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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 667 records · Page 37

Investigation of electrodynamic stabilization and control of long orbiting tethers

An algorithm for using electric currents to control pendular oscillations induced by various perturbing forces on the Skyhook wire is considered. Transverse and vertical forces on the tether; tether instability modes and causes during retrieval by space shuttle; simple and spherical pendulum motion and vector damping; and current generation and control are discussed. A computer program for numerical integration of the in-plane and out-of-plane displacements of the tether vs time was developed for heuristic study. Some techniques for controlling instabilities during payload retrieval and methods for employing the tether for launching satellites from the space shuttle are considered. Derivations and analyses of a general nature used in all of the areas studied are included.

Arnold, D. A.↗

Extension to an analysis of turbulent swirling compressible flow for application to axisymmetric small gas turbine ducts

An existing computer program, the Axisymmetric Diffuser Duct Code (ADD code), which calculates compressible turbulent swirling flow through axisymmetric ducts was modified to permit calculation of flows through small gas turbine ducts with struts, guide vanes and large degrees of turning. The improvements include a coordinate generator, an end-wall loss model, and a generalized geometry capability to describe struts and guide vanes in ducts which turn more than 90 degrees. An improved output format was developed to provide the solution on any arbitrary plane in the duct and an extensive literature survey of calculation procedures used in gas turbine technology was completed which suggests improvements in the computer code. Calculations are presented for the flow through the AGT101 small gas turbine inlet duct and turbine exhaust diffuser which demonstrate the ADD code modifications implemented in the investigation. The computed results compare favorably with experimental results.

Anderson, O. L.↗

Computation of turbulent boundary layer flows with an algebraic stress turbulence model

An algebraic stress turbulence model is presented, characterized by the following: (1) the eddy viscosity expression is derived from the Reynolds stress turbulence model; (2) the turbulent kinetic energy dissipation rate equation is improved by including a production range time scale; and (3) the diffusion coefficients for turbulence equations are adjusted so that the kinetic energy profile extends further into the free stream region found in most experimental data. The turbulent flow equations were solved using a finite element method. Examples include: fully developed channel flow, fully developed pipe flow, flat plate boundary layer flow, plane jet exhausting into a moving stream, circular jet exhausting into a moving stream, and wall jet flow. Computational results compare favorably with experimental data for most of the examples considered. Significantly improved results were obtained for the plane jet flow, the circular jet flow, and the wall jet flow; whereas the remainder are comparable to those obtained by finite difference methods using the standard kappa-epsilon turbulence model. The latter seems to be promising with further improvement of the expression for the eddy viscosity coefficient.

Kim, Sang-Wook↗

Fracture mechanics

The application of fracture mechanics to the design of ceramic structures will require the precise measurement of crack growth and fracture resistance of these materials over their entire range of anticipated service temperatures and standardized test methods for making such measurements. The development of a standard test for measuring the plane strain fracture toughness is sought. Stress intensity factor coefficients were determined for three varieties of chevron-notch specimens, and fracture toughness measurements were made on silicon nitrides, silicon carbides, and aluminum oxides to assess the performance of each specimen variety. It was determined that silicon nitride and silicon carbides have flat crack growth resistance curves, but aluminum oxide does not. Additionally, batch-to-batch differences were noticed for the aluminum oxide. Experiments are continuing to explain the rising crack growth resistance and batch-to-batch variations for the aluminum oxide.

Shannon, John L., Jr.↗

Analysis of interlaminar fracture in composites under combined loading

Delamination is a predominant failure mode in continuous fiber reinforced laminated composite structures. One type of delamination is the transverse crack tip delamination which originates at the tip of transverse matrix cracks. An analytical model based on the sublaminate approach and fracture mechanics is developed to study the growth of such delaminations. Plane strain conditions are assumed and estimates are provided for the total strain energy release rate as well as the mode 1 and mode 2 contribution. The energy release rate estimates are used in combination with a simple failure law to predict critical delamination growth strains and stresses. These predictions are compared with experimental data on T300/934 Graphite Epoxy (+ or - 25/90 nano seconds) laminates in the range n=.5 to 8. A good agreement is demonstrated for the range of n where the experimental observations indicate transverse crack tip delamination to be the predominant failure mode.

Armanios, Erian A.↗

Scramjet testing from Mach 4 to 20 - Present capability and needs for the nineties

Consideration is given to the development of testing methods for the National Aerospace Plane (NASP) program, focusing on techniques for simulating and testing scramjet engines and scramjet engine components during flight from Mach 4 to Mach 20. The flight conditions associated with the NASP are outlined and the stagnation temperatures and pressures required to simulate flight up to orbital speeds are presented. The types of test facilities needed for scramjet propulsion testing are discussed, including combustion-heated facilities, electric arc-heated facilities, convection-heated facilities, shock tunnels, and expansion tubes. Issues related to assessing the flow quality in the scramjet ground testing facilities are examined, including uniformity, contamination, dissociation, ionization, nonequilibrium, and turbulence. Direct-connect combustor versus free-jet scramjet engine tests are compared and the capabilities of current and near-future scramjet engine test facilities are evaluated.

Thomas, Scott R.↗

Turbulence modeling in supersonic combusting flows

To support the National Aerospace Plane project, the RPLUS3D CFD code has been developed at NASA Lewis. The code has the ability to solve three-dimensional flowfields with finite rate combustion of hydrogen and air. The combustion processes of the hydrogen-air system are simulated by an 18-reaction path, 8-species chemical kinetic mechanism. The code uses a Lower-Upper (LU) decomposition numerical algorithm as its basis, making it a very efficient and robust code. Except for the Jacobian matrix for the implicit chemistry source terms, there is no inversion of a matrix even though it uses a fully implicit numerical algorithm. A k-epsilon (two equation) turbulence model is incorporated into the RPLUS3D code.

Chitsomboon, Tawit↗

Periodic structures with time-varying loads

A new method is described for generating and predicting the power scattered at multiple frequencies from a periodic array excited by a monochromatic incident plane wave. The scattered power is predicted by developing an equivalent circuit representation from the exact method of moment solution of the unloaded periodic problem and then employing a modified method of moments approach to predict the scattered fields due to time-varying loads. An advantage of this method is that the generality of the equivalent circuit can be exploited to study the effects of different loads on the power scattered in the harmonic frequencies with a minimal amount of effort. The method can also be applied to multiple incident frequencies.

Epp, Larry W.↗

On absorbing boundary conditions for linearized Euler equations by a perfectly matched layer

Recently, Berenger introduced a Perfectly Matched Layer (PML) technique for absorbing electromagnetic waves. In the present paper, a perfectly matched layer is proposed for absorbing out-going two-dimensional waves in a uniform mean flow, generated by linearized Euler equations. It is well known that the linearized Euler equations support acoustic waves, which travel with the speed of sound relative to the mean flow, and vorticity and entropy waves, which travel with the mean flow. The PML equations to be used at a region adjacent to the artificial boundary for absorbing these linear waves are defined. Plane waves solutions to the PML equations are developed and wave propagation and absorption properties are given. It is shown that the theoretical reflection coefficients at an interface between the Euler and PML domains are zero, independent of the angle of incidence and frequency of the waves. As such, the present study points out a possible alternative approach for absorbing out-going waves of the Euler equations with little or no reflection in computation. Numerical examples that demonstrate the validity of the proposed PML equations are also presented.

Hu, Fang Q.↗

Reduction of Blade-Vortex Interaction (BVI) noise through X-force control

Momentum theory and the longitudinal force balance equations of a single rotor helicopter are used to develop simple expressions to describe tip-path-plane tilt and uniform inflow to the rotor. The uniform inflow is adjusted to represent the inflow at certain azimuthal locations where strong Blade-Vortex Interaction (BVI) is likely to occur. This theoretical model is then used to describe the flight conditions where BVI is likely to occur and to explore those flight variables that can be used to minimize BVI noise radiation. A new X-force control is introduced to help minimize BVI noise. Several methods of generating the X-force are presented that can be used to alter the inflow to the rotor and thus increasing the likelihood of avoiding BVI during approaches to a landing.

Schmitz, Fredric H.↗

A Ka-band helical antenna

A number of Ka-band satellite systems have been proposed to provide wide band personal communications services to terminals employing small antennas. Work has been performed looking at the use of helical antenna elements to provide an efficient low-cost antenna at these frequencies. Although helices are not planar, at Ka-band they are less than 2.5 cm in height and thus are suitable for these applications. This paper describes the design and development of a broadband helical antenna element. The work performed on: feed configurations, matching techniques, ground plane structures, broadbanding, and the effects of dielectric loading are discussed. Experimental measurements performed during the development of the helical antenna element are presented.

deLeseleuc, Michel↗

Quantum Well Infrared Photodetectors (QWIPs) for Astronomy

In recent years, many research groups in the world have demonstrated large format Quantum Well Infrared Photodetector (QWIP) focal plane arrays for various thermal imaging applications. QWIPs as opposed to conventional low bandgap infrared detectors, are limited by thermionic dark current and not tunneling currents down to 30K or less. As a result the performance of QWIPs can be substantially improved (orders of magnitude) by cooling from 70K to 30K. Cooling does not induce any nonuniformity or 1/f noise in QWIP focal plane arrays. In this paper, we discuss the development of highly uniform long- wavelength QWIPs for astronomical applications.

Gunapala, S. D.↗

Dependence of the Element Patterns of HYDROSTAR on Mutual Coupling

HYDROSTAR is a hybrid synthesis radiometer, intended for spaceborne applications, which employs a real aperture (waveguide stick antenna) for resolution along track and employs aperture synthesis to obtain resolution across track. This L-band system is an extension of the successful aircraft prototype, the Electronically Steered Thinned Array Radiometer (ESTAR). A proof-of-concept, full size system was constructed (45 wavelength in the synthesis direction) and subjected to extensive antenna pattern and associated microwave component measurements in an outdoor antenna test facility. HYDROSTAR employs a thinned array of 16 elements each 5.8 inches long in the along track dimension and spanning 9.5 inches across track. Each element is a narrow-wall shunt slot array with 36 slots. The polarization is linear (along-track). In the across track dimension, the antennas are deployed in a minimum redundancy array which has 90 independent baselines spaced in integer multiples of half a wavelength. The closest spacings used are for the first three elements at each end, which are spaced by only one-half wavelength. This study was intended to assess how closely each of 16 stick element patterns compare with their nominal values (as an individual, isolated radiator), when installed in their intended composite thinned array configuration. Extensive pattern measurements of the 16 elements that constitute the HYDROSTAR antenna subsystem were conducted to observe their relative features along the synthesis plane. Data was also collected for the mutual coupling between pairs of selected antennas. All antenna patterns had features different from that of an isolated element indicating some level of interaction among neighboring radiators. Those elements which had nearest neighbors at least five wavelengths away were located near the middle of the array. Their radiation patterns displayed sonic small, symmetric ripple across their full azimuth range. The patterns of elements that lie within 2.5 wavelengths of their neighbors showed stronger and asymmetric features. These are believed to be caused by mutual coupling among these structures. Evidence for this was seen when an antenna position was displaced by 0.05 wavelengths, Its pattern and those of its near neighbors were seen to change. Displacement within the plane of the array were observed to have different effects than displacements out-of-plane. A program of data analysis and theoretical development is in progress to provide a physical interpretation of the properties of these antenna patterns and to develop methods which can optimize the performance of this synthetic aperture imaging system. This includes compensation for pattern asymmetries and element position perturbation.

Weissman, David E.↗

Spinning Rocket Simulator Turntable Design

Contained herein is the research and data acquired from the Turntable Design portion of the Spinning Rocket Simulator (SRS) project. The SRS Project studies and eliminates the effect of coning on thrust-propelled spacecraft. This design and construction of the turntable adds a structural support for the SRS model and two degrees of freedom. The two degrees of freedom, radial and circumferential, will help develop a simulated thrust force perpendicular to the plane of the spacecraft model while undergoing an unstable coning motion. The Turntable consists of a ten-foot linear track mounted to a sprocket and press-fit to a thrust bearing. A two-inch high column grounded by a Triangular Baseplate supports this bearing and houses the slip rings and pressurized, air-line swivel. The thrust bearing allows the entire system to rotate under the moment applied through the chain-driven sprocket producing a circumferential degree of freedom. The radial degree of freedom is given to the model through the helically threaded linear track. This track allows the Model Support and Counter Balance to simultaneously reposition according to the coning motion of the Model. Two design factors that hinder the linear track are bending and twist due to torsion. A Standard Aluminum "C" channel significantly reduces these two deflections. Safety considerations dictate the design of all the components involved in this project.

Miles, Robert W.↗

Preliminary Planar Formation-Flight Dynamics Near Sun-Earth L2 Point

A few space agencies are planning missions to the vicinity of the Sun-Earth L(sub 2) point, some involving a distributed space system of telescope spacecraft, configured in a plane about a hub. An improved understanding is developed of the relative motion of such objects in formation flight. The telescope equations of motion are written relative to the hub, in terms of the hub s distance from L(sub 2), and an analytical solution is developed, useful for performing orbit control analysis. A halo telescope orbit is investigated, with initial conditions selected to avoid resonance excitation. An example case of the resulting solution is presented.

Segerman, Alan M.↗

Investigations of Spacecraft Orbits Around the L(2) Sun-Earth Libration Point

Space agencies are planning missions to the vicinity of the Sun-Earth L sub 2 point, some involving a distributed system of telescope spacecraft, configured in a plane about a hub. An improved understanding is developed of their relative motion. First, the telescope equations of motion are written relative to L sub 2 in the context of the classical circular restricted three-body problem, and expanded in terms of the distance from L sub 2. A basic examination is presented of the spacecraft configuration requirements and effects of small orbit insertion errors. Next, the telescope equations of motion relative to the hub are written and further expanded in terms of the hub-L sub 2 and hub-telescope distances. An analytical solution is developed and a halo telescope orbit investigated, with appropriate initial conditions. Then, the force model is extended to include perturbations to an accuracy of 10 to 20 m, east as additive contributions to the circular restricted problem. Perturbations include Earth's orbital eccentricity, lunar motion, solar radiation pressure, and small thrusting forces. Simulations are presented, along with solution sensitivity to errors in hub position knowledge.

RELATIVE MOTION↗

Cassini-Huygens maneuver automation for navigation

Many times during the Cassini-Huygens mission to Saturn, propulsive maneuvers must be spaced so closely together that there isn't enough time or workforce to execute the maneuver-related software manually, one subsystem at a time. Automation is required. Automating the maneuver design process has involved close cooperation between teams. We present the contribution from the Navigation system. In scope, this includes trajectory propagation and search, generation of ephemerides, general tasks such as email notification and file transfer, and presentation materials. The software has been used to help understand maneuver optimization results, Huygens probe delivery statistics, and Saturn ring-plane crossing geometry. The Maneuver Automation Software (MAS), developed for the Cassini-Huygens program enables frequent maneuvers by handling mundane tasks such as creation of deliverable files, file delivery, generation and transmission of email announcements, generation of presentation material and other supporting documentation. By hand, these tasks took up hours, if not days, of work for each maneuver. Automated, these tasks may be completed in under an hour. During the cruise trajectory the spacing of maneuvers was such that development of a maneuver design could span about a month, involving several other processes in addition to that described, above. Often, about the last five days of this process covered the generation of a final design using an updated orbit-determination estimate. To support the tour trajectory, the orbit determination data cut-off of five days before the maneuver needed to be reduced to approximately one day and the whole maneuver development process needed to be reduced to less than a week..

propulsion maneuvers↗

Icing Simulation

A grid block transformation scheme which allows the input of grids in arbitrary reference frames, the use of mirror planes, and grids with relative velocities has been developed. A simple ice crystal and sand particle bouncing scheme has been included.. Added an SLD splashing model based on that developed by William Wright for the LEWICE 3.2.2 software. A new area based collection efficiency algorithm will be incorporated which calculates trajectories from inflow block boundaries to outflow block boundaries. This method will be used for calculating and passing collection efficiency data between blade rows for turbo-machinery calculations.

Bidwell, Colin↗