Dynamics of an Inflatable Structure in Vacuum and Ambient Conditions

Abstract

In the dynamic modal testing of extremely lightweight inflatable structures, the results may vary dramatically according to the presence of either thermal vacuum or ambient atmospheric conditions. Unique aspects of modal testing techniques for an inflatable solar concentrator are identified, including the use of a noncontacting laser vibrometer measurement system. For the thermal vacuum environment, mode shapes and frequency response functions are compared for three different test article inflation pressures at room temperature. Modes that persist through all inflation pressure regimes are identified, as are modes that are unique for each pressure. In atmospheric pressure and room temperature conditions, dynamic measurements were obtained for the expected operational inflation pressure. Experimental mode shapes and frequency response functions for ambient conditions are described and compared to the results from the thermal vacuum tests. There is a surprising lack of correlation in test results between the two test conditions, which may be explained by damping and air mass considerations. Results of this investigation point out the necessity of testing inflatable space structures in vacuum conditions before they can be launched.