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At least 595 records · Page 33

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NASA NTRS1972

Damage of thermal control coating properties by energetic mercury ion bombardment.

The thermophysical properties (absorptance and emittance) of five commonly used spacecraft thermal control coatings have been measured in situ after exposure to 1.5 and 3 KeV mercury ions. Sample temperature was varied from 20 to 150 C. The results showed negligible change in emittance but significant degradation of the solar absorptance at fluences of 10 to the 17th power ions/sq cm. Plots of absorptance as a function of ion fluence are presented, along with pre- and post-exposure ex situ spectral absorptance measurements. The widely used Z93 white paint exhibited quantitative degradation from mercury ions similar to that reported by other experimenters for hydrogen and helium ions.

Kelley, L. R.↗

NASA NTRS1972

Polarization mismatch errors in radio phase interferometers.

An analysis is presented which deals with the effects of polarization mismatch errors on the accuracy of a phase interferometer used for position location of unknown emitters relative to known calibration emitters. Closed-form expressions for the induced phase difference between interferometer antennas are derived for several combinations of receiving and transmitting antenna polarizations. Errors contributed by mechanical misalignment between antennas, as well as effects of power loss attributable to polarization mismatch, are also considered. The analysis leads to the conclusion that circularly polarized interferometer and transmitter antennas are best suited for the position location application, if it is assumed that polarization tracking of the interferometer antennas is not available. It is shown that a reasonable amount of ellipticity can be tolerated before the phase error becomes significant.

Muehldorf, E. I.↗

NASA NTRS1971

A comparison of closed loop thermionic reactor control schemes.

The dynamics of the constant-voltage and constant-emitter-temperature control schemes are described. The closed loop responses to load changes, converter failures and reactivity perturbations are shown. Both control schemes inherently allow load voltages to vary during transients. An alternative control scheme is proposed which allows constant load voltage while preserving constant emitter temperatures.

Sawyer, C. D.↗

NASA NTRS1971

Design of an external-fueled thermionic diode for in-pile testing.

Description of an external-fueled thermionic diode suitable for in-pile testing in a research reactor. The active electrode area is 94 sq cm. The 10-in. long, 1.5-in.-OD emitter body is tungsten 2% thoria. The fuel is contained in six 0.4-in.-diam holes equally spaced about the 0.5-in. central emitter hole. The collector is niobium-1% zirconium. The expected diode performance is 6 W/sq cm at 2000 K. In addition to following the constraints imposed by the in-pile testing and the electrically heated performance mapping prior to insertion in-pile, the diode will have end configurations prototypical of those anticipated for a flow-through, NaK-cooled, external-fuel thermionic reactor.

Ernst, D. M.↗

NASA NTRS1972

Cesium oxide-cesium solution as a source of cesium and oxygen.

Examination of the feasibility of using the solution as a source of cesium and oxygen, with description of an experimental system designed for a systematic investigation. Preliminary data are presented. Thermionic performance of the converter was recorded before the injection of oxygen at emitter temperatures of 1600, 1700, 1800, and 1900 K. The power of this converter, which had a polycrystalline tungsten emitter and a polycrystalline molybdenum collector, is compared with other converters. Experimental results show that cesium oxide mole factors higher than 0.18 are needed to achieve oxygen effects. It appears not to be necessary to be concerned about avoiding oxygen impurities in the cesium reservoir, since mole fractions as high as 0.18 have failed to influence the performance.

Gunther, B.↗

NASA NTRS1973

Correlation of experimental performance data for a CVD tungsten-niobium, planar thermionic converter

Approximate expressions are presented which correlate experimental performance data from a CVD tungsten-niobium, planar thermionic converter. The current voltage characteristics are given as functions of emitter and collector temperatures and cesium pressure for currents below the knee in the ignited mode. The correlation covers the temperature ranges of 1700 to 1950 K for the emitter, 900 to 1050 K for the collector, and 580 to 645 K for the cesium reservoir.

Sockol, P. M.↗

NASA NTRS1972

Performance tests of a thermionic converter with an oxygenated cesium reservoir

Cesium oxide/cesium solutions as a possible source of oxygen and cesium are investigated. A converter-reservoir system which allows multiple additions of oxygen with intervening thermionic performance tests was developed. The experimental data cover the cesium oxide mole fraction range of 0 to 0.018 with thermionic performance measurements in the emitter temperature range of 1600 to 1900 K. The data showed that the thermionic performance was unaffected by the presence of cesium oxide in this mole fraction range and higher mole fractions appear to be required for the proper oxygen arrival rate at the emitter surface.

Gunther, B.↗

NASA NTRS1972

Electrodes for the thermionic converters, phase 2 - 3

The characteristics of electrodes for thermionic converters are discussed. A metal/ceramic Marchuk tube was constructed and operated with a quiescent cesium vapor plasma corresponding to cesium liquid reservoir temperatures from 376 to 427 K. The work function characteristics of tantalum test emitters coated with anodic films were measured to evaluate the applicability of the materials as the collector surface in a plasma mode thermionic converter. Work function minimum values between 1.4 and 1.5 electron volts were observed. The effect of the electronegative surface layers was partially obscured by additive vapor impurities detected by the clean tantalum reference emitter.

Desteese, J. G.↗

NASA NTRS1973

Effects of sea salts on the physical characteristics of reusable surface insulation

Test results show that sea salt contamination and thermal cycles cause significant changes in the morphology of surface insulation coatings with increased melting. Sea salt contamination and thermal cycles do not have a significant effect on emittance of mullite and silica composite coatings at high temperatures (1300 K). At the lower temperatures there is apparently some effect on emittance but the evidence is inconclusive because of the absence of multiple tests.

Ransone, P. O.↗

NASA NTRS1973

Development of a ruggedized 20-watt, 2.3- gigahertz transistor

This program was to develop a ruggedized transistor capable of meeting the following CW objectives at 2.3 gigahertz: a power output of 20 watts, a power gain of 6 db, and an efficiency of 40 percent. Devices developed produced the following CW performance at 2.3 gigahertz: a power output of 23 watts, a power gain of 7.7 db, and an efficiently of 40.0 percent. This performance was achieved with an eight-cell TA8407 transistor design having the following modifications: a thin pellet, high-frequency diffusion, high-level emitter ballasting, an optimized emitter-bonding configuration for uniform power sharing, and a package that provides for some internal matching through the use of distributed line techniques.

Source record↗

NASA NTRS1974

Evaluation of commercially supplied silver coated Teflon for spacecraft temperature control usage

A series of tests are described which were performed to evaluate the acceptability of a commercially supplied silver backed teflon thermal control coating relative to teflon previously coated at GSFC. Optical measurements made on numerous samples indicate that the commercial material possesses an average solar absorptance of 0.085, an emittance of 0.76 and an average alpha/epsilon equal to 0.112, all of which are equivalent to the GSFC coated teflon. The emittance of the protective inconel backing was found to be 0.037. The coating is shown to have good adhesion at the Ag-teflon interface and exposure to UV irradiation uncovered no coating irregularities. Temperature cycling over the range -135 C to +200 C produced crazing in the evaporated Ag layer as expected but no delamination was observed. The suitability of Mystik no. 7366 and 3M no. 467 adhesives as bonding agents for the metallized polymer is demonstrated. Various problems associated with production reproducibility and selection of a proper bonding process are discussed.

Heaney, J. B.↗

NASA NTRS1973

Optical modeling of agricultural fields and rough-textured rock and mineral surfaces

Review was made of past models for describing the reflectance and/or emittance properties of agricultural/forestry and geological targets in an effort to select the best theoretical models. An extension of the six parameter Allen-Gayle-Richardson model was chosen as the agricultural plant canopy model. The model is used to predict the bidirectional reflectance of a field crop from known laboratory spectra of crop components and approximate plant geometry. The selected geological model is based on Mie theory and radiative transfer equations, and will assess the effect of textural variations of the spectral emittance of natural rock surfaces.

Suits, G. H.↗

NASA NTRS1974

LC-oscillator with automatic stabilized amplitude via bias current control

A stable excitation supply for measurement transducers is described. It consists of a single-transistor oscillator with a coil connected to the collector and a capacitor connected from the collector to the emitter. The output of the oscillator is rectified and the rectified signal acts as one input to a differential amplifier; the other input being a reference potential. The output of the amplifier is connected at a point between the emitter of the transistor and ground. When the rectified signal is greater than the reference signal, the differential amplifier produces a signal of polarity to reduce bias current and, consequently, amplification.

Hamlet, J. F.↗

NASA NTRS1974

Thermionic performance of a cesium diminiode with relatively impure 110-tungsten electrodes

Thermionic performance data from a miniature plane cesium diode (diminiode) with 110-tungsten electrodes are presented. The diminiode has a guard-ringed collector and a spacing of 0.23 mm. The data were obtained by using a computerized acquisition system. The diode was tested at increments between 1700 and 1900 K for the emitter, 694 and 1101 K for the collector, and 519 and 650 K for the reservoir. A maximum power density of 4.5 W/sq cm was obtained at an emitter temperature of 1900 K. This relatively low output probably results from high carbon and sodium impurities in the electrode materials.

Smith, A. L.↗

NASA NTRS1974

Thermophysical properties of lunar materials. I - Thermal radiation properties of lunar materials from the Apollo missions

The successful landings on the moon of the Apollo flights and the return of samples of lunar surface material has permitted the measurement of the thermophysical properties necessary for heat transfer calculations. The characteristics of the Apollo samples are discussed along with remote sensing results which made it possible to deduce many of the thermophysical properties of the lunar surface. Definitions considered in connection with thermal radiation measurements include the bond albedo, the geometric albedo, the normal albedo, the directional reflectance, the bidirectional reflectance, and the directional emittance. The measurement techniques make use of a directional reflectance apparatus, a bidirectional reflectance apparatus, and a spectral emittance apparatus.

Birkebak, R. C.↗

NASA NTRS1974

New theoretical models and ratio imaging techniques associated with the NASA earth resources spectral information system

Four independent investigations are reported; in general these are concerned with improving and utilizing the correlation between the physical properties of natural materials as evidenced in laboratory spectra and spectral data collected by multispectral scanners. In one investigation, two theoretical models were devised that permit the calculation of spectral emittance spectra for rock and mineral surfaces of various particle sizes. The simpler of the two models can be used to qualitatively predict the effect of texture on the spectral emittance of rocks and minerals; it is also potentially useful as an aid in predicting the identification of natural atmospheric aerosol constituents. The second investigation determined, via an infrared ratio imaging technique, the best pair of infrared filters for silicate rock-type discrimination. In a third investigation, laboratory spectra of natural materials were compressed into 11-digit ratio codes for use in feature selection, in searches for false alarm candidates, and eventually for use as training sets in completely automatic data processors. In the fourth investigation, general outlines of a ratio preprocessor and an automatic recognition map processor are developed for on-board data processing in the space shuttle era.

Vincent, R. K.↗

NASA NTRS1975

Electrodes for thermionic energy conversion

Problems concerning an application of thermionic energy conversion methods are related to the high heat source temperatures currently required for practical power densities and efficiencies. A description is given of advances made in the development of improved emitter and collector surfaces as a basis for the reduction of operating temperatures. The controlled addition of oxygen has resulted in a considerable improvement of emitter performance. Improvements in converter performance have been obtained by a reduction of the collector work function. Attention is given to fundamental studies, simulated converter environment tests, and variable spacing converter experiments.

Rufeh, F.↗

NASA NTRS1976

Design and evaluation of thin metal surface insulation for hypersonic flight

An all-metal insulation was studied as a thermal protection system for hypersonic vehicles. Key program goals included fabricating the insulation in thin packages which are optimized for high temperature insulation of an actively cooled aluminum structure, and the use of state-of-the-art alloys. The insulation was fabricated from 300 series stainless steel in thicknesses of 0.8 to 12 mm. The outer, 0.127 mm thick, skin was textured to accommodate thermal expansion and oxidized to increase emittance. The thin insulating package was achieved using an insulation concept consisting of foil radiation shields spaced within the package, and conical foil supports to carry loads from the skin and maintain package dimensions. Samples of the metal-insulation were tested to evaluate thermal insulation capability, rain and sand erosion resistance, high temperature oxidation resistance, applied load capability, and high temperature emittance.

Miller, R. C.↗