Dragon Cargo Spacecraft Berths with International Space Station
To Sam DiMaggio, Jessica Jensen and all my colleagues at SpaceX… Very Well Done! Great day for America’s commerical space industry!
I am pleased that Sam and his engineering staff use my Vibrationdata software and tutorials in their analysis work. :)
Count me as a proud supporter!
– Tom Irvine
Launch vehicle avionics components must be designed and tested to withstand random vibration environments. These environments are often derived from flight accelerometer data of previous vehicles. This data tends to be nonstationary as shown in the figure above.
The typical method for post-processing is to divide the data into short-duration segments. The segments may overlap. This is termed piecewise stationary analysis.
A power spectral density (PSD) is then taken for each segment. The maximum envelope is then taken from the individual PSD curves.
The maximum envelope for a completed mission can be used to check the test levels for components which flew on that mission.
In addition, the maximum expected flight level (MEFL) for a future mission can be derived from the maximum envelope with the addition of an appropriate statistical margin. The component acceptance and qualification test levels can then be derived from the MEFL.
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A more realistic enveloping method is to use the damage potential based on rainflow cycles, which accounts for fatigue. This method is described in: Nonstationary Damage Potential. I developed this method in collaboration with Sam DiMaggio of SpaceX and with Vince Grillo of NASA Kennedy Space Center.
Here is the PowerPoint version.
The software programs for this method, both source code and executable files, are given at: Vibrationdata Nonstationary Page. The software is available on a subscription basis.
PS: After writing this paper, I learned that Scot McNeill had previously published a similar paper: FDS_FDET_McNeill.pdf. So I may have reinvented the wheel on this one. But I whimsically noticed that Scot used two of my previous papers as references in his own paper.
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Fatigue Damage Spectrum
Optimized PSD Envelope for Nonstationary Vibration
Some NASA colleagues and I visited SpaceX in Hawthorne, California, on Thursday. Thanks to SpaceX engineers Sam DiMaggio and Jessica Jensen for the guided tour. We saw a Dragon spacecraft being built, as well as several rocket engines.
Sam DiMaggio, who leads the loads and environments groups, said they have a policy that no margin is added to any loads at any step in the process. Then at the end, all disciplines get together and decide what the overall factor should be.
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I wish that I were 25 years younger and working at SpaceX.
But I take satisfaction knowing that Sam, Jessica, and their SpaceX colleagues are using my Vibrationdata software. :)