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At least 1,009 records · Page 56

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NASA NTRS1989

Periodic amplitude variations as precursors of plumes of equatorial ionospheric irregularities

Periodic amplitude fluctuations of VHF signals from a geostationary satellite monitored from near the magnetic equator have been observed in the evening hours as precursors of strong Rayleigh fading associated with plumes of irregularities. These periodic fluctuations called amplitude waves exhibit amplitude changes of only 1 to 2 dB and have been observed for up to 30 minutes before the onset of strong scintillations. Individual fades are correlated over distances of at least 120 km in the magnetic east-west direction. The velocity of these wavelike disturbances has been found to be approximately 140 m/s eastward with a corresponding wavelength of 25 km.

Klobuchar, J. A.↗

NASA NTRS1990

Analytical modeling of nonradial expansion plumes

The 'Modified Simons' model presented allows the nonradial nature of axisymmetric rocket and thruster plume flowfields having a large exit Mach number and/or a large nozzle exit half-angle to be successfully predicted. The model is applied to monatomic and polyatomic gas (N, Ar, tetrafluoromethane) expansions; the nonradial density decay observed experimentally is successfully predicted.

Boyd, Iain D.↗

NASA NTRS1989

Three-dimensional simulation of underexpanded plumes using upwind algorithms

Different numerical techniques have been examined for simulating the underexpanded plumes in supersonic flows. The results revealed that the monotone differentiable limiter is superior to the TVD (total variation diminishing) minmod limiter when the MUSCL (monotonic upstream schemes for conservation laws) approach is used for Roe's upwind scheme. Roe's upwind scheme is found to be superior to the central difference scheme not only for capturing shock waves, but also for capturing shear flows.

Obayashi, Shigeru↗

NASA NTRS1990

A feasibility study and mission analysis for the Hybrid Plume Plasma Rocket

The Hybrid Plume Plasma Rocket (HPPR) is a high power electric propulsion concept which is being developed at the MIT Plasma Fusion Center. This paper presents a theoretical overview of the concept as well as the results and conclusions of an independent study which has been conducted to identify and categorize those technologies which require significant development before the HPPR can be considered a viable electric propulsion device. It has been determined that the technologies which require the most development are high power radio-frequency and microwave generation for space applications and the associated power processing units, low mass superconducting magnets, a reliable, long duration, multi-megawatt space nuclear power source, and long term storage of liquid hydrogen propellant. In addition to this, a mission analysis of a one-way transfer from low earth orbit (LEO) to Mars indicates that a constant acceleration thrust profile, which can be obtained using the HPPR, results in faster trip times and greater payload capacities than those afforded by more conventional constant thrust profiles.

Sullivan, Daniel J.↗

NASA NTRS1990

Fabry-Perot interferometer development for rocket engine plume spectroscopy

This paper describes a new rugged high-resolution Fabry-Perot interferometer (FPI) designed for rocket engine plume spectroscopy, which is capable of detecting spectral signatures of eroding engine components during rocket engine tests and/or flight operations. The FPI system will make it possible to predict and to respond to the incipient rocket engine failures and to indicate the presence of rocket components degradation. The design diagram of the FPI spectrometer is presented.

Bickford, R. L.↗

NASA NTRS1991

Engine throat/nozzle optics for plume spectroscopy

The Task 2.0 Engine Throat/Nozzle Optics for Plume Spectroscopy, effort was performed under the NASA LeRC Development of Life Prediction Capabilities for Liquid Propellant Rocket Engines program. This Task produced the engineering design of an optical probe to enable spectroscopic measurements within the SSME main chamber. The probe mounts on the SSME nozzle aft manifold and collects light emitted from the throat plane and chamber. Light collected by the probe is transferred to a spectrometer through a fiber optic cable. The design analyses indicate that the probe will function throughout the engine operating cycle and is suitable for both test stand and flight operations. By detecting metallic emissions that are indicative of component degradation or incipient failure, engine shutdown can be initiated before catastrophic failure. This capability will protect valuable test stand hardware and provide enhanced mission safety.

Bickford, R. L.↗

NASA NTRS1990

Surface and airborne evidence for plumes and winds on Triton

Aeolian features on Triton that were imaged during the Voyager Mission have been grouped. The term 'aeolian feature' is broadly defined as features produced by or blown by the wind, including surface and airborne materials. Observations of the latitudinal distributions of the features probably associated with current activity (known plumes, crescent streaks, fixed terminator clouds, and limb haze with overshoot) all occur from latitude -37 deg to latitude -62 deg. Likely indicators of previous activity (dark surface streaks) occur from latitude -5 deg to -70 deg, but are most abundant from -15 deg to -45 deg, generally north of currently active features. Those indicators which give information on wind direction and speed have been measured. Wind direction is a function of altitude. The predominant direction of the surface wind streaks is found to be between 40 deg and 80 deg measured clockwise from north. The average orientation of streaks in the northeast quadrant is 59 deg. Winds at 1- to 3-kilometer altitude are eastward, while those at more than 8 kilometers blow west.

Hansen, C. J.↗

NASA NTRS1990

Arcjet plasma plume effect on a microwave reflector antenna

The effect of plasma produced by an arc-jet thruster on the performance of on-board reflector antennas was investigated using a configuration in which the arc-jet plume is modeled as an inhomogeneous scatterer with a smoothly varying refractive index. On the basis of laws of geometrical optics, used to calculate the wave propagation, the trajectory, phase, amplitude, and polarization of the high-frequency ray field are computed numerically. The experiments performed to validate the calculated results experimentally are described.

Birkner, B. W.↗

NASA NTRS1991

Energy balance and plume dynamics in Triton's lower atmosphere

The present study of the thermal balance-affecting relationships among Triton lower atmosphere thermal conduction, eddy mixing, condensation, and radiative heating indicates that, while the temperature gradient is negative in the lower atmosphere, it becomes positive at higher altitudes due to the downward conduction of ionospheric heat. This temperature profile is essentially consistent with radio-occultation experiment data; the geyser-like plumes observed by Voyager suggest that the Trioton atmosphere's convective and conductive regions join near 10-km altitude, and that the values inferred for the eddy diffusion and heat-transport coefficients indicate a profile reminiscent of the earth's.

Yelle, Roger V.↗

NASA NTRS1991

Numerical and experimental investigations of rarefied nozzle and plume flows of nitrogen

Numerical and experimental investigations are performed for the rarefied flow of nitrogen through a small nozzle which is expanded into near-vacuum conditions. Two different numerical studies are undertaken: the first employs a continuum approach in solving the Navier-Stokes equations, and the second employs a particle approach through use of the direct simulation Monte Carlo method (DSMC). The experimental investigation concerns the measurement of pressure, using a Pitot tube, in the nozzle exit plane and near-field of the plume. Comparison of the experimental and numerical data at the nozzle exit reveals that the DSMC technique provides the more accurate description of the expanding flow. It is discovered that the DSMC solutions are quite sensitive to the model employed to simulate the interaction between the gas and the nozzle wall surface. It is concluded that the simplistic fully diffuse model is quite satisfactory for the present application.

Boyd, Iain D.↗

NASA NTRS1991

Rocket engine plume diagnostics using video digitization and image processing - Analysis of start-up

Video digitization techniques have been developed to analyze the exhaust plume of the Space Shuttle Main Engine. Temporal averaging and a frame-by-frame analysis provide data used to evaluate the capabilities of image processing techniques for use as measurement tools. Capabilities include the determination of the necessary time requirement for the Mach disk to obtain a fully-developed state. Other results show the Mach disk tracks the nozzle for short time intervals, and that dominate frequencies exist for the nozzle and Mach disk movement.

Disimile, P. J.↗

NASA NTRS1991

Are the Equatorial Highlands on Venus formed by mantle plume diapirs?

Several origins have been proposed for the Equatorial Highlands on Venus, including spreading centers and plume-related uplift. Recently, the spreading center hypothesis has been shown to be incompatible with the measured geoid and topography variations over the highlands. It is also difficult to reconcile the range of geoid anomalies over the highlands with a steady-state plum model. There is a large variation in admittance values (geoid/topography ratios) among highland regions. This variation suggests that different uplifted regions represent distinct stages in a time dependent process. It has been proposed that the Beta Regio, Thetis Regio, Ovda Regio, and Artemis Plateau Equatorial Highland Regions are formed by large mantel diapirs. According to this model, topography and geoid height decrease with increasing age of the highland, as the diapir spreads out beneath the lithosphere. In order to determine if the diapir model is compatible with the sequence of tectonic and volcanic events recorded in the surface geology of the highlands, a series of finite difference calculations were made of the ascent and partial melting of a spherical thermal diapir in an incompressible, infinite Prandtl number, isoviscous fluid.

Olson, Peter↗

NASA NTRS1992

An overview of in-flight plume diagnostics for rocket engines

An overview and progress report of the work performed or sponsored by LeRC toward the development of in-flight plume spectroscopy technology for health and performance monitoring of liquid propellant rocket engines are presented. The primary objective of this effort is to develop technology that can be utilized on any flight engine. This technology will be validated by a hardware demonstration of a system capable of being retrofitted onto the Space Shuttle Main Engines for spectroscopic measurements during flight. The philosophy on system definition and status on the development of instrumentation, optics, and signal processing with respect to implementation on a flight engine are discussed.

Madzsar, G. C.↗

NASA NTRS1992

Rocket plume flowfield characterization using laser Rayleigh scattering

A Doppler-resolved laser Rayleigh scattering diagnostic was applied to a 111 N thrust, regenerative and fuel-film cooled, gaseous hydrogen/gaseous oxygen rocket engine. The axial and radial mean gas velocities were measured from the net Doppler shifts observed for two different scattering angles. Translational temperatures and number densities were estimated from the Doppler widths and scattered intensities, respectively, by assuming that water was the dominant scattering species in the exhaust. The experimental results are compared with theoretical predictions from a full Navier-Stokes code (RD/RPLUS) and the JANNAF Two-Dimensional Kinetics (TDK) and Standardized Plume Flowfield (SPF-II) codes. Discrepancies between the measured and predicted axial velocities, temperatures, and number densities are evident. Radial velocity measurements, however, show excellent agreement with predictions. The discrepancies are attributed primarily to inefficient mixing and combustion caused by the injection of excessive oxidizer along one side of the thrust chamber. Thrust and mass flow rate estimates obtained from the Rayleigh measurements show excellent agreement with the globally measured values.

Zupanc, Frank J.↗

NASA NTRS1991

Simulations of microphysical, radiative, and dynamical processes in a continental-scale forest fire smoke plume

The impact of a large forest fire smoke plume on atmospheric processes is studied through a numerical model of meteorology, aerosols, and radiative transfer. The simulated smoke optical depths at 0.63-micron wavelength are in agreement with analyses of satellite data and show values as high as 1.8. The smoke has an albedo of 35 percent, or more than double the clear-sky value, and cools the surface by as much as 5 K. An imaginary refractive index, n sub im, of 0.01 yields results which closely match the observed cooling, single scattering albedo, and the Angstrom wavelength exponent. An n exp im of 0.1, typical of smoke from urban fires, produces 9 K cooling. Coagulation causes the geometric mean radius by number to increase from the initial value of 0.08 micron to a final value of 0.15 micron, while the specific extinction and absorption increase by 40 and 25 percent, respectively.

Westphal, Douglas L.↗

NASA NTRS1992

Preliminary analysis of observations of the Pinatubo volcanic plume with a polarization-sensitive lidar

A dual-polarization lidar aboard the NASA Electra aircraft was used in July 1991 to survey the stratospheric plume from the recent Mt. Pinatubo eruption. Both depolarizing and non-depolarizing volcanic layers were observed, ranging from 17 to 26 km in altitude. Differences in the depolarization signatures of the layers indicates differences in the composition or physical state of the particles in the layers.

Winker, D. M.↗

NASA NTRS1992

Airborne observations of SO2, HCl, and O3 in the stratospheric plume of the Pinatubo volcano in July 1991

A high-resolution IR spectrometer aboard the NASA Electra aircraft to measure the total column amount of SO2, O3, and HCl above the aircraft while flying over the Caribbean three weeks after the June 15 eruption of Mt. Pinatubo in the Philippines. South of 20 deg N latitude columns of SO2 were observed ranging from 2.0-3.7 x 10 exp 16 molecules/sq cm. In addition, the column amount of HCl averaged 1.5 x 10 exp 15 molecules/sq cm in the region of the plume. This represents a small increase in HCl above the amount, estimated from the previous measurements, that would have been presented had there been no volcanic eruption, but the increase is substantially less than that seen following the 1982 eruptions of El Chichon.

Mankin, William G.↗

NASA NTRS1991

Aerosol characterization in smoke plumes from a wetlands fire

Results are presented from airborne measurements of aerosol mass loading, size distribution, and elemental composition obtained in a smoke plume from a fire at a Florida wildlife refuge. It is found that there was a high concentration of carbon-containing aerosols and salt crystals in the 0.1-0.2 micron size range; the composition and morphology of the aerosols differed with size. Aerosol mass concentrations are used in conjunction with those obtained for CO2 in order to calculate ratios of aerosol and CO2. The ratios are noted to be higher for the smoldering phase of the fire than for its flaming phase.

Woods, David C.↗