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At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Anti-gravity device

An educational toy useful in demonstrating fundamental concepts regarding the laws of gravity is described. The device comprises a sphere 10 of radius r resting on top of sphere 12 of radius R. The center of gravity of sphere 10 is displaced from its geometrical center by distance D. The dimensions are so related that D((R+r)/r) is greater than r. With the center of gravity of sphere 10 lying on a vertical line, the device is in equilibrium. When sphere 10 is rolled on the surface of sphere 12 it will return to its equilibrium position upon release. This creates an illusion that sphere 10 is defying the laws of gravity. In reality, due to the above noted relationship of D, R, and r, the center of gravity of sphere 10 rises from its equilibrium position as it rolls a short distance up or down the surface of sphere 12.

Palsingh, S.↗
NASA NTRS1975

Centaur zero gravity coast and engine restart demonstration on the Titan/Centaur (TC-2) extended mission

The Centaur propellant management and thermal control techniques required for zero gravity coasting were successfully demonstrated during an extended mission following spacecraft separation on the TC-2 flight. As part of the demonstration, two successful engine starts were accomplished. The first engine start followed a one-hour zero gravity coast, and the second engine start followed a three-hour zero gravity coast. All of the Centaur systems performed satisfactorily, the design parameters for zero gravity coasting were verified, and no significant problems were encountered. The flight results showed that the propellant location and behavior, propellant heating, and tank pressure rise rates observed during the zero-gravity coasts were less severe than expected. Consequently, the majority of the propellants remained at the tank bottom, the propellant collection times were very short, and more than 7 hours of coast could have been achieved before a tank venting was required. The tank pressurization prior to the engine starts provided boost pump Net Positive Suction Head values well in excess of the values required. The LO2 tank pressurization was accomplished by a new bubbler method that greatly reduced the helium usage.

Lacovic, R. F.↗
NASA NTRS1976

Gravity effects on flame spreading over solid surfaces

The effects of gravity on the spreading of a flame over a solid combustible surface were determined. Flame propagation rates were measured from specimens of thin cellulose acetate sheets burning in both normal gravity (1 g) and reduced gravity (0 g) environments; the specimens were burned in various quiescent mixtures of oxygen, helium, argon, and nitrogen. A correlation for normal gravity and reduced gravity burning was obtained based on theoretical models of previous investigators.

Andracchio, C. R.↗
NASA NTRS1977

Lunar gravity - A harmonic analysis

A sixteenth-degree and sixteenth-order spherical harmonic lunar gravity field has been derived from the long-term Keplerian variations in the orbits of the Apollo subsatellites and Lunar Orbiter 5. This model resolves the major mascon gravity anomalies of the lunar near side and is in very good agreement with line-of-sight acceleration results. The far-side map shows the major ringed basins to be strong localized negative anomalies located in broad regions of positive gravity which correspond closely to the highlands. The rms pressure levels calculated from equivalent-surface height variations show that the moon and earth support nearly equal pressures, whereas Mars is appreciably stronger. The moon appears to support larger loads than earth owing to its weaker central gravity field and perhaps a colder upper lithosphere. Significant differences between the low-degree gravity and topography spectra indicate that the longer-wavelength topographic features are isostatically compensated.

Ferrari, A. J.↗
NASA NTRS1977

Lunar gravity - A long-term Keplerian rate method

Nearside tracking data from Apollo subsatellites and Lunar Orbiter 5 are used to construct a model of the farside lunar gravity field. An indirect method of calculating the average effect of all the gravity anomalies which affect spacecraft orbits is reduced to a linear estimation problem by eliminating the integration of spacecraft dynamics. Doppler frequency shifts between the lunar satellite and earth-based tracking station are used to measure the line of sight velocity component between the orbiter and the tracker; gravity determination is then performed in a weighted least squares estimator which employs the orbital rates as data and formulates the normal equations using the associated Keplerian state. Simulations performed demonstrate that farside gravity features can be economically determined by use of gravity estimates obtained from long-period models of nearside orbiting tracks.

Ferrari, A. J.↗
NASA NTRS1978

Simplified model of statistically stationary spacecraft rotation and associated induced gravity environments

A stochastic model of spacecraft motion was developed based on the assumption that the net torque vector due to crew activity and rocket thruster firings is a statistically stationary Gaussian vector process. The process had zero ensemble mean value, and the components of the torque vector were mutually stochastically independent. The linearized rigid-body equations of motion were used to derive the autospectral density functions of the components of the spacecraft rotation vector. The cross-spectral density functions of the components of the rotation vector vanish for all frequencies so that the components of rotation were mutually stochastically independent. The autospectral and cross-spectral density functions of the induced gravity environment imparted to scientific apparatus rigidly attached to the spacecraft were calculated from the rotation rate spectral density functions via linearized inertial frame to body-fixed principal axis frame transformation formulae. The induced gravity process was a Gaussian one with zero mean value. Transformation formulae were used to rotate the principal axis body-fixed frame to which the rotation rate and induced gravity vector were referred to a body-fixed frame in which the components of the induced gravity vector were stochastically independent. Rice's theory of exceedances was used to calculate expected exceedance rates of the components of the rotation and induced gravity vector processes.

Fichtl, G. H.↗
NASA NTRS1975

Farside lunar gravity from a mass point model

A mass point representation of the lunar gravity field was determined from the long-period orbital variations of the Apollo 15 and 16 subsatellites and Lunar Orbiter V. A radial acceleration contour map, evaluated at 100 km altitude from the lunar surface, shows that the nearside is in close agreement with the result derived from the line of sight method by Muller and Sjogren. The farside map shows the highland regions as broad positive gravity anomaly areas and the basins such as Korolev, Hertzsprung, Moscoviense, Mendeleev, and Tsiolkovsky as localized, negative gravity anomaly regions. The farside map has a first-order agreement with the result derived from the harmonic field method by Ferrari. The mass points analysis indicates that the nearside is almost all negative gravity anomaly regions except for the known positive mass anomaly basins (mascons) and the farside is almost all positive gravity anomaly regions except for some localized negative areas near the basins.

Ananda, M.↗
NASA NTRS1978

Gravity field, geoid and ocean surface by space techniques

Knowledge of the earth's gravity field continued to increase during the last four years. Altimetry data from the GEOS-3 satellite has provided the geoid over most of the ocean to an accuracy of about one meter. Increasing amounts of laser data has permitted the solution for 566 terms in the gravity field with which orbits of the GEOS-3 satellite have been computed to an accuracy of about one to two meters. The combination of satellite tracking data, altimetry and gravimetry has yielded a solution for 1360 terms in the earth's gravity field. A number of problems remain to be solved to increase the accuracy of the gravity field determination. New satellite systems would provide gravity data in unsurveyed areas and correction for topographic features of the ocean and improved computational procedures together with a more extensive laser network will considerably improve the accuracy of the results.

Anderle, R. J.↗
NASA NTRS1979

Time-dependent gravity in southern California, May 1974 - Apr 1979

Gravity measurements were coordinated with the long baseline three dimensional geodetic measurements of the Astronomical Radio Interferometric Earth Surveying project which used radio interferometry with extra-galactic radio sources. Gravity data from 28 of the stations had a single reading standard deviation of 11 microgal which gives a relative single determination between stations a standard deviation of 16 microgal. The largest gravity variation observed, 80 microgal, correlated with nearby waterwell variations and with smoothed rainfall. Smoothed rainfall data appeared to be a good indicator of the qualitative response of gravity to changing groundwater levels at other suprasediment stations, but frequent measurement of gravity at a station was essential until the quantitative calibration of the station's response to groundwater variations was accomplished.

Whitcomb, J. H.↗
NASA NTRS1978

Simulation gravity modeling to spacecraft-tracking data - Analysis and application

It is proposed that line-of-sight gravity measurements derived from spacecraft-tracking data can be used for quantitative subsurface density modeling by suitable orbit simulation procedures. Such an approach avoids complex dynamic reductions and is analogous to the modeling of conventional surface gravity data. This procedure utilizes the vector calculations of a given gravity model in a simplified trajectory integration program that simulates the line-of-sight gravity. Solutions from an orbit simulation inversion and a dynamic inversion on Doppler observables compare well (within 1% in mass and size), and the error sources in the simulation approximation are shown to be quite small. An application of this technique is made to lunar crater gravity anomalies by simulating the complete Bouguer correction to several large young lunar craters. It is shown that the craters all have negative Bouguer anomalies.

Phillips, R. J.↗
NASA NTRS1979

Gravity wave propagation in a diffusively separated atmosphere with height-dependent collision frequencies

Numerical calculations of gravity wave propagation through a two-component thermosphere with vertically-varying inter-species collision frequencies are performed using a direct integration method. Reflection of upgoing gravity wave energy into downgoing gravity waves appears to be small though nonnegligible at typical thermospheric periods and wavelengths. Coupling into diffusion waves is completely insignificant in most cases and may be determined in part by the relative abundance of the minor species. Temperature perturbation differences between species can be greater than 10% above 300 km altitude even though coupling into diffusion waves is small. Diffusive dissipation of gravity wave energy can be significant in the lower thermosphere and may be comparable to dissipation by viscosity and heat conduction below about 225 km altitude. Heating by diffusive dissipation may also be comparable to direct heating by solar radiation in the lower thermosphere. As a result of dissipative filtering of long-period and short-wavelength waves and the sensitivity of He-N2 density perturbation phase differences to diffusion, the AE-C satellite wave observations can only be fit by internal gravity waves with periods no greater than about 25 min and horizontal wavelengths no less than about 200 km.

Del Genio, A. D.↗
NASA NTRS1980

Baroclinic instability with variable gravity: A perturbation analysis

Solutions for a quasigeostrophic baroclinic stability problem in which gravity is a function of height were obtained. Curvature and horizontal shear of the basic state flow were omitted and the vertical and horizontal temperature gradients of the basic state were taken as constant. The effect of a variable dielectric body force, analogous to gravity, on baroclinic instability for the design of a spherical, baroclinic model for Spacelab was determined. Such modeling could not be performed in a laboratory on the Earth's surface because the body force could not be made strong enough to dominate terrestrial gravity. A consequence of the body force variation and the preceding assumptions was that the potential vorticity gradient of the basic state vanished. The problem was solved using a perturbation method. The solution gives results which are qualitatively similar to Eady's results for constant gravity; a short wavelength cutoff and a wavelength of maximum growth rate were observed. The averaged values of the basic state indicate that both the wavelength range of the instability and the growth rate at maximum instability are increased. Results indicate that the presence of the variable body force will not significantly alter the dynamics of the Spacelab experiment. The solutions are also relevant to other geophysical fluid flows where gravity is constant but the static stability or Brunt-Vaisala frequency is a function of height.

Giere, A. C.↗
NASA NTRS1980

A harmonic analysis of lunar gravity

An improved model of lunar global gravity has been obtained by fitting a sixteenth-degree harmonic series to a combination of Doppler tracking data from Apollo missions 8, 12, 15, and 16, and Lunar Orbiters 1, 2, 3, 4, and 5, and laser ranging data to the lunar surface. To compensate for the irregular selenographic distribution of these data, the solution algorithm has also incorporated a semi-empirical a priori covariance function. Maps of the free-air gravity disturbance and its formal error are presented, as are free-air anomaly and Bouguer anomaly maps. The lunar gravitational variance spectrum has the form V(G; n) = O(n to the -4th power), as do the corresponding terrestrial and martian spectra. The variance spectra of the Bouguer corrections (topography converted to equivalent gravity) for these bodies have the same basic form as the observed gravity; and, in fact, the spectral ratios are nearly constant throughout the observed spectral range for each body. Despite this spectral compatibility, the correlation between gravity and topography is generally quite poor on a global scale.

Bills, B. G.↗
NASA NTRS1982

Gravity and geoid anomalies of the Philippine Sea: Evidence on the depth of compensation for the negative residual water depth anomaly

A negative free-air gravity anomaly which occurs in the central part of the Philippine Sea was examined to determine the distribution and nature of possible regional mass excesses or deficiencies. Geoid anomalies from GEOS-3 observation were positive. A negative residual geoid anomaly consistent with the area of negative free-air gravity anomalies were found. Theoretical gravity-topography and geoid-topography admittance functions indicated that high density mantle at about 60 km dept could account for the magnitudes of the gravity and residual geoid anomaly and the 1 km residual water depth anomaly in the Philippine Sea. The negative residual depth anomaly may be compensated for by excess density in the uppermost mantle, but the residual geoid and regional free-air gravity anomalies and a slow surface wave velocity structure might result from low-density warm upper mantle material lying beneath the zone of high-density uppermost mantle. From a horizontal disk approximation, the depth of the low-density warm mantle was estimated to be on the order of 200 km.

Bowin, C.↗
NASA NTRS1981

Structure of the midcontinent basement. Topography, gravity, seismic, and remote sensing

Some 600,000 discrete Bouguer gravity estimates of the continental United States were spatially filtered to produce a continuous tone image. The filtered data were also digitally painted in color coded form onto a shaded relief map. The resultant image is a colored shaded relief map where the hue and saturation of a given image element is controlled by the value of the Bouguer anomaly. Major structural features (e.g., midcontinent gravity high) are readily discernible in these data, as are a number of subtle and previously unrecognized features. A linear gravity low that is approximately 120 to 150 km wide extends from southeastern Nebraska, at a break in the midcontinent gravity high, through the Ozark Plateau, and across the Mississippi embayment. The low is also aligned with the Lewis and Clark lineament (Montana to Washington), forming a linear feature of approximately 2800 km in length. In southeastern Missouri the gravity low has an amplitude of 30 milligals, a value that is too high to be explained by simple valley fill by sedimentary rocks.

Guinness, E. A.↗
NASA NTRS1981

Internal gravity waves in the solar atmosphere. I - Adiabatic waves in the chromosphere

The properties of adiabatic and linear internal gravity waves propagating in a solar wind model are discussed, using nonlinearity criteria unique to gravity waves to estimate wave-breaking heights. The results are used to deduce information on the possible role of gravity waves in the chromospheric energy balance. Maximum vertical velocity amplitudes for gravity waves are estimated to be on the order of 2 km/sec or less, and maximum horizontal velocity amplitudes are less than 6 km/sec, with temperature perturbations as large as 1000-2000 K. It is also estimated that gravity waves with an incident energy flux of one million ergs/sq cm-sec can propagate upward to a maximum height of 900-1000 km above the visible surface before nonlinearities lead to wave breaking, while those with an energy flux of 100,000 ergs/sq cm-sec can reach maximum heights of 1400-1600 km.

Mihalas, B. W.↗
NASA NTRS1982

Mean gravity anomalies from a combination of Apollo/ATS 6 and GEOS 3/ATS 6 SST tracking campaigns

Advances in satellite tracking data accuracy and coverage over the past 15 years have led to major improvements in global geopotential models. But the spacial resolution of the gravity field obtained solely from satellite dynamics sensed by tracking data is still of the order of 1000 km. Attention is given to an approach which will provide information regarding the fine structure of the gravity field on the basis of an application of local corrections to the global field. According to this approach, a basic satellite to satellite tracked (SST) range-rate measurement is constructed from the link between a ground station, a geosynchronous satellite (ATS 6), and a near-earth satellite (Apollo or GEOS 3). Attention is given to a mathematical model, the simulation of SST gravity anomaly estimation accuracies, a gravity anomaly estimation from GEOS 3/ATS 6 and Apollo/ATS 6 SST observations, and an evaluation of the mean gravity anomalies determined from SST.

Kahn, W. D.↗
NASA NTRS1981

Long wavelength gravity and topography anomalies

It is shown that gravity and topography anomalies on the earth's surface may provide new information about deep processes occurring in the earth, such as those associated with mantle convection. Two main reasons are cited for this. The first is the steady improvement that has occurred in the resolution of the long wavelength gravity field, particularly in the wavelength range of a few hundred to a few thousand km, mainly due to increased coverage of terrestrial gravity measurements and the development of radar altimeters in orbiting satellites. The second reason is the large number of numerical and laboratory experiments of convection in the earth, including some with deformable upper and lower boundaries and temperature-dependent viscosity. The oceans are thought to hold the most promise for determining long wavelength gravity and topography anomalies, since their evolution has been relatively simple in comparison with that of the continents. It is also shown that good correlation between long wavelength gravity and topography anomalies exists over some portions of the ocean floor

Watts, A. B.↗