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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6

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NASA NTRS1988

Mechanisms for the rigid rotation of coronal holes

The rotation of coronal holes is modeled in terms of a current-free coronal magnetic field in which holes are the footpoint locations of open field lines. It is shown that ongoing field-line reconnection allows the holes to rotate quasi-rigidly with their outer-coronal extensions. The results suggest that a strong axisymmetric field component may be responsible for the prolonged rigid rotation of large meridional holes during the declining phase of the sunspot cycle.

Nash, A. G.↗

NASA NTRS1989

A control approach for robots with flexible links and rigid end-effectors

Multiarm flexible robots with dexterous end effectors are currently being considered in such tasks as satellite retrieval, servicing and repair where a two phase problem can be identified: Phase 1, robot positioning in space; Phase 2, object retrieval. Some issues in Phase 1 regarding modelling and control strategies for a robotic system comprised of along flexible arm and a rigid three-link end effector are presented. The control objective is to maintain the last (rigid) link stationary in space in the presence of an additive disturbance caused by the flexible energy in the first link after a positioning maneuver has been accomplished. Several configuration strategies can be considered, and optimal decentralized servocompensators can be designed. Preliminary computer simulations are included for a simple proportional controller to illustrate the approach.

Barbieri, Enrique↗

NASA NTRS1990

State-variable models of structures having rigid-body modes

In cases where the equations of motion of a structure having rigid-body freedom are cast in state-variable form, generalized state rigid-body modes may be needed. It is possible to find a linearly-independent set of generalized vectors which transform an n x n matrix into the almost-diagonal Jordan form. Attention is presently given to equations governing these generalized eigenvectors, together with illustrative examples of the damped and undamped structure cases.

Craig, Roy R., Jr.↗

NASA NTRS1992

Thermal response of rigid and flexible insulations and reflective coating in an aeroconvective heating environment

Described here is the thermal performance of rigid and flexible thermal protection systems considered for potential use in future Aeroassist Space Transfer Vehicles. The thermal response of these materials subjected to aeroconvective heating from a plasma arc is described. Properties that were measured included the thermal conductivity of both rigid and flexible insulations at various temperatures and pressures and the emissivity of the fabrics used in the flexible insulations. The results from computerized thermal analysis models describing the thermal response of these materials subjected to flight conditions are included.

Kourtides, D. A.↗

NASA NTRS1992

Pneumatically erected rigid habitat

The pneumatically erected rigid habitat concept consists of a structure based on an overexpanded metal bellows. The basic concept incorporates the advantages of both inflatable and rigid structures. The design and erection detail are presented with viewgraphs.

Salles, Bradley↗

NASA NTRS1992

Rigid body mode identification of the PAH-2 helicopter using the eigensystem realization algorithm

The rigid body modes of the PAH-2 'Tiger' helicopter were identified using the Eigensystem Realization Algorithm (ERA). This work complements ground vibration tests performed using DLR's traditional phase resonance technique and the ISSPA (Identification of Structural System Parameters) method. Rigid body modal parameters are important for ground resonance prediction. Time-domain data for ERA were obtained by inverse Fourier transformation of frequency response functions measured with stepped-sine excitation. Mode purity (based on the Phase Resonance Criterion) was generally equal to or greater than corresponding results obtained in the ground vibration tests. All identified natural frequencies and mode shapes correlate well with corresponding ground vibration test results. The modal identification approach discussed in this report has become increasingly attractive in recent years due to the steadily declining cost and increased performance of scientific computers. As illustrated in this application, modern time-domain methods can be successfully applied to data acquired using DLR's existing test equipment. Some suggestions are made for future applications of time domain modal identification in this manner.

Schenk, Axel↗

NASA NTRS1993

Rigid fibrous ceramics for entry systems

The topics addressed are: (1) high payoff areas with reusable surface insulation; (2) technology opportunities/gap; (3) coatings for rigid fibrous ceramics; (4) challenges for reusable rigid fibrous ceramics - Lunar/Mars aerobraking heatshield; (5) comparison of LI-900 and HTP properties; and (6) comparison of microstructures.

Banas, Ronald P.↗

NASA NTRS1993

Attitude stabilization of a rigid spacecraft using two momentum wheel actuators

It is well known that three momentum wheel actuators can be used to control the attitude of a rigid spacecraft and that arbitrary reorientation maneuvers of the spacecraft can be accomplished using smooth feedback. If failure of one of the momentum wheel actuators occurs, it is demonstrated that two momentum wheel actuators can be used to control the attitude of a rigid spacecraft and that arbitrary reorientation maneuvers of the spacecraft can be accomplished. Although the complete spacecraft equations are not controllable, the spacecraft equations are small time locally controllable in a reduced nonlinear sense. The reduced spacecraft dynamics cannot be asymptotically stabilized to any equilibrium attitude using a time-variant continuous feedback control law, but discontinuous feedback control strategies are constructed which stabilize any equilibrium attitude of the spacecraft in finite time. Consequently, reorientation of the spacecraft can be accomplished using discontinuous feedback control.

Krishnan, Hariharan↗

NASA NTRS1991

Rigid lightweight optical bench for a spaceborne FUV spatial heterodyne interferometer

Requirements for spaceflight optical instruments usually dictate that for the structures be rigid, lightweight, and thermally stable. In addition, for interferometric far ultraviolet (FUV) spectrometers, the requirements for torsional deflection are more severe than with conventional spectrometers. To meet the challenge for rigid and lightweight optical instruments, this paper explores the design of a high-stiffness structure for the support of an FUV spatial heterodyne interferometer where the torsional deflection of the instrument is on the order of 10 arc seconds. The structure is based on use of a thin, hollow section beam with weight-relieving between optical elements. The design also uses a modular and self-contained positioning mechanism that is removed after final optical alignment. Several specific material properties are presented as criteria for material selection. The parameters which affect the particular design requirements are identified with respect to the desired material properties and physical design features. Although large thin sections are susceptible to thermal gradients, this could be minimized by a trade-off for weight, where adequate margin exists. This paper describes the preliminary design for the structure and presents an analysis to verify compliance with the requirements.

Tom, James L.↗

NASA NTRS1993

The problem of exact interior solutions for rotating rigid bodies in general relativity

The (3 + 1) dyadic formalism for timelike congruences is applied to derive interior solutions for stationary, axisymmetric, rigidly rotating bodies. In this approach the mathematics is formulated in terms of three-space-covariant, first-order, vector-dyadic, differential equations for a and Omega, the acceleration and angular velocity three-vectors of the rigid body; for T, the stress dyadic of the matter; and for A and B, the 'electric' and 'magnetic' Weyl curvature dyadics which describe the gravitational field. It is shown how an appropriate ansatz for the forms of these dyadics can be used to discover exact rotating interior solutions such as the perfect fluid solution first published in 1968. By incorporating anisotropic stresses, a generalization is found of that previous solution and, in addition, a very simple new solution that can only exist in toroidal configurations.

Wahlquist, H. D.↗

NASA NTRS1990

Cosmic ray proton spectra at low rigidities

The cosmic ray proton rigidity spectra have been investigated with data collected in the Low Energy Antiproton (LEAP) balloon flight experiment flown from Prince Albert, Canada in 1987. The LEAP apparatus was designed to measure antiprotons using a superconducting magnet spectrometer with ancillary scintillator, time-of-flight, and liquid Cherenkov detectors. After reaching float altitude the balloon drifted south and west to higher geomagnetic cutoffs. The effect of the changing geomagnetic cutoff on the observed spectra was observed during analysis of the proton data along the balloon trajectory. This is the first measurement of the primary and splash albedo spectra over a wide rigidity range (few hundred MV to about 100 GV) with a single instrument.

Seo, E. S.↗

NASA NTRS1948

Compressive buckling of flat rectangular plates supported by rigid posts

Results of a theoretical investigation of the compressive buckling of flat, rectangular, simply supported plates supported in the interior by equally spaced rows of rigid posts are presented. It is found that the plates buckle as if simply supported along all of the transverse lines or else all of the longitudinal lines passing through the rigid posts, the occurrence of the one buckling mode or the other depending on the number and spacing of the posts.

LOADS AND STRESSES, STRUCTURAL - COMPRESSION↗

NASA NTRS1929

Torsional rigidity of cantilever wings with constant spar and rib sections

The present paper treats less of the effect of the union (of spars and ribs, namely, the reduction of the bending moment at the fixed ends of the spars) than of its influence on the torsional rigidity of the wing. The calculations are carried out for a two-spar wing of constant cross section, in which the ribs are replaced by a continuous member of constant rigidity.

WINGS, TWO-SPAR - TORSION↗

NASA NTRS1931

Temperature Coefficient of the Modulus of Rigidity of Aircraft Instrument Diaphragm and Spring Materials

Experimental data are presented on the variation of the modulus of rigidity in the temperature range -20 to +50 degrees C. of a number of metals which are of possible use for elastic elements for aircraft and other instruments. The methods of the torsional pendulum was used to determine the modulus of rigidity and its temperature coefficient for aluminum, duralumin, monel metal, brass, phosphor bronze, coin silver, nickel silver, three high carbon steels, and three alloy steels. It was observed that tensile stress affected the values of the modulus by amounts of 1 per cent or less.

Brombacher, W G↗

NASA NTRS1999

Transonic Dynamics Tunnel Force and Pressure Data Acquired on the HSR Rigid Semispan Model

This report describes the aerodynamic data acquired on the High Speed Research Rigid Semispan Model (HSR-RSM) during NASA Langley Transonic Dynamics Tunnel (TDT) Test 520 conducted from 18 March to 4 April, 1996. The purpose of this test was to assess the aerodynamic character of a rigid high speed civil transport wing. The wing was fitted with a single trailing edge control surface which was both steadily deflected and oscillated during the test to investigate the response of the aerodynamic data to steady and unsteady control motion. Angle-of-attack and control surface deflection polars at subsonic, transonic and low-supersonic Mach numbers were obtained in the tunnel?s heavy gas configuration. Unsteady pressure and steady loads data were acquired on the wing, while steady pressures were measured on the fuselage. These data were reduced using a variety of methods, programs and computer systems. The reduced data was ultimately compiled onto a CD-ROM volume which was distributed to HSR industry team members in July, 1996. This report documents the methods used to acquire and reduce the data, and provides an assessment of the quality, repeatability, and overall character of the aerodynamic data measured during this test.

Schuster, David M.↗

NASA NTRS2001

Structural Modeling of a Five-Meter Thin Film Inflatable Antenna/Concentrator With Rigidized Support Struts

Dynamic characterization of a non-rigidized thin film inflatable antenna/solar concentrator structure with rigidized composite support struts is described in detail. A two-step finite element modeling approach in MSC/NASTRAN is utilized, consisting of: (1) a nonlinear static pressurization procedure used to obtain the updated stiffness matrix, and (2) a modal "restart" eigen solution that uses the modified stiffness matrix. Unique problems encountered in modeling of this large 5-m lightweight inflatable are identified, including considerable difficulty in obtaining convergence in the nonlinear pressurization solution. It was found that the extremely thin polyimide film material (.001 in or I mil) presents tremendous problems in obtaining a converged solution when internal pressure loading is applied. It was concluded that the ratios of film thickness to other geometric dimensions such as torus cross-sectional and ring diameter and lenticular diameter are the critical parameters for convergence of the pressurization procedure. Comparison of finite element predictions for frequency and mode shapes with experimental results indicated reasonable agreement considering the complexity of the structure, the film-to-air interaction, and the nonlinear material properties of the film. It was also concluded that analysis should be done using different finite element to codes to determine if a more robust and stable solution can be obtained.

Smalley, Kurt B.↗

NASA NTRS1995

A Network Model for the Effective Thermal Conductivity of Rigid Fibrous Refractory Insulations

A procedure is described for computing the effective thermal conductivity of a rigid fibrous refractory insulation. The insulation is modeled as a 3-dimensional Cartesian network of thermal conductance. The values and volume distributions of the conductance are assigned to reflect the physical properties of the insulation, its constituent fibers, and any permeating gas. The effective thermal conductivity is computed by considering the simultaneous energy transport by solid conduction, gas conduction and radiation through a cubic volume of model insulation; thus the coupling between heat transfer modes is retained (within the simplifications inherent to the model), rather than suppressed by treating these heat transfer modes as independent. The model takes into account insulation composition, density and fiber anisotropy, as well as the geometric and material properties of the constituent fibers. A relatively good agreement, between calculated and experimentally derived thermal conductivity values, is obtained for a variety of rigid fibrous insulations.

Marschall, Jochen↗

NASA NTRS1976

Rigidized Metal Panels

Plasteel Products Corporation approached the Industrial Applications Center at the University of Pittsburgh to search for ideas on "rigidized" metal sheeting. Thinner-gage metal with strength of heavier sheeting can be obtained through cold bending, stamping, dimpling, or embossing - referred to as "rigidizing." Pittsburgh came up with 36 reports relevant to Plasteel's needs. On the basis of this search the company purchased an embossing machine with which it hopes to provide thinner gage metal panels where permitted by building codes.

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