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At least 1,225 records · Page 68
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Stellar fibril magnetic systems. II - Two-dimensional magnetohydrodynamic equations. III - Convective counterflow

The dynamics of magnetic fibrils in the convective zone of a star is investigated analytically, deriving mean-field equations for the two-dimensional transverse motion of an incompressible fluid containing numerous small widely spaced circular cylinders. The equations of Parker (1982) are extended to account for the inertial effects of local flow around the cylinders. The linear field equation for the stream function at the onset of convection is then rewritten, neglecting large-scale heat transport, and used to construct a model of convective counterflow. The Kelvin impulse and fluid momentum, convective motion initiated by a horizontal impulse, and the effects of a viscous boundary layer are considered in appendices.

Parker, E. N.↗
NASA NTRS1985

Tidal interactions between spherical galaxies

The impulse approximation and full N-body experiments are used to investigate the changes in mass and binding energy which result from gravitational encounters between spherical galaxies. Models with King (1966) and de Vaucouleurs (1948) surface density profiles, and radial, tangential, and isotropic velocity distributions. Results based on the impulse approximation depend on two collision parameters (the impact parameter and the ratio of the perturbing mass to the relative velocity at infinity); they agree well with N-body results. The asymptotic limit, valid for distant encounters, is found to approximate the correct results only for impact parameters larger than 10 effective radii. It is therefore of little use in most situations of interest. No simple 'linear' or 'quadratic' dependence of energy and mass loss on collision strength is found, but rather, a smooth intermediate behavior. Cross sections depend very little on the internal velocity structure of the galaxy, but quite strongly on the surface density profile in its outer parts. The reasons why these results disagree with the conclusions of previous workers are discussed.

Aguilar, L. A.↗
NASA NTRS1985

Experimental performance of a 1-kilowatt arcjet thruster

A formerly unused cathode and anode/nozzle assembly from a flight model arcjet was tested with nitrogen, hydrogen, and nitrogen-hydrogen mixture simulating ammonia decomposition products at arc power levels from about 300 to 950 W. Two different power sources and two nozzle configurations were tested at low back-ground pressures to exclude facility effects. Increased nozzle expansion ratio improved cold flow nozzle efficiency from 0.8 to 0.9. Hydrogen thrust efficiency of 0.26 at 872 sec specific impulse matched some 1964 performance on a similar device. Simulated ammonia thrust efficiency was 0.31 at 422 sec. Spontaneously occurring voltage mode changes at constant arc current could be partially stabilized with appropriate power source characteristics. In the higher voltage mode specific impulse was higher, but thrust efficiency changed only slightly from that of the lower voltage mode. Sustained tests of up to 2 hr duration exhibited no apparent performance degradation with time.

Nakanishi, S.↗
NASA NTRS1985

Scramjet sidewall burning: Preliminary shock tunnel results

Experiments performed with a two dimensional model scramjet with particular emphasis on the effect of fuel injection from a wall are reported. Air low with a nominal Mach number of 3.5 and varied enthalpies was produced. It was found that neither hydrogen injection angle nor combustor divergence angle had any appreciable effect on thrust values while increased combustor length appeared to increase thrust levels. Specific impulse was observed to peak when hydrogen was injected at an equivalence ratio of about 2. Lowering the Mach number of the injected hydrogen at low equivalence ratios, less than 4, appeared to benefit specific impulse while hydrogen Mach number had little effect at higher equivalence ratios. When a 1:1 mixture by volume of nitrogen and oxygen is used instead of air as a test gas, it is found that hydrogen combustion is enhanced but only at high enthalpies.

Morgan, R. G.↗
NASA NTRS1985

A control system design approach for flexible spacecraft

A control system design approach for flexible spacecraft is presented. The control system design is carried out in two steps. The first step consists of determining the ideal control system in terms of a desirable dynamic performance. The second step consists of designing a control system using a limited number of actuators that possess a dynamic performance that is close to the ideal dynamic performance. The effects of using a limited number of actuators is that the actual closed-loop eigenvalues differ from the ideal closed-loop eigenvalues. A method is presented to approximate the actual closed-loop eigenvalues so that the calculation of the actual closed-loop eigenvalues can be avoided. Depending on the application, it also may be desirable to apply the control forces as impulses. The effect of digitizing the control to produce the appropriate impulses is also examined.

Silverberg, L. M.↗
NASA NTRS1986

Hard X-ray imaging and the relative contribution of thermal and nonthermal emission in flares

The question of whether the impulsive 25 to 100 keV X-ray emission from solar flares is thermal or nonthermal has been a long-standing controversy. Both thermal and nonthermal (beam) models have been developed and applied to the hard X-ray data. It now seems likely that both thermal and nonthermal emission have been observed at hard X-ray energies. The Hinotori classification scheme, for example, is an attempt to associate the thermal-nonthermal characteristics of flare hard X-ray emission with other flare properties. From a theoretical point of view, it is difficult to generate energetic, nonthermal electrons without dumping an equal or greater amount of energy into plasma heating. On the other hand, any impulsive heating process will invariably generate at least some nonthermal particles. Hence, strictly speaking, although thermal or nonthermal emission may dominate the hard X-ray emission in a given energy range for a given flare, there is no such thing as a purely thermal or nonthermal flare mechanism.

Holman, G. D.↗
NASA NTRS1986

Lower power dc arcjet operations with hydrogen hydrogen/nitrogen propellant mixtures

The arcjet assembly from a flight model system was modified with a new thoriated tungsten nozzle insert and has been tested with hydrogen-nitrogen mixtures simulating the decomposition products of ammonia and hydrazine. Arcjet power consumption ranged from 0.7 to 1.15 kW depending on low rate, input current, and mixture composition. At a nominal 1 kW power level the ammonia mixtures thrust efficiency was about 0.31 at specific impulse values ranging between 460 and 500 sec. Hydrazine mixtures gave similar thrust efficiencies at the same power level with specific impulse values between 395 and 430 sec. Large, spontaneous voltage mode changes were not observed once the thruster had passed a period of instability immediately following start up. This period of instability, and the startup at low pressure, were seen as major causes of constrictor damage during the tests.

Curran, F. M.↗
NASA NTRS1985

The acoustic gravity wave induced by a point source in the middle atmosphere

Acoustic gravity wave (AGW) results computed for a stationary impulsive point source and a moving point source in the middle atmosphere are presented. For a stationary impulsive point Row's far field formula of the AGW was extended into the near field one, which comprises the Zeroth order Bessel function and its derivative terms. When (t-t sub o) is not large, the contribution of the derivative terms is important. The computed results agree with the experimental ones. For a moving point source with supersonic velocity, AGW is calculated using the moving point theory. Two solar eclipses that occurred in the lower latitude and over the ocean on Feb. 16, 1980, and June 11, 1983, were compared. The results show that the theoretical curve of AGW is fairly consistent with the observed ones.

Zhang, X. J.↗
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 fucntion of the ratio of compressor air to rocket propellant flow and rocket champer 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, C. A.↗
NASA NTRS1986

Solar filament eruptions and energetic particle events

The 1981 December 5 solar filament eruption that is associated with an energetic (E greater than 50 MeV) particle event observed at 1 AU. The eruption was photographed in H-alpha and was observed by the Solwind whitelight coronagraph on P78-1. It occurred well away from any solar active region and was not associated with an impulsive microwave burst, indicating that magnetic complexity and a detectable impulsive phase are not required for the production of a solar energetic particle (SEP) event. No metric type II or IV emission was observed, but an associated interplanetary type II burst was detected by the low-frequency radio experiment on ISEE 3. The December 5 and two other SEP events lacking evidence for low coronal shocks had unusually steep energy spectra (gamma greater than 3.5). In terms of shock acceleration, this suggests that shocks formed relatively high in the corona may produce steeper energy spectra than those formed at lower altitudes. It is noted that the filament itself maybe one source of the ions accelerated to high energies, since it is the only plausible coronal source of the He(+) ions observed in SEP events.

Kahler, S. W.↗
NASA NTRS1985

Experimental performance of a 1-kilowatt arcjet thruster

A formerly unused cathode and anode/nozzle assembly from a flight model arcjet was tested with nitrogen, hydrogen, and nitrogen-hydrogen mixture simulating ammonia decomposition products at arc power levels from about 300 to 950 W. Two different power sources and two nozzle configurations were tested at low background pressures to exclude facility effects. Increased nozzle expansion ratio improved cold flow nozzle efficiency from 0.8 to 0.9. Hydrogen thrust efficiency of 0.26 at 872 sec specific impulse matched some 1964 performance on a similar device. Simulated ammonia thrust efficiency was 0.31 at 422 sec. Spontaneously occurring voltage mode changes at constant arc current could be partially stabilized with appropriate power source characteristics. In the higher voltage mode specific impulse was higher, but thrust efficiency changed only slightly from that of the lower voltage mode. Sustained tests of up to 2 hr duration exhibited no apparent performance degradation with time.

Nakanishi, S.↗
NASA NTRS1986

Dispersive ducting of MHD waves in the plasma sheet - A source of Pi2 wave bursts

Fast magnetoacoustic waves can be ducted by plasma inhomogeneities such as the plasma sheet. As this ducting is dispersive an impulsive source will give rise to a well-defined, quasi-periodic wave packet with time-scales determined by the width of the inhomogeneity and characteristic speeds in the wave duct and surrounding medium. The duration of the wave packet depends upon the distance from the source. It is argued that an impulsive source in the plasma sheet at substorm onset will produce a wave packet near earth with characteristics similar to pi2 wave bursts and put this idea forward as a mechanism for the generation of pi2 pulsations.

Edwin, P. M.↗
NASA NTRS1986

Characteristic wave approach in controlled large space structures

The modal approach in structural dynamics usually implies a truncation technique in the course of which the contribution of high frequencies is lost. This can lead to significant error in the case of impulsive loads. As an alternative to modal (or spectral) methods, the characteristic wave approach is developed. It appears that the application of this approach is most beneficial in the domains where spectral methods fail. That is why it can be used as a supplement to modal methods when the loads contain impulsive components.

Zak, M.↗
NASA NTRS1986

A review of the multimegawatt MPD thruster and current mission applications

The magnetoplasmadynamic (MPD) thruster is potentially capable of providing 20-200 N of steady-state thrust, at 1000-20,000 s specific impulse while consuming a variety of propellants and megawatts of DC power. The specific impulse and power capacity put the MPD thruster in a class separate from other electric propulsion engines. Several types of missions are enhanced by the unique capabilities of a nuclear-electric driven MPD thruster and these include earth-orbit raising, cis-lunar transportation, and planetary exploration. This paper describes current MPD thruster technology, and several missions typical of the above cited categories.

King, D. Q.↗
NASA NTRS1986

Low power dc arcjet operation with hydrogen/nitrogen propellant mixtures

The arcjet assembly from a flight model system was modified with a new thoriated tungsten nozzle insert and has been tested with hydrogen-nitrogen mixtures simulating the decomposition products of ammonia and hygrazine. Arcjet power consumption ranged from 0.7 to 1.15 kW depending on low rate, input current, and mixture composition. At a nominal 1 kW power level the ammonia mixtures thrust efficiency was about 0.31 at specific impulse values ranging between 460 and 500 sec. Hydrazine mixtures gave similar thrust efficiencies at the same power level with specific impulse values between 395 and 430 sec. Large, spontaneous voltage mode changes were not observed once the thruster had passed a period of instability immediately following start up. This period of instability, and the startup at low pressure, were seen as major causes of constrictor damage during the tests.

Curran, F. M.↗
NASA NTRS1986

The solar flare extreme ultraviolet to hard X-ray ratio

Simultaneous measurements of the peak 10-1030 A extreme ultraviolet (EUV) flux enhancement and more than 10 keV hard X-ray (HXR) peak flux of many solar flare bursts, ranging over about four orders of magnitude in HXR intensity, are studied. A real departure from linearity is found in the relationship between the peak EUV and HXR fluxes in impulsive flare bursts. This relationship is well described by a given power law. Comparison of the predictions of the impulsive nonthermal thick-target electron beam model with observations shows that the model satisfactorily predicts the observed time differences between the HXR and EUV peaks and explains the data very well under given specific assumptions. It is concluded that the high-energy fluxes implied by the invariant area thick-target model cannot be completely ruled out, while the invariant area model with smaller low cutoff requires impossibly large beam densities. A later alternative thick-target model is suggested.

Mcclymont, A. N.↗
NASA NTRS1986

Maneuver strategies for the Magellan (Venus radar mapper) mission

The paper describes the maneuver strategies for the Venus Orbit Insertion and the Orbit Trim Maneuvers for the Magellan mission. An optimum three impulse strategy has been developed for the Venus Orbit insertion maneuver that minimizes the fuel consumed during the trim maneuvers. In addition an optimum two impulse strategy has been designed specifically for the two trim maneuvers that minimizes the fuel consumed while correcting for period, periapsis altitude, argument of periapsis, and inclination.

Dsouza, C. N.↗
NASA NTRS1986

Manned Mars mission transfer from Mars parking orbit to Phobos or Deimos

This paper addresses the problem of orbit transfers from a Mars parking orbit with an inclination of 165 degrees to the Mars moons. The transfer can be accomplished using a three impulse transfer. The current 1999 baseline manned Mars mission requires a Mars parking orbit with an inclination of 165 degrees. This orbit inclination is necessary due to the direction of the Mars arrival and departure asymptotes of the interplanetary trajectory. The selection of this inclination for the parking orbit minimized the delta velocity requirements at Mars arrival and departure. This presents a problem In making transfers from this orbit to either Phobos or Deimos since it is a retrograde orbit. It is possible to make this transfer efficiently using a three impulse transfer and an intermediate transfer orbit with a very large apogee altitude. This paper will show how the intermediate transfer orbit apogee can be determined based on a preselected transfer time, the delta velocities required as a function of transfer time, and the propellant required as a function of mission module weight for a transfer time of 5 days. The data presented in this paper Is specifically for the 1999 opposition class mission but the methods outlined are applicable to any other mission which requires a high inclination parking orbit.

Jack Mulqueen↗