1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-04-18a71 Center Point of Contact: GSFCa71 Submitted by: Wil HarkinsSubject: Flight Loads Anaylysis as a Spacecraft Design Tool Practice: The determination of accurate spacecraft loads via coupled flight loads analysis is used
2、 throughout the entire spacecraft development cycle, from conceptual design to final verification loads calculations.Programs that Certify Usage: This practice has been used on the Hubble Space Telescope, Gamma Ray Observatory, Superfluid Helium On-Orbit Transfer and Get Away Special (GAS) programs.
3、Center to Contact for Information: GSFCImplementation Method: This Lesson Learned is based on Reliability Practice number PD-AP-1317, from NASA Technical Memorandum 4322A, Reliability Preferred Practices for Design and Test.Benefit:Flight loads analysis, when used throughout the spacecraft developme
4、nt cycle, will 1) provide a mission specific set of loads, 2) provide a balanced structural design, 3) reduce conservatism inherent in bounding quasi-static design load calculations, 4) provide early problem definition, and 5) reduce surprises at the final verification loads cycle.Provided by IHSNot
5、 for ResaleNo reproduction or networking permitted without license from IHS-,-,-Implementation Method:The accurate determination of structural loads during all phases of STS (Space Transportation System) and/or ELV (Expendable Launch Vehicle) flight environments is crucial to spacecraft development.
6、 The procedure to calculate these loads involves 1) creation of an accurate spacecraft loads model and its corresponding validation by test, and 2) using this model in conjunction with vehicle/spacecraft coupled flight forcing functions during the development process. The model is linear and cannot
7、be accurate unless the structure it represents is essentially linear.Finite Element Model Creation:The development of an accurate finite element model (FEM) is crucial to the successful structural design of a spacecraft. It can be used to predict nodal and relative accelerations, member loads and st
8、resses, critical displacements, mechanically transmitted random loads, and thermal distortion loads. In addition, it can be used to perform weight saving studies, deployment studies, static test loads calculations, and control and stability studies.The creation of a FEM should begin early in the des
9、ign phase of a spacecraft. Even a simple model made from sketches can be useful for preliminary predictions of mass properties, frequency calculations, and primary structure design. As the design matures, more details become available (i.e., mass properties, materials, section properties). At this p
10、oint, it is essential to understand the analyses for which the model is intended, since this determines the complexity required and the assumptions made in modeling. With the recent advent of very high speed computers (Work-stations, RISC machines, etc.) which are available to todays engineers, the
11、temptation is to create large models which in effect when displayed, look almost like a photograph of the structure. This is often not necessary or efficient, and many times incorrect, especially in the interpretation of results. The engineer should have a clear understanding of the structural load
12、paths, including knowledge about how the actual structure will behave under static and dynamic loading. This is the only way he/she will be capable to later interpret transient events in the structure.Finite Element Model ChecksModel validity must be validated both mathematically and structurally. T
13、hroughout the design cycle some of the checks will directly indicate modeling problems, many requiring engineering insight and judgment to assess model validity. The following checks should be performed:1. Line-by-line check of input data to ensure input accuracy.2. Static run with fixed boundary co
14、nditions and 1G loads applied separately in each primary structural direction, reviewing resultant displacements, forces and stresses for reasonableness, and symmetry (if possible), etc.3. Fixed base modal run to calculate resultant structural shapes and frequency modes. Results should be consistent
15、 with the structural design. Hand calculations should be made to verify Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-simple characteristics such as panel modes. Modal effective weights and participation factors should be tabulated and studied alon
16、g with animated mode shape displays to gain insight and ensure consistency with design expectations. Modal strain and kinetic energy calculations also yield insight into structural characteristics. The sum of the effective weights for the modes considered should account for almost the total weight i
17、n each direction.4. Free-free modal run to calculate rigid body modes. These modes should be two orders of magnitude less than the first flexible modes, generally less than .001 Hz, to ensure that there is no inadvertent grounding of the structure. Equilibrium checks of the model are calculated by m
18、ultiplying the free-free stiffness matrix by geometrically derived rigid body modes. Nodes at which the structure is grounded should be displayed in a tabular form.5. For those models which will be used to predict thermally induced loads and deflections, a thermal equilibrium check is performed in w
19、hich a bulk temperature change is imposed in a kinematically constrained model which has all thermal expansion coefficients changed to a single value. Negligible element forces should be generated for this case. All rigid elements must be replaced with stiff elastic elements to facilitate proper tem
20、perature calculations.6. Plot element rotations to assure conformity (if applicable). Also generate a boundary plot to verify proper element locations. It is also strongly recommended that extensive verification using a preprocessor with graphic output be used to verify proper load orientation, mode
21、l construction, etc.7. Inspection of all FEM software message and warnings which might indicate among other things, improperly shaped elements, ill-conditioning, mechanisms, massless degrees of freedom, and so forth. Mass properties should be compared to current spacecraft weight statements. Comment
22、s relative to automatically imposed restraints, in particular, must be verified to be appropriate.8. Dynamic models reduced by the Guyan reduction technique to facilitate manageable normal modes calculations should be checked with a full-size matrix eigenvalue extraction technique such as Lanczos to
23、 assess the validity of the Guyan reduced eigenvalues and mode shapes. An improperly chosen Guyan reduction set may miss modes and result in frequency error.Model Validation:The finite element model is verified via a series of mechanical tests. Instrumentation data acquired during these tests are th
24、en compared to the FEM predicted behavior in order to assess the accuracy of the model. Subsequent adjustments are typically made to improve the model correlation to the test data before the model is considered test verified.The static and dynamic response results and the corresponding strength asse
25、ssment of the structure are solely dependent on the accuracy of the FEM used to calculate these responses. A model uncertainty factor (MUF) is applied to early loads results when coupled loads analysis is used for structural design. The magnitude of this factor may be reduced to 1.0 as confidence in
26、 the model is increased through the verification program.Mass Properties Verification:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-The initial step in the model correlation should be the measurement and correlation of the rigid body mass propertie
27、s. At the very least, this should include the structural weight. The center of gravity location and rotary inertia values should also be verified to the greatest extent practicable. In specific instances, where the structure has an axis of symmetry or a simple geometry it may be permissible to rely
28、on analytical values at the discretion of the project management.Typically, the mass property adjustments are made by accounting for non-structural mass items such as harnessing, thermal blanketing, or attachment hardware. This adjustment can be included in the model via the use of the non-structura
29、l mass parameter on the element property designation or by distributing concentrated mass elements in the region associated with these items.The mass property adjustments will affect both the static and the dynamic characteristics of the structural model making it essential to perform this correlati
30、on first. Accurate representation of the mass properties will have the greatest influence on the rigid body component of response which can be the dominant contributor to the total response. Further, for small components or very stiff structures which can be considered rigid bodies, the mass propert
31、ies correlation is the only verification necessary.Structural Dynamic VerificationThe modal survey test is the primary test for verifying the ability of the FEM to represent the dynamic behavior of the structure. The goal of this test is to measure the significant natural modes of the structure with
32、in a frequency range defined by the frequency content of the loading environment as transmitted by the launch vehicle. If no modes are predicted within this frequency range, a limited modal survey or shaker based sine sweep test should still be performed in order to verify that no modes exist.A pre-
33、test analysis should be conducted in order to predict the natural frequencies and mode shapes and to determine appropriate locations for accelerometers. To assess the choice of a set of potential accelerometer locations, a Guyan reduction, consisting only of the instrumentation locations, should be
34、performed and mode shapes and frequencies calculated. These results should then be compared to the previously calculated modal quantities, for the full model, to assess the adequacy of the test instrumentation set.The principal method of quantifying the similarity of mode shapes is the cross orthogo
35、nality matrix. This method of comparison should be used to make the previously discussed assessment of instrumentation and finally used to evaluate the correlation to the measured test data. If the model correlation is perfect, this matrix should be equal to the identity matrix. The correlation is c
36、onsidered adequate when the diagonal and off-diagonal elements of the matrix are within specified values.Once the mass properties are verified, adjustments to the FEM for modal correlation are usually Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-m
37、ade by the introduction or removal of local flexibilities which may have been based on poor assumptions. These errors can be detected through visual comparison of the test and analysis mode shapes as well as a combination of strain energy or modal effective weight analysis. Spring elements are often
38、 included to represent local flexibilities at joints which have originally been assumed rigid. Conversely, rigid elements may be used in instances where electronic boxes or other equipment appear to stiffen an area in which no stiffness contribution was originally assumed. The correct values for the
39、se local changes are typically found by iteration and determination of the cross orthogonality matrix. The changes should be demonstrated to be realistic by local detail analysis and not just chosen to match the sparse test values. Automated methods based on sensitivity and optimization techniques a
40、re useful to identify areas of likely error. Within this context, all changes should reflect physical reality.Static Verification:Although the purpose of the static test is to demonstrate the strength of the structure, it may also be used as a secondary model verification test. This is usually not p
41、ossible if the strength test is performed via the sine burst method, but may be appropriate if actual static pulls are used.A pre-test analysis is always performed in order to determine the appropriate test levels and therefore member loads and stress predictions should be available. The actual memb
42、er loads as experienced in the test are then acquired through strain gage measurements. The locations of this instrumentation should be chosen in order to characterize the primary load paths and are necessary to prove that the qualification levels were attained.Assuming that the measured levels are
43、not in accordance with pre-test predictions, adjustments to the FEM for correlation to the static test data may be applied in a similar manner as discussed pertaining to the modal survey test. It should be recognized that the need for implementation of a significant model change indicated by the sta
44、tic test data could result in an invalidation of the modal test correlation. This situation rarely occurs because the modal test should adequately exercise the structure and the static test correlation should only indicate minor adjustments. Both the modal and static correlation must be based on the
45、 model after all changes are made.Coupled Models:The NASA centers have the luxury of having available for themselves and their contractors use, a library of government and commercial launch vehicle FEMs with the appropriate forcing functions to simulate liftoff, staging, on-orbit and in the case of
46、STS, landing events. These models and their forcing functions, when used in conjunction with the spacecraft model, can be used to determine all spacecraft transient responses. This leads to the most accurate prediction of spacecraft loads possible. In addition, time-phased responses can be used to r
47、educe the conservatism resulting from the application of simultaneous bounding quasi-static design loads.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-The creation of system level models for coupled loads analysis almost always requires the techniq
48、ues of modal synthesis. This is due to both the size and complexity of the individual instrument, spacecraft, and launch, landing or stage models. Modal synthesis involves 1) reduction of the individual models to manageable and yet dynamically accurate sizes, and 2) coupling of these reduced models
49、to form the overall system level model, ready for forced response calculations. Modal synthesis requires matrix manipulations and transformations, eigenvalue analysis, and extensive bookkeeping to track the structural parameters through the transformations. Typically, a Craig-Bampton modal model of the spacecraft is made which has as its major characteristics, the physical boundary points and cantilever modes defining the vehicle flexibility wi
