssure wave are shown to propagate at local sonic velocities.

Oppenheim, A. K.↗