Response of a shroud-enclosed spacecraft to combined acoustic-vacuum environments

Tests were conducted in the launch phase simulator test facility, with the Orbiting Geophysical Observatory structural model and the Nimbus-type shroud being used as test items. Data are given for the acoustic vibratory response of the spacecraft as the ambient pressure external to the shroud was reduced. The results show that at most spacecraft locations, the reduction in ambient pressure causes a significant reduction in the vibration response. There are, however, certain locations on the spacecraft and spacecraft adapter trusses where the reduction in ambient pressure has no effect. The differences in vibration responses obtained in this test can be directly related to two important factors: (1) excitation path - the path through which the external acoustic energy flows (either the mechanical path through structural members, or the air path directly from shroud to spacecraft); and (2) noise reduction of the shroud as a function of ambient pressure. As the pressure external to the shroud is reduced, the vented shroud has a corresponding decrease in internal pressure. The change in internal air density means less energy will be transmitted via the air path.