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At least 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

A miniaturized pointing mount for Spacelab missions

A Miniaturized Pointing Mount (MPM) for Spacelab missions is defined and simulation results are described. This mount is proposed to complement the Spacelab Instrument Pointing System (IPS). It uses the same mount isolator concept as the Spacelab IPS but is much more efficient and economical for the accommodation of small shuttle payloads. The MPM is built from star tracker assemblies left over from the Apollo Telescope Mount program thereby assuring low cost and development risk. Simulation results indicate a high level of instrument stability can be expected. The short development time of the MPM would permit it to serve as a precursor to the Spacelab IPS for verifying critical new concepts such as the mount isolation and hold down mechanisms.

Fritz, C. G.↗
NASA NTRS1975

Experiment Pointing Subsystems (EPS) requirements for Spacelab missions

The goal of the experiment pointing subsystems (EPS) is to accommodate a broad spectrum of instrument types by providing a number of stability and control functions that greatly exceed the capability of the shuttle. These functions include target acquisition, target tracking through wide gimbal ranges, stabilization, simultaneous pointing to one or more targets, instrument rastering, and on-orbit calibration. The experiments will vary widely in size, weight, geometry, and instrument types, and many have not been completely defined. This great diversity of requirements reflects the long term plans of the user community and establishes challenging performance requirements for the EPS.

Nein, M. E.↗
NASA NTRS1976

Isothermal elastohydrodynamic lubrication of point contacts. 2: Ellipticity parameter results

A numerical solution of the isothermal elastohydrodynamic problem for point contacts is presented which reproduces all the essential features of experimental observations based upon optical interferometry. In particular, the two side lobes, in which minimum film thickness regions occur, emerge in the theoretical solutions. The influence of the ellipticity parameter on solutions to the point contact problem is explored. The ellipticity parameter k was varied from 1 (a ball on a plate) to 8 (a configuration approaching line contact). It is shown that the minimum film thickness can be related to the well known line contact solutions by a remarkably simple expression involving either k or the effective radius of curvature ratio R sub y/R sub x.

Hamrock, B. J.↗
NASA NTRS1976

Simplified solution for point contact deformation between two elastic solids

A linear-regression by the method of least squares is made on the geometric variables that occur in the equation for point contact deformation. The ellipticity and the complete eliptic integrals of the first and second kind are expressed as a function of the x, y-plane principal radii. The ellipticity was varied from 1 (circular contact) to 10 (a configuration approaching line contact). These simplified equations enable one to calculate easily the point-contact deformation to within 3 percent without resorting to charts or numerical methods.

Brewe, D. E.↗
NASA NTRS1975

A uniformly valid solution for motion about the interior libration point of the perturbed elliptic-restricted problem

Bounded motion about Lagrange collinear libration points is considered for a perturbed elliptic-restricted problem. A practical application is the motion of a satellite near a libration point collinear with the sun and the earth-moon barycenter. Such a system is treated here as an earth-sun-satellite elliptic restricted problem with lunar perturbations. The method of dual time scales is utilized to develop a uniformly valid three-dimensional analytical solution to the satellite's equations of motion. The analytical development applies somewhat generally to that class of four-body problems where the second primary mass is much greater than the first, and the third primary mass is much greater than the second.

Richardson, D. L.↗
NASA NTRS1975

A small instrument pointing system for shuttle sortie missions

A small instrument pointing system (SIPS) has been conceptually designed as a means of interfacing smaller astronomical instruments with the ESA spacelab system on shuttle sortie missions. Small instruments are defined as being up to 0.9 meter in diameter, three meters in length, and 340 kilograms in mass. Two independently supported instruments are deployed on a common pedestal to give a hemispherical field of view. The instruments are each supported at their centers-of-mass in bi-axial gimbal systems for fine pointing. An optional roll gimbal can be used to give roll about the line of sight. Instruments are held in environmental canisters which protect them thermally, and acoustically as well as from contamination. The canister facilitates instrument integration and provides interfaces to all spacelab subsystems. The SIPS is a total instrument support system for small sortie mission instruments.

Henrikson, C. C.↗
NASA NTRS1975

Isothermal elastohydrodynamic lubrication of point contacts. II - Ellipticity parameter results

A numerical solution of the isothermal elastohydrodynamic problem for point contacts has been presented which reproduces all the essential features of the previously reported experimental observations based upon optical interferometry. In particular, the two 'side lobes' in which minimum film thickness regions occur are shown to emerge in the theoretical solutions. The influence of the ellipticity parameter upon solutions to the point contact problem has been explored in the present paper. The ellipticity parameter (k) was varied from one (a ball on a plate) to eight (a configuration approaching line contact), and it has been shown that both the central and minimum film thicknesses can be related to the well known line contact solutions by remarkably simple expressions involving either (k) or the effective radius of curvature ratio (Ry/Rx).

Hamrock, B. J.↗
NASA NTRS1975

Isothermal elastohydrodynamic lubrication of point contacts. I - Theoretical formulation

The analysis of an isothermal elastohydrodynamic lubrication (EHL) point contact was evaluated numerically. This required the simultaneous solution of the elasticity and Reynolds equations. In the elasticity analysis the contact zone is divided into equal rectangular areas and it is assumed that a uniform pressure is applied over each element. In the numerical analysis of the Reynolds' equation a phi analysis where phi is equal to the pressure times the film thickness to the 3/2 power is used to help the relaxation process. The EHL point contact analysis is applicable for the entire range of elliptical parameters and is valid for any combination of rolling and sliding within the contact.

Hamrock, B. J.↗
NASA NTRS1975

ATS-6 - Spacecraft Attitude Precision Pointing and Slewing Adaptive Control Experiment

The primary objective of the Spacecraft Attitude Precision Pointing and Slewing Adaptive Control (SAPPSAC) experiment is to establish feasibility and evaluate capabilities of a ground-based spacecraft attitude control system, wherein RF command and telemetry links, together with a ground station on-line minicomputer, perform closed loop attitude control of the Applications Technology Satellite-6 (ATS-6). The ground processor is described, including operational characteristics and the controller software. Attitude maneuvers include precision pointing to fixed targets, slewing between targets, and generation of prescribed ground tracks. Test results show high performance and reliability for over 30 hours of on-line control with no serious anomalies. Attitude stabilization relative to a prescribed target has been achieved to better than 0.007 deg in pitch and roll and 0.02 deg in yaw for a period of 43 min. Ground tracks were generated which had maximum latitude/longitude deviations less than 0.15 deg from reference.

Isley, W. C.↗
NASA NTRS1975

Optical atmospheric scattering and absorption limitations on offset pointing from Earth Observatory Satellite /EOS/ sensors

The Braslau-Dave atmospheric model which calculates the upward monochromatic light fluxes leaving the top of the atmosphere as a function of viewing angle, sun angle, and ground reflectance was employed to study the effect of atmospheric scattering and attenuation on universal apparent contrast for two EOS remote sensors operated at very large offset or pointing angles: the Thematic Mapper (TM) and the High Resolution Pointable Imager (HRPI). The TM offset off nadir could be plus or minus 20 degrees with an 11 degree scan angle and the HRPI pointing angle off nadir could be plus or minus 45 degrees with a 3 degree scan angle. The reduction of universal apparent contrast of EOS imagery is studied as a function of sun elevation angle, atmospheric aerosol loading, radiation wavelength and sensor look angles.

Egan, W. G.↗
NASA NTRS1976

The quarter-point quadratic isoparametric element as a singular element for crack problems

The quadratic isoparametric elements which embody the inverse square root singularity are used for calculating the stress intensity factors at tips of cracks. The strain singularity at a point or an edge is obtained in a simple manner by placing the mid-side nodes at quarter points in the vicinity of the crack tip or an edge. These elements are implemented in NASTRAN as dummy elements. The method eliminates the use of special crack tip elements and in addition, these elements satisfy the constant strain and rigid body modes required for convergence.

Hussain, M. A.↗
NASA NTRS1977

Development of a character, line and point display system

A compact graphics terminal for use as the input to a computerized medical records system is described. The principal mode of communication between the terminal and the records system is by checklists and menu selection. However, the terminal accepts short, handwritten messages as well as conventional alphanumeric input. The terminal consists of an electronic tablet, a display, a microcomputer controller, a character generator, and a refresh memory for the display. An Intel SBC 80/10 microcomputer controls the flow of information and a 16 kilobyte memory stores the point-by-point array of information to be displayed. A specially designed interface continuously generates the raster display without the intervention of the microcomputer.

Owen, E. W.↗
NASA NTRS1976

Comparison of modal test results - Multipoint sine versus single-point random

The Mariner Jupiter/Saturn (MJS) spacecraft was subjected to the traditional multipoint sine dwell (MPSD) modal test using 111 accelerometer channels, and also to single-point random (SPR) testing using 26 accelerometer channels, and the two methods are compared according to cost, schedule, and technical criteria. A measure of comparison between the systems was devised in terms of the cumulative difference in the kinetic energy distribution of the common accelerometers. The SPR and MPSD method show acceptable agreement with respect to frequencies and mode damping. The merit of the SPR method is that the excitation points are minimized and the test article can be committed to other uses while data analysis is performed. The MPSD approach allows validity of the data to be determined as the test progresses. Costs are about the same for the two methods.

Leppert, E. L.↗
NASA NTRS1976

Optimal maneuvering and fine pointing control of large space telescope with a new magnetically suspended, single gimballed momentum storage device

This paper considers the application of an Annular Momentum Control Device (AMCD) to both fine pointing and large-angle maneuvering of a large space telescope (LST). The AMCD, which consists principally of a spinning rim suspended in noncontacting electromagnetic bearings, represents a new development in momentum storage devices. A nonlinear mathematical model of the AMCD/LST system is derived. An optimal stochastic fine-pointing controller is designed via LQG theory and the minimum-energy maneuvering problem is solved via a gradient technique. Number of state variable and control variable constraints, as well as all trigonometric nonlinearities, are considered in the latter part.

Nadkarni, A. A.↗
NASA NTRS1977

A search for microwave emission from solar X-ray bright point flares

An attempt was made to detect 9.4-cm radio emission from flaring X-ray bright points with the 46-m telescope at the Algonquin Radio Observatory. Observations from the X-ray heliometer aboard OSO-8 were combined with optical and magnetic data to substantiate possible events. Reduction of 52 h of radio data has revealed one event which is a candidate for radio emission from a flaring X-ray bright point.

Avery, L. W.↗
NASA NTRS1977

Lunar gravity - A mass point model

A point-mass representation of a quasi-global lunar gravity field is developed by processing Apollo 15 and 16 subsatellite and Lunar Orbiter 5 Doppler tracking data. The model is generated by reducing the long-period variations in the mean orbital elements. The model consists of 117 point masses distributed over the region of + or - 30 deg in latitude about the lunar equator and resolves all the previously known 'mascons' on the near side as broad positive-gravity regions. The near-side acceleration map evaluated at 100 km above the lunar surface shows good agreement with line-of-sight acceleration results. The lunar far-side gravity map shows strong broad positive-gravity regions for the highland areas. However, all the major ringed basins are resolved as localized negative anomalies in contrast with the near-side basins. This model does not indicate any evidence for the existence of any mascon-type feature on the lunar far-side.

Ananda, M. P.↗
NASA NTRS1977

The effects of the atmospheric point-spread 'seeing' function on spatially resolved spectra of Jupiter

We present the results of an analysis of the effects of atmospheric seeing and of instrumental spectral and spatial resolution on the observed variation of absorption-line profiles across the disk of Jupiter. The technique described may be applied equally well to the analysis of observations of any extended astronomical source. These results show the necessity of obtaining accurate point-spread-function information during the course of observations of this nature. We also point out that in order to avoid the uncertainties and ambiguities inherent in attempts at deconvolution of observational data, one must properly convolve the appropriate spatial and spectral resolution functions with the models being tested and then compare the results with the observational data.

Gelfand, J.↗