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NASA NTRS1987

Experimental study of low Reynolds number nozzles

High-performance electrothermal thrusters operate in a low nozzle-throat Reynolds number regime. Under these conditions, the flow boundary layer occupies a large volume inside the nozzle, contributing to large viscous losses. Four nozzles (conical, bell, trumpet, and modified trumpet) and a sharp-edged orifice were evaluated over a Reynolds number range of 500 to 9000 with unheated nitrogen and hydrogen. The nozzles showed significant decreases in specific impulse efficiency with decreasing Reynolds number. At Reynolds numbers less than 1000, all four nozzles were probably filled with a large boundary layer. The discharge coefficient decreased with Reynolds number in the same manner as the specific impulse efficiency. The bell and modified trumpet nozzles had discharge coefficients 4 to 8 percent higher than those of the cone or trumpet nozzles. The Two-Dimensional Kinetics (TDK) nozzle analysis computer program was used to predict nozzle performance. The results were then compared to the experimental results in order to determine the accuracy of the program within this flow regime.

Grisnik, Stanley P.↗

NASA NTRS1986

Rapid fluctuations in solar flares

Study of rapid fluctuations in the emission of radiation from solar flares provides a promising approach for probing the magneto-plasma structure and plasma processes that are responsible for a flare. It is proposed that elementary flare bursts in X-ray and microwave emission may be attributed to fine structure of the coronal magnetic field, related to the aggregation of photospheric magnetic field into magnetic knots. Fluctuations that occur on a subsecond time-scale may be due to magnetic islands that develop in current sheets during magnetic reconnection. The impulsive phase may sometimes represent the superposition of a large number of the elementary energy-release processes responsible for elementary flare bursts. If so, the challenge of trying to explain the properties of the impulsive phase in terms of the properties of the elementary processes must be faced. Magnetic field configurations that might produce solar flares are divided into a number of categories, depending on: whether or not there is a filament; whether there is no current sheet, a closed current sheet, or an open current sheet; and whether the filament erupts into the corona, or is ejected completely from the Sun's atmosphere. Analysis of the properties of these possible configurations is compared with different types of flares, and to Bai's subdivision of gamma-ray/proton events.

Sturrock, Peter A.↗

NASA NTRS1986

Hard X-ray delays

High time resolution hard X-ray rates with good counting statistics over 5 energy intervals were obtained using a large area balloon-borne scintillation detector during the 27 June 1980 solar flare. The impulsive phase of the flare was comprised of a series of major bursts of several to several tens of seconds long. Superimposed on these longer bursts are numerous smaller approximately 0.5 to 1.0 second spikes. The time profiles for different energies were cross-correlated for the major bursts. The rapid burst decay rates and the simultaneous peaks below 120 keV both indicate a rapid electron energy loss process. Thus, the flux profiles reflect the electron acceleration/injection process. The fast rate data was obtained by a burst memory in 8 and 32 msec resolution over the entire main impulsive phase. These rates will be cross-correlated to look for short time delays and to find rapid fluctuations. However, a cursory examination shows that almost all fluctuations, down to the 5% level, were resolved with 256 msec bins.

Schwartz, Richard A.↗

NASA NTRS1986

Relation between solar narrow-band decimeter wave bursts and associated X-ray bursts

Results are reported of an initial comparison of solar narrow-band decimeter wave spike bursts with corresponding X-ray events for 25 solar flares. Contrary to the rather straighforward temporal and spatial relations which exist between typical impulsive microwave and X-ray bursts, narrow-band decimeter wave spike bursts show complex temporal structure relative to the associated X-ray features. The time delay between the hard X-ray and microwave emission peaks varies from -2 to 8 seconds. The associated X-ray events are very weak in intensity, are spectrally very soft, and do not show any peculiar spectral features. Although the total number of bursts is small, it is found that decimeter spike bursts are 100 to 1000 times stronger than normal impulsive microwave bursts for the same observed amount of hard X-ray emission. This feature, supplemented by the fact that 100% circular polarization is observed, strongly supports a maser origin for the decimeter spike bursts.

Enome, Shinzo↗

NASA NTRS1986

Signatures of current loop coalescence in solar flares

The nonlinear coalescence instability of current carrying solar loops can explain many of the characteristics of the solar flares such as their impulsive nature, heating and high energy particle acceleration, amplitude oscillations of electromagnetic emission as well as the characteristics of 2-D microwave images obtained during a solar flare. The physical characteristics of the explosive coalescence of currents are presented in detail through computer simulation and theory. Canonical characteristics of the explosive coalescence are: (1) a large amount of impulsive increase of kinetic energies of electrons and ions; (2) simultaneous heating and acceleration of electrons and ions in high and low energy spectra; (3) ensuing quasi-periodic amplitude oscillations in fields and particle quantities; and (4) the double peak (or triple peak) structure in these profiles, participate in the coalescence process, yielding varieties of phenomena.

Sakai, J.↗

NASA NTRS1987

A detailed description of the uncertainty analysis for High Area Ratio Rocket Nozzle tests at the NASA Lewis Research Center

A preliminary uncertainty analysis has been performed for the High Area Ratio Rocket Nozzle test program which took place at the altitude test capsule of the Rocket Engine Test Facility at the NASA Lewis Research Center. Results from the study establish the uncertainty of measured and calculated parameters required for the calculation of rocket engine specific impulse. A generalized description of the uncertainty methodology used is provided. Specific equations and a detailed description of the analysis are presented. Verification of the uncertainty analysis model was performed by comparison with results from the experimental program's data reduction code. Final results include an uncertainty for specific impulse of 1.30 percent. The largest contributors to this uncertainty were calibration errors from the test capsule pressure and thrust measurement devices.

Kenneth J. Davidian↗

NASA NTRS1986

The identification of solar He-3-rich events and the study of particle acceleration at the sun

Kilometric radio observations are to associate type III radio bursts with solar He-3-rich events and to identify and study the sources of those events at the sun and the transport of the particles outward toward earth. The events exhibit an impulsive behavior that earns them a natural role in the event classification scheme based upon the time scale of the particle acceleration process. Multiple He-3-rich events are observed frequently from a single active region where they provide a convenient signature to test theories of particle storage and transport. These multiple events act as impulsive probes of the condition of the interplanetary medium. Their profiles contrast with the source-acceleration-induced profiles of the long-duration events from the same active region that are interspersed among them.

Reames, D. V.↗

NASA NTRS1986

Spectral line profiles of Fe XXI 1354.1 A from the Solar Maximum Mission

Observations of the Fe XXI 1354.1 A line were obtained for several flares using the SMM-UVSP instrument with varying spectral and spatial resolution. Of special interest are spectral line profiles from the footpoints of flare loops taken during the impulsive phase. These data show blueshifted Fe XXI profiles coincident and cospatial with the impulsive brightening of chromospheric material. The present analysis supports the hypothesis that the blueshifted component of the high temperature emission is an integral part of the flare, possibly associated with chromospheric evaporation.

Mason, H. E.↗

NASA NTRS1987

Turbulence as a contributor to intermediate energy storage during solar flares

Turbulence is considered as a method for converting the energy observed as mass motions during the impulsive phase into thermal energy observed during the gradual phase of solar flares. The kinetic energy of the large-scale eddies driven by the upflowing material continuously cascades to smaller scale eddies until viscosity is able to convert it into thermal energy. The general properties of steady state, homogeneous, fluid turbulence is a nonmagnetic plasma and the properties of turbulent decay are reviewed. The time-dependent behavior of the velocities and energies observed by the X-Ray Polychromator (XRP) instrument on the SMM during the November 5, 1980 flare are compared with the properties of turbulence. This study indicates that turbulence may play a role in flare energies and may account for a fraction of the total amount of thermal energy observed during the gradual phase. The rate at which the observed flare velocities decrease is consistent with the decay of turbulent energy but may be too rapid to account for the entire time delay between the impulsive and gradual phases.

Bornmann, P. L.↗

NASA NTRS1987

Sound radiation from single and annular stream nozzles, with modal decomposition of in-duct acoustic power

An experimental program was carried out to study the acoustic characteristics of single and annular stream duct-nozzle systems at various flow conditions by using a refined acoustic impulse technique. In this technique, signal synthesis and signal averaging processes are incorporated to generate a desired impulsive signal from acoustic driver(s) and to eliminate background noise (flow noise) from in-duct and far field signals, respectively. The contribution of higher order modes to incident reflected and transmitted acoustic powers is accounted for by using a modal decomposition process. The annular stream terminations were tested statically at various annular stream flow velocities with no inner stream flow. The results derived from the experiments include in-duct acoustic powers, termination reflection coefficients, transmission coefficients, far field power, and acoustic dissipation.

Salikuddin, M.↗

NASA NTRS1987

Performance characterization of a low power hydrazine arcjet

Hydrazine arcjets, which offer substantial performance advantages over alternatives in geosynchronous satellite stationkeeping applications, have undergone startup, materials compatibility, lifetime, and power conditioning unit design issues. Devices in the 1000-3000 W output range have been characterized for several different electrode configurations. Constrictor length and diameter, electrode gap setting, and vortex strength have been parametrically studied in order to ascertain the influence of each on specific impulse and efficiency; specific impulse levels greater than 700 sec have been achieved.

Knowles, S. C.↗

NASA NTRS1986

Solar flare precursors

Recent progress in the study of flare precursors and onset is reviewed. New theoretical results on filament formation and erution are presented, including especially the dynamical effects of magnetic fields. Ample evidence is found that energetic processes are already at work in the onset phase, a few minutes before the rapid rise of the hard-radiation impulse. In particular, the prevalence of soft and hard X-ray preheating (to less than 100 million K) is detailed, along with a connection to the launch of coronal mass ejections. A possible interpretation of the empirical time profiles near onset is that 'preheating' signifies that the flare has slowly started, and the transition to the impulsive stage then represents a change of phase in the flare-instability process. Finally, the problem of the interpretation of microwave preflare data is delineated.

Van Hoven, G.↗

NASA NTRS1986

Evidence for explosive chromospheric evaporation in a solar flare observed with SMM

SMM soft X-ray data and Sacramento Peak Observatory H-alpha observations are combined in a study of the impulsive phase of a solar flare. A blue asymmetry, indicative of upflow motions, was observed in the coronal Ca XIX line during the soft X-ray rise phase. H-alpha redshifts, indicative of downward motions, were observed simultaneously in bright flare kernels during the period of hard X-ray emission. It is shown that, to within observational errors, the impulsive phase momentum transported by the upflowing soft X-ray plasma is equivalent to that of the downward moving chromospheric material.

Zarro, D. M.↗

NASA NTRS1987

Experimental study of low Reynolds number nozzles

Grisnik, Stanley P.↗

NASA NTRS1987

Operating characteristics of a 10 kW xenon ion propulsion module

Performance testing of a two-engine functional model xenon ion propulsion module is described. Use of highly modified J-series 30 cm ion engines reconfigured for xenon propellant, a computer controlled operating system, and precise flow control system are shown to result in very reliable ion module operation at high input power levels. Ion engine operation at a nominal 4.0 ampere beam current and 30.0 volt discharge gives a specific impulse of 3310 sec, a total engine efficiency of 64.3 percent, and a thrust-to-power ratio of 39.5 mN/kW at an input power level of 5.10 kW. These modified J series ion engines are shown to be capable of throttling over an 8:1 range from a power level of 5.48 kW at 3285 sec to a power level of 0.70 kW at 1856 sec. In addition, complete ion engine performance mapping of important system level parameters such as thrust, specific impulse, efficiency and thrust-to-power ratio are presented.

Aston, Graeme↗

NASA NTRS1986

A parametric study of a gas-generator airturbo ramjet (ATR)

Parametric engine performance calculations were carried out for an airturbo ramjet (ATR). A LOX-LH2 rocket powered turbine powered the compressor. The engine was flown over a typical flight path up to Mach 5 to show the effect of engine off design operation. The compressor design efficiency, compressor pressure ratio, rocket turbine efficiency, rocket turbine inlet temperature, and rocket chamber pressure were varied to show their effect on engine net thrust and specific impulse at Mach 5 cruise. Estimates of engine weights as a function of the ratio of compressor air to rocket propellant flow and rocket chamber pressure are also included. In general, the Mach 5 results indicate that increasing the amount of rocket gas produced increased thrust but decreased the specific impulse. The engine performance was fairly sensitive to rocket chamber pressure, especially at higher compressor pressure ratios. At higher compressor pressure ratios, the engine thrust was sensitive to turbine inlet temperature. At all compressor pressure ratios, the engine performance was not sensitive to compressor or turbine efficiency.

Snyder, Christopher A.↗

NASA NTRS1987

The effect of ambient pressure on the performance of a resistojet

Tests were conducted to investigate the effect of ambient pressure on the measured thrust of a prototype resistojet. The tests were performed using both heated and unheated nitrogen as the propellant at flow rates of 0.0379 and 0.0759 g/s. For the cold flow tests there was no effect on specific impulse over a range of ambient pressures from 1 to 400 microns. The results for the hot flow tests did show as much as a 7 percent variation in specific impulse over the same range of ambient pressures.

Manzella, D. H.↗

NASA NTRS1987

Magnetospheric electric fields and currents

The progress made in the years 1983-1986 in understanding the character and operation of magnetospheric electric fields and electric currents is discussed, with emphasis placed on the connection with the interior regions. Special attention is given to determinations of global electric-field configurations, measurements of the response of magnetospheric particle populations to the electric-field configurations, and observations of the magnetospheric currents at high altitude and during northward IMF. Global simulations of current distributions are discussed, and the sources of global electric fields and currents are examined. The topics discussed in the area of impulsive and small-scale phenomena include substorm current systems, impulsive electric fields and associated currents, and field-aligned electrodynamics. A key finding of these studies is that the electric fields and currents are interrelated and cannot be viewed as separate entities.

Mauk, B. H.↗