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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 361 records · Page 20
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NASA NTRS1987

Microgravity nucleation and particle coagulation experiments support

A preliminary model for diffusion between concentric hemispheres was adapted to the cylindrical geometry of a microgravity nucleation apparatus, and extended to include the effects of radiation and conduction through the containment walls. Computer programs were developed to calculate first the temperature distribution and then the evolving concentration field using a finite difference formulation of the transient diffusion and radiation processes. The following estimations are made: (1) it takes approximately 35 minutes to establish a steady temperature field; (2) magnesium vapors released into the argon environment at the steady temperature distribution will reach a maximum supersaturation ratio of approximately 10,000 in the 20-second period at a distance of 15 cm from the source of vapors; and (3) approximately 750W electrical power will be required to maintain steady operating temperatures within the chamber.

Lilleleht, L. U.↗
NASA NTRS1987

Effect of atmospheric turbulence on the bit error probability of a space to ground near infrared laser communications link using binary pulse position modulation and an avalanche photodiode detector

The effect of atmospheric turbulence on the bit error rate of a space-to-ground near infrared laser communications link is investigated, for a link using binary pulse position modulation and an avalanche photodiode detector. Formulas are presented for the mean and variance of the bit error rate as a function of signal strength. Because these formulas require numerical integration, they are of limited practical use. Approximate formulas are derived which are easy to compute and sufficiently accurate for system feasibility studies, as shown by numerical comparison with the exact formulas. A very simple formula is derived for the bit error rate as a function of signal strength, which requires only the evaluation of an error function. It is shown by numerical calculations that, for realistic values of the system parameters, the increase in the bit error rate due to turbulence does not exceed about thirty percent for signal strengths of four hundred photons per bit or less. The increase in signal strength required to maintain an error rate of one in 10 million is about one or two tenths of a db.

Safren, H. G.↗
NASA NTRS1988

Intraseasonal variability in atmospheric surface pressure and relationship to polar motion

Two techniques were used to validate the theoretical relationship between polar motion and P sub s variations. The first method, reproduced from Hide et al. (1984), compares the fluctuations in physical space of the polar spirals and the high frequencies about them. Although the agreement appears reasonable, the lack of a clear measure of closeness and the presence of the large Chandler signal itself makes a comparison at higher frequencies difficult. To overcome this difficulty, the authors present the X functions required to maintain the observed fluctuations at roughly 20 to 70 days and those calculated from the National Meteorological Center (NMC) P sub s data. These curves are in relatively good agreement, indicating that polar motions are indeed driven, at least in large part, by fluctuations in P sub s.

Salstein, David A.↗
NASA NTRS1987

Electrodeposition and codeposition under low gravity/nonconvecting conditions

An experimental electrodeposition system was developed for modeling the behavior of inert particles codepositing in an electroplating matrix under low-gravity conditions. The device consists of a Co-electrodeposition cell operating in a convectionless mode (cathode over anode) and containing polystyrene particles with density approximating that of the electroplating solution. Data were obtained in shielded cells at 1 g, and the experiment was duplicated at 0.01 g on a KC-135 flight. No difference was found between convection-free bench experiments and the 0.01 g KC-135 experiments with 0.0900 M CuSO4. Codeposition experiments using 11.8-micron matched-density polystyrene spheres in 1 M CoSO4 have revealed that a noticeable particle gradient is created as the solution density mismatches in the vicinity of the cathode; gentle stirring was required to maintain a homogeneous particle suspension. Cr3C2 dust, which readily disperses at 1 g, tended to coagulate into spherical globules at 0.01 g, when stirred.

Riley, Clyde↗
NASA NTRS1988

Heating requirements and nonadiabatic surface effects for a model in the NTF cryogenic wind tunnel

A theoretical study has been made of the severity of nonadiabatic surface conditions arising from internal heat sources within a model in a cryogenic wind tunnel. Local surface heating is recognized as having an effect on the development of the boundary layer, which can introduce changes in the flow about the model and affect the wind tunnel data. The geometry was based on the NTF Pathfinder I wind tunnel model. A finite element heat transfer computer code was developed and used to compute the steady state temperature distribution within the body of the model, from which the surface temperature distribution was extracted. Particular three dimensional characteristics of the model were represented with various axisymmetric approximations of the geometry. This analysis identified regions on the surface of the model susceptible to surface heating and the magnitude of the respective surface temperatures. It was found that severe surface heating may occur in particular instances, but could be alleviated with adequate insulating material. The heat flux through the surface of the model was integrated to determine the net heat required to maintain the instrumentation cavity at the prescribed temperature. The influence of the nonadiabatic condition on boundary layer properties and on the validity of the wind tunnel simulation was also investigated.

Macha, J. M.↗
NASA NTRS1989

A seal test facility for the measurement of isotropic and anisotropic linear rotordynamic characteristics

A new seal test facility for measuring high-pressure seal rotor-dynamic characteristics has recently been made operational at Case Western Reserve University (CWRU). This work is being sponsored by the Electric Power Research Institute (EPRI). The fundamental concept embodied in this test apparatus is a double-spool-shaft spindle which permits independent control over the spin speed and the frequency of an adjustable circular vibration orbit for both forward and backward whirl. Also, the static eccentricity between the rotating and non-rotating test seal parts is easily adjustable to desired values. By accurately measuring both dynamic radial displacement and dynamic radial force signals, over a wide range of circular orbit frequency, one is able to solve for the full linear-anisotropic model's 12 coefficients rather than the 6 coefficients of the more restrictive isotropic linear model. Of course, one may also impose the isotropic assumption in reducing test data, thereby providing a valid qualification of which seal configurations are well represented by the isotropic model and which are not. In fact, as argued in reference (1), the requirement for maintaining a symmetric total system mass matrix means that the resulting isotropic model needs 5 coefficients and the anisotropic model needs 11 coefficients.

Adams, M. L.↗
NASA NTRS1989

Digital robust control law synthesis using constrained optimization

Development of digital robust control laws for active control of high performance flexible aircraft and large space structures is a research area of significant practical importance. The flexible system is typically modeled by a large order state space system of equations in order to accurately represent the dynamics. The active control law must satisy multiple conflicting design requirements and maintain certain stability margins, yet should be simple enough to be implementable on an onboard digital computer. Described here is an application of a generic digital control law synthesis procedure for such a system, using optimal control theory and constrained optimization technique. A linear quadratic Gaussian type cost function is minimized by updating the free parameters of the digital control law, while trying to satisfy a set of constraints on the design loads, responses and stability margins. Analytical expressions for the gradients of the cost function and the constraints with respect to the control law design variables are used to facilitate rapid numerical convergence. These gradients can be used for sensitivity study and may be integrated into a simultaneous structure and control optimization scheme.

Mukhopadhyay, Vivekananda↗
NASA NTRS1988

Semi-Lagrangian integration of a grid-point shallow water model on the sphere

This paper describes a semi-Lagrangian technique for integrating the equations of motion on the global domain. The technique uses an auxiliary spherical coordinate system at each near-polar gridpoint of the latitude-longitude grid; the auxiliary system is obtained by a rotation such that the new equator passes through the gridpoint in question and the new coordinate directions coincide with those of the original system at that point. The technique was applied to the shallow water equations, incorporating a semiimplicit treatment of the adjustment terms on a C-grid, with two-time levels. A five day integration was successfully carried out for a situation involving strong cross-polar flow. No filtering or diffusion was required to maintain stability over a five day period.

Mcdonald, A.↗
NASA NTRS1989

Crustal diffusion of gases out of Mercury and the moon

An upper limit to the rate of diffusion of sodium and potassium out of Mercury and the moon was calculated, and the resulting flux was compared to that required to maintain the known exospheres. It was found that diffusion rates are inadequate by 12 orders of magnitude on the moon and 9 orders of magnitude on Mercury for diffusion out of orthoclase minerals. Diffusion will be more rapid out of pure glass by five to six orders of magnitude and out of shocked basalt by an amount depending on the microstructure of the mineral. The observed abundance and distribution of volatiles in small glass spherules on the moon indicates that diffusion is very inefficient after solidification and cooling. At Mercury, the limitation on sodium flux to the atmosphere is shown to be the rate at which new regolith is created. The discrepancy between the observed column abundance of sodium in the Mercurean atmosphere and the known sources may indicate that either Mercury's crust has a larger volatile content than the moon or that a recycling mechanism exists in the Mercurean atmosphere which is not present for the moon.

Killen, R. M.↗
NASA NTRS1989

Computationally generated velocity taper for efficiency enhancement in a coupled-cavity traveling-wave tube

A computational routine has been created to generate velocity tapers for efficiency enhancement in coupled-cavity TWTs. Programmed into the NASA multidimensional large-signal coupled-cavity TWT computer code, the routine generates the gradually decreasing cavity periods required to maintain a prescribed relationship between the circuit phase velocity and the electron-bunch velocity. Computational results for several computer-generated tapers are compared to those for an existing coupled-cavity TWT with a three-step taper. Guidelines are developed for prescribing the bunch-phase profile to produce a taper for efficiency. The resulting taper provides a calculated RF efficiency 45 percent higher than the step taper at center frequency and at least 37 percent higher over the bandwidth.

Wilson, Jeffrey D.↗
NASA NTRS1990

Erosional development of bedrock spur and gully topography in the Valles Marineris, Mars

Gully networks separated by resistant bedrock spurs are a common erosional feature along the escarpments that border the Valles Marineris. The resistant spur topography is best developed where the base of the slope is truncated by linear scarps interpreted as fault scarps. Regional variations in slope morphology imply that spur and gully topography undergoes a systematic progressive degradation through time associated with the erosional destruction of the basal fault scarps. The comparative morphometry of the divide networks indicates that the density of the spur networks and the number of first-order unbranched spurs decreases as the basal slope break becomes more sinuous. Abstraction of the spurs occurs through regolith storage in adjacent gullies at the slope base and the most degraded slope forms are entirely buried in talus. The basal fault scarps apparently control regolith transport by allowing debris to drain from the slope. As these basal scarps decay the slope base becomes increasingly sinuous and the slopes become transport limited. Dry mass-wasting may be the most important process acting on these slopes where a continually lowered base level is required to maintain the spur topography. In contrast to the Martian slopes, range front fault escarpments in the western U.S. show no systematic trend in spur network geometry as they are eroded. These weathering limited slopes are controlled by the more efficient removal of regolith through fluvial processes which rapidly create quasi-equilibrium drainage networks.

Patton, Peter C.↗
NASA NTRS1989

Experiments in identification and control of flexible-link manipulators

Interest in the study of flexible-link manipulators for space-based applications has risen strongly in recent years. Moreover, numerous experimental results have appeared for the various problems in the modeling, identification and control of such systems. Nevertheless, relatively little literature has appeared involving laboratory verification of tuning controllers for certain types of realistic flexible-link manipulators. Specifically flexible-link manipulators which are required to maintain endpoint accuracy while manipulating loads that are possibly unknown and varying as they undergo disturbance effects from the environment and workspace. Endpoint position control of flexible-link manipulators in these areas are discussed, with laboratory setups consisting of one and two-link manipulators.

Yurkovich, Stephen↗
NASA NTRS1989

Optical design for SOFIA

A preliminary first-order optical design for the Stratospheric Observatory for Infrared Astronomy (SOFIA) is presented. This is a Cassegrain design with a 3 meter diameter, approximately f/1 primary mirror. Phenomena limiting the image quality of the telescope are divided into 'seeing', optics, and guidance. An error budget is presented for these categories and specific effects contributing to each. The seeing effects from the shear layer between the telescope cavity and the external air are expected to be dominant. Results are presented on the necessary thermal, optical, structural and guidance requirements to maintain contributions of these phenomena below that of the shear-layer seeing.

Davis, Paul K.↗
NASA NTRS1990

Upward ion flow in ionospheric holes on Venus

The nature of ion flow within two ionospheric holes on the nightside of Venus was investigated using ion composition measurements made by the ion mass spectrometer on the Pioneer Venus Orbiter. A comparison of the altitude profiles of the observed ion densities with those expected under diffusive equilibrium conditions indicates that the major ions O(+), NO(+), and O2(+), as well as the minor ions H(+) and He(+) flow upward and away from Venus along the axes of the holes. This result agrees with a quantitative evaluation of the ion flow speeds appearing in expressions derived from the equations for conservation of mass and momentum of the ions and electrons. The analysis shows that all ion species flow upward in the holes because the upward force produced by the plasma pressure gradient exceeds all downward forces. However, the nature of the ion source required to maintain such flow is not known.

Hartle, R. E.↗
NASA NTRS1989

Measurement of the effects of particulate contamination on X-ray reflectivity

Because particles of sizes larger than a few tenths microns adversely affect high resolution X-ray telescopes by scattering and absorbing X-rays, the cleanliness required to maintain the about 1 percent overall calibration precision desired for the Advanced X-ray Astrophysics Facility (AXAF) is being investigated. At the grazing angles used for the AXAF mirrors, each particle shadows a surface area about 100 times its geometric area, necessitating glass occlusion specifications much more stringent than typically stipulated for visible-light particulate contamination. On test flats coated with gold, controlled levels of contamination have been deposited spanning the range from 5 x 10 to the -5th to 0.005 fractional area covered, and the absorption component of extinction has been measured over a range of grazing angles and X-ray energies to verify the predicted effects of particulate contamination.

Slane, P.↗
NASA NTRS1990

Effects of stick dynamics on helicopter flying qualities

An experiment that investigated the influence of typical helicopter force-feel system dynamics on roll-axis handling qualities was conducted in concurrent ground and inflight simulations. Variations in lateral control natural frequency and damping ratio, effected by changes in inertia and damping, were evaluated in a disturbance-rejection task. Pilot ratings indicated a preference for low-inertia feel systems, although measured performance was relatively constant over the range of stick characteristics. Force-sensing was compared with position sensing as the input to the control system. Force-sensing improved performance but did not improve pilot ratings. Overall, the results indicated that control-stick dynamics, at least within a reasonable range, did not have a significant effect on pilot-vehicle performance. However, the physical effort required to maintain a desired pilot/manipulator bandwidth became objectionable as the stick inertia increased beyond 5-7 lbm, which was reflected in the pilot ratings and comments.

Watson, Douglas C.↗
NASA NTRS1991

Radiation protection for human missions to the Moon and Mars

Radiation protection assessments are performed for advanced Lunar and Mars manned missions. The Langley cosmic ray transport code and the nucleon transport code are used to quantify the transport and attenuation of galactic cosmic rays and solar proton flares through various shielding media. Galactic cosmic radiation at solar maximum and minimum, as well as various flare scenarios are considered. Propagation data for water, aluminum, liquid hydrogen, lithium hydride, lead, and lunar and Martian regolith (soil) are included. Shield thickness and shield mass estimates required to maintain incurred doses below 30 day and annual limits (as set for Space Station Freedom and used as a guide for space exploration) are determined for simple geometry transfer vehicles. On the surface of Mars, dose estimates are presented for crews with their only protection being the carbon dioxide atmosphere and for crews protected by shielding provided by Martian regolith for a candidate habitat.

Simonsen, Lisa C.↗
NASA NTRS1991

Expert system decision support for low-cost launch vehicle operations

Progress in assessing the feasibility, benefits, and risks associated with AI expert systems applied to low cost expendable launch vehicle systems is described. Part one identified potential application areas in vehicle operations and on-board functions, assessed measures of cost benefit, and identified key technologies to aid in the implementation of decision support systems in this environment. Part two of the program began the development of prototypes to demonstrate real-time vehicle checkout with controller and diagnostic/analysis intelligent systems and to gather true measures of cost savings vs. conventional software, verification and validation requirements, and maintainability improvement. The main objective of the expert advanced development projects was to provide a robust intelligent system for control/analysis that must be performed within a specified real-time window in order to meet the demands of the given application. The efforts to develop the two prototypes are described. Prime emphasis was on a controller expert system to show real-time performance in a cryogenic propellant loading application and safety validation implementation of this system experimentally, using commercial-off-the-shelf software tools and object oriented programming techniques. This smart ground support equipment prototype is based in C with imbedded expert system rules written in the CLIPS protocol. The relational database, ORACLE, provides non-real-time data support. The second demonstration develops the vehicle/ground intelligent automation concept, from phase one, to show cooperation between multiple expert systems. This automated test conductor (ATC) prototype utilizes a knowledge-bus approach for intelligent information processing by use of virtual sensors and blackboards to solve complex problems. It incorporates distributed processing of real-time data and object-oriented techniques for command, configuration control, and auto-code generation.

Szatkowski, G. P.↗