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NASA NTRS1980

Torque equilibrium attitude control for Skylab reentry

The method of torque equilibrium attitude control used to control the reentry of Skylab to an altitude below 150 km without the use of thruster fuel once the attitude was established is discussed. The Skylab attitude and pointing control system, which included rate gyros, sun sensors, star tracker, the Apollo telescope mount digital computer, control moment gyros and cold-gas attitude thrusters, is presented. The 12 torque equilibrium attitudes found at which aerodynamic, gravity gradient and gyroscopic torques would balance are indicated, and the three of those at which the solar power supply would be adequate for attitude control are illustrated. The equilibrium seeking method employed is then examined, and the operation and performance of the torque equilibrium attitude control system during the three weeks prior to Skylab reentry are discussed. It is concluded that the torque equilibrium attitude control method developed for Skylab was successful in performing its assigned mission, and will be valuable for the design of future, low-altitude spacecraft or tethered vehicles.

Glaese, J. R.↗

NASA NTRS2004

Circular Data Images for Directional Data

Directional data includes vectors, points on a unit sphere, axis orientation, angular direction, and circular or periodic data. The theoretical statistics for circular data (random points on a unit circle) or spherical data (random points on a unit sphere) are a recent development. An overview of existing graphical methods for the display of directional data is given. Cross-over occurs when periodic data are measured on a scale for the measurement of linear variables. For example, if angle is represented by a linear color gradient changing uniformly from dark blue at -180 degrees to bright red at +180 degrees, the color image will be discontinuous at +180 degrees and -180 degrees, which are the same location. The resultant color would depend on the direction of approach to the cross-over point. A new graphical method for imaging directional data is described, which affords high resolution without color discontinuity from "cross-over". It is called the circular data image. The circular data image uses a circular color scale in which colors repeat periodically. Some examples of the circular data image include direction of earth winds on a global scale, rocket motor internal flow, earth global magnetic field direction, and rocket motor nozzle vector direction vs. time.

Morpet, William J.↗

NASA NTRS1975

Recent developments in density, temperature and momentum fluctuation measurement

Attention is drawn to recent advancements in the fluctuating line-reversal temperature measurement, the development of the two-dimensional drag-sensing probe into a three-dimensional drag-sensing probe, and the fluctuating density gradient cross beam Schlieren technique. An experimental apparatus is explained whereby the temperature fluctuation in a pulsating air-fuel ratio Bunsen burner is measured by means of sodium D-line reversal methods with a new photoelectric circuit to obtain both fluctuating and mean temperature data. The three-D drag probe is made possible by a newly invented differential and total magnetic sensing system that separates signals due to three orthogonal movements so that the vectorial momentum fluctuation can be measured. In the case of the Schlieren technique, the effect of anisotropic density gradients with respect to the angle between the polarization plane and the knife edge of a laser Schlieren system is studied.

Battle, T. M.↗

NASA NTRS1974

Some results of track method of investigating Luna 16 lunar material

Preliminary data from an investigation of tracks in olivine crystals, separated from the five zones of a lunar surface material core, are reported. The gradients of track densities, their lengths, and their angular distribution were measured with an optical microscope. Throughout the core depth (35 cm) crystals bearing traces of exposure to low energy solar cosmic rays were found, indicating the occurrence of mixing processes in the surficial layer of lunar surface material. The age of the occurrence of the samples investigated on the lunar surface, calculated from the track density, is in the interval 0.7 to 16 million years.

Kashkarov, L. L.↗

NASA NTRS2004

Enhanced Boiling on Micro-Configured Composite Surfaces Under Microgravity Conditions

Two experimental setups: High Heat Flux Pool Boiling Setup and Optical system of Measuring Contact Angle and Spreading Parameters were established and a series of experiments was conducted. The experimental work on the nucleate boiling performance of both Freon-113 and water on the composite surfaces has been finished and the results show that the composite surface with 25% volume fraction of graphite fibers in the copper matrix has the best enhancing performance. Stronger enhancement was obtained for water compared to Freon-113. The wall superheat initiating boiling was reduced 40% and 35% and the heat flux of the nucleate boiling was augmented up to ten times and 6.7 times, respectively. A bubble departure model has been presented to explain the enhancement mechanism of the boiling heat transfer on the composite surfaces. A powerful optical method to measure contact angle and spreading parameters of liquids on solid surfaces, including both transparent and non-transparent substrates, has been developed and successfully used in the measurements for various liquid-solid systems. The experimental work on identification of profiles near three-phase line by laser shadowgraphy method has been finished and the characteristics of the three-phase line for different liquid-solid systems were unveiled, which is very important to understand the mechanism of boiling under microgravity conditions. The four basic patterns of sessile profile were discovered through caustics and caustic-diffraction analysis by applying wave theory in shadowgraphy technology, which contributes to clearly explain the different characteristics of wetting and spreading of various liquids on a solid surface, and therefore to further understand the bubble departure mechanisms. A new working pattern of heat pipes using working fluids with positive surface-tension gradient against temperature was discovered and analyzed, and then the new heat pipe systems using this kind of working fluids have been presented. Two patent devices, i.e. Shadowgraphic Method to Measure Properties of Sessile Drops and Heat Pipe Systems Using New Working Fluids, have been developed; the former has been approved by The United States Patent & Trademark Office and the latter is being examined.

Source record↗

NASA NTRS2006

Multiclass Reduced-Set Support Vector Machines

There are well-established methods for reducing the number of support vectors in a trained binary support vector machine, often with minimal impact on accuracy. We show how reduced-set methods can be applied to multiclass SVMs made up of several binary SVMs, with significantly better results than reducing each binary SVM independently. Our approach is based on Burges' approach that constructs each reduced-set vector as the pre-image of a vector in kernel space, but we extend this by recomputing the SVM weights and bias optimally using the original SVM objective function. This leads to greater accuracy for a binary reduced-set SVM, and also allows vectors to be 'shared' between multiple binary SVMs for greater multiclass accuracy with fewer reduced-set vectors. We also propose computing pre-images using differential evolution, which we have found to be more robust than gradient descent alone. We show experimental results on a variety of problems and find that this new approach is consistently better than previous multiclass reduced-set methods, sometimes with a dramatic difference.

reduced set methods↗

NASA NTRS1970

One-Dimensional Shock Wave Formation by an Accelerating Piston

The formation of a shock wave by a solid accelerating piston was studied. A theoretical solution using the method of characteristics for a perfect gas showed that a complex wave system exists, and that the compressed gas can have large gradients in temperature, density and entropy. Experiments were performed with a piston tube where piston speed, shock speed and pressure were measured. The comparison of theory and experiment was good.

Mann, M. J.↗

NASA NTRS1976

Stability of a viscous fluid in a rectangular cavity in the presence of a magnetic field

The stability of an electrically conducting fluid subjected to two dimensional disturbance was investigated. A physical system consisting of two parallel infinite vertical plates which are thermally insulated was studied. An external magnetic field of constant strength was applied to normal plates. The fluid was heated from below so that a steady temperature gradient was maintained in the fluid. The governing equations were derived by perturbation technique, and solutions were obtained by a modified Galerkin method. It was found that the presence of the magnetic field increases the stability of the physical system and instability can occur in the form of neutral or oscillatory instability.

Liang, C. Y.↗

NASA NTRS1977

Investigation of supersonic turbulent boundary-layer separation on a compression ramp by an integral method

An investigation was made to determine the feasibility of using a boundary layer integral method to study the separation of a turbulent boundary layer on a two dimensional ramp at supersonic speeds. The numerical calculations were made for a free stream Mach number of 3, a Reynolds number of 10 million, and over a ramp angle range from 0 deg to 30 deg. For ramp angles where no flow separation was indicated, theoretical calculations were in reasonable agreement with experimental data except for a somewhat belated rise in pressure. For larger ramp angles, where separation was present, the investigation produced results that were not in agreement with experiment or with results calculated by time dependent Navier-Stokes methods. This apparently was true because no provision had been made for a proper shock boundary layer interaction where strong normal pressure gradients are induced within the boundary layer under the shock independent of surface curvature effects.

Patel, D. K.↗

NASA NTRS1977

Theory of viscous transonic flow over airfoils at high Reynolds number

This paper considers viscous flows with unseparated turbulent boundary layers over two-dimensional airfoils at transonic speeds. Conventional theoretical methods are based on boundary layer formulations which do not account for the effect of the curved wake and static pressure variations across the boundary layer in the trailing edge region. In this investigation an extended viscous theory is developed that accounts for both effects. The theory is based on a rational analysis of the strong turbulent interaction at airfoil trailing edges. The method of matched asymptotic expansions is employed to develop formal series solutions of the full Reynolds equations in the limit of Reynolds numbers tending to infinity. Procedures are developed for combining the local trailing edge solution with numerical methods for solving the full potential flow and boundary layer equations. Theoretical results indicate that conventional boundary layer methods account for only about 50% of the viscous effect on lift, the remaining contribution arising from wake curvature and normal pressure gradient effects.

Melnik, R. E.↗

NASA NTRS1978

Simulation of distributed microprocessor-based flight control systems

The aim of the present paper is to demonstrate, within the framework of a digital flight control system, the method of simulating the information exchange between a microcomputer and a supervisory computer, and between microcomputers working on separate control tasks. A gradient technique is described that considers the trade-off between the objectives of the control system and the information exchange requirements.

Lee, P. S.↗

NASA NTRS1981

A viscous-inviscid interaction model of jet entrainment

A viscous-inviscid interaction model for predicting jet entrainment effects on axisymmetric, nozzle afterbodies at subsonic speeds is presented. The model is based on a displacement thickness correction to the inviscid jet boundary that accounts for mixing-induced streamline deflections in the inviscid region. The displacement correction is shown to be related to the local mass entrainment rate and, for thin mixing layers, the model is shown to be analogous to displacement models used in conventional boundary-layer interaction theory. A method is presented for computing the entrainment rate by an overlaid mixing layer model that accounts for the nonsimilar behavior and pressure gradients occurring in the near field region. An iterative scheme for coupling the model to analyses for the external inviscid flow, the external boundary layer, and the inviscid jet exhaust is also given. Results are presented that illustrate the qualitative behavior of the entrainment interaction under various flow conditions and that demonstrate the validity of the model by comparisons with experiment.

Wilmoth, R. G.↗

NASA NTRS1981

Present growth technology of silicon germanium alloys and possible advantages of microgravity growth

The growth technology of Silicon-germanium (Si-Ge) alloys and the possible advantages of growth in microgravity is reviewed. The Si-Ge alloys have a continuous variation of bandgap energy from the germanium bandgap to the silicon bandgap. The unusual two slope behavior of Eg versus composition is due to the differences in the conduction band structure between Si and Ge. Below 17% (atomic), the germanium band structure dominates; and above it, the bands are "silicon like". It is found that the growth of Si-Ge alloys in microgravity is very attractive. In particular, the float zone method, in which a liquid zone of controlled starting composition, used to grow a large amount of useful alloy crystal. Large temperature gradients and relatively flat growth interfaces are necessary to obtain homogeneous crystal growth.

Stafsudd, O. M.↗

NASA NTRS1983

Summary Proceedings of a Wind Shear Workshop

A number of recent program results and current issues were addressed: the data collection phase of the highly successful Joint Airport Weather Study (JAWS) Project and the NASA-B5f7B Gust Gradient Program, the use of these data for flight crew training through educational programs (e.g., films) and with manned flight training simulators, methods for post-accident determination of wind conditions from flight data recorders, the microburst wind shear phenomenon which was positively measured and described the ring vortex as a possible generating mechanism, the optimum flight procedure for use during an unexpected wind shear encounter, evaluation of the low-level wind shear alert system (LLWSAS), and assessment of the demonstrated and viable application of Doppler radar as an operational wind shear warning and detection system.

Enders, J. H.↗

NASA NTRS1985

A minimum time control algorithm for linear and nonlinear systems

The minimum time control problem with bounded control has long been of interest to control engineers. Much of the theoretical study of this problem has been limited to linear systems and the results are usually problem-specific. This paper presents a new computational method for solving the minimum-time control problem when the control action is assumed to be bang-bang. A gradient-based algorithm is developed where the switching times are updated to minimize the final state missed distance. The algorithm is applicable to linear and nonlinear problems, including multi-input systems.

Wen, J.↗

NASA NTRS1987

High resolution upwind schemes for the three-dimensional incompressible Navier-Stokes equations

Based on flux-difference splitting, implicit high resolution schemes are constructed for efficient computations of steady-state solutions to the three-dimensional, incompressible Navier-Stokes equations in curvilinear coordinates. These schemes use first-order accurate Euler backward-time differencing and second-order central differencing for the viscous shear fluxes. Up to third-order accurate upwind differencing is achieved through a reconstruction of the solution from its cell averages. The reconstruction is accomplished by linear interpolation, where the node stencils are selected such that in regions of smooth solution the flow is highly resolved while spurious oscillations in regions of rapid changes in gradient are still suppressed. Fairly rapid convergence to steady-state solutions is attained with a completely vectorizable hybrid time-marching method. Flows around a sharp-edged delta wing are computed with the maximum accuracy of the upwind-differencing restricted to first-, second-, and third-order, to illustrate the effect of accuracy on the global and on the local vortical flow fields. The results are validated with experimental data.

Hartwich, PETER-M.↗

NASA NTRS1989

Analysis of the low gravity tolerance of Bridgman-Stockbarger crystal growth. I - Steady and impulse accelerations

The effects of steady and impulse-type residual accelerations on dopant distributions during directional solidification in 2D and 3D 'generic' models of the Bridgman-Stockbarger technique are investigated using numerical methods. The calculations are based on the thermophysical properties of molten germanium doped with a low concentration of gallium. A Chebyshev collocation pseudospectral method is used for the solution of the governing momentum-, mass-, species-, and heat-transfer equations. Only convection caused by temperature gradients is considered. It is found that lateral nonuniformity in composition is very sensitive to the orientation of the steady component of the residual gravity vector and to the particular operating conditions under consideration. It is also found that laterally or radially averaged composition profiles are alone insufficient to describe the extent of residual convection in a spacecraft environment. The effects of impulse-type disturbances can be severe and can extend for times on the order of 1000 sec after the termination of the impulse.

Alexander, J. Iwan D.↗

NASA NTRS1989

NASA-UVA light aerospace alloy and structures technology program

The report on progress achieved in accomplishing of the NASA-UVA Light Aerospace Alloy and Structures Technology Program is presented. The objective is to conduct interdisciplinary graduate student research on the performance of next generation, light weight aerospace alloys and associated thermal gradient structures in close collaboration with researchers. The efforts will produce basic understanding of material behavior, new monolithic and composite alloys, processing methods, solid and fluid mechanics analyses, measurement advances, and a pool of educated graduate students. The presented accomplishments include: research on corrosion fatigue of Al-Li-Cu alloy 2090; research on the strengthening effect of small In additions to Al-Li-Cu alloys; research on localized corrosion of Al-Li alloys; research on stress corrosion cracking of Al-Li-Cu alloys; research on fiber-matrix reaction studies (Ti-1100 and Ti-15-3 matrices containing SCS-6, SCS-9, and SCS-10 fibers); and research on methods for quantifying non-random particle distribution in materials that has led to generation of a set of computer programs that can detect and characterize clusters in particles.

Gangloff, Richard P.↗