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At least 307 records · Page 17

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NASA NTRS1973

Studies of dished accelerator grids for 30-cm ion thrusters

Eighteen geometrically different sets of dished accelerator grids were tested on five 30-cm thrusters. The geometric variation of the grids included the grid-to-grid spacing, the screen and accelerator hole diameters and thicknesses, the screen and accelerator open area fractions, ratio of dish depth to dish diameter, compensation, and aperture shape. In general, the data taken over a range of beam currents for each grid set included the minimum total accelerating voltage required to extract a given beam current and the minimum accelerator grid voltage required to prevent electron backstreaming.

Rawlin, V. K.↗

NASA NTRS1974

Lunar orbit determination in the presence of unmodeled accelerations

A technique for estimating the state of an artificial satellite in the presence of unmodeled accelerations is presented. The unmodeled acceleration is approximated by a first-order Gauss-Markov sequence which can be separated into a timewise-correlated component and a purely random component. Using this approximation, a sequential procedure for estimating the position, velocity, and the unmodeled acceleration is developed. The method is evaluated by reducing range-rate observations obtained by tracking the Apollo 10 and 11 spacecraft during the lunar-orbit phase of the mission. Numerical results are presented which show that the observation residual pattern lies within the observation noise standard deviation. The values of the estimated components of the unmodeled acceleration are repeatable from orbit to orbit within a given mission and from mission to mission when the same ground track is covered. Finally, the variation in the radial component of the unmodeled acceleration shows a high correlation with the reported location of the lunar surface mascons.

Ingram, D. S.↗

NASA NTRS1975

The acceleration of heavy nuclei in solar flares

The overabundance of heavy nuclei in solar cosmic rays of energy below about 10 MeV/nucleon is explained by taking into account the pre-flare ionization states of these nuclei in the region where they are accelerated. A model is proposed which considers two-step accelerations associated with the initial development of solar flares. The first step is closely related to the triggering process of flares, while the second one starts with the development of the explosive phase. Further ionization of medium and heavy nuclei occurs through their interaction with keV electrons accelerated by the first-step acceleration. It is suggested that the role of these electrons is important in producing fully ionized atoms in the acceleration regions.

Sakurai, K.↗

NASA NTRS1975

Propellant reorientation with off-axis accelerations

Reorientation of propellant within a tank is important to the design of many acquisition and vent systems. A drop tower test program was conducted to determine the influence of off-axis accelerations on the motion of the liquid. A series of 39 tests was performed with a spherical tank and a cylindrical tank with hemispherical end domes, varying tank orientation, liquid volume and the magnitude of the acceleration. It was shown that reorientation under the effect of a pure axial acceleration, which has been studied in prior test programs, is a special case. The lateral component of the acceleration caused the liquid to reorient along one side of the tank. The motion of the liquid appeared to be independent of the magnitude of the lateral acceleration for the range of values considered.

Tegart, J.↗

NASA NTRS1976

Determination of the cosmological rate of change of G and the tidal accelerations of earth and moon from ancient and modern astronomical data

The theory and numerical analysis of ancient astronomical observations (1374 to 1715) are combined with modern data in a simultaneous solution for: the tidal acceleration of the lunar longitude; the observed apparent acceleration of the earth's rotation; the true nontidal geophysical part of this acceleration; and the rate of change in the gravitational constant. Provided are three independent determinations of a rate of change of G consistent with the Hubble Constant and a near zero nontidal rotational acceleration of the earth. The tidal accelerations are shown to have remained constant during the historical period within uncertainties. Ancient and modern solar system data, and extragalactic observations provided a completely consistent astronomical and cosmological scheme.

Muller, P. M.↗

NASA NTRS1975

Visual reaction times during prolonged angular acceleration parallel the subjective perception of rotation

The effect of prolonged angular acceleration on choice reaction time to an accelerating visual stimulus was investigated, with 10 commercial airline pilots serving as subjects. The pattern of reaction times during and following acceleration was compared with the pattern of velocity estimates reported during identical trials. Both reaction times and velocity estimates increased at the onset of acceleration, declined prior to the termination of acceleration, and showed an aftereffect. These results are inconsistent with the torsion-pendulum theory of semicircular canal function and suggest that the vestibular adaptation is of central origin.

Mattson, D. L.↗

NASA NTRS1976

Characteristics of auroral electron acceleration regions observed by Atmosphere Explorer C

Satellite measurements of electron precipitation and ion drift velocities showed that electron acceleration regions (or inverted V's) in the 1200 to 1800 MLT quadrant exhibit the following systematic behavior: electron distribution functions in the accelerated region can be well described by Maxwellian primary electron beams accelerated through an electrostatic potential; the typical inverted V latitudinal structure is always observed in the accelerated regions, the electrostatic potential reaching a maximum and consequently decreasing to near zero over distances of 100 to 250 km; the Maxwellian temperature of the primary electron beam increases systematically with increasing electrostatic potential; rather weak acceleration regions, characterized by values of the electrostatic potential below 1 keV and values of the Maxwellian temperature between 100 and 350 eV, occur in the cusp and in the highest-latitude portion of the dusk side electron precipitation zone.

Burch, J. L.↗

NASA NTRS1976

Subjective detection of vertical acceleration: A velocity dependent response?

Difficulty in the subjective tracking of the whole body vertical accelerative movement parallel to the long axis of the body is discussed which identified specifically the direction of imposed acceleration to be the difficulty rather than low sensitivity to vertical acceleration. The product of angular acceleration and time to detect proves to be constant over a wide range of suprathreshold step changes of angular acceleration. It is concluded that threshold conditions are determined by the velocity attained rather than the acceleration amplitude for the semicircular canals.

Jones, G. M.↗

NASA NTRS1977

Studies on solar hard X-Rays and gamma-rays: Compton backscatter, anisotropy, polarization and evidence for two phases of acceleration

Observations of solar X-rays and gamma-rays from large flares show that the hard X-ray spectrum extends into the gamma ray region, where a flattening in the spectrum of the continuum emission is observed above about 1 MeV. This emission is believed to be due to bremsstrahlung. In addition to electron-proton collisions, at energies greater than approximately 500 keV, bremsstrahlung due to electron-electron collisions becomes significant. Bremsstrahlung production was calculated for a variety of electron spectra extending from the nonrelativistic region to relativistic energies and electron-electron bremsstrahlung is taken into account. By comparing these calculations with data, it is shown that the flattening in the spectrum of the continuum emission can be best explained by an electron spectrum consisting of two distinctive components. This evidence, together with information on the X-ray and gamma ray time profiles, implied the existence of two phases of acceleration. The first phase accelerates electrons mainly up to about several hundred keV; the second phase accelerates a small fraction of the electrons accelerated in the first phase to relativistic energies and accelerates protons to tens and hundreds of MeV.

Bai, T.↗

NASA NTRS1976

Effect of hypovolemia, infusion, and oral rehydration on gradual onset +Gz acceleration tolerance

The purpose of this study was to determine the effect of blood withdrawal, blood infusion, and oral fluid intake on +Gz tolerance at an acceleration rate of 0.5 G/min. Six healthy men aged 21-27 yr were centrifuged after the withdrawal of 400 ml of blood (hypovolemia) from each man; they were centrifuged again following blood infusion (Phase I). Three weeks later the men were accelerated after similar hypovolemia and again after consuming 800 ml of an isotonic NaCl drink (Phase II). Phase I hypovolemia resulted in a reduction in tolerance in all subjects from a mean control level of 6.42 + or - 0.35 min to 5.45 + or - 0.17 min (-15.1%, p less than 0.05). Both infusion and drinking returned tolerances to control levels. During acceleration there were significant (p less than 0.05) increases in plasma vasopressin levels to 35 pg/ml; these were not influenced appreciably by infusion or drinking. In all acceleration runs there was an obligatory shift (loss) of plasma volume and electrolytes, especially potassium, regardless of the experimental treatments. Oral rehydration is shown to be as effective as blood replacement in restoring +Gz acceleration tolerance decrements due to hypovolemia.

Greenleaf, J. E.↗

NASA NTRS1978

Studies of acceleration processes in the corona using ion measurements on the solar probe mission

The energy spectra and composition of particles escaping from the Sun provide essential information on mechanisms responsible for their acceleration, and may also be used to characterize the regions where they are accelerated and confined and through which they propagate. The suprathermal energy range, which extends from solar wind energies (approximately 1 KeV) to about 1 MeV/nucleon, is of special interest to studies of nonthermal acceleration processes because a large fraction of particles is likely to be accelerated into this energy range. Data obtained from near earth observations of particles in the suprathermal energy range are reviewed. The necessary capabilities of an a ion composition experiment in the solar probe mission and the required ion measurements are discussed. A possible configuration of an instrument consisting of an electrostatic deflection system, modest post acceleration, and a time of flight versus energy system is described as well as its possible location on the spacecraft.

Gloeckler, G.↗

NASA NTRS1980

The relation between tilt table and acceleration-tolerance and their dependence on stature and physical fitness

A group of 12 highly trained athletes and a group of 12untrained students were subjected to passive changes of position on a tilt table and positive accelerations in a centrifuge. During a 20 min tilt, including two additional respiratory maneuvers, the number of faints and average cardiovascular responses did not differ significantly between the groups. During linear increase of acceleration, the average blackout level was almost identical in both groups. Statistically significant coefficients of product-moment correlation for various relations were obtained. The coefficient of multiple determination computed for the dependence of acceleration tolerance on heart-eye distance and systolic blood pressure at rest allows the explanation of almost 50% of the variation of acceleration tolerance. The maximum oxygen uptake showed the expected significant correlation to the heart rate at rest, but not the acceleration tolerance, or to the cardiovascular responses to tilting.

Klein, K. E.↗

NASA NTRS1980

First phase acceleration mechanisms and implications for hard X-ray burst models in solar flares

The paper considers the number of nonthermal electrons which must be accelerated in the impulsive phase of a flare, as well as possible acceleration mechanisms. Direct electric field acceleration in a plasma is considered, taking explicit account of magnetohydrodynamic processes; it is shown that significant heating is involved. Attention is also given to acceleration by electron plasma waves, which requires the waves to be driven to large amplitude by already streaming electrons or anisotropic ion-acoustic waves. It is concluded that at most 0.1% of the flare energy can be deposited into nonthermal streaming electrons with the proposed acceleration and energy conversion mechanisms. Therefore, hard X-ray production above 10 keV primarily by hot thermal electrons is the only choice compatible with existing primary energy release models.

Smith, D. F.↗

NASA NTRS1980

Generation of auroral kilometric radiation and the structure of auroral acceleration region

Generation of auroral kilometric radiation (AKR) in the auroral acceleration region is studied. It is shown that auroral kilometric radiation can be generated by backscattered electrons trapped in the acceleration region via a cyclotron maser process. The parallel electric field in the acceleration region is required to be distributed over 1-2 earth radii. The observed AKR frequency spectrum can be used to estimate the altitude range of the auroral acceleration region. The altitudes of the lower and upper boundaries of the acceleration region determined from the AKR data are respectively approximately 2000 and 9000 km.

Lee, L. C.↗

NASA NTRS1979

Ion acceleration at the earth's bow shock - A review of observations in the upstream region

Positive ions are accelerated at or near the earth's bow shock and propagate into the upstream region. Two distinctly different populations of these ions, distinguished by their greatly different spectral and angular widths, can be identified there. The type of ion population observed in the upstream region is strongly correlated with the presence or absence of long-period compressive waves in the solar wind. Very few ions are accelerated in the vicinity of the shock to energies much above about 100 keV. It is not yet clear whether the most energetic ions (i.e., those near 100 keV) are accelerated at the shock or in the broad disturbed region upstream from the shock. In either case stochastic acceleration by turbulent electrostatic fields seems to be the most viable candidate for the acceleration of the most energetic particles.

Gosling, J. T.↗

NASA NTRS1980

Body acceleration distribution and O2 uptake in humans during running and jumping

The distribution of body acceleration and associated oxygen uptake and heart rate responses are investigated in treadmill running and trampoline jumping. Accelerations in the +Gz direction were measured at the lateral ankle, lumbosacral region and forehead of eight young men during level treadmill walking and running at four speeds and trampoline jumping at four heights, together with corresponding oxygen uptake and heart rate. With increasing treadmill speed, peak acceleration at the ankle is found always to exceed that at the back and forehead, and acceleration profiles with higher frequency components than those observed during jumping are observed. Acceleration levels are found to be more uniformly distributed with increasing height in jumping, although comparable oxygen uptake and heat rates are obtained. Results indicate that the magnitude of the biomechanical stimuli is greater in trampoline jumping than in running, which finding could be of use in the design of procedures to avert deconditioning in persons exposed to weightlessness.

Bhattacharya, A.↗

NASA NTRS1981

Exceedance statistics of accelerations resulting from thruster firings on the Apollo-Soyuz mission

Spacecraft acceleration resulting from firings of vernier control system thrusters is an important consideration in the design, planning, execution and post-flight analysis of laboratory experiments in space. In particular, scientists and technologists involved with the development of experiments to be performed in space in many instances required statistical information on the magnitude and rate of occurrence of spacecraft accelerations. Typically, these accelerations are stochastic in nature, so that it is useful to characterize these accelerations in statistical terms. Statistics of spacecraft accelerations are summarized.

Fichtl, G. H.↗

NASA NTRS1981

On the acceleration of thermal coronal ions by flare induced shock waves

The energy spectra of solar flare ions are calculated by assuming that the process which accelerates solar wind ions to MeV/ nucleon energies in the interplanetary corotating interaction region (CIR) also occurs in flare induced magnetosonic fast-mode (MFM) shocks in the corona. Solar wind ions are considered to be accelerated to MeV/nucleon energies by wave-particle interactions in the shock front and the downstream flow, being compressed between upstream and downstream magnetic field irregularities, and then accelerated by the shock drift acceleration mechanism. The energy spectra of the accelerated ions is calculated from the number of shock encounters as a function of the post- and preacceleration energies. A best fit by an exponential in momentum is determined for ions in the 50 MeV to a few GeV range, and from 20-80 MeV by a suitable power law in kinetic energy with a mean spectral index. Comparisons with observed solar protons show good agreement.

Decker, R. B.↗