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At least 307 records · Page 17

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NASA NTRS1976

Real eigenvalue analysis in NASTRAN by the tridiagonal reduction (FEER) method

Implementation of the tridiagonal reduction method for real eigenvalue extraction in structural vibration and buckling problems is described. The basic concepts underlying the method are summarized and special features, such as the computation of error bounds and default modes of operation are discussed. In addition, the new user information and error messages and optional diagnostic output relating to the tridiagonal reduction method are presented. Some numerical results and initial experiences relating to usage in the NASTRAN environment are provided, including comparisons with other existing NASTRAN eigenvalue methods.

Newman, M.↗

NASA NTRS1977

FLUT - A program for aeroelastic stability analysis

A computer program (FLUT) that can be used to evaluate the aeroelastic stability of aircraft structures in subsonic flow is described. The algorithm synthesizes data from a structural vibration analysis with an unsteady aerodynamics analysis and then performs a complex eigenvalue analysis to assess the system stability. The theoretical basis of the program is discussed with special emphasis placed on some innovative techniques which improve the efficiency of the analysis. User information needed to efficiently and successfully utilize the program is provided. In addition to identifying the required input, the flow of the program execution and some possible sources of difficulty are included. The use of the program is demonstrated with a listing of the input and output for a simple example.

Johnson, E. H.↗

NASA NTRS1978

Effects of entrained water and strong turbulence on afterburning within solid rocket motor plumes

During the first few seconds of the space shuttle trajectory, the solid rocket boosters will be in the proximity of the launch pad. Because of the launch pad structures and the surface of the earth, the turbulent mixing experienced by the exhaust gases will be greatly increased over that for the free flight situation. In addition, a system will be present, designed to protect the lifting vehicle from launch structure vibrations, which will inject quantities of liquid water into the hot plume. The effects of these two phenomena on the temperatures, chemical composition, and flow field present in the afterburning solid rocket motor exhaust plumes of the space shuttle were studied. Results are included from both a computational model of the afterburning and supporting measurements from Titan 3 exhaust plumes taken at Kennedy Space Center with infrared scanned radiometers.

Gomberg, R. I.↗

NASA NTRS1977

Torque-while-turnaround scan mirror assembly

A scan mirror assembly which is part of a thematic mapper system is described with emphasis on mechanical aspects of the design. Features of the oscillating scan mirror mechanism include: a low level of structural vibration for the impact energies involved in mirror oscillation and return of energy lost during impact to the mirror by applying torque during the instant of impact.

Starkus, C. J.↗

NASA NTRS1978

Entry flight control system downmoding evaluation

A method to desensitize the entry flight control system to structural vibration feedback which might induce an oscillatory instability is described. Trends in vehicle response and handling characteristics as a function of gain combinations in the FCS forward and rate feedback loops were described as observed in a man-in-the-loop simulation. Among the flight conditions considered are the effects of downmoding with APU failures, off-nominal trajectory conditions, sensed angle of attack errors, the impact on RCS fuel consumption, performance in the presence of aero variations, recovery from large FCS upsets, and default gains.

Barnes, H. A.↗

NASA NTRS1977

Torque-while-turnaround scan mirror assembly

Mechanical aspects in the development of an oscillating scan mirror mechanism were reviewed, that featured a remarkably low level of structural vibration for the impact energies involved in mirror oscillation. Another feature was that energy lost during impact was returned to the mirror by applying torque only during the instant of impact. Because the duration of impact was only about 0.010 second, it was critical that energy losses be minimal because there was not much time to restore them.

Starkus, C. J.↗

NASA NTRS1979

Spectral emission of nuclear excited XeBr*

The vacuum-ultraviolet spectroscopy system and MCFIG capsule were filled with a He-3:Ar:Xe:Br (534:195:12:6) torr mixture and irradiatied by a CW flux of 17 x 10 to the 11th power neutrons/sq cm sec. The XeBr* emission was observed, but the Br2 emission normally observed in argon-bromine mixtures predominates. This spectrum is typical of these mixtures with XeBr* probably peaking with 0.1 percent Bromine in 2 atmosphere argon with 2 percent xenon. The addition of a large partial pressure of He-3 produces additional unknowns. The typical vibrational structure of XeBr* was not observed. Sapphire emission continuums were present. Energy deposition was 0.43 watts and the capsule output under the XeBr* and Br2 bands was 0.0016 watts, giving an efficiency of 0.37 percent.

Walters, R. A.↗

NASA NTRS1980

OH/A 2 Sigma + - X 2 Pi sub i/ yield from H2O photodissociation in 1050-1370 A

The cross section and quantum yield for the production of the OH(A 2 Sigma + - X 2 Pi sub i) emission from H2O photodissociation by synchrotron radiation in the region 1050-1370 A are determined. Measurements of the photoemission and photoabsorption cross sections obtained for incident radiation bandwidths of 2 A reveal different vibrational structures in the range 1250-1370 A, and the photoemission is attributed to the 3a1-4a1 valence and 3a1-3sa1 Rydberg transitions. Photoemission quantum yields determined from the ratio of photoemission to photoabsorption cross sections are found to increase sharply at a threshold of 1362-5 A to a peak of 11% at 1300 A, then to decrease slowly with decreasing wavelength. Implications of the results for the dissociation processes of H2O are considered.

Lee, L. C.↗

NASA NTRS1981

Evaluation of emergency-locator-transmitter performance in real and simulated crash tests

Emergency locator transmitter (ELT) activation problems were investigated by testing a sampling of ELT units in actual airplane crashes and in a special test apparatus which simulated longitudinal crash pulses with superimposed local structural resonances. Probable causes of excessive false alarms and nonactivations of ELT's during crash situations were determined and solutions to the current operational and technical problems were obtained. The results, which considered placement, mounting, and activation of ELT's under simulated crash impacts, and an evaluation of the sensitivity of ELT impact switches to orientation and to local structural vibrations are discussed.

Carden, H. D.↗

NASA NTRS1981

Evaluation of Emergency-Locator-Transmitter performance in real and simulated crash tests

Emergency locator transmitter (ELT) activation problems were investigated by testing a sampling of ELT units in actual crashes and in a special test apparatus which simulated longitudinal crash pulses with superimposed local structural resonances. The probable causes of excessive false alarms and nonactivation of ELT's during crash situations were determined. Solutions to operational and technical problems were also examined as well as the sensitivity of ELT impact switches to orientation and to local structural vibrations.

Carden, H. D.↗

NASA NTRS1981

Peculiarities of spectra of induced emission of polymethine dye solutions at low temperatures

Results from studying the induced emission spectra with monochromatic excitation (superluminescent regime) within a temperature range of 300 K - 5.2 K are presented. The significant feature of such operations is the appearance of quasi-lines whose minimum width is realized at 4.2 K. The frequency distribution of the quasi-lines unambiguously characterizes the dye investigated. When interpreting these results, the idea of an inhomogeneous broadening of the polymethine dye spectra in the solution being studied experimentally was used for the first time. The quasi-lines observed are interpreted as a manifestation of the electron vibration structure of singlet states of a complex molecule.

Melishchuk, M. V.↗

NASA NTRS1981

The algebraic theory of latent projectors in lambda matrices

Multivariable systems such as a finite-element model of vibrating structures, control systems, and large-scale systems are often formulated in terms of differential equations which give rise to lambda matrices. The present investigation is concerned with the formulation of the algebraic theory of lambda matrices and the relationship of latent roots, latent vectors, and latent projectors to the eigenvalues, eigenvectors, and eigenprojectors of the companion form. The chain rule for latent projectors and eigenprojectors for the repeated latent root or eigenvalues is given.

Denman, E. D.↗

NASA NTRS1983

Fluorescence yield from photodissociation of CH4 at 1060-1420 A

The photoabsorption and fluorescence cross sections of CH4 were measured using synchrotron radiation at 1060-1420 A. The fluorescence starts to appear at 1330 A, and its cross section increases with decreasing photon wavelengths. The fluorescence cross section at 1060 A is 0.38 x 10 to the -18th sq cm, which is much smaller than the theoretical expectation. It is unlikely that the CH2 production from CH4 photodissociation will have a population inversion in the singlet system. A vibrational structure observed at 1060-1120 A is probably the 1t2-3p Rydberg transition of CH4.

Lee, L. C.↗

NASA NTRS1983

Electron impact excitation from a 1 Delta g state of molecular oxygen

First measurements of electron impact excitation on the O2 a delta-one sub g metastable state to a higher bound state, pi-one sub u, is reported. The data were taken at an incident electron energy of 500 eV and scattering angles of 0 deg to 15 deg. A superelastic electron scattering experiment was performed on the pure oxygen target gas in order to establish the composition of the discharged oxygen and to confirm the existence of delta-one sub g molecules in the interaction region. The electron-impact energy loss spectrum obtained is shown. The electronic assignment of the upper state which produces the vibrational structure was determined to be pi-one sub u by examining the potential energy curves of O2 and applying the electric dipole selection rules. The relative line intensities were obtained and used to determine the Morse potential for the pi-one sub u state.

Khakoo, M. A.↗

NASA NTRS1984

Research opportunities for rotorcraft

The control of structural vibration is helicopters is discussed. Active control concepts, wind tunnel tests, rotor vibratory loads, systems integration, and engine control are among the topics discussed.

Blake, B. B.↗

NASA NTRS1985

Multishaker Modal Testing

Report prepared for Marshall Space Flight Center summarizes recent advances in mathematical analysis of structural vibrations based on experimental data. Also includes summary of research by author as well as abstracts of technical reports and papers presented at meetings or otherwise published.

Graig, R. R., Jr.↗

NASA NTRS1985

Finite Element Prediction of Loss Factors for Structures with Frequency-dependent Damping Treatments

A finite element procedure is described for calculating the loss factors for elastic structures to which frequency-dependent viscoelastic damping treatments were applied. The frequency dependence of the viscoelastic damping material is treated by approximating its shear modulus with a second-order polynomial so that the stiffnesses associated with the constant, linear, and quadratic terms can be combined, respectively, with the stiffness, damping, and mass matrices assembled for the rest of the structure. A single complex eigenvalue analysis is then performed in which the eigenvalues are purely imaginary. The loss factor is computed by the modal strain energy (MSE) approach. In the the MSE approach, the loss factor of a composite structure vibrating in one of its natural modes may be visualized as a weighted average of the loss factors of the component parts, with the relative stored energies as weighting constants. The finite element procedure, which can treat very general geometries, is illustrated for the case of a vibrating constrained-layer damped plate.

Everstine, G. C.↗

NASA NTRS1985

Rationale for an experimental test for flexible space structure attitude control

The problems of large flexible spacecraft control are characterized by the infinite bandwidth of structural vibrations, which cannot be accounted for in the dynamic design model. This may lead to instability even, if ideal control hardware is assumed, which can be concluded from preceding numerical investigations. Additional performance limitations are expected to occur due to hardware constraints. A laboratory experiment is proposed to investigate the key problems in more detail. The test setup requirements being defined by the idealized control system are extremely high demanding a high speed processor and special hardware component developments. The test element is a wire suspended plate being controlled by an array processor via high performance sensors and actuators. First tests on component level indicate the feasibility of the system presently being developed.

Lange, T.↗