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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Short-lived nuclides in the early solar system and general isotopic anomalies in meteorites

Evidence for short-lived nuclides in the early solar system is reviewed. The presence of these nuclides requires the addition to the interstellar medium, from which the solar system formed, of freshly synthesized materials just prior to the formation of the solar system. The best evidence is obtained for Al26. The time scale implied by the Al26 effects is equated with the free-fall time for the collapse of the protosolar cloud. From the observation of ratios of short-lived parent to stable isotopes for nuclides with different half-lives, it is apparent that addition of exotic material to the solar system occurred just prior to its formation. Evidence for a diffuse galactic gamma-ray line emission attributed to Al26 provides support for the existence of Al26 in the protosolar cloud. The time scale for isolation of the protosolar cloud would still be restricted on a timescale comparable to tau 1/2 (Al26).

Papanastassiou, D. A.↗
NASA NTRS1985

Evolution of the innermost stable orbits around accreting neutron stars

The surface of most neutron stars with 'soft' equations of state lies within the innermost stable circular orbit predicted by general relativity. In disk accretion onto a weakly magnetized neutron star, the disk will reach the stellar surface for 'stiff' equations of state, but for soft equations of state the matter will hit the surface in free fall at an angle of about 0.001 to 0.1 radians. All calculations are carried out through first order in the angular momentum of the star.

Kluzniak, W.↗
NASA NTRS1986

Interface stability in a slowly rotating, low gravity tank Experiments

Analytical models of liquid in partially-filled rotating tanks predict both the shape of the interface between the liquid and its vapor, and the stability of that interface. The models are of necessity incomplete and experimental data are needed to assess the approximations made. Presented are preliminary experimental studies both in the laboratory and in the low-gravity environment of a free-falling aircraft. Emphasis is placed on bubbles which intersect the container boundaries. Measurements of rotating equilibrium bubble shapes are in agreement with theoretical profiles derived from Laplace's formula. The interface shape depends on the contact angle, the radius of intersection with container, and the ratio of centrifugal force to surface tension.

Leslie, F.↗
NASA NTRS1985

Measurements of rotating bubble shapes in low-gravity environment

Measurements of rotating equilibrium bubble shapes in the low-gravity environment of a free-falling aircraft are presented. Emphasis is placed on bubbles which intersect the container boundaries. These data are compared with theoretical profiles derived from Laplace's formula and are in good agreement with the measurements. The interface shape depends on the contact angle, the radius of intersection with the container, and the parameter F, which is a measure of the relative importance of centrifugal force to surface tension. For isolated bubbles F has a maximum value of 1/2. A further increase in F causes the bubble to break contact with the axis of rotation. For large values of F the bubble becomes more cylindrical and the capillary rise occurs over a thinner layer in order that the small radius of curvature can generate a sufficient pressure drop to account for the increased hydrostatic contribution.

Leslie, F.↗
NASA NTRS1986

Material damping of simple structures in a simulated space environment

The need for accurate, quantitative knowledge of the vibration dissipation of large space structures is explained. The sources of experimental error in vibrational measurements are detailed. A new method for such testing based on the use of a miniature telemetry system is presented, which allows the testing of structures in a simulated space environment consisting of free-fall inside of a vacuum chamber. Theoretical relations are given for the damping ratios of metals and composites. Measured damping ratios for both aluminum and composite beams and plates, and aluminum planar trusses, are presented. Experimental results are used to evaluate the theoretical damping relations.

Edberg, D. L.↗
NASA NTRS1986

A procedure for calculating the damping in multi-element space structures

A procedure for analyzing the damping in a multielement space structure connected by joints is described, in which distributed material damping and discrete nonlinear joint properties are incorporated into a linear analysis. The procedure involves four steps: (1) creation of a linear undamped finite element model; (2) experimental measurements of the transient response of a truss member in free fall tests to obtain material damping properties; with these properties incorporated into a linear damped finite element model of the structure; (3) the identification of the nonlinear joint properties using the force-state mapping technique; and (4) linearization of the identified nonlinear components, which are then incorporated into the linear damped model to create the linearized damped finite element model.

Crawley, E. F.↗
NASA NTRS1986

A dynamical model for the central engine of QSOs and active galactic nuclei

A model for the energy generation in quasars and active galaxies is proposed based on (quasi)spherical accretion and a shock as a means for randomizing the inflowing kinetic energy. According to the model, most of the accretion energy is converted into relativistic protons at the shock, which in turn can provide the necessary pressure to self-consistently support it if their energy loss time scale by nuclear collisions is longer than the free fall time scale. The shock can thus be characterized as a 'relativistic proton radiative shock' in analogy with similar accretion shocks on white dwarfs.

Kazanas, Demosthenes↗
NASA NTRS1987

The behavior of surface tension on steady-state rotating fluids in the low gravity environments

The effect of surface tension on steady-state rotating fluids in a low gravity environment is studied. All the values of the physical parameters used in these calculations, except in the low gravity environments, are based on the measurements carried out by Leslie (1985) in the low gravity environment of a free-falling aircraft. The profile of the interface of two fluids is derived from Laplace's equation relating the pressure drop across an interface to the radii of curvature which has been applied to a low gravity rotating bubble that contacts the container boundary. The interface shape depends on the ratio of gravity to surface tension forces, the ratio of centrifugal to surface tension forces, the contact radius of the interface to the boundary, and the contact angle. The shape of the bubble is symmetric about its equator in a zero-gravity environment. This symmetry disappears and gradually shifts to parabolic profiles as the gravity environment becomes non-zero. The location of the maximum radius of the bubble moves upward from the center of the depth toward the top boundary of the cylinder as gravity increases. The contact radius of interface to the boundary r0 at the top side of cylinder increases and r0 at the bottom side of the cylinder decreases as the gravity environment increases from zero to 1 g.

Hung, R. J.↗
NASA NTRS1986

Solidification of Nb-Ge alloys in long drop tubes

The 30-m and 100 m-long drop tubes at the Marshall Space Flight Center have been used to obtain large undercooling in Nb-Ge alloys. Electron beam melting has been used to obtain drops approximately 2.5 mm in diam. In the 30-m tube, many specimens fell the length of the tube without solidifying, and were ultimately liquid-quenched in oil. The amount of undercooling prior to the quench was usually around 0.13 T(m). In the 100-m tube, freezing generally initiated during free fall, and the maximum undercooling was around 0.22 T(m). Microstructures were characterized by a combination of X-ray diffraction, optical microscopy, and scanning electron microscopy with energy dispersive analysis by X-rays. A variety of interesting microstructures was observed.

Bayuzick, R. J.↗
NASA NTRS1987

Numerical simulations of adiabatic axisymmetric accretion flow. I - A new mechanism for the formation of jets

Numerical simulations of the uniform axisymmetric flow past a gravitating sphere have been studied. It is found that the structure of the flow is extremely sensitive to the boundary condition at the surface of the gravitating object. For the case in which the boundary is totally absorbing, a steady state flow is reached. However, for a boundary which is not totally absorbing, steady state flows are not obtained. The morphology of the flow is also sensitive to the Mach number at infinity and to the ratio of the free-fall velocity at the surface of the gravitating object to the flow velocity at inifinity. A new mechanism for the formation of jets is identified in which a fraction of the accretion energy is tapped to drive an anisotropic supersonic outflow with collimation provided by a combination of the inertia of matter which surrounds the beam and the development of multiple shock structures.

Fryxell, B. A.↗
NASA NTRS1987

Material damping in aluminum and metal matrix composites

The material damping in beam-like specimens of aluminum and metal matrix composites was measured. A unique apparatus to determine damping by free decay while the specimens are in free fall in a vacuum was used. The specimens tested include 2024-T3 and 6061-T4 aluminum, and unidirectional graphite/metal matrix specimens with P55 and P100 fibers and 6061 Aluminum and AZ91C Magnesium as matrix materials. Tests were conducted to determine the dependence of damping on frequency and stress level. For the aluminum specimens, the material damping followed the Zener model at very low stress levels. Below the Zener relaxation frequency, a strong dependence of damping on stress was found for even moderate stress levels. Damping for the aluminum matrix materials was slightly above that predicted by the Zener model for a homogeneous bar of the matrix aluminum. For the magnesium matrix specimens, damping significantly above the Zener prediction for the homogeneous matrix material was observed.

Crawley, Edward F.↗
NASA NTRS1987

Study of the production of some superconducting and magnetic materials by solidification in the drop tube and drop tower

A systematic study on the relationship between the microstructure and physical properties of several superconducting materials prepared by solidification in low gravity was conducted. Further study of the materials, such as the applications of hydrostatic pressure which is known to be an effective mean to vary the electronic structure of materials, in conjunction with the detailed microstructure analysis of the samples was also performed to better understand the low gravity effects on the enhancement of the electronic properties. Results of the studies on the directionally solidified AlInSn alloys processed in the KC-135 aircraft and immiscible GaBi alloy prepared during free fall in the Marshall Space Flight Center Drop Tower are presented.

Wu, M. K.↗
NASA NTRS1987

Method of evaporation

Liquids, such as juices, milk, molten metal and the like are concentrated by forming uniformly-sized, small droplets in a precision droplet forming assembly and deploying the droplets in free fall downwardly as a central column within an evacuated column with cool walls. A portion of the solvent evaporates. The vapor flows to the wall, condenses, and usually flows down the wall as a film to condensate collector and drain. The vertical column of freely falling droplets enters the splash guard. The condensate can be collected, sent to other towers or recycled.

Dufresne, Eugene R.↗
NASA NTRS1988

Noncontact temperature measurement: Requirements and applications for metals and alloys research

Temperature measurement is an essential capability for almost all areas of metals and alloys research. In the microgravity environment many of the science priorities that have been identified for metals and alloys also require noncontact temperature measurement capability. For example, in order to exploit the full potential of containerless processing, it is critical to have available a suitable noncontact temperature measurement system. This system is needed to track continuously the thermal history, including melt undercooling and rapid recalescence, of relatively small metal spheres during free-fall motion in drop tube systems. During containerless processing with levitation-based equipment, accurate noncontact temperature measurement is required to monitor one or more quasi-static samples with sufficient spatial and thermal resolution to follow the progress of solidification fronts originating in undercooled melts. In crystal growth, thermal migration, coarsening and other experiments high resolution thermal maps would be a valuable asset in the understanding and modeling of solidification processes, fluid flows and microstructure development. The science and applications requirements place several constraints on the spatial resolution, response time and accuracy of suitable instrumentation.

Perepezko, J. H.↗
NASA NTRS1987

The influence of gravitoinertial force level on oculomotor and perceptual responses to sudden stop stimulation

The goal of this investigation was to determine whether the vestibular response to vertical, z-axis body rotation in the dark is influenced by the magnitude of gravitoinertial force. The parameters measured were the nystagmus and the duration of illusory self-motion elicited in blindfolded subjects by cessation of such rotation during the free-fall, high, and terrestrial force phases of parabolic flight maneuvers. The pattern of findings is consistent with the responses that were observed earlier to constant levels of Coriolis cross-coupled stimulation during parabolic flight maneuvers both in terms of the mode of nystagmus suppression and the effect of G-level.

Dizio, Paul↗
NASA NTRS1988

Recent developments in gravity gradiometry from the Space-Shuttle-borne tethered satellite system

This paper describes the current development of a noncryogenic gravity gradiometer for future use on board the tethered satellite system (TSS). The paper also proposes a way of testing a single-axis prototype of such an instrument that makes use of the free-fall technique in vacuum. This gravity gradiometer has a design sensitivity of the order of 0.01 EU in 10-s integration time. Because of the small size and light weight, a noncryogenic gravity gradiometer is a valid candidate to fit the limited accommodation capability of the TSS deployed from the Shuttle and maintained, for several days, at an altitude of 120-130 km. This unique capability improves the resolution and accuracy achievable in reconstructing the earth gravity field, in particular at short wavelengths. A preliminary analysis of the dynamics of the TSS satellite indicates the potential of such satellite as a low-altitude research platform for gravity gradiometry.

Lorenzini, Enrico C.↗
NASA NTRS1987

Containerless processing of undercooled melts

Containerless drop tube processing allows for significant levels of liquid undercooling through control of parameters such as sample size, surface coating and cooling rate. A laboratory scale (3 m) drop tube has been developed which allows the undercooling and solidification behavior of powder samples to be evaluated under low gravity free-fall conditions. The level of undercooling obtained in an InSb-Sb eutectic alloy has been evaluated by comparing the eutectic spacing in drop tube samples with a spacing/undercooling relationship established using thermal analysis techniques. Undercoolings of 0.17 and 0.23 T(e) were produced by processing under vacuum and He gas conditions respectively. Alternatively, the formation of an amorphous phase in a Ni-Nb eutectic alloy indicates that undercooling levels of approximately 500 C were obtained by drop tube processing. The influence of droplet size and gas environment on undercooling behavior in the Ni-Nb eutectic was evaluated through their effect on the amorphous/crystalline phase ratio. To supplement the structural analysis, heat flow modeling has been developed to describe the undercooling history during drop tube processing, and the model has been tested experimentally.

Shong, D. S.↗
NASA NTRS1987

Pressure dependence of the electrical properties of GaBi solidified in low gravity

Immiscible GaBi alloys were solidified during free fall in the NASA Marshall Space Flight Center drop tower, which provides about 4.5 seconds of low gravity. The electrical resistivity and magnetic susceptibility were measured as a function of pressure (up to 18 kbar) and temperature (300 K to 4.2 K) of drop tower (DT) and ground control (GC) samples prepared under identical conditions, except for gravity. At ambient pressure the electrical resistance of the DT sample exhibits a broad maximum at 100 K, while that of GC sample decreases rapidly as temperature decreases. Both DT and GC samples become superconducting at 7.7 K. However, a minor second superconducting phase with a transition temperature at 8.3 K is observed only in the DT samples.

Wu, M. K.↗