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At least 757 records · Page 42

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NASA NTRS1973

Plate motion and the secular shift of the mean pole

The global plate motion indicates that changes in the products of inertia of the earth due to tectonic plate movement may provide a secular shift of the mean pole. A mathematical procedure for calculating this shift based on the plate theory is presented. Explicit expressions were obtained for the dependence of the secular polar shift on the dimensions and locations of the plate boundaries. Numerical results show that the secular motion of the mean pole is 0.0002 sec/year in the direction of 67 W. Hence, it is deduced that the influence of the plate motion on the secular polar shift may account for 10% of the observed value.

Liu, H.↗

NASA NTRS1973

Diurnal polar motion

An analytical theory is developed to describe diurnal polar motion in the earth which arises as a forced response due to lunisolar torques and tidal deformation. Doodson's expansion of the tide generating potential is used to represent the lunisolar torques. Both the magnitudes and the rates of change of perturbations in the earth's inertia tensor are included in the dynamical equations for the polar motion so as to account for rotational and tidal deformation. It is found that in a deformable earth with Love's number k = 0.29, the angular momentum vector departs by as much as 20 cm from the rotation axis rather than remaining within 1 or 2 cm as it would in a rigid earth. This 20 cm separation is significant in the interpretation of submeter polar motion observations because it necessitates an additional coordinate transformation in order to remove what would otherwise be a 20 cm error source in the conversion between inertial and terrestrial reference systems.

Mcclure, P.↗

NASA NTRS1973

Estimates of the moon's geometry using lunar orbiter imagery and Apollo laser altimeter data

Selenographic coordinates for about 6000 lunar points identified on the Lunar Orbiter photographs are tabulated and have been combined with those lunar radii derived from the Apollo 15 laser altimeter data. These coordinates were used to derive that triaxial ellipsoid which best fits the moon's irregular surface. Fits were obtaind for different constraints on both the axial orientations and the displacement of the center of the ellipsoid. The semiaxes for the unconstrained ellipsoid were a = 1737.6 km, b = 1735.6 km, and c = 1735.0 km which correspond to a mean radius of about 1736.1 km. These axes were found to be nearly parallel to the moon's principal axes of inertia, and the origin was displaced about 2.0 km from the moon's center of gravity in a direction away from the earth and to the south of the lunar equator.

Jones, R. L.↗

NASA NTRS1973

Rapid iterative reanalysis for automated design

A method for iterative reanalysis in automated structural design is presented for a finite-element analysis using the direct stiffness approach. A basic feature of the method is that the generalized stiffness and inertia matrices are expressed as functions of structural design parameters, and these generalized matrices are expanded in Taylor series about the initial design. Only the linear terms are retained in the expansions. The method is approximate because it uses static condensation, modal reduction, and the linear Taylor series expansions. The exact linear representation of the expansions of the generalized matrices is also described and a basis for the present method is established. Results of applications of the present method to the recalculation of the natural frequencies of two simple platelike structural models are presented and compared with results obtained by using a commonly applied analysis procedure used as a reference. In general, the results are in good agreement. A comparison of the computer times required for the use of the present method and the reference method indicated that the present method required substantially less time for reanalysis. Although the results presented are for relatively small-order problems, the present method will become more efficient relative to the reference method as the problem size increases. An extension of the present method to static reanalysis is described, ana a basis for unifying the static and dynamic reanalysis procedures is presented.

Bhatia, K. G.↗

NASA NTRS1971

Applications of laser ranging and VLBI observations for selenodetic control

The observation equations necessary to utilize lunar laser ranging and very long baseline interferometry measurements were developed for the establishment of a primary control network on the moon. The network consists of coordinates of moon points in the selenodetic Cartesian coordinate system, which is fixed to the lunar body, oriented along the three principal axes of inertia of the moon, and centered at the lunar center of mass. The observation equations derived are based on a general model in which the unknown parameters included: the selenodetic Cartesian coordinates, the geocentric coordinates of earth stations, parameters of the orientation of the selenodetic coordinate system with respect to a fixed celestial system, the parameters of the orientation of the average terrestrial coordinate system with respect to a fixed celestial coordinate system, and the geocentric coordinates of the center of mass of the moon, given by a lunar ephemeris.

Fajemirokun, F. A.↗

NASA NTRS1972

Density of the lunar interior.

It is attempted to derive the constraints that can be placed on the density of the lunar interior. The moment of inertia of the moon and its mean density are being considered in the investigation together with the mass and density of the lunar crust that have been inferred from the seismic refraction data recorded by the passive seismometer. The calculations presented show that the density of the lunar interior can easily approach values as high as 3.5 for a fraction of the lunar mass which lies in the range from 1/2 to 2/3.

Gast, P. W.↗

NASA NTRS1972

The dynamic liquid squeeze film.

Experiments were conducted during which plane circular surfaces bounding a liquid squeeze film were subjected to impulsive loads. Pressure measurements within the liquid film and the boundary motion were recorded. These experimental results were compared with theoretical analyses which predict the bonding strength of dynamic squeeze films. The data show that the fluid inertia contributes significantly to the value of the bonding strength. It was also found that, for high loading rates, the liquid squeeze film can resist tensile loads. When in tension, liquid cavitation was observed. A photographic study was conducted which indicated that the inception of cavitation is directly related to the squeeze film liquid viscosity and inversely related to the film thickness.

Riepe, R.↗

NASA NTRS1972

Identification of rotating assembly inertial and bearing parameters.

The problem of estimating the state of a turbopump rotating assembly and identifying unknown products of inertia and unknown bearing parameters is considered. A linearized, extended Kalman filtering approach has been used and found to be successful for both state estimation and parameter identification in an inherently nonlinear problem. The input data were simulated Bently test data obtained from a verified 12-dimensional state-space model and were compared with real Bently test data obtained from NASA. The results prove the feasibility of using this model and this type of test data to obtain the hidden parameters of a typical turbopump rotating assembly.

Alspach, D. L.↗

NASA NTRS1972

A quasi-inertial attitude mode for orbiting spacecraft.

A method is presented for achieving quasi-inertial orientation of an orbiting spacecraft with minimal control effort. Spacecraft motion is a small-amplitude oscillation about the orbit normal with one principal axis held in the orbital plane. The nominal orientation of this axis is arbitrary, so that any spacecraft axis normal to it may be pointed arbitrarily on the celestial sphere and remain within a small neighborhood of the nominal orientation. Oscillation amplitude can be chosen to minimize control requirements for maintaining the mode and may range up to 18.8 deg depending on spacecraft inertia. The quasi-inertial mode would be an efficient means for maintaining a nominal solar orientation for NASA's Skylab configuration if the control-moment-gyros fail in the primary attitude control system. This approach, which utilizes the backup reaction thrust system, affords a substantial reduction in propellant requirements compared to that for a true solar inertial attitude hold.

Elrod, B. D.↗

NASA NTRS1972

Thermal conductivity and resonant multipole interactions.

Investigation of the influence of resonant multipole interactions on exchanges of rotational energy in molecular collisions, by means of a simple two-state impact-parameter approximation. It is found that dipole-quadrupole and quadrupole-quadrupole interactions can have a significant effect for molecules with low moments of inertia.

Nyeland, C.↗

NASA NTRS1972

The effects of trajectory characteristics on scientific objectives for major planetary orbiters.

Outline of specific solutions in the areas of attitude control, interplanetary trajectory parameters, and orbital configurations of major planetary spacecraft. It is shown that inertia wheel supplementation to a three-axis, stabilized attitude control system will substantially reduce propellant weight and attitude jet firings, especially when considering three-year orbital lifetimes as well as interplanetary trip times of several years. Eight interplanetary trajectory parameters including the earth injection energy and the declination of the launch asymptote are shown for a 1980 earth launch mission to Jupiter. The effects of the interplanetary trajectory parameters on the deboost velocities and resulting orbits about Jupiter are summarized. Orbital mode and science interfaces are defined, and a celestial mechanics experiment independent of earth tracking is briefly outlined.

Paul, C. K.↗

NASA NTRS1973

Venus - Radar determination of gravity potential.

We describe a method for the determination of the gravity potential of Venus from multiple-frequency radar measurements. The method is based on the strong frequency dependence of the absorption of radio waves in Venus' atmosphere. Comparison of the differing radar reflection intensities at several frequencies yields the height of the surface relative to a reference pressure contour; combination with measurements of round-trip echo delays allows the pressure, and hence the gravity potential contour, to be mapped relative to the mean planet radius. Since calibration data from other frequencies are unavailable, the absorption-sensitive Haystack Observatory data have been analyzed under the assumption of uniform surface reflectivity to yield a gravity equipotential contour for the equatorial region and a tentative upper bound of .0006 on the fractional difference of Venus' principal equatorial moments of inertia.

Shapiro, I. I.↗

NASA NTRS1972

Apollo 14 mineral ages and the thermal history of the Fra Mauro formation.

The thermal inertia of the Fra Mauro formation limits its rate of cooling. The calculations of Jaeger (1961) for the cooling of an extrusive sheet are used to estimate the time required for a central temperature drop of 10%. This time is found to be about 45 years. For a 90% temperature drop, the time is about 200 years. Analytical methods used in the investigations are discussed together with the diffusion of Rb and Sr within lunar basalt. Isochron diagrams for various minerals are presented.

Compston, W.↗

NASA NTRS1972

Orientation and resonance locks for satellites in the elliptic orbit.

In order to achieve the maximum strength of higher resonance locks for satellites in the elliptic orbit, the condition of satellite orientation during the process of deployment is established. It is shown that for maximum strength locks the axis of the minimum moment of inertia of satellites should point toward the attracting body at plus or minus (5/8) pi and 0 values of the true anomaly f. This condition of deployment is applicable to all cases of resonance rotation regardless of the value of lock number k and orbit eccentricity e.

Liu, H.-S.↗

NASA NTRS1973

Analysis of swimming motions.

This paper presents an analysis of swimming motion with specific attention given to the flutter kick, the breast-stroke kick, and the breast stroke. The analysis is completely theoretical. It employs a mathematical model of the human body consisting of frustrums of elliptical cones. Dynamical equations are written for this model including both viscous and inertia forces. These equations are then applied with approximated swimming strokes and solved numerically using a digital computer. The procedure is to specify the input of the swimming motion. The computer solution then provides the output displacement, velocity, and rotation or body roll of the swimmer.

Gallenstein, J.↗

NASA NTRS1973

Thermal properties of the Galilean satellites.

Radiometry in the 20-micron band of eclipses of each of the four Galilean satellites of Jupiter provides information about the thermal properties of the uppermost surface layers of these bodies. Their thermal inertias are all smaller than those of the moon or of Mercury; there is no evidence for atmospheres, and where the data are of high quality vertically homogeneous thermal properties are excluded. The thermal properties, together with other observational evidence, suggest that the surfaces of these satellites are largely composed of ices. A thin coating of frost provides the upper, low-conductivity layer, while the subsurface material maintains a high thermal conductivity by fusion of a mixture of ice and rock.

Morrison, D.↗

NASA NTRS1973

Gravity field of Mars from Mariner 9 tracking data.

Further reduction of Doppler tracking data from Mariner 9 confirms our earlier conclusion that the gravity field of Mars is considerably rougher than the fields of either the earth or the moon. The largest positive gravity anomaly uncovered is in the Tharsis region which is also topographically high and geologically unusual. The value obtained for the inverse mass of Mars is in good agreement with prior determinations from Mariner fly by trajectories. The direction found for the rotational pole of Mars is in excellent agreement with Sinclair's recent value, determined from earth-based observations of Mars' satellites. Other important physical constants that have either been refined or confirmed by the Mariner 9 data include: (1) the dynamical flattening, (2) the maximum principal moment of inertia, and (3) the period of precession of Mars' pole.

Lorell, J.↗

NASA NTRS1973

Inertial magnetic field reconnection and magnetospheric substorms.

We describe and calculate the growth rate of a magnetohydrodynamic neutral-sheet instability due to electron-inertia terms in the infinite-conductivity Ohm's law. The results are compared with an approximate Vlasov-equation calculation, and are shown to be particularly germane to the geomagnetic-tail instability.

Van Hoven, G.↗