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NASA NTRS1989

A brief review of JPL's electric propulsion technology activities

Near-term objectives and recent technological progress of JPL's electric propulsion program are discussed. Particular attention is given to accomplishments for ion, magnetoplasmadynamic (MPD), electron-cyclotron resonance (ECR), and arcjet thrusters. Xenon ion thruster erosion tests indicate a 15-fold reduction in tantalum baffle erosion when nitrogen is added to the xenon propellant and steady-state cylindrical MPD thruster tests at powers up to 72 kW show distinct self-constricted and diffuse discharge modes. An ECR thruster was operated at up to 7 kW with plasma acceleration at energies up to 7 kW; there was plasma acceleration at energies approaching 100 electron volts.

Barnett, John W.↗

NASA NTRS1989

Stripline feed for a microstrip array of patch elements with teardrop shaped probes

A circularly polarized microstrip array antenna utilizing a honeycomb substrate made of dielectric material to support on one side the microstrip patch elements in an array, and on the other side a stripline circuit for feeding the patch elements in subarray groups of four with angular orientation and phase for producing circularly polarized radiation, preferably at a 0, 90, 180, and 270 degree relationship is described. The probe used for coupling each feed point in the stripline circuit to a microstrip patch element is teardrop shaped in order to introduce capacitance between the coupling probe and the metal sheet of the stripline circuit that serves as an antenna ground plane. The capacitance thus introduced tunes out inductance of the probe. The shape of the teardrop probe is not critical. The probe capacitance required is controlled by the maximum diameter for the teardrop shaped probe, which can be empirically determined for the operating frequency. An aluminum baffle around each subarray blocks out surface wave between subarrays.

Huang, John↗

NASA NTRS1990

Hollow cathode operation at high discharge currents

It was shown that ion thruster hollow cathode operation at high discharge current levels can induce reduced thruster lifetimes by causing cathode insert overheating and/or erosion of surfaces located downstream of the cathode. The erosion problem has been particularly baffling because the mechanism by which it occurs has not been understood. The experimental investigation described reveals the energies of the ions produced close to the cathode orifice can be several times the anode-to-cathode potential difference generally considered available to accelerate them. These energies (of order 50 eV) are sufficient to cause the observed erosion rates. The effects of discharge current (to 60 A), magnetic field configuration and the cathode flowrate, orifice diameter and insert design on the energies and current densities of these jet ions are examined. A model describing the mechanism by which the high energy ions could be produced when the anode-cathode potential difference is insufficient is proposed. The effects of discharge current on cathode temperature and internal pressure are also examined experimentally and described phenomenologically.

Friedly, Verlin Joe↗

NASA NTRS1990

Interaction of acoustic waves generated by coupled plate

When two substructures are coupled, the acoustic field generated by the motion of each of the substructures will interact with the motion of the other substructure. This would be the case of a structure enclosing an acoustic cavity. A technique to model the interaction of the generated sound fields from the two components of a coupled structure, and the influence of this interaction on the vibration of the structural components is presented. Using a mobility power flow approach, each element of the substructure is treated independently both when developing the structural response and when determining the acoustic field generated by this component. The presence of the other substructural components is introduced by assuming these components to be rigid baffles. The excitation of one of the substructures is assumed to be by an incident acoustic wave which is dependent of the motion of the substructure. The sound field generated by the motion of the substructure is included in the solution of the response.

Cuschieri, J. M.↗

NASA NTRS1989

Design and performance analysis of the CLAES Ne/CO2 cryostat

The design of the dual-stage Ne/CO2 cryostat, constructed for the Cryogenic Limb Array Etalon Spectrometer (CLAES) for the cooling, to their respective required operation temperatures, of the focal plane (to less than 15 K), spectrometer (to less than 30 K), telescope (to less than 150 K), and baffles (to less than 180 K) is described, and the cryostat's performance is analyzed. The results of this study show that the current cryostat will meet all sensor cooling requirements. The cryostat weighs 2276 lbs, of which 988 lbs are charged to the neon and 484 lbs are charged to the CO2. Schematic diagrams of the CLAES cooler/sensor and of the external plumbing are presented together with the test data.

Naes, L. G.↗

NASA NTRS1990

A cryogenically cooled photofragment fluorescence instrument for measuring stratospheric water vapor

An instrument developed for high-resolution daytime measurements of water vapor in the stratosphere using the technique of photofragment fluorescence is examined. A detailed description of all aspects of the instrument, as well as the results of its first two flights, are presented. The main areas of concern were optical baffling, cryogen transfer, water vapor measurement without contamination, and a dual path absorption measurement. Results of the second flight test indicate that the problems of instrument and gondola contamination, identified in the first flight test, were solved. A signal-to-noise ratio of about 50:1 for 10 sec of averaging throughout the stratosphere is achieved, as well as an altitude resolution of better than 100 m.

Weinstock, Elliot M.↗

NASA NTRS1990

Assessment of shock capturing schemes for resonant flows in nonlinear instability analysis

The paper presents computational assessment of advanced numerical schemes for nonlinear acoustic problems related to combustion instabilities in liquid rocket engines. Several time-accurate, shock capturing schemes have been evaluated on a benchmark, closed-end resonant pipe flow problem. It involves the numerical solution of inviscid, compressible gas dynamics equations to predict acoustic wave propagation, wave steepening, formation of shocks, acoustic energy dissipation and wave-wall reflection for several hundred wave cycles. It was demonstrated that high accuracy TVD type schemes can be used for direct, exact nonlinear analysis of combustion instability problems, preserving high harmonic energy content for long periods of time. The selected scheme was then applied to analyze the acoustic responses of resonant pipe-resonator, radial acoustic modes and hub-baffle configurations. Interesting observations of wave shape and damping characteristics have been drawn from presented computational studies.

Przekwas, A. J.↗

NASA NTRS1990

The effect of nitrogen on xenon ion engine erosion

Erosion studies were performed on a 30-cm diameter J-series ion engine modified for operation on xenon propellant. The erosion rates of molybdenum and tantalum badges placed at different locations within the discharge chamber were measured as a function of the percentage of nitrogen (by mass) added to the xenon propellant. Reductions in the erosion rates of these badges of a factor of 8 to 50 were observed at nitrogen addition fractions between 0.5 to 2.0 percent. Reductions in cathode-side baffle erosion were achieved by adding nitrogen to the xenon propellant or by increasing the cathode orifice diameter. Analyses show that no significant degradation in ion engine performance should be expected at these nitrogen mass fractions. XRD, XPS and Auger analyses indicate the existence of nitrogen and nitrides in the surface of some but not all of the badges used in the tests where nitrogen was added to the xenon. Difficulty in identifying surface nitrides in the samples may be due to the existence of surface oxides and contaminants, or to the small thicknesses of the nitride layers.

Garner, Charles E.↗

NASA NTRS1989

New measurements of the far ultraviolet scattering properties of interstellar dust

An analysis is presented for the spectra of the diffuse ultraviolet background taken during shuttle flight STS-61C (January 1986). Eight regions of the sky were observed for approximately 20 minutes each, using a spectrograph designed specifically to perform measurements of the UV background. The field of view was 3.8 deg x 8 min, with imaging along the slit to confine stellar contamination. The instrument featured a shutter mechanism to measure internal background during flight, a low-scatter holographically ruled diffraction grating, photon counting microchannel plate detectors, through baffling, and a crystal window to further attenuate stray light. The spectra covered the range 1400 to 1850 A and was binned in 50 A bands. The procedure for substracting the contribution of stars too faint to be detected as discrete sources during the observations (in general this represents a small fraction of the total intensity detected except at the longest wavelengths). A radiative transfer model used to interpret the data and set confidence intervals on the relevant parameters is described. It was found that the continuum component of the diffuse ultraviolet background arises primarily from two sources. One source is scattering of starlight by interstellar dust with an albedo of about 12 percent and a relatively isotropic phase function. A second source consists of about 150 photons/cm/sec/ster/A of extragalactic light which is attenuated by the dust in the galaxy. Although emission features possibly associated with molecular H2 are detected in one look direction, fluorescence of H2 is not a major contributor to the diffuse UV background, at least at galactic latitudes greater than -10 degrees.

Hurwitz, Mark↗

NASA NTRS1990

Contamination control program for the Cosmic Background Explorer: An overview

Each of the three state of the art instruments flown aboard NASA's Cosmic Background Explorer (COBE) were designed, fabricated, and integrated using unique contamination control procedures to ensure accurate characterization of the diffuse radiation in the universe. The most stringent surface level cleanliness specifications ever attempted by NASA were required by the Diffuse Infrared Background Experiment (DRIBE) which is located inside a liquid helium cooled dewar along with the Far Infrared Absolute Spectrophotometer (FIRAS). The DRIBE instrument required complex stray radiation suppression that defined a cold primary optical baffle system surface cleanliness level of 100A. The cleanliness levels of the cryogenic FIRAS instrument and the Differential Microwave Radiometer (DMR) which were positioned symmetrically around the dewar were less stringent ranging from 300 to 500A. To achieve these instrument cleanliness levels, the entire flight spacecraft was maintained at level 500A throughout each phase of development. The COBE contamination control program is described along with the difficulties experienced in maintaining the cleanliness quality of personnel and flight hardware throughout instrument assembly.

Barney, Richard D.↗

NASA NTRS1991

Laboratory investigation of visible shuttle glow mechanisms

Laboratory experiments designed to uncover mechanistic information about the spectral and spatial characteristics of shuttle glow were conducted. The luminescence was created when a pulse of O atoms traveling at orbital velocities was directed toward NO molecules previously adsorbed to aluminum, nickel, and Z306 Chemglaz (a common baffle black) coated surfaces held at various temperatures. Spectral and spatial measurements were made using a CCD imaging spectrometer. Corroborative spectral information was recorded in separate measurements using a scanning monochromator and gated photomultiplier arrangement. The e-folding distance at several temperatures was calculated from images of the surface glow using the photometrics image processing capability of the imaging spectrometer. The e-folding distance was not altered as a function of incoming O beam velocity. The results are presented and the observations provide direct evidence that the visible shuttle glow results from recombination of oxygen atoms and surface bound NO.

Leone, A.↗

NASA NTRS1991

Acoustical analysis of gear housing vibration

The modal and acoustical analysis of the NASA gear-noise rig is described. Experimental modal analysis techniques were used to determine the modes of vibration of the transmission housing. The resulting modal data were then used in a boundary element method (BEM) analysis to calculate the sound pressure and sound intensity on the surface of the housing as well as the radiation efficiency of each mode. The radiation efficiencies of the transmission housing modes are compared with theoretical results for finite, baffled plates. A method that uses the measured mode shapes and the BEM to predict the effect of simple structural changes on the sound radiation efficiency of the modes of vibration is also described.

Seybert, A. F.↗

NASA NTRS1989

Scatter from optical components; Proceedings of the Meeting, San Diego, CA, Aug. 8-10, 1989

Various papers on scatter from optical components are presented. Individual topics addressed include: BRDF of SiC and Al foam compared to black paint at 3.39 microns, characterization of optical baffle materials, bidirectional transmittance distribution function of several IR materials at 3.39 microns, thermal cycling effects on the BRDF of beryllium mirrors, BTDV of ZnSe with multilayer coatings at 3.39 microns, scattering from contaminated surfaces, cleanliness correlation by BRDF and PFO instruments, contamination effects on optical surfaces, means of eliminating the effects of particulate contamination on scatter measurements of superfine optical surfaces, vacuum BRDF measurement of cryogenic optical surfaces, Monte Carlo simulation of contaminant transport to and deposition on complex spacecraft surfaces, surface particle observation and BRDF predictions, satellite material contaminant optical properties, dark field photographic techniques for documenting optical surface contamination, design of a laboratory study of contaminant film darkening in space, contamination monitoring approaches for EUV space optics.

Stover, John C.↗

NASA NTRS1989

Precision engineering and optomechanics; Proceedings of the Meeting, San Diego, CA, Aug. 10, 11, 1989

Various papers on precision engineering and optomechanics are presented. Individual topics addressed include: performance optimization of an IR sensor optical system, design and development of a rugged airborne scanning optical assembly for a calibrated IR imaging radiometer, numerical method for Cassegrain telescope baffle design, precision alignment of an IR imaging system, optical alignment of a pupil imaging spectrometer, optical alignment equipment and methods for visible and IR imaging meteorological radiometer. Also addressed are: vibration-design aspects of an IR-scanning earth sensor, chopping secondary mirror for the Kuiper Airborne Observatory, advanced flexure suspension systems, subminiature IR detector translation stage, thermal/optical test setup for the Geostationary Operational Environmental Satellite Telescope, design of the GOES telescope secondary mirror mounting.

Vukobratovich, Daniel↗

NASA NTRS1991

Contamination control program for the Cosmic Background Explorer

Barney, Richard D.↗

NASA NTRS1991

Investigations of ice formation in the Space Shuttle Main Engine 0209 main injector coolant cavity

Severe main combustion chamber wall and main injector baffle element deterioration occurred during tests of Space Shuttle Main Engine 0209. One of the possible causes considered is ice formation and blockage of coolant to these components, resulting from the mixing of leaking hot turbine exhaust gas (hydrogen rich steam) and hydrogen coolant in the injector coolant cavity. The plausibility of ice blockage is investigated through simple mixing calculations for hot gas and hydrogen, investigation of condensation and water droplet formation, calculation of the freezing times for droplets, and the prediction of ice layer thicknesses. It is concluded that condensation and droplet formation can occur, and small water droplets that form can freeze very quickly when in contact with the cold coolant cavity surfaces. Copnservative analysis predicts, however, that the maximum thickness of the ice layers formed is too small to result in significant blockage of the coolant flow.

Richards, D. R.↗

NASA NTRS1991

Stability testing of a modified Space Shuttle Main Engine

The testing of the combustion stability characteristics of Space Shuttle Main Engine (SSME) 0208 is described in terms of augmenting the technology base for large O/H thrust-chamber assemblies. The throat area is increased by 12 percent over that of the flight SSMEs, and the thrust chamber assembly does not include stability aids. Acoustic modes in the chamber are excited by means of rapid pressure generators employed in the start-transient through mainstage operations. Stability characteristics are determined by damp times which are facilitated by high-frequency instrumentation measuring oscillations and locating stable operating regions. All vibration modes are damped to within the requirements for a chamber mode set forth by the Chemical Propulsion Information Agency. No sustained chamber acoustic oscillations are exhibited in engine 0208's combustion chamber configuration in spite of the absence of baffles and acoustic cavities.

Dennis, H.↗

NASA NTRS1991

Analysis of cryogenic propellant behavior in microgravity and low thrust environments

Predictions of a CFD program calculating a fluid-free surface shape and motion as a function of imposed acceleration are validated against the drop-tower test data collected to support design and performance assessments of the Saturn S-IVB stage liquid-hydrogen tank. The drop-tower facility, experimental package, and experiment procedures are outlined, and the program is described. It is noted that the validation analysis confirms the program's suitability for predicting low-g fluid slosh behavior, and that a similar analysis could examine the effect of incorporating baffles and screens to impede initiation of any unwanted side loads due to slosh. It is concluded that in actual vehicle applications, the engine thrust tailoff profile should be included in computer simulations if the precise interface versus time definition is needed.

Fisher, Mark F.↗