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At least 505 records · Page 28

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NASA NTRS1981

Electrical behavior of a Shuttle Electrodynamic Tether System /SETS/

Many novel electrodynamic and space plasma physics experiments can be done from the space shuttle using a gravity gradient stabilized subsatellite tethered 10-30 km above the Orbiter via a long, conducting wire which is insulated from the ionospheric plasma. This system, called the Shuttle Electrodynamic Tether System (SETS), is described in the present paper with emphasis upon the various electrical processes which determine its coupling to the ambient plasma. The three most important physical effects include sheath formation and electron collection by the subsatellite, the generation of a large emf through the orbital motion of the tether across geomagnetic field lines, and the active ejection of electrons from the Orbiter into the surrounding ionosphere. An electrical circuit analogy is presented for SETS together with a brief outline of possible areas related to the artificial generation of magnetohydrodynamic waves.

Banks, P. M.↗

NASA NTRS1981

Microwave solar limb brightening

Previous models of microwave limb brightening have omitted the alignment of spicules along supergranule boundaries, have neglected the high temperature sheath around spicules, and have assumed an interspicular medium which was averaged over chromospheric network and nonnetwork regions. A model is presented here which includes these factors. Constraining the model to conform to results from earlier UV and optical studies in effect leaves two free parameters: the temperature at the core of the spicules and (at solar minimum) the interspicular chromospheric network density model of the lower transition zone. The absence of limb brightening at the short millimeter wavelengths implies a temperature at the core of the spicules less than approximately 6000 K. Differences between the model and certain deconvolved observations near 9 mm are expected as a consequence of an extension of emission beyond the optical limb, predicted by the model, which affects the accuracy of the deconvolution technique. Unlike models which assume homogeneous spicules in a random distribution, the model presented here does not require an abnormally high spicule area.

Ahmad, I. A.↗

NASA NTRS1981

Plasma waves near Saturn - Initial results from Voyager 1

The Voyager 1 plasma wave instrument detected many familiar types of plasma waves during the encounter with Saturn, including ion-acoustic waves and electron plasma oscillations upstream of the bow shock, an intense burst of electrostatic noise at the shock, and chorus, hiss, electrostatic electron cyclotron waves, and upper hybrid resonance emissions in the inner magnetosphere. A clocklike Saturn rotational control of low-frequency radio emissions was observed, and evidence was obtained of possible control by the moon Dione. Strong plasma wave emissions were detected at the Titan encounter indicating the presence of a turbulent sheath extending around Titan, and upper hybrid resonance measurements of the electron density show the existence of a dense plume of plasma being carried downstream of Titan by the interaction with the rapidly rotating magnetosphere of Saturn.

Gurnett, D. A.↗

NASA NTRS1981

Inert-gas thruster technology

Attention is given to recent advances in component technology for inert-gas thrusters. It is noted that the maximum electron emission of a hollow cathode with Ar can be increased 60-70% by using an enclosed keeper configuration. Operation with Ar but without emissive oxide has also been attained. A 30-cm thruster operated with Ar at moderate discharge voltages is found to give double-ion measurements consistent with a double-ion correlation developed earlier on the basis of 15-cm thruster data. An attempt is made to reduce discharge losses by biasing anodes positive of the discharge plasma. The performance of a single-grid ion-optics configuration is assessed. The ion impingement on the single-grid accelerator is found to approach the value expected from the projected blockage when the sheath thickness next to the accelerator is 2-3 times the aperture diameter.

Kaufman, H. R.↗

NASA NTRS1981

Parasitic current losses due to solar electric propulsion generated plasmas

Solar electric propulsion is a leading candidate for many upcoming space missions. Under many circumstances plasma produced by charge-exchange reactions within the ion beam dominates the ambient environment near the spacecraft. The calculations presented here contain a predictive hydrodynamic model for the charge-exchange plasma expansion, and a fully three-dimensional model for the structure of the plasma sheath around the solar array wing. Results of calculations for several configurations and voltage levels indicate that with kilovolt biases power losses of approximately 10 percent or more are likely, even with only one engine in operation, and that ameliorative measures should focus on the inboard portion of the solar arrays.

Katz, I.↗

NASA NTRS1981

Oxidation-induced contraction and strengthening of boron fibers

An investigation of the physical and mechanical effects of thermal treatment in a controlled oxygen-argon atmosphere on boron fibers is reported, with attention to the optimization of such treatment as a secondary processing method for improvement of fiber strength. The strengthening mechanism is comprised of an oxidation-induced axial contraction of the fiber, accompanied by axial compression of strength-limiting flaws within the fiber's tungsten boride core. It was found that after an oxidation contraction of 0.3% near 900 C, and a slight surface etch near 100 C, the average tensile strength of 203-micron fibers increased from 500 to 800 ksi. Various physical observations are used to develop mechanistic models of oxidation, contraction, and the formation of new flaws in the boron sheath at contractions greater than 0.3%.

Dicarlo, J. A.↗

NASA NTRS1980

Flight evidence of spacecraft surface contamination rate enhancement by spacecraft charging obtained with a quartz crystal microbalance

The significance of the fraction of the mass outgassed by a negatively charged space vehicle which is ionized within the vehicle plasma sheath and electrostatically reattracted to the space vehicle was determined. The ML-12 retarding potential analyzer/temperature controlled quartz crystal microbalances (RPA/TQCMs) distinguishes between charged and neutral molecules and investigates contamination mass transport mechanism. Two long term, quick look flight data sets indicate that on the average a significant fraction of mass arriving at one RPA/TQCM is ionized. It is assumed that vehicle frame charging during these periods was approximately uniformly distributed in degree and frequency. It is shown that electrostatic reattraction of ionized molecules is an important contamination mechanism at and near geosynchronous altitudes.

Clark, D. M.↗

NASA NTRS1982

Micromechanical predictions of crack propagation and fracture energy in a single fiber boron/aluminum model composite

The axisymmetric finite element model and associated computer program developed for the analysis of crack propagation in a composite consisting of a single broken fiber in an annular sheath of matrix material was extended to include a constant displacement boundary condition during an increment of crack propagation. The constant displacement condition permits the growth of a stable crack, as opposed to the catastropic failure in an earlier version. The finite element model was refined to respond more accurately to the high stresses and steep stress gradients near the broken fiber end. The accuracy and effectiveness of the conventional constant strain axisymmetric element for crack problems was established by solving the classical problem of a penny-shaped crack in a thick cylindrical rod under axial tension. The stress intensity factors predicted by the present finite element model are compared with existing continuum results.

Adams, D. F.↗

NASA NTRS1982

Investigation of soot and carbon formation in small gas turbine combustors

An investigation of hardware configurations which attempt to minimize carbon and soot-production without sacrificing performance in small gas turbine combustors was conducted. Four fuel injectors, employing either airblast atomization, pressure atomization, or fuel vaporization techniques were combined with nozzle air swirlers and injector sheaths. Eight configurations were screened at sea-level takeoff and idle test conditions. Selected configurations were focused upon in an attempt to quantify the influence of combustor pressure, inlet temperature, primary zone operation, and combustor loading on soot and carbon formation. Cycle tests were also performed. It was found that smoke emission levels depended on the combustor fluid mechanics, the atomization quality of the injector and the fuel hydrogen content.

Rosfjord, T. J.↗

NASA NTRS1981

Analysis of I-V characteristics of negatively biased panels in a magneto-plasma

Current voltage characteristics of negatively biased panels placed in a plasma environment were analyzed for panel size, electron density, and superimposed magnetic field. It is indicated that the thickness of the surrounding sheath and its rate of growth with bias potential depends on all these parameters in a nonlinear manner.

Miller, D. D.↗

NASA NTRS1982

One-step dual purpose joining technique

This fastener used in induction heating is a wire screen basically of an eddy current carrying material such as carbon steel. Selected wires in the screen are copper, sheathed in an insulating material. The screen is placed between two sheets of thermoplastics. When inductively heated, the composite softens and flows around the apertures of the screen. After this heating and joining, the copper wires may be used to conduct electricity.

Buckley, J. D.↗

NASA NTRS1981

A survey of the plasma electron environment of Jupiter - A view from Voyager

A survey of the plasma environment within Jupiter's bow shock is given in terms of the in situ calibrated electron plasma measurements made between 10 eV and 5.95 keV by the Voyager Plasma Science Experiment (PLS). The measurements are analyzed and corrected for spacecraft potential variations; the data are reduced to nearly model independent macroscopic parameters of the local electron density and temperature. The electron parameters are derived without reference to or internal calibration from the positive ion measurements made in the PLS experiment. Extensive statistical and direct comparisons with other determinations of the local plasma charge density indicate clearly that the analysis procedures have successfully and routinely discriminated between spacecraft sheath and ambient plasmas.

Scudder, J. D.↗

NASA NTRS1981

Electric propulsion and its applications to space missions

Consideration is given the NASA, Air Force and European electric propulsion programs, the characteristics of primary electric propulsion systems, nuclear electric orbit transfer vehicles, and such topics in the fundamental processes of electrostatic thrusters as sputtering in mercury ion thrusters, the screen hole plasma sheath of an ion accelerator system, and the modelling of ion beam neutralization and nitrogen chemisorption. Also considered are electrostatic thruster components and systems, electromagnetic thrusters such as MPD and RIT systems, electric rail guns and mass drivers, power sources which include solar and nuclear alternatives, power conversion systems and their cooling apparatus, and the environmental interactions between spacecraft and their electric propulsion systems.

Finke, R. C.↗

NASA NTRS1981

The plasma wake of mesosonic conducting bodies. II - An experimental parametric study of the mid-wake ion density peak

An experimental investigation of the disturbed flow field created by conducting bodies in a mesosonic, collisionless plasma stream is reported. The mid-wake region is investigated, where, for bodies of the order of a Debye length in size, the focused ion streams converge to form a significant current density peak on the wake axis. A parametric description is obtained of the behavior of the amplitude, width, and position of this peak. The results also indicate that portions of the axial ion peak are created by additional mechanisms and that body geometry affects the mid-wake structure only when the sheath is sufficiently thin to conform to the shape of the body.

Stone, N. H.↗

NASA NTRS1982

Evaluation of fuel injection configurations to control carbon and soot formation in small GT combustors

An experimental program to investigate hardware configurations which attempt to minimize carbon formation and soot production without sacrificing performance in small gas turbine combustors has been conducted at the United Technologies Research Center. Four fuel injectors, embodying either airblast atomization, pressure atomization, or fuel vaporization techniques, were combined with nozzle air swirlers and injector sheaths, and evaluated at test conditions which included and extended beyond standard small gas turbine combustor operation. Extensive testing was accomplished with configurations embodying either a spill return or a T-vaporizer injector. Minimal carbon deposits were observed on the spill return nozzle for tests using either Jet A or ERBS test fuel. A more extensive film of soft carbon was observed on the vaporizer after operation at standard engine conditions, with large carbonaceous growths forming on the device during off-design operation at low combustor inlet temperature. Test results indicated that smoke emission levels depended on the combustor fluid mechanics (especially the mixing rates near the injector), the atomization quality of the injector and the fuel hydrogen content.

Rosfjord, T. J.↗

NASA NTRS1982

Plasma flow measurements in a simulated low earth orbit plasma

The employment of large, higher power solar arrays for space operation has been considered, taking into account a utilization of high operating voltages. In connection with the consideration of such arrays, attention must be given to the fact that the ambient environment of space contains a tenuous low energy plasma which can interact with the high voltage array causing power 'leakage' and arcing. An investigation has been conducted with the aim to simulate the behavior of such an array in low-earth-orbit (LEO). During the experiments, local concentrations of the 'leakage' current were observed when the panel was at a high voltage. These concentrations could overload or damage a small area of cells in a large string. It was hypothesized that this effect was produced by electrostatic focusing of the particles by the sheath fields. To verify this experimentally, an end-effect Langmuir probe was employed. The obtained results are discussed.

Gabriel, S. B.↗

NASA NTRS1982

Gravity perception and asymmetric growth in plants - A model derived from the grass pulvinus

It is pointed out that gravitropic responses in plants involve asymmetric growth. On the basis of the geometry of growth response in grass leaf sheath pulvinus, a general model is proposed for gravitropism in multicellular plant organs. The negative gravitropic response of a pulvinus is a result of cell elongation involving all but the uppermost region of a horizontally placed organ. Whereas the uppermost region does not grow, the lowermost region elongates maximally. The regions between elongate to intermediate extents. An expression is given relating the angle of curvature of the organ to the diameter and initial and final lengths of the organ. It is shown that the response of the individual cells can be expressed as inherent sensitivity to gravitational stimulus according to a particular equation.

Dayanandan, P.↗

NASA NTRS1982

DE-2 photoelectron measurements consistent with a large scale parallel electric field over the polar cap

Winningham and Heikkila (1974) presented observations of various polar cap particle morphologies. They interpreted observations of 'anomalous' photoelectron angular distribution over the polar caps to be indicative of a large scale, outwardly directed, parallel electric field over the polar cap. The parallel field was observed to be spatially and/or temporarily variable. However, results obtained by Winningham and Heikkila have one weakness, which is related to the lack of simultaneous observations at many pitch angles. The present investigation is, therefore, concerned with the presentation of results from Dynamics Explorer 2 (DE-2) which confirm the experimental results of Winningham and Heikkila. It is concluded that the earth's polar caps act much like any conductor immersed in a plasma and illuminated by sunlight. DE-1 and DE-2 would then represent tiny point probes examining the internal details in the sheath region of the polar cap.

Winningham, J. D.↗