Pyrotechnic devices such as linear shaped charge and frangible joints are used for stage and fairing separation in launch vehicles. These devices generate high-frequency mechanical shock energy. Avionics components mounted in the vehicle must be designed and tested to withstand this shock energy.
Thus, ground testing is needed to measure the pyrotechnic shock levels so that component test levels can be derived. Measurements are typically taken near the source and at locations away from the source where components are to be mounted.
Ideally, this ground testing is performed using flight-like structures with connected cylindrical modules, fairings, etc. Cost and schedule concerns, however, may drive program managers to perform alternate tests instead, using curved or even flat panels with partial segments of the actual separation device. In some cases, these subscale tests are used for source shock measurement only.
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Stage separation events have the potential to excite the ring frequency of cylindrical structures as well as other cylindrical modes.
Note that wave propagation in a cylinder is governed by the Donnell-Mushtari-Vlasov eighth-order partial differential equation (or by an equivalent pair of coupled lower-order equations.) This equation covers both bending and membrane effects.
On the other hand, bending waves in rectangular plates are governed by a single, fourth-order equation.
All of this is a fancy way of saying that a cylinder responds to pyrotechnic shock much differently than a rectangular panel does.
Furthermore, cylindrical shells are much more representative of actual flight structures that are separated by pyrotechnic devices than rectangular panels are.
So I recommend testing with full, cylindrical structures. The structures should also have mass simulators for the avionics components.
An example of a ground test where a ring mode was apparently excited is given at: Ring Vibration Modes. See Appendix B in this paper.
See also:
Rings, Cylinders, Shells & Cones Page
– Tom Irvine
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