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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 343 records · Page 19

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NASA NTRS2001

Dynamic Characterization of an Inflatable Concentrator for Solar Thermal Propulsion

An inflatable structural system that is a technology demonstrator for solar thermal propulsion and other applications is characterized for structural dynamic behavior both experimentally and computationally. The inflatable structure is a pressurized assembly developed for use in orbit to support a Fresnel lens or inflatable lenticular element for focusing sunlight into a solar thermal rocket engine. When the engine temperature reaches a pre-set level, the propellant is injected into the engine, absorbs heat from an exchanger, and is expanded through the nozzle to produce thrust. The inflatable structure is a passively adaptive system in that a regulator and relief valve are utilized to maintain pressure within design limits during the full range of orbital conditions. Modeling and test activities are complicated by the fact that the polyimide film material used for construction of the inflatable is nonlinear, with modulus varying as a function of frequency, temperature, and level of excitation. Modal vibration testing and finite element modeling are described in detail in this paper. The test database is used for validation and modification of the model. This work is highly significant because of the current interest in inflatable structures for space application, and because of the difficulty in accurately modeling such systems.

Leigh, Larry↗

NASA NTRS1984

Galileo spacecraft modal identification using an eigensystem realization algorithm

A modal parameter identification technique referred to as the Eigensystem Realization Algorithm (ERA) was applied to free-response measurements from the Galileo spacecraft modal survey test. The data were recorded following single-point random excitation of the structure. This work is one phase in a research project coordinated by the Jet Propulsion Laboratory to compare the performance of various contemporary identification techniques using Galileo data. Principal emphasis is placed on estimating the accuracy of the ERA-identified modal parameters. Various accuracy indicators, such as Modal Amplitude Coherence and Modal Phase Collinearity, are discussed. More than 20 modes of the spacecraft were identified, demonstrating the ability of the ERA method to determine the dynamics of such complex structures using only a few seconds of test data.

Pappa, R. S.↗

NASA NTRS1990

Nonlinear Modeling of Joint Dominated Structures

The development and verification of an accurate structural model of the nonlinear joint-dominated NASA Langley Mini-Mast truss are described. The approach is to characterize the structural behavior of the Mini-Mast joints and struts using a test configuration that can directly measure the struts' overall stiffness and damping properties, incorporate this data into the structural model using the residual force technique, and then compare the predicted response with empirical data taken by NASA/LaRC during the modal survey tests of the Mini-Mast. A new testing technique, referred to as 'link' testing, was developed and used to test prototype struts of the Mini-Masts. Appreciable nonlinearities including the free-play and hysteresis were demonstrated. Since static and dynamic tests performed on the Mini-Mast also exhibited behavior consistent with joints having free-play and hysteresis, nonlinear models of the Mini-Mast were constructed and analyzed. The Residual Force Technique was used to analyze the nonlinear model of the Mini-Mast having joint free-play and hysteresis.

Chapman, J. M.↗

NASA NTRS1988

Initial test results on state estimation on the SCOLE mast

Modal state estimation tests are performed on the SCOLE mast for the fixed Shuttle platform case. Kalman filter state estimation results from a five mode computer model of the SCOLE mast, developed from a finite element analysis, are compared with those state estimates obtained from laboratory tests. Two comparison runs are presented, one an excitation of the first two bending modes, another, an excitation of the first torsional mode of the mast. Results from both runs show poor agreement in modal estimation between the computer model simulations and the laboratory test data. At present, the reason(s) for this poor performance is unknown. Both the laboratory hardware and software and the computer model are being checked for possible sources of errors. Further computer simulations as well as laboratory testing will be performed.

Sparks, D., Jr.↗

NASA NTRS1979

An introduction to a unified approach to flexible rotor balancing

Two types of technique for flexible-rotor balancing are examined: the influence coefficient method, and the modal balancing method. Briefly, the influence coefficient method seeks those correction masses in a predetermined set of planes which will minimize measured vibration (readings) at a series of sensors and speeds, as predicted by influence coefficients relating vibration readings to mass additions; the influence coefficients are normally determined by a series of trial mass tests. The modal balancing method seeks to balance the rotor, one mode at a time, with a set of masses specifically selected not to disturb previously balanced lower modes, the sensitivity to this combination of masses is determined empirically by a series of trial mass tests. The two approaches are compared in supercritical-shaft balancing tests, and some common features are stressed for ultimate incorporation into a unified approach toward flexible-rotor balancing.

Parkinson, A. G.↗

NASA NTRS2020

Feasibility Study of SDAS Instrumentation's Ability to Identify Mobile Launcher (ML)/Crawler-Transporter (CT) Modes During Rollout Operations

The Space Launch System (SLS) and its Mobile Launcher (ML) will be transported to the launch pad via the Crawler-Transporter (CT) system. Rollout (i.e., transportation) loads produce structural loads on the integrated SLS/Orion Multi-Purpose Crew Vehicle (MPCV) launch vehicle which are of a concern with respect to fatigue. As part of the risk reduction process and in addition to the modal building block test approach that has been adopted by the SLS Program, acceleration data will be obtained during rollout for use in modal parameter estimation. There are several occurrences where the ML/CT will be transported either into the Vertical Assembly Building (VAB) or to the launch pad and back without the SLS stack as part of the Kennedy Space Center (KSC) Exploration Ground Systems (EGS) Integrated Test and Checkout (ITCO). NASA KSC EGS has instrumentation installed on both the ML and CT to record data during rollout, at the launch pad, and during liftoff. The EGS instrumentation on the ML, which includes accelerometers, is referred to as the Sensor Data Acquisition System (SDAS). The EGS instrumentation on the CT, which also includes accelerometers, is referred to as the CT Data Acquisition System (CTDAS). The forces and accelerations applied to the ML and CT during a rollout event will be higher than any of the planned building block modal tests. This can be very beneficial in helping identify nonlinear behavior in the structure. Developing modal parameters from the same test hardware in multiple boundary conditions and under multiple levels of excitation is a key step in developing a well correlated FEM. The purpose of this study was three fold. First, determine the target modes of the ML/CT in its rollout configuration. Second, determine if the test degrees of freedom (DOF) corresponding to the layout of the SDAS/CTDAS accelerometers (i.e. position and orientation) is sufficient to identify the target modes. Third, determine if the Generic Rollout Forcing Functions (GRFF's) is sufficient for identifying the ML/CT target modes accounting for variations in CT speed, modal damping, and sensor/ambient background noise levels. The finding from the first part of this study identified 28 target modes of the ML/CT rollout configuration based upon Modal Effective Mass Fractions (MEFF) and engineering judgement. The finding from the second part of this study showed that the SDAS/CTDAS accelerometers (i.e. position and orientation) are able to identify a sufficient number of the target modes to support model correlation of the ML/CT FEM. The finding from the third part of this study confirms the GRFFs sufficiently excite the ML/CT such that varying quantities of the defined target modes should be able to be extracted when utilizing an Experimental Modal Analysis (EMA) Multi-Input Multi-Output (MIMO) analysis approach. An EMA analysis approach was used because Operational Modal Analysis (OMA) tools were not available and the GRFFs were sufficiently uncorrelated. Two key findings from this third part of the study are that the CT speed does not show a significant impact on the ability to extract the modal parameters and that keeping the ambient background noise observed at each accelerometer location at or below 30 µgrms is essential to the success of this approach.

Winkel, James P.↗

NASA NTRS1986

Illustration of the use of modal assurance criterion to detect structural changes in an Orbiter test specimen

The analysis of empirical modal models of structural components using techniques based on the modal assurance criterion (MAC) defined by Ewins (1984) is discussed and demonstrated for the case of the Space Shuttle Orbiter aft bulkhead. Modal models of the bulkhead are constructed before and after environmental acoustic tests equivalent to 30 or 100 missions and compared (by computing their MACs) to detect significant structural changes; subdivision of the modal vector followed by computation of the MAC for each part is used to localize the changes. Numerical data demonstrating the usefulness of the MAC approach are presented in tables and diagrams.

West, Walter M.↗

NASA NTRS1998

TACT: A Set of MSC/PATRAN- and MSC/NASTRAN- based Modal Correlation Tools

This paper describes the functionality and demonstrates the utility of the Test Analysis Correlation Tools (TACT), a suite of MSC/PATRAN Command Language (PCL) tools which automate the process of correlating finite element models to modal survey test data. The initial release of TACT provides a basic yet complete set of tools for performing correlation totally inside the PATRAN/NASTRAN environment. Features include a step-by-step menu structure, pre-test accelerometer set evaluation and selection, analysis and test result export/import in Universal File Format, calculation of frequency percent difference and cross-orthogonality correlation results using NASTRAN, creation and manipulation of mode pairs, and five different ways of viewing synchronized animations of analysis and test modal results. For the PATRAN-based analyst, TACT eliminates the repetitive, time-consuming and error-prone steps associated with transferring finite element data to a third-party modal correlation package, which allows the analyst to spend more time on the more challenging task of model updating. The usefulness of this software is presented using a case history, the correlation for a NASA Langley Research Center (LaRC) low aspect ratio research wind tunnel model. To demonstrate the improvements that TACT offers the MSC/PATRAN- and MSC/DIASTRAN- based structural analysis community, a comparison of the modal correlation process using TACT within PATRAN versus external third-party modal correlation packages is presented.

Marlowe, Jill M.↗

NASA NTRS2019

Ground Vibration Testing at NASA Armstrong, Emphasizing on Passive Aeroelastic Tailored Wing Ground Vibration Test Using Fixed Base Correction Method

How does experimental mechanics contribute to aircraft aeroelastic airworthiness? The aviation community is always improving analysis and testing techniques to realize quicker, cheaper, reliable solutions. A ground vibration test (GVT) is conducted to identify structural mode shapes, frequencies, and damping values to validate analytical models used for flutter analysis, which shows whether a structure has acceptable aeroelastic flutter margins for airworthiness. The FLL recently conducted a GVT of the Passive Aeroelastic Tailored (PAT) Wing using an experimental modal technique called Fixed Base Correction (FBC). The GVT objective was to obtain the PAT wing modal characteristics to compare test results with finite element model (FEM) results for the tow-steered wingbox.

loads testing↗

NASA NTRS2018

Bit Error Rate Performance on Passive Alignment in Free Space Optical Links Using Large Core Fibers

A 20-meter free-space optical link (FSOL) is proposed for data transmission between external ISS payload sites and the main cabin at a target rate of 10 Gbps (gigabits per second). Motion between a payload site and the main cabin is predicted to cause up to 5 cm in lateral misalignment and 0.2 degrees of angular misalignment. Due to the harsh environment of space it is advantageous to locate the optical transceivers inside the spacecraft or in a controlled environment. With the optical components and transceivers in separate locations, a fiber optic cable will be required to carry light between the two. In our past work we found that the use of large-core fibers provide an increased misalignment tolerance for such systems and could eliminate the need for active control of the optics. In that work, it was shown that a 105 ïm core diameter fiber optic cable offered a viable low SWaP (size, weight, and power) solution for the ISS application; however, the effects of modal dispersion were not investigated. This paper will present bit error rate performance of the FSOL using these large-core fibers.

Optical Bit Error Rate Testing↗

NASA NTRS2018

Bit Error Rate Performance on Passive Alignment in Free Space Optical Links Using Large Core Fibers

Free Space Optical Link↗

NASA NTRSDate not supplied

Error Localization Examples: Looking for a Needle in a Hay-stack

Finite element models (FEM) are routinely developed and used during fabrication of high dollar-value hardware. NASA as part of the pre-flight certification of launch vehicles routinely conducts vibration and static tests to calibrate models used for flight-risk assessments. As part of the calibration process, certain areas in the model are modified, using engineering judgment and sensitivity analysis, to match the test results. Unfortunately, tools to identify problem areas in the FEM using test data directly are scarce. Over the years, Error Localization Algorithms (ELA) have been proposed with very limited success. Recently, the Analytical Dynamics Model Improvement (ADMI) algorithm, which computes closed-form mass and stiffness corrections to match the test data exactly, have been shown to be effective for error localization. The paper will present several FEM example problems where ELA is used with simulated test data to determine FEM problem areas. For each example, the correct answer is shown along with ELA results. It is shown that the ELA process is able to identify general problem areas in the FEM, which are consistent with known model perturbations. However, in most cases the ELA identified area of improvement is larger than the true answer. Nonetheless, with proper optimization tools, calibration results using the ELA identified areas provide excellent results.

error localization↗

NASA NTRS2017

The Effects of Fiber Orientation and Adhesives on Tensile Properties of Carbon Fiber Reinforced Polymer Matrix Composite with Embedded Nickel-Titanium Shape Memory Alloys

Nickel-titanium (NiTi) shape memory alloy (SMA) sections were embedded within carbon fiber reinforced polymer matrix composite (CFRPPMC) laminates and their tensile properties were evaluated with simultaneous monitoring of modal acoustic emissions. The test specimens were fabricated in three different layup configurations and two different thin film adhesives were applied to bond the SMA with the PMC. A trio of acoustic sensors were attached to the specimens during tensile testing to monitor the modal acoustic emission (AE) as the materials experienced mechanical failure. The values of ultimate tensile strengths, strains, and moduli were obtained. Cumulative AE energy of events and specimen failure location were determined. In conjunction, optical and scanning electron microscopy techniques were used to examine the break areas of the specimens. The analysis of AE data revealed failure locations within the specimens which were validated from the microscopic images. The placement of 90 deg plies in the outer ply gave the strongest acoustic signals during break as well as the cleanest break of the samples tested. Overlapping 0 deg ply layers surrounding the SMA was found to be the best scenario to prevent failure of the specimen itself.

shape memory alloy↗

NASA NTRS1975

Results of NASTRAN modal analyses and ground vibration tests on the YF-12A airplane

The YF-12A aircraft, a delta-winged vehicle powered by two jet engines, was utilized in an investigation of the structural dynamic characteristics of a large, flexible, supersonic research vehicle. A large NASA structural analysis (NASTRAN) finite-element model was used to compute the ten lowest frequency symmetric and ten lowest frequency antisymmetric modes for the YF-12A aircraft. The results of the analysis were compared with experimental data obtained in a ground vibration test conducted with the completed aircraft. It was found that the finite-element structural model employed provides an adequate prediction of the dynamic behavior of the aircraft structure in the case of basic wing and body modes.

Kordes, E. E.↗

NASA NTRS1977

A method for experimental modal separation

A method is described for the numerical simulation of multiple-shaker modal survey testing using simulated experimental data to optimize the shaker force-amplitude distribution for the purpose of isolating individual modes of vibration. Inertia, damping, stiffness, and model data are stored on magnetic disks, available by direct access to the interactive FORTRAN programs which perform all computations required by this relative force amplitude distribution method.

Hallauer, W. L., Jr.↗

NASA NTRS1980

Active damping of modal vibrations by force apportioning

Force apportioning, a method of active structural damping based on that used in modal vibration testing of isolating modes by multiple shaker excitation, was analyzed and numerically simulated. A distribution of as few forces as possible on the structure is chosen so as to maximally affect selected vibration modes while minimally exciting all other modes. The accuracy of numerical simulations of active damping, active damping of higher-frequency modes, and studies of imperfection sensitivity are discussed. The computer programs developed are described and possible refinements of the research are examined.

Hallauer, W. L., Jr.↗

NASA NTRS1981

Analytical testing

Analytical methods for combining flight acceleration and strain data with shake test mobility data to predict the effects of structural changes on flight vibrations and strains are presented. This integration of structural dynamic analysis with flight performance is referred to as analytical testing. The objective of this methodology is to analytically estimate the results of flight testing contemplated structural changes with minimum flying and change trials. The category of changes to the aircraft includes mass, stiffness, absorbers, isolators, and active suppressors. Examples of applying the analytical testing methodology using flight test and shake test data measured on an AH-1G helicopter are included. The techniques and procedures for vibration testing and modal analysis are also described.

Flannelly, W. G.↗

NASA NTRS1981

Automated dynamic analytical model improvement

A method is developed and illustrated which finds minimum changes in analytical mass and stiffness matrices to make them consistent with a set of measured normal modes and natural frequencies. The corrected model is an improved base for studies of physical changes, changes in boundary conditions, and for prediction of forced responses. Features of the method are: efficient procedures not requiring solutions of the eigenproblem; the model may have more degrees of freedom than the test data; modal displacements at all the analytical degrees of freedom are obtained; the frequency dependence of the coordinate transformations are properly treated.

Berman, A.↗