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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 235 records · Page 13

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NASA NTRS1982

NASA geodynamics program

The goals of the NASA geodynamics program comprise the study of the interior structure and composition of the earth, its crustal motions and deformations, the rotational dynamics, and the terrestrial potential fields. Laser ranging an VLBI techniques are being applied in cooperation with 16 other countries. Reflectors on the moon are illumined with laser beams from earth, which are also bounced off the Lageos and French Starlette satellites, permitting monitoring of polar motion, earth rotation, and distance to the moon within 5 cm. Mobile laser systems are being deployed for developing a gravity field model and examining crustal deformation regionally. The VLBI installations use quasars as reference objects and the GPS satellites will give position accuracy within 1-2 cm. Satellite-to-satellite tracking, Magsat, and Shuttle timing experiment data are providing geopotential field data and timing synchronization.

Fischetti, T. L.↗

NASA NTRS1982

The rings of Uranus

Stellar occultation data on the Uranian rings are summarized. Spatial resolution is 0.1 km, for positions of ring segments, 4 km for structural details. The system consists of 9 narrow rings with average geometric albedo of 0.02. The data are well fitted by a kinematic model that describes the rings as ellipses, inclined to the equatorial plane of Uranus and precess due to harmonic terms in the Uranian potential fields. The main characteristics of the Uranian ring system that need a confirmed theoretical explanation are: narrow rings; sharp edges; uniform orbital precession; origin of the eccentricities and inclinations; the structure of the epsilon ring; the structure of the eta ring; and the origin of the ring system.

Elliot, J. L.↗

NASA NTRS1984

The three-dimensional compressible flow in a radial inflow turbine scroll

This work presents the results of an analytical study and an experimental investigation of the three-dimensional flow in a turbine scroll. The finite element method is used in the iterative numerical solution of the locally linearized governing equations for the three-dimensional velocity potential field. The results of the numerical computations are compared with the experimental measurements in the scroll cross sections, which were obtained using laser Doppler velocimetry and hot wire techniques. The results of the computations show a variation in the flow conditions around the rotor periphery which was found to depend on the scroll geometry.

Hamed, A.↗

NASA NTRS1985

The satellite magnetic anomaly of Ahaggar - Evidence for African Plate motion

The Ahaggar volcanic province of North Central Africa is considered a region of excess heat flow (hot spot) and hence elevated Curie isotherm. Using a modified version of the Parker FFT potential field representation, magnetic signals were calculated at Magsat altitudes for models in which the African Plate is both fixed and moving. The moving-plate model extends the Curie isotherm anomaly in the direction of plate motion and provides a satisfactory match to vertical component anomaly data when the magnitude of plate velocity is 0.75 cm/yr. Although the signal levels are marginal for the scalar component anomalies of this region, the same model provides an adequate match to this data set and is clearly preferable to a fixed-plate model.

Phillips, R. J.↗

NASA NTRS1986

Gravity gradient torque for an arbitrary potential function

To permit investigation of the relative effects of higher-order terms in the potential function and allow computation of the gravity gradient torques consistent with the gravitational acceleration for a given potential function, a simple formula is derived for computing the gravity gradient torque on a vehicle in terms of the eigenvalues and eigenvectors of the gravity gradient matrix and the inertia properties of the vehicle. The formulation is applicable to any potential function for which the gravity matrix can be computed for a given point in the potential field.

Glandorf, D. R.↗

NASA NTRS1986

Evaluation of spacecraft navigation using the Tracking and Data Relay Satellite System (TDRSS)

The navigational capabilities of the TDRSS for the operational support of user spacecraft is evaluated. A batch-weighted least-squares algorithm is used to fit tracking measurements and to generate orbit solutions for the TDRS-E spacecraft and the following user spacecraft: (1) the SMM, (2) the Landsat-5, (3) the Earth Radiation Budget Satellite (ERBS), and (4) the Solar Mesosphere Explorer (SME). TDRS-E orbit accuracy was evaluated by using the consistency of consecutive orbit solutions as a measure; user orbit accuracy was estimated by comparing orbits determined from TDRSS tracking with orbits obtained from ground tracking for the same time periods. It was found that the predicted accuracy of the TDRS-E orbit propagation was improved by as much as seven times by refining the spherical harmonic expansion model of the earth's potential field. The results obtained for SMM were fairly consistent, while those obtained for Landsat-5 were consistent to better than 80 meters. The orbits obtained using ERBS tracking were consistent to better than 60 meters and the SME orbits for two arcs with 14 and 20 passes of TDRSS tracking had a maximum position difference of 140 meters.

Samii, M. V.↗

NASA NTRS1986

Three-dimensional magnetostatic models of the large-scale corona

A special class of magnetostatic equilibria is described, which are mathematically simple and yet sufficiently versatile so as to fit any arbitrary normal magnetic flux prescribed at the photosphere. With these solutions, the corona can be modeled with precisely the same mathematically simple procedure as has previously been done with potential fields. The magnetostatic model predicts, in addition to the coronal magnetic field, the three dimensional coronal density which can be compared with coronagraph observations.

Bogdan, T. J.↗

NASA NTRS1986

On transient rheology and glacial isostasy

The effect of transient creep on the inference of long-term mantle viscosity is investigated using theoretical predictions from self-gravitating, layered earth models with Maxwell, Burgers' body, and standard linear solid rheologies. The interaction between transient and steady-state rheologies is studied. The responses of the standard linear solid and Burgers' body models to transient creep in the entire mantle, and of the Burgers' body and Maxwell models to creep in the lower mantle are described. The models' responses are examined in terms of the surface displacement, free air gravity anomaly, wander of the rotation pole, and the secular variation of the degree 2 zonal coefficient of the earth's gravitational potential field. The data reveal that transient creep cannot operate throughout the entire mantle.

Yuen, David A.↗

NASA NTRS1987

The role of orbital dynamics and cloud-cloud collisions in the formation of giant molecular clouds in global spiral structures

The role of orbit crowding and cloud-cloud collisions in the formation of GMCs and their organization in global spiral structure is investigated. Both N-body simulations of the cloud system and a detailed analysis of individual particle orbits are used to develop a conceptual understanding of how individual clouds participate in the collective density response. Detailed comparisons are made between a representative cloud-particle simulation in which the cloud particles collide inelastically with one another and give birth to and subsequently interact with young star associations and stripped down simulations in which the cloud particles are allowed to follow ballistic orbits in the absence of cloud-cloud collisions or any star formation processes. Orbit crowding is then related to the behavior of individual particle trajectories in the galactic potential field. The conceptual picture of how GMCs are formed in the clumpy ISMs of spiral galaxies is formulated, and the results are compared in detail with those published by other authors.

Roberts, William W., Jr.↗

NASA NTRS1988

Solar cycle variations of the corona and their implications for dynamics of the interior and the heliosphere

Using a simple potential field approximation to compute the coronal magnetic field from the surface magnetic field, it is argued that six distinctive solar cycle-associated variations of the solar corona can be understood in terms of solar cycle variations in the formation and dynamics of open and closed field line configurations. It is proposed that the X-ray emission of the sun should show solar cycle variations, and that it is possible to detect and study stellar magnetic cycles by monitoring X-ray emission of the stellar coronae over a long time span.

Yoshimura, Hirokazu↗

NASA NTRS1990

Advanced electric propulsion research, 1989

Results of an experimental study of the characteristics of ion thruster hollow cathodes operating at high discharge currents (up to 60 A) are presented in a companion report. This work shows that ions produced near the cathode orifice can acquire sufficient energy to induce the high sputter erosion rates on cathode potential surfaces that have been observed in ion thrusters. A mechanism by which these ions could be produced is also described. A second, brief study showing how a discharge chamber model developed previously can be applied to determine optimal values for one or more discharge chamber design parameters is presented. The experimental approach being used to study the plasma potential field and charge-exchange ion production rate downstream of the accelerator grid of an ion thruster is discussed and preliminary results are presented.

Wilbur, Paul J.↗

NASA NTRS1990

Numerical approach of collision avoidance and optimal control on robotic manipulators

Collision-free optimal motion and trajectory planning for robotic manipulators are solved by a method of sequential gradient restoration algorithm. Numerical examples of a two degree-of-freedom (DOF) robotic manipulator are demonstrated to show the excellence of the optimization technique and obstacle avoidance scheme. The obstacle is put on the midway, or even further inward on purpose, of the previous no-obstacle optimal trajectory. For the minimum-time purpose, the trajectory grazes by the obstacle and the minimum-time motion successfully avoids the obstacle. The minimum-time is longer for the obstacle avoidance cases than the one without obstacle. The obstacle avoidance scheme can deal with multiple obstacles in any ellipsoid forms by using artificial potential fields as penalty functions via distance functions. The method is promising in solving collision-free optimal control problems for robotics and can be applied to any DOF robotic manipulators with any performance indices and mobile robots as well. Since this method generates optimum solution based on Pontryagin Extremum Principle, rather than based on assumptions, the results provide a benchmark against which any optimization techniques can be measured.

Wang, Jyhshing Jack↗

NASA NTRS1990

Microwave background distortions from domain walls

Domain walls arising in a cosmic phase transition after decoupling were recently proposed as seeds for the formation of large scale structure. The distortion induced in the microwave background radiation is calculated in dependence of the wall thickness, surface density, scalar field potential, cosmic redshift and the velocity of the wall. It was found that the maximal redshift distortion for both spherical and planar walls is of the order pi G sigma H(sup -1)(sub 0), where sigma is the surface energy density and H(sup -1)(sub 0) the Hubble parameter. It was also found that, for a wall thickness smaller than the horizon, walls can be treated as infinitely thin, i.e., the redshift distortion is independent of the wall thickness and the specific form of the scalar potential. For planar walls moving with a Lorentz-factor gamma the redshift distortion is enhanced by gamma cubed.

Goetz, Guenter↗

NASA NTRS1991

Optical Isolators With Transverse Magnets

New design for isolator includes zigzag, forward-and-backward-pass beam path and use of transverse rather than longitudinal magnetic field. Design choices produce isolator with as large an aperture as desired using low-Verdet-constant glass rather than more expensive crystals. Uses commercially available permanent magnets in Faraday rotator. More compact and less expensive. Designed to transmit rectangular beam. Square cross section of beam extended to rectangular shape by increasing one dimension of glass without having to increase magnetic field. Potentially useful in laser systems involving slab lasers and amplifiers. Has applications to study of very-high-power lasers for fusion research.

Fan, Yuan X.↗

NASA NTRS1990

CO mapping of the nuclear region of NGC 6946 and IC 342 with Nobeyama millimeter array

CO observations of nearby galaxies with nuclear active star forming regions (and starburst galaxies) with angular resolutions around 7 seconds revealed that molecular bars with a length of a few kiloparsecs have been formed in the central regions of the galaxies. The molecular bar is interpreted as part of shock waves induced by an oval or barred potential field. By shock dissipation or dissipative cloud-cloud collisions, the molecular gas gains an infall motion and the nuclear star formation activity is fueled. But the distribution and kinematics of the molecular gas in the nuclear regions, which are sites of active star formation, remain unknown. Higher angular resolutions are needed to investigate the gas in the nuclear regions. Researchers made aperture synthesis observations of the nuclear region of the late-type spiral galaxies NGC 6946 and IC 342 with resolutions of 7.6 seconds x 4.2 seconds (P.A. = 147 deg) and 2.4 seconds x 2.3 seconds (P.A. = 149 deg), respectively. The distances to NGC 6496 and IC 342 are assumed to be 5.5 Mpc and 3.9 Mpc, respectively. Researchers have found 100-300 pc nuclear gas disk and ring inside a few kpc molecular gas bars. Researchers present the results of the observations and propose a possible mechanism of active star formation in the nuclear region.

Ishizuki, Sumio↗

NASA NTRS1991

Measurement and calculation of forces in a magnetic journal bearing actuator

Numerical calculations and experimental measurements of forces from an actuator of the type used in active magnetic journal bearings are presented. The calculations are based on solution of the scalar magnetic potential field in and near the gap regions. The predicted forces from single magnet with steady current are compared with experimental measurements in the same geometry. The measured forces are smaller than calculated ones in the principal direction but are larger than calculated in the normal direction. This combination of results indicate that material and spatial effects other than saturation play roles in determining the force available from an actuator.

Knight, Josiah↗

NASA NTRS1991

Avoiding space robot collisions utilizing the NASA/GSFC tri-mode skin sensor

Sensor based robot motion planning research has primarily focused on mobile robots. Consider, however, the case of a robot manipulator expected to operate autonomously in a dynamic environment where unexpected collisions can occur with many parts of the robot. Only a sensor based system capable of generating collision free paths would be acceptable in such situations. Recently, work in this area has been reported in which a deterministic solution for 2DOF systems has been generated. The arm was sensitized with 'skin' of infra-red sensors. We have proposed a heuristic (potential field based) methodology for redundant robots with large DOF's. The key concepts are solving the path planning problem by cooperating global and local planning modules, the use of complete information from the sensors and partial (but appropriate) information from a world model, representation of objects with hyper-ellipsoids in the world model, and the use of variational planning. We intend to sensitize the robot arm with a 'skin' of capacitive proximity sensors. These sensors were developed at NASA, and are exceptionally suited for the space application. In the first part of the report, we discuss the development and modeling of the capacitive proximity sensor. In the second part we discuss the motion planning algorithm.

Prinz, F. B.↗

NASA NTRS1992

Ion collection from a plasma by a pinhole

Ion focusing by a biased pinhole is studied numerically. Laplace's equation is solved in 3-D for cylindrical symmetry on a constant grid to determine the potential field produced by a biased pinhole in a dielectric material. Focusing factors are studied for ions of uniform incident velocity with a 3-D Maxwellian distribution superimposed. Ion currents to the pinhole are found by particle tracking. The focusing factor of positive ions as a function of initial velocity, temperature, injection radius, and hole size is reported. For a typical Space Station Freedom environment (oxygen ions having a 4.5 eV ram energy, 0.1 eV temperature, and a -140 V biased pinhole), a focusing factor of 13.35 is found for a 1.5 mm radius pinhole.

Snyder, David B.↗