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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 1,225 records · Page 68
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Cosmic structure from radiation-blown bubbles

Absorbing material in an expanding universe filled with sources of radiation is subject to an instability driven by radiation pressure. In the optically thick limit, this instability takes the form of rapidly growing cavities or bubbles in the absorbing material. No matter how small they are when they begin, such bubbles grow to approach a limiting size which depends only on the ratio of photon pressure to absorber inertia. For a nonprimordial submillimeter radiation background as strong as that recently detected by the Berkely-Nagoya group, the bubbles grow to a comoving radius of about 20 Mpc, comparable in scale to the voids seen in the large-scale galaxy distribution.

Hogan, Craig J.↗
NASA NTRS1989

Radiometry of near-earth asteroids

Infrared photometry is reported for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-earth asteroids.

Veeder, G. J.↗
NASA NTRS1988

The equations of motion for a general orbiting large space flexible system

General motion equations in compact form using dyadic notation are developed for an orbiting flexible body of arbitrary shape. The equations are applied to three typical flexible space systems: a flexible beam, a flexible plate, and a flexible shallow spherical shell in orbit. The inertia dyadic of these flexible bodies may be calculated using the equations which have been obtained.

Xing, Guangqian↗
NASA NTRS1989

Closure of field-aligned currents carried by super-Alfvenic auroral electrons

A linear analysis is carried out to investigate how the return current is generated for the field-aligned current carried by a super-Alfvenic auroral electron beam. If the electron beam is a thin sheet with thickness less than or equal to the electron inertia length, the negative space charge of the beam is neutralized by the ion polarization current produced by the beam. The return current in this case is generated on the two outsides of the beam connected to the polarization current to form a closed current loop. This current structure can explain the thin (a few km) current sheets in the auroral arcs observed by satellites.

Nakamura, Tadas↗
NASA NTRS1989

Sensitivity of mesoscale-model forecast skill to some initial-data characteristics, data density, data position, analysis procedure and measurement error

The effects of horizontal and vertical data resolution, data density, data location, different objective analysis algorithms, and measurement error on mesoscale-forecast accuracy are studied with observing-system simulation experiments. Domain-averaged errors are shown to generally decrease with time. It is found that the vertical distribution of error growth depends on the initial vertical distribution of the error itself. Larger gravity-inertia wave noise is produced in forecasts with coarser vertical data resolution. The use of a low vertical resolution observing system with three data levels leads to more forecast errors than moderate and high vertical resolution observing systems with 8 and 14 data levels. Also, with poor vertical resolution in soundings, the initial and forecast errors are not affected by the horizontal data resolution.

Warner, Thomas T.↗
NASA NTRS1989

Random vibrations of laminated plates modeled within the first order shear deformation theory

This paper deals with the dynamic response of anisotropic laminated composite plates to stationary random excitation. The theory of laminated plates used here includes shear deformation and rotary inertia effects in the same manner as Mindlin's theory for isotropic homogeneous plates. Two cases of random pressure fields are considered in this analysis. In the first case, it is modeled as a uniformly distributed load, random in time with an exponential decaying correlation function, while in the second one as a jet noise, fully correlated in one direction and homogeneous in the perpendicular one. The analysis presented herein, as well as the obtained response characteristics, expressed in terms of mean-squares and angle-dependent mean-squares, may be useful in the design of composite plates subjected to random pressure fields and in the evaluation of their reliability.

Cederbaum, G.↗
NASA NTRS1989

Collisional and inertial dynamics of the ionospheric interchange instability

The dynamical properties and spectral characteristics of the ionospheric interchange instability in the collisional and inertial regimes are investigated numerically and analytically, using a two-dimensional fluid model. This model, which describes the collisional and inertial dynamics of the F region ionosphere, is similar to the models of Ott (1978) and Kintner and Seyler (1985). By numerically integrating the model equations, the nonlinear evolution of three interchange unstable configurations in the collisional and inertial regimes are followed, and their dynamical and spectral properties are studied. The solutions indicate that the dynamical properties of a collision-dominated regime are distinctly different from those of an inertia-dominated regime. The qualitative characteristics of a plasma bubble in both regime-types are described.

Zargham, S.↗
NASA NTRS1989

A new adaptive control approach for aerospace vehicles with parameter uncertainties

A new stochastic adaptive control structure is developed for the problem of combined parameter estimation and control of aerospace vehicles with changing parameters. Parameter uncertainties are modeled as first-order Gauss-Markov processes, and are introduced to the system dynamics through a small parameter. It is assumed that an accurate inertial measurement unit gives perfect measurements of the state variables. Since the stochastic system is assumed to be Gauss-Markov, the density function of the parameters given these measurements is conditionally Gaussian. Based on this conditionally Gaussian density, the problem of minimizing a quadratic cost over an infinite time horizon can be set up within the framework of stochastic optimal control theory. The optimal feedback control law is derived from a straightforward expansion of the Hamilton-Jacobi-Bellman equation, based on the LQG solution. The resulting nonlinear controller is applied to the pitch axis control of a space platform with uncertain moments of inertia and is shown to produce marked improvement over a fixed controller.

Hahn, Yungsun↗
NASA NTRS1990

Markov Chains For Testing Redundant Software

Preliminary design developed for validation experiment that addresses problems unique to assuring extremely high quality of multiple-version programs in process-control software. Approach takes into account inertia of controlled system in sense it takes more than one failure of control program to cause controlled system to fail. Verification procedure consists of two steps: experimentation (numerical simulation) and computation, with Markov model for each step.

White, Allan L.↗
NASA NTRS1990

Checking Automated-Welder Programs By Computer

Computer system detects and displays actual and potential errors in programs for computer-controlled electron-beam welder. Uses personal computer, separate from welding computer. Programmed specifically to highlight errors in welding program. Eliminates need for preweld full-power test run reducing checkout time to about half hour. Assures much higher quality, damage-free welding. Used to create and edit new programs for welder. Control computer thereby free for production. Useful for checking programs for such other computer-controlled equipment as inertia welders, robots, machine tools, and heat treaters.

Damicone, L. O.↗
NASA NTRS1990

Liquid-Ring Attitude-Control System For Spacecraft

Angular momentum transferred to liquid flowing in rings of tubing. Proposed fluid-ring actuator used to orient spacecraft. Used instead of rotating inertia wheels. Meets need for pointing system of low mass, low power, and high resolution. Advantage of actuator, rings of tubing routed through existing structures, minimizing required space and mass.

Lurie, Boris J.↗
NASA NTRS1989

An adaptive controller for enhancing operator performance during teleoperation

An adaptive controller is developed for adjusting robot arm parameters while manipulating payloads of unknown mass and inertia. The controller is tested experimentally in a master/slave configuration where the adaptive slave arm is commanded via human operator inputs from a master. Kinematically similar six-joint master and slave arms are used with the last three joints locked for simplification. After a brief initial adaptation period for the unloaded arm, the slave arm retrieves different size payloads and maneuvers them about the workspace. Comparisons are then drawn with similar tasks where the adaptation is turned off. Several simplifications of the controller dynamics are also addressed and experimentally verified.

Carignan, Craig R.↗
NASA NTRS1989

Advanced Turbine Technology Applications Project (ATTAP)

Work to develop and demonstrate the technology of structural ceramics for automotive engines and similar applications is described. Long-range technology is being sought to produce gas turbine engines for automobiles with reduced fuel consumption and reduced environmental impact. The Advanced Turbine Technology Application Project (ATTAP) test bed engine is designed such that, when installed in a 3,000 pound inertia weight automobile, it will provide low emissions, 42 miles per gallon fuel economy on diesel fuel, multifuel capability, costs competitive with current spark ignition engines, and noise and safety characteristics that meet Federal standards.

Source record↗
NASA NTRS1989

High-frequency instability of the sheath-plasma resonance

Coherent high frequency oscillations near the electron plasma frequency (omega approx. less than omega sub p) are generated by electrodes with positive dc bias immersed in a uniform Maxwellian afterglow plasma. The instability occurs at the sheath-plasma resonance and is driven by a negative RF sheath resistance associated with the electron inertia in the diode-like electron-rich sheath. With increasing dc bias, i.e., electron transit time, the instability exhibits a hard threshold, downward frequency pulling, line broadening and copious harmonics. The fundamental instability is a bounded oscillation due to wave evanescence, but the harmonics are radiated as electromagnetic waves from the electrodes acting like antennas. Wavelength and polarization measurements confirm the emission process. Electromagnetic waves are excited by electrodes of various geometries (planes, cylinders, spheres) which excludes other radiation mechanisms such as orbitrons or beam-plasma instabilities. The line broadening mechanism was identified as a frequency modulation via the electron transit time by dynamic ions. Ion oscillations at the sheath edge give rise to burst-like RF emissions. These laboratory observations of a new instability are important for antennas in space plasmas, generation of coherent beams with diodes, and plasma diagnostics.

Stenzel, R. L.↗
NASA NTRS1990

Architectural study of the design and operation of advanced force feedback manual controllers

A teleoperator system consists of a manual controller, control hardware/software, and a remote manipulator. It was employed in either hazardous or unstructured, and/or remote environments. In teleoperation, the main-in-the-loop is the central concept that brings human intelligence to the teleoperator system. When teleoperation involves contact with an uncertain environment, providing the feeling of telepresence to the human operator is one of desired characteristics of the teleoperator system. Unfortunately, most available manual controllers in bilateral or force-reflecting teleoperator systems can be characterized by their bulky size, high costs, or lack of smoothness and transparency, and elementary architectures. To investigate other alternatives, a force-reflecting, 3 degree of freedom (dof) spherical manual controller is designed, analyzed, and implemented as a test bed demonstration in this research effort. To achieve an improved level of design to meet criteria such as compactness, portability, and a somewhat enhanced force-reflecting capability, the demonstration manual controller employs high gear-ratio reducers. To reduce the effects of the inertia and friction on the system, various force control strategies are applied and their performance investigated. The spherical manual controller uses a parallel geometry to minimize inertial and gravitational effects on its primary task of transparent information transfer. As an alternative to the spherical 3-dof manual controller, a new conceptual (or parallel) spherical 3-dof module is introduced with a full kinematic analysis. Also, the resulting kinematic properties are compared to those of other typical spherical 3-dof systems. The conceptual design of a parallel 6-dof manual controller and its kinematic analysis is presented. This 6-dof manual controller is similar to the Stewart Platform with the actuators located on the base to minimize the dynamic effects. Finally, a combination of the new 3-dof and 6-dof concepts is presented as a feasible test-bed for enhanced performance in a 9-dof system.

Tesar, Delbert↗
NASA NTRS1990

Dynamic analysis of geared rotors by finite elements

A finite-element model of a geared rotor system on flexible bearings was developed. The model includes the rotary inertia of shaft elements, the axial loading on shafts, flexibility and damping of bearings, material damping of shafts and the stiffness and the damping of gear mesh. The coupling between the torsional and transverse vibrations of gears were considered in the model. A constant mesh stiffness was assumed. The analysis procedure can be used for forced vibration analysis of geared rotors by calculating the critical speeds and determining the response of any point on the shaft to mass unbalances, geometric eccentricities of gears and displacement transmission error excitation at the mesh point. The dynamic mesh forces due to these excitations can also be calculated. The model has been applied to several systems for the demonstration of its accuracy and for studying the effect of bearing compliances on system dynamics.

Kahraman, Ahmet↗
NASA NTRS1990

Analysis and testing of axial compression in imperfect slender truss struts

The axial compression of imperfect slender struts for large space structures is addressed. The load-shortening behavior of struts with initially imperfect shapes and eccentric compressive end loading is analyzed using linear beam-column theory and results are compared with geometrically nonlinear solutions to determine the applicability of linear analysis. A set of developmental aluminum clad graphite/epoxy struts sized for application to the Space Station Freedom truss are measured to determine their initial imperfection magnitude, load eccentricity, and cross sectional area and moment of inertia. Load-shortening curves are determined from axial compression tests of these specimens and are correlated with theoretical curves generated using linear analysis.

Lake, Mark S.↗
NASA NTRS1989

LAGEOS 3 and the gravitomagnetic field

The importance of the gravitomagnetic field is discussed. A never-measured field of nature, the foundations of inertia in Einstein General Relativity, and a key role in theories of quasars and active galactic nuclei are important aspects of this field and are discussed. In high energy astrophysics, some theories of energy storage, power generation, jet formation and jet alignment of quasars and active galactic nuclei are based on the existence of the gravitomagnetic field of a supermassive black hole (Thorne et al. 1986). LAGEOS 3 is discussed in terms of laser ranged satellites to detect the gravitomagnetic field and supplementary inclination satellites to avoid gravity field uncertainties. Many experiments have been proposed to measure the gravitomagnetic field. The GPB experiment intends to measure the Lense-Thirring-Schiff precession of gyroscopes orbiting the earth. Polar satellites have been proposed to measure the Lense-Thirring precession of the orbital plane (an enormous gyroscope and two guided, drag-free, counter-rotating, polar satellites have been suggested to avoid orbital inclination errors.) The new idea to measure the gravitomagnetic drag of the nodes of two nonpolar, supplementary inclination, satellites is summarized.

Ciufolini, Ignazio↗