Critique of Ballistic Vehicle Vibration Measurements

Vibration measurements made during a reentry vehicle flight test program were complicated by intense high-frequency excitation of the vehicle shell structure during the reentry phase of the flights. Evidence obtained from standard data processing results, and from some that were non-standard, indicated filtering after the transducer signal conditioning process obscured the effect of this high frequency excitation on the reported acceleration measurement. The intensity of aerodynamic excitation at frequencies beyond the bandlimit of the measurement system caused occasional overloading of signal conditioning amplifiers. In many instances, acceleration waveforms obtained from overloaded channels were believable, but yielded unbelieveable power spectral densities. In some cases, both the acceleration waveform and the power spectral density exhibited credible characteristics, but were subtly inconsistent with the results of measurements from other vehicle locations. Filtering between accelerometers and signal conditioning amplifiers proved to be absolutely essential for this type of measurement.

In this paper, some of the evidence leading to these assertions is presented and recommendations for the design of future reentry vibration measurement systems are offered. The ideas, while discussed in the context of reentry vehicle vibration measurements, are applicable to any measurement system using “active” electronic components.