1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-02a71 Center Point of Contact: JPLa71 Submitted by: Wil HarkinsSubject: Combination Methods for Deriving Structural Design Loads Considering Vibro-Acoustic, etc., Responses Practice: Practice: Design primary and secondary
2、structural components to accommodate loads which include steady-state, transient dynamic, and vibro-acoustic contributions at liftoff.Abstract: Preferred Practice for Design from NASA Technical Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Memorand
3、um 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:The probability of structural failure during launch and landing is significantly reduced.Implementation Method:Loads due to various sources (steady-state acceleration, transient dynamic, vibro-acoustic) may be computed separa
4、tely. Then use one of the combination methods listed below to derive the combined load. For acoustically sensitive components, direct acoustic load should be included as well.Technical Rationale:Vibration which causes structural loads can be classified as follows (see Table 1):1. Vibration due to tr
5、ansient events (liftoff, staging, etc.), typically below 60 Hz, including steady-state acceleration;2. Random vibration transmitted through mechanical interfaces, typically from 20 to 2000 Hz;3. Random vibration caused by direct acoustic loading on the surface of the structure, typically from 50 to
6、10,000 Hz.For primary structure, the steady-state and transient loads typically dominate the vibro-acoustic loads, and the latter are often ignored in practice. For secondary structure, however, the vibro-acoustic loads can be comparable to, or larger than, the steady-state and transient loads. Acou
7、stically sensitive components may have loads which are dominated by their response to direct acoustic excitation.Because the transient and vibro-acoustic loads can be of comparable magnitude, and both are present simultaneously at liftoff, it can be unconservative to design the structure to the tran
8、sient and vibro-acoustic loads separately. A number of methods are available for assessing the combined load. The following methods are considered acceptable.Method 1: Coupled Transient Analysis with Base Drive RandomDepending on the launch vehicle, coupled transient analysis predicts structural loa
9、ds up to 60 Hz, although in most cases the frequency cutoff is much lower (35 Hz for STS liftoff). Forcing functions for these analyses are adjusted, based on flight data, to assure that the loads envelop the actual flight loads (including transient and mechanically transmitted random vibrations wit
10、hin the frequency range of analysis).Above the cutoff frequency of the coupled transient analysis, mechanically transmitted random Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-vibration loads may be computed using base drive random analysis of the
11、 payload structure. The base vibration is specified in terms of power spectral densities of the acceleration in each direction. If possible, the analysis should be performed using input accelerations corresponding to the time of peak transient loads, rather than the maximum random vibration over the
12、 entire flight. Accelerations in different directions should be considered uncorrelated, and may be applied simultaneously or one direction at a time. A higher level of damping is acceptable for the random analysis than for the coupled transient. Common practice is to use 3 times the RMS (3 sigma) a
13、s the peak load prediction.Peak loads from the coupled transient and base drive random analyses may be combined by a root-sum-square (RSS) approach.When direct acoustic loading on the payload structure is non-negligible, it may be combined with the above load using an RSS approach. Methods for predi
14、cting acoustic loading include finite-element based approaches, which are limited to low frequency predictions, and statistical energy methods, which are limited to higher frequency predictions.Method 2: Mass Acceleration CurveA typical mass acceleration curve (MAC) is shown in Figure 1. The MAC is
15、an upper bound acceleration level for all components of a given mass, regardless of location, orientation, or frequency. Applicability is limited to appendage masses up to 500 kg, with frequencies up to approximately 100 Hz. Such a curve can be derived based on analytical and flight data, and includ
16、es the effects of both transient and mechanically transmitted random vibration. That is, the load predicted by the curve is already a combination of transient and random vibration.When direct acoustic loading is non-negligible, it may be combined with the MAC load using an RSS approach.Method 3: Cou
17、pled Transient Analysis with Modal MACIt is generally accepted that base drive random analysis can be very conservative, because it does not account for impedance effects. These effects can be very significant for the payload modes with large effective mass.An approach which accounts for impedance i
18、n an approximate way is based on application of the MAC to the modes of the payload structure. Each mode of the payload may be assigned an acceleration level based on its effective mass. The acceleration level is taken from a curve similar to Figure 1, but the modal MAC is typically lower in level t
19、han the MAC which applies to physical appendage masses. Physical loads corresponding to each mode are then derived by scaling the mode shape according to this level. The combined load is obtained as the RSS of the transient load with the modal loads above the transient cutoff frequency. It can be se
20、en that this method is the same as Method 1, except that the base drive random approach is replaced by an RSS of modal loads scaled Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-to the modal MAC.As in the previous two methods, when direct acoustic
21、loading is non-negligible, it should be computed by an appropriate acoustic analysis method, and combined with the transient and random load using the RSS approach.Table 1. Sources of Structural Loads Type of vibration Frequencies (Hz) Types of Analysis From To 1. Steady-state: Transient Vibration0
22、60 Coupled transient MAC 2. Mechanically transmitted random20 2,000 Base drive random Modal MAC 3. Direct acoustic50 10,000 Finite element (low frequency) Statistical engergy (high frequency) Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-refer to D
23、 descriptionD Impact of Non-Practice: Impact of Nonpractice: The probability of structural failure during launch will be increased, particularly for secondary structure.Related Practices: N/AAdditional Info: Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Approval Info: a71 Approval Date: 2000-03-02a71 Approval Name: Eric Raynora71 Approval Organization: QSa71 Approval Phone Number: 202-358-4738Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
