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The modal survey vibration test conducted on an 80 ton open hopper freight car is described. The test data, the post-test update of the modal survey test requirements and procedure, and an index to the test data are presented. Photographs of actual measurement locations and the test historical log are included.
Preceding program flight tests, a ground vibration test and modal test analysis of a UH-60A Black Hawk helicopter was conducted by Sikorsky Aircraft to complement the UH-60A test plan and NASA/ARMY Modern Technology Rotor Airloads Program. The 'NASA/AEFA' shake test configuration was tested for modal frequencies and shapes and compared with its NASTRAN finite element model counterpart to give correlative results. Based upon previous findings, significant differences in modal data existed and were attributed to assumptions regarding the influence of secondary structure contributions in the preliminary NASTRAN modeling. An analysis of an updated finite element model including several secondary structural additions has confirmed that the inclusion of specific secondary components produces a significant effect on modal frequency and free-response shapes and improves correlations at lower frequencies with shake test data.
Using model dispersions as a starting point allows us to quickly adjust a model to reflect new test data: a) The analyst does a lot of work before the test to save time post-test. b) Creating 1000s of model dispersions to provide "coarse tuning," then use Attune to provide the "fine tuning." Successful model tuning on three structures: a) TAURUS. b) Ares I-X C) Cart (in backup charts). Mode weighting factors, matrix norm method, and XOR vs. MAC all play key roles in determining the BME. The BME process will be used on future tests: a) ISPE modal test (ongoing work). b) SLS modal test (mid 2018).
This paper describes the testing aspects and the problems encountered during the free-free and fixed base modal surveys completed on the original Space Station Common Module Prototype (CMP). The CMP is a 40-ft long by 14.5-ft diameter 'waffle-grid' cylinder built by the Boeing Company and housed at the Marshall Space Flight Center (MSFC) near Huntsville, AL. The CMP modal survey tests were conducted at MSFC by the Dynamics Test Branch. The free-free modal survey tests (June '90 to Sept. '90) included interface verification tests (IFVT), often referred to as impedance measurements, mass-additive testing and linearity studies. The fixed base modal survey tests (Feb. '91 to April '91), including linearity studies, were conducted in a fixture designed to constrain the CMP in 7 total degrees-of-freedom at five trunnion interfaces (two primary, two secondary, and the keel). The fixture also incorporated an airbag off-load system designed to alleviate the non-linear effects of friction in the primary and secondary trunnion interfaces. Numerous test configurations were performed with the objective of providing a modal data base for evaluating the various testing methodologies to verify dynamic finite element models used for input to coupled load analysis.
Loads analysis for structural dynamic systems is usually performed by finite element models. Because of the complexity of the structural system, the model contains large number of degree-of-freedom. The large model is necessary since details of the stress, loads and responses due to mission environments are computed. However, a simplified model is needed for other tasks such as pre-test analysis for modal testing, and control-structural interaction studies. A systematic method of model reduction for modal test analysis is presented. Perhaps it will be of some help in developing a simplified model for the control studies.
Test/analysis correlation is an important aspect of the verification of analysis models which are used to predict on-orbit response characteristics of large space structures. This paper presents results of a study using reduced analysis models for performing dynamic test/analysis correlation. The reduced test-analysis model (TAM) has the same number and orientation of DOF as the test measurements. Two reduction methods, static (Guyan) reduction and the Improved Reduced System reduction, are applied to the test/analysis correlation of a laboratory truss structure. Simulated test results and modal test data are used to examine the performance of each method. It is shown that selection of DOF to be retained in the TAM is critical when large structural masses are involved. In addition, the use of modal test results may provide difficulties in TAM accuracy even if a large number of DOF are retained in the TAM.
Test/analysis correlation is an important aspect of the verification of analysis models which are used to predict on-orbit response characteristics of large space structures. This paper presents results of a study using reduced analysis models for performing dynamic test/analysis correlation. The reduced test-analysis model (TAM) has the same number and orientation of DOF as the test measurements. Two reduction methods, static (Guyan) reduction and the Improved Reduced System (IRS) reduction, are applied to the test/analysis correlation of a laboratory truss structure. Simulated test results and modal test data are used to examine the performance of each method. It is shown that selection of DOF to be retained in the TAM is critical when large structural masses are involved. In addition, the use of modal test results may provide difficulties in TAM accuracy even if a large number of DOF are retained in the TAM.
The objective of the Mars Micromission program being managed by the Jet Propulsion Laboratory (JPL) for NASA is to develop a common spacecraft that can carry telecommunications equipment and a variety of science payloads for exploration of Mars. The spacecraft will be capable of carrying robot landers and rovers, cameras, probes, balloons, gliders or aircraft, and telecommunications equipment to Mars at much lower cost than recent NASA Mars missions. The lightweight spacecraft (about 220 Kg mass) will be launched in a cooperative venture with CNES as a TWIN auxiliary payload on the Ariane 5 launch vehicle. Two or more Mars Micromission launches are planned for each Mars launch opportunity, which occur every 26 months. The Mars launch window for the first mission is November 1, 2002 through April 2003, which is planned to be a Mars airplane technology demonstration mission to coincide with the 100 year anniversary of the Kittyhawk flight. Several subsequent launches will create a telecommunications network orbiting Mars, which will provide for continuous communication with lenders and rovers on the Martian surface. Dedicated science payload flights to Mars are slated to start in 2005. This new cheaper and faster approach to Mars exploration calls for innovative approaches to the qualification of the Mars Micromission spacecraft for the Ariane 5 launch vibration and acoustic environments. JPL has in recent years implemented new approaches to spacecraft testing that may be effectively applied to the Mars Micromission. These include 1) force limited vibration testing, 2) combined loads, vibration and modal testing, and 3) direct acoustic testing. JPL has performed nearly 200 force limited vibration tests in the past 9 years; several of the tests were on spacecraft and large instruments, including the Cassini and Deep Space One spacecraft. Force limiting, which measures and limits the spacecraft base reaction force using triaxial force gages sandwiched between the spacecraft and the test fixture, alleviates the severe overtest at spacecraft resonances inherent in rigid fixture vibration tests. It has the distinct advantage over response limiting that the method is not dependent on the accuracy of a detailed dynamic model of the spacecraft. Combined loads, vibration, and modal testing were recently performed on the QuikSCAT spacecraft. The combined tests were performed in a single test setup per axis on a vibration shaker, reducing test time by a factor of two or three. Force gages were employed to measure the true c.g. acceleration of the spacecraft for structural loads verification using a sine burst test, to automatically notch random vibration test input accelerations at spacecraft resonances based on predetermined force limits, and to directly measure modal masses in a base drive modal test. In addition to these combined tests on the shaker, the QuikSCAT spacecraft was subjected to a direct field acoustic test by surrounding the spacecraft, still on the vibration shaker, with rock concert type acoustic speakers. Since the spacecraft contractor does not have a reverberant field acoustic test facility, performing a direct field acoustic test -saved the program nearly two weeks schedule time that would have been required for packing / unpacking and shipping of the spacecraft. This paper discusses the rationale behind and advantages of the above test approaches and provides examples of their actual implementation and comparisons to flight data. The applicability of the test approaches to Mars Micromission spacecraft qualification is discussed.
NASA's Marshall Space Flight Center has had several projects involving inflatable space structures. Projects in solar thermal propulsion have had the most involvement, primarily inflatable concentrators. A flight project called Shooting Star Experiment initiated the first detailed design, analysis and testing effort involving an inflatable concentrator that supported a Fresnel lens. The lens was to concentrate the sun's rays to provide an extremely large heat transfer for an experimental solar propulsion engine. Since the conclusion of this experiment, research and development activities for solar propulsion at Marshall Space Flight Center have continued both in the solar propulsion engine technology as well as inflatable space structures. Experience gained in conducting modal survey tests of inflatable structures for the Shooting Star Experiment has been used by dynamic test engineers at Marshall Space Flight Center to conduct a modal survey test on a Solar Orbital Transfer Vehicle (SOTV) off-axis inflatable concentrator. This paper describes how both previously learned test methods and new test methods that address the unique test requirements for inflatable structures were used. Effects of the inherent nonlinear response of the inflatable concentrator on test methods and test results are noted as well. Nine analytical mode shapes were successfully correlated to test mode shapes. The paper concludes with several "lessons learned" applicable to future dynamics testing and shows how Marshall Space Flight Center has utilized traditional and new methods for modal survey testing of inflatable space structures.
Finite element model correlation as part of a spacecraft program has always been a challenge. For any NASA mission, the coupled system response of the spacecraft and launch vehicle can be determined analytically through a Coupled Loads Analysis (CLA), as it is not possible to test the spacecraft and launch vehicle coupled system before launch. The value of the CLA is highly dependent on the accuracy of the frequencies and mode shapes extracted from the spacecraft model. NASA standards require the spacecraft model used in the final Verification Loads Cycle to be correlated by either a modal test or by comparison of the model with Frequency Response Functions (FRFs) obtained during the environmental qualification test. Due to budgetary and time constraints, most programs opt to correlate the spacecraft dynamic model during the environmental qualification test, conducted on a large shaker table. For any model correlation effort, the key has always been finding a proper definition of the boundary conditions. This paper is a correlation case study to investigate the difference in responses of a simple structure using a free-free boundary, a fixed boundary on the shaker table, and a base-drive vibration test, all using identical instrumentation. The NAVCON Jim Beam test structure, featured in the IMAC round robin modal test of 2009, was selected as a simple, well recognized and well characterized structure to conduct this investigation. First, a free-free impact modal test of the Jim Beam was done as an experimental control. Second, the Jim Beam was mounted to a large 20,000 lbf shaker, and an impact modal test in this fixed configuration was conducted. Lastly, a vibration test of the Jim Beam was conducted on the shaker table. The free-free impact test, the fixed impact test, and the base-drive test were used to assess the effect of the shaker modes, evaluate the validity of fixed-base modeling assumptions, and compare final model correlation results between these boundary conditions.
In order to support the environmental test needs for our new Orion and Constellation program, NASA is developing unique world-class test facilities. To optimize this testing of spaceflight hardware while minimizing transportation issues, a one-stop, under one roof test capability is being developed at the Space Power Facility at the NASA Glenn Research Center's Plum Brook Station. This facility will provide the capability to perform the following environmental testing: (1) reverberation acoustic testing, (2) mechanical base-shake sine testing, (3) modal testing, (4) thermal-vacuum testing, and (5) EMI/EMC (electromagnetic interference and compatibility) testing. An overview of this test capability will be provided in this presentation, with special focus on the two new vibroacoustic test facilities currently being designed and built, the Reverberant Acoustic Test Facility (RATF) and the Mechanical Vibration Facility (MVF). Testing of the engineering developmental hardware and qualification hardware of the Orion (Crew Exploration Vehicle) will commence shortly after the facilities are commissioned.
The Tiltrotor Test Rig (TTR) is a NASA project, joint with the U.S. Army and Air Force, to develop a new, large scale proprotor test system for the National Full-Scale Aerodynamics Complex (NFAC). The first wind-tunnel entry was completed in November 2018 with a modern, 26-ft diameter proprotor. The primary purpose was to complete the development of the TTR, including systems integration with the NFAC. The TTR and rotor were tested up to 273 knots in axial flow. This is the highest airspeed ever achieved by a full-scale proprotor in any wind tunnel. Extensive conversion-mode data were also acquired, and hover/climb conditions were explored. Additional testing included aerodynamic tares, motor tests, thermal tests, modal vibration tests, and other checkout activities. This paper summarizes the results of the test, including examples of the most significant data.
The Tiltrotor Test Rig (TTR) is a new, large-scale proprotor test system for the National Full-Scale Aerodynamics Complex (NFAC). The TTR was jointly developed by NASA, the U. S. Army and the U. S. Air Force. The first wind-tunnel entry was completed in November 2018 with a modern, 26-ft diameter proprotor. The primary purpose was to complete the development of the TTR, including systems integration with the NFAC. The TTR and rotor were tested up to 273 knots in axial flow, which is the highest airspeed ever achieved by a full-scale proprotor in any wind tunnel. The entry constitutes the first full-scale wind-tunnel test of a gimballed, coning-flexure rotor. Extensive conversion-mode data were also acquired, and hover/climb conditions were explored. Additional testing included aerodynamic tares, motor tests, thermal tests, modal vibration tests, and other checkout activities. This report summarizes the results of the test, including examples of the most significant data.
The Tiltrotor Test Rig (TTR) is a large-scale proprotor test system for the National Full-Scale Aerodynamics Complex (NFAC). The first wind-tunnel entry was completed in November 2018 with a modern, 26-ft diameter proprotor. The rotor tested was the Bell Model 699, an AW609 rotor modified specifically for wind-tunnel testing. The primary purpose was to complete the development of the TTR, including systems integration with the NFAC. Testing included axial flow up to 273 knots, conversion mode from 30 to 100 deg nacelle angle, and hover/vertical climb conditions. Additional testing included aerodynamic tares, motor tests, thermal tests, modal vibration tests, and other checkout activities. This report lists the test conditions achieved and the types of data acquired and saved in the database.
Major issues in system identification are summarized and recent advances are reviewed. Modal testing and system identification used in control theory are examined, and the mathematical relationships and conversions of the models appropriate to modal testing and those appropriate to modern control design methods are discussed. The importance of obtaining input and output matrices in modal testing is emphasized, and the changes that may be needed in modal testing procedures to meet the needs of the control system designer are addressed. Directions for future research are considered.
Variations in the modal characteristics of different Space Shuttle Main Engines (SSMEs) are presented herein to study their structural dynamics vibrations, especially under fluid/structure interaction. The results of extensive modal survey tests and analysis relative to significant features in the modal characteristics of different powerheads and components are discussed.
New NASA launch vehicles will require development of robust systems in a fiscally-constrained environment. NASA, Department of Defense (DoD), and commercial space companies routinely conduct ground vibration tests as an essential part of math model validation and launch vehicle certification. Although ground vibration testing must be a part of the integrated test planning process, more affordable approaches must also be considered. A study evaluated several ground vibration test options for the NASA Constellation Program flight test vehicles, Orion-1 and Orion-2, which concluded that more affordable ground vibration test options are available. The motivation for ground vibration testing is supported by historical examples from NASA and DoD. The approach used in the present study employed surveys of ground vibration test subject-matter experts that provided data to qualitatively rank six test options. Twenty-five experts from NASA, DoD, and industry provided scoring and comments for this study. The current study determined that both element-level modal tests and integrated vehicle modal tests have technical merits. Both have been successful in validating structural dynamic math models of launch vehicles. However, element-level testing has less overall cost and schedule risk as compared to integrated vehicle testing. Future NASA launch vehicle development programs should anticipate that some structural dynamics testing will be necessary. Analysis alone will be inadequate to certify a crew-capable launch vehicle. At a minimum, component and element structural dynamic tests are recommended for new vehicle elements. Three viable structural dynamic test options were identified. Modal testing of the new vehicle elements and an integrated vehicle test on the mobile launcher provided the optimal trade between technical, cost, and schedule.
This paper describes a series of modal tests that were performed on a cantilevered truss structure. The goal of the tests was to assemble a large database of high quality modal test data for use in verification of proposed methods for on orbit model verification and damage detection in flexible truss structures. A description of the hardware is provided along with details of the experimental setup and procedures for 16 damage cases. Results from selected cases are presented and discussed. Differences between ground vibration testing and on orbit modal testing are also described.