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NASA NTRS1987

The auroral kilometric radiation from Uranus and its magnetospheric implications

The Gulkis and Carr (1987) emission beam model for the kilometric radio emission from Uranus and types of information regarding the Uranian inner magnetosphere that may be derivable from its application are discussed. A modeled electron density profile is presented. The manner in which the hollow-cone beam from each element of the distributed auroral-zone source varies with frequency is determined. It is suggested that the observed effects might be due to refraction by a smooth component of the electron density variation with distance, together with multiple small refractions by electron density inhomogeneities along the ray path.

Carr, Thomas D.↗

NASA NTRS1998

A Feasibility Study of Synthesizing Subsurfaces Modeled with Computational Neural Networks

This paper investigates the feasibility of synthesizing substructures modeled with computational neural networks. Substructures are modeled individually with computational neural networks and the response of the assembled structure is predicted by synthesizing the neural networks. A superposition approach is applied to synthesize models for statically determinate substructures while an interface displacement collocation approach is used to synthesize statically indeterminate substructure models. Beam and plate substructures along with components of a complicated Next Generation Space Telescope (NGST) model are used in this feasibility study. In this paper, the limitations and difficulties of synthesizing substructures modeled with neural networks are also discussed.

Wang, John T.↗

NASA NTRS1988

Preliminary investigation of stability of a fin-stiffened slender strut

A fin-stiffened strut concept which possesses high bending stiffness and low packaged volume has been identified for use on large space structures. The concept incorporates three curved fins which deploy from a core tube to increase the effective cross-sectional moment of inertia, the buckling load, and the vibration frequency of the strut. A strut design incorporating welded fin connections provides an upper bound of the strut buckling load. A strut design which allows the individual fins and core to move independently in the strut axial direction provides a lower bound of the strut buckling load which is approximately 20 to 25 percent of the upper bound. A practical strut design, incorporating hinge assemblies which constrain the fins to move together but independently of the core tube in the strut axial direction, provides a buckling load which is 75 to 80 percent of the upper bound. Euler's equation can be used to accurately predict buckling loads for the bounding designs. Flat plate finite element models of all designs give results that agree to within 10 percent of the experimental values. Equivalent beam models of the strut give results which are slightly less accurate.

Lake, Mark S.↗

NASA NTRS1992

Neptune's non-thermal radio emissions - Phenomenology and source locations

During the inbound and the outbound leg of Voyager 2's encounter with Neptune, the Planetary Radio Astronomy (PRA) experiment aboard the spacecraft detected short radio bursts at frequencies within the range of about 500-1300 kHz, and broad-banded smoothly varying emission patterns within the frequency range from about 40-800 kHz. Both emissions can be described in terms of a period of 16.1 hours determining Neptune's rotation period. Furthermore, just near closest approach, a narrow-banded smoothly varying radio component was observed occurring between 600 and 800 kHz. After giving a brief overview about some general characteristics of Neptune's nonthermal radio emission, the source locations of Neptune's emission components are determined, using an offset tilted dipole model for Neptune's magnetic field. Assuming that the emission originates near the electron gyrofrequency a geometrical beaming model is developed in order to fit the observed emission episodes.

Rabl, Gerald K. F.↗

NASA NTRS1987

Structural modeling for multicell composite rotor blades

Composite material systems are currently good candidates for aerospace structures, primarily for the design flexibility they offer, i.e., it is possible to tailor the material and manufacturing approach to the application. A working definition of elastic or structural tailoring is the use of structural concept, fiber orientation, ply stacking sequence, and a blend of materials to achieve specific performance goals. In the design process, choices of materials and dimensions are made which produce specific response characteristics, and which permit the selected goals to be achieved. Common choices for tailoring goals are preventing instabilities or vibration resonances or enhancing damage tolerance. An essential, enabling factor in the design of tailored composite structures is structural modeling that accurately, but simply, characterizes response. The objective of this paper is to present a new multicell beam model for composite rotor blades and to validate predictions based on the new model by comparison with a finite element simulation in three benchmark static load cases.

Rehfield, Lawrence W.↗

NASA NTRS1987

A structural model for composite rotor blades and lifting surfaces

Composite material systems are currently candidates for aerospace structures, primarily for the design flexibiity they offer i.e., it is possible to tailor the material and manufacturing approach to the application. Two notable examples are the wing of the Grumman/USAF/DARPA X-29 and rotor blades under development by the U.S.A. Aerostructures Directorate (AVSCOM), Langley Research Center. A working definition of elastic or structural tailoring is the use of structural concept, fiber orientation, ply stacking sequence, and a blend of materials to achieve specific performance goals. In the design process, choices of materials and dimensions are made which produce specific response characteristics which permit the selected goals to be achieved. Common choices for tailoring goals are preventing instabilities or vibration resonances or enhancing damage tolerance. An essential, enabling factor in the design of tailored composite structures is structural modeling that accurately, but simply, characterizes response. The objective of this paper is to improve the single-cell beam model for composite rotor blades or lifting surfaces and to demonstrate its usefullness in applications.

Rehfield, Lawrence W.↗

NASA NTRS1983

Acoustic responses of coupled fluid-structure system by acoustic-structural analogy

The use of an analogy between structural mechanics and acoustics makes it possible to solve fluid-structural interaction (FSI) problems using an existing structural analysis computer program. This method was implemented in MSC/NASTRAN program and the FSI analysis was performed using two dimensional coupled fluid beam model to assess and evaluate the adequacy of this approach. The coupled modal analysis of 3-D model is also briefly discussed. The normal mode, modal frequency response and transient response analysis of 2-D coupled fluid beam system is presented. The significant reduction of the acoustic pressure response at the fluid structure interface is observed as a result of fluid structure interaction.

Shin, Y. S.↗

NASA NTRS1991

Distributed parameter estimation for NASA Mini-Mast truss through displacement measurements

Most methods of system identification of large flexible structures by far are based on the lumped parameter approach. Because of the considerable computational burden due to the large number of unknown parameters, distributed parameter approach, which greatly decreases the number of unknowns, has being investigated. In this paper a distributed parameter model for the estimation of modal characteristics of NASA Mini-Mast truss has been formulated. Both Bernoulli-Euler beam and Timoshenko beam equations are used to characterize the lateral bending vibrations of the truss. The measurement of the lateral displacement at the tip of the truss is provided to the maximum likelihood estimator. Closed-form solutions of the partial differential equations and closed-form expressions of the sensitivity functions are derived so that the estimation algorithm is highly efficient. The resulting estimates from test data by using Timoshenko beam model are found to be comparable to those derived from finite element analysis.

Huang, Jen-Kuang↗

NASA NTRS1999

Multi-Body Analysis of the 1/5 Scale Wind Tunnel Model of the V-22 Tiltrotor

The paper presents a multi-body analysis of the 1/5 scale wind tunnel model of the V-22 tiltrotor, the Wing and Rotor Aeroelastic Testing System (WRATS), currently tested at NASA Langley Research Center. An original multi-body formulation has been developed at the Dipartimento di Ingegneria Aerospaziale of the Politecnico di Milano, Italy. It is based on the direct writing of the equilibrium equations of independent rigid bodies, connected by kinematic constraints that result in the addition of algebraic constraint equations, and by dynamic constraints, that directly contribute to the equilibrium equations. The formulation has been extended to the simultaneous solution of interdisciplinary problems by modeling electric and hydraulic networks, for aeroservoelastic problems. The code has been tailored to the modeling of rotorcrafts while preserving a complete generality. A family of aerodynamic elements has been introduced to model high aspect aerodynamic surfaces, based on the strip theory, with quasi-steady aerodynamic coefficients, compressibility, post-stall interpolation of experimental data, dynamic stall modeling, and radial flow drag. Different models for the induced velocity of the rotor can be used, from uniform velocity to dynamic in flow. A complete dynamic and aeroelastic analysis of the model of the V-22 tiltrotor has been performed, to assess the validity of the formulation and to exploit the unique features of multi-body analysis with respect to conventional comprehensive rotorcraft codes; These are the ability to model the exact kinematics of mechanical systems, and the possibility to simulate unusual maneuvers and unusual flight conditions, that are particular to the tiltrotor, e.g. the conversion maneuver. A complete modal validation of the analytical model has been performed, to assess the ability to reproduce the correct dynamics of the system with a relatively coarse beam model of the semispan wing, pylon and rotor. Particular care has been used to model the kinematics of the gimbal joint, that characterizes the rotor hub, and of the control system, consisting in the entire swashplate mechanism. The kinematics of the fixed and the rotating plates have been modeled, with variable length control links used to input the controls, the rotating flexible links, the pitch horns and the pitch bearings. The investigations took advantage of concurring wind tunnel test runs, that were performed in August 1998, and allowed the acquisition of data specific to the multi-body analysis.

Ghiringhelli, G. L.↗

NASA NTRSDate not supplied

Study of Trailing Edge Mini Flaps Interactions on a Wind Tunnel Scale Aspect Ratio 13.5 Common Research Model

This paper presents a computation study of an aspect ratio 13.5 wind tunnel scale Common Research Model (CRM) with trailing edge mini-flaps. A vortex-lattice model of the CRM is developed with transonic small disturbance and integral boundary-layer corrections coupled with NASTRAN equivalent beam model for the rapid CRM aeroelastic simulation. The simulations are performed with individual trailing edge mini-flap deflected and a series of groups of two adjacent trailing edge mini-flaps deflected at the same time. A surrogate model is developed to estimate the aerodynamic interactions between the flaps. To validate the surrogate model, static aeroelastic simulations are performed using FUN3D CFD solver.

Interaction↗

NASA NTRS1984

Fast variations in high-energy X-rays from solar flares and their constraints on nonthermal models

The results of a quantitatve analysis of quantitative the temporal properties of over 7000 hard X-ray solar flares are presented. The data were obtained with the Hard X-ray Burst Spectrometer of the SMM satellite. The data reveal examples of hundreds of fast spikes with durations of less than 1 s. The spectra of a well-observed fast spike were compared with the predicted spectra of a detailed nonthermal electron beam model, in order to determine upper limits for electron beam parameters and path lengths. The theoretical spectra were found to be consistent with the observed spectra when the separation of between the accretion site and the thick target was greater than 10 to the 8th cm. It is shown that if the separation is much greater than 3 x 10 to the 9th cm, the electrons with high pitch angles (greater than 60 deg) are either not emitted or are mirrored before they reach the footpoints.

Kiplinger, A. L.↗

NASA NTRS1990

Bremsstrahlung X-rays from Jovian auroral electrons

The spectrum of X-rays from the planet Jupiter is calculated according to an auroral electron beam model. The electrons are assumed to be accelerated by a field-aligned potential drop and penetrate into the atmosphere as a Maxwellian beam of primaries which are scattered, degraded in energy, and merged with a population of ionization secondaries having a power law energy distribution. The soft X-rays observed by the Einstein Observatory satellite are due to bremsstrahlung from the secondary electrons in the H2 atmosphere. The X-ray intensity is best reconciled with a beam of primaries having a characteristic energy 30-100 keV and penetrating the homopause with an auroral energy flux typically of 10-20 ergs/sq cm s but no greater than 50 ergs/sq cm s.

Barbosa, D. D.↗

NASA NTRS1986

Effects of warm streaming electrons on electrostatic shock solutions

A plasma model consisting of warm downward streaming electrons, cool Boltzmann electrons, cold upward streaming ions, and hot Boltzmann ions is considered. It is shown that there is little difference between having a finite temperature in the electron beam and leaving the beam cold. This implies a greater utility for a cold electron beam model, and adds weight to the idea that the smaller amplitude electrostatic shocks seen in conjunction with ion beams are nonlinear ion acoustic modes.

Witt, Earl↗

NASA NTRS1989

A comparison of two trusses for the space station structure

The structural performance of two truss configurations, the orthogonal tetrahedral and a Warren-type, are compared using finite element models representing the November Reference Phase 1 Space Station. The truss torsional stiffness properties and fundamental torsion frequency are determined using cantilever truss-beam models. Frequencies, mode shapes, transient response, and truss strut compressive loads are compared for the two space station models. The performance benefit resulting from using a high modulus truss strut is also presented. Finally, assembly and logistics characteristics of the two truss configurations are evaluated.

Sutter, Thomas R.↗

NASA NTRS1987

Optimization procedure to control the coupling of vibration modes in flexible space structures

As spacecraft structural concepts increase in size and flexibility, the vibration frequencies become more closely-spaced. The identification and control of such closely-spaced frequencies present a significant challenge. To validate system identification and control methods prior to actual flight, simpler space structures will be flown. To challenge the above technologies, it will be necessary to design these structures with closely-spaced or coupled vibration modes. Thus, there exists a need to develop a systematic method to design a structure which has closely-spaced vibration frequencies. This paper describes an optimization procedure which is used to design a large flexible structure to have closely-spaced vibration frequencies. The procedure uses a general-purpose finite element analysis program for the vibration and sensitivity analyses and a general-purpose optimization program. Results are presented from two studies. The first study uses a detailed model of a large flexible structure to design a structure with one pair of closely-spaced frequencies. The second study uses a simple equivalent beam model of a large flexible structure to obtain a design with two pairs of closely-spaced frequencies.

Walsh, Joanne L.↗

NASA NTRS1987

Optimization procedure to control the coupling of vibration modes in flexible space structures

As spacecraft structural concepts increase in size and flexibility, the vibration frequencies become more closely-spaced. The identification and control of such closely-spaced frequencies present a significant challenge. To validate system identification and control methods prior to actual flight, simpler space structures will be flown. To challenge the above technologies it will be necessary to design these structures with closely-spaced or coupled vibration modes. Thus there exists a need to develop a systematic method to design a structure which has closely-spaced vibration frequencies. This paper describes an optimization procedure which is used to design a large flexible structure to have closely-spaced vibration frequencies. The procedure uses a general-purpose finite-element analysis program for the vibration and sensitivity analyses and a general-purpose optimization program. Results are presented from two studies. The first study uses a detailed model of a large flexible structure to design a structure with one pair of closely-spaced frequencies. The second study uses a simple equivalent beam model of a large flexible structure to obtain a design with two pairs of closely-spaced frequencies.

Walsh, Joanne L.↗

NASA NTRS1991

Modes of interconnected lattice trusses using continuum models, part 1

This represents a continuing systematic attempt to explore the use of continuum models--in contrast to the Finite Element Models currently universally in use--to develop feedback control laws for stability enhancement of structures, particularly large structures, for deployment in space. We shall show that for the control objective, continuum models do offer unique advantages. It must be admitted of course that developing continuum models for arbitrary structures is no easy task. In this paper we take advantage of the special nature of current Large Space Structures--typified by the NASA-LaRC Evolutionary Model which will be our main concern--which consists of interconnected orthogonal lattice trusses each with identical bays. Using an equivalent one-dimensional Timoshenko beam model, we develop an almost complete continuum model for the evolutionary structure. We do this in stages, beginning only with the main bus as flexible and then going on to make all the appendages also flexible-except for the antenna structure. Based on these models we proceed to develop formulas for mode frequencies and shapes. These are shown to be the roots of the determinant of a matrix of small dimension compared with mode calculations using Finite Element Models, even though the matrix involves transcendental functions. The formulas allow us to study asymptotic properties of the modes and how they evolve as we increase the number of bodies which are treated as flexible. The asymptotics, in fact, become simpler.

Balakrishnan, A. V.↗

NASA NTRS1984

Modelling issues of the SCOLE control problem

Nonlinear and linear equations of motions were derived. The preliminary investigation consisted of model beam as truss structure, effects of truss structure on control design, and effects of reflector offset on control design. It was concluded that the offset of the reflector c.g. from the beam reflector attach point is dynamically significant. Also, truss effects may also significantly effect the performance of the controller if ignored. If the truss is included in the modeling of the NASA/SCOLE configuration, a practically implementable scheme is available to reduce the model order.

Hotz, A. F.↗