PICOMETER SCALE SPONTANEOUS VIBRATIONS IN A PRECISION DEPLOYABLE BOOM UNDER THERMAL LOADING

INTRODUCTION This paper reports observations and analysis of picometer scale spontaneous vibrations in a precision deployable boom under thermal loading. The structural test article is a deployable boom previously flown in space that exhibited spontaneous vibrations during the temperature rise following a night to day transition on orbit. In an attempt to reproduce the orbital behavior on the ground, the test article was thermally loaded within a stabilized test environment. Spontaneous vibrations were also induced in these experiments. The amplitudes of these vibrations were on the order of a few dozen picometers, and the frequency was near 1500 Hz. Evidence of wave dispersion was detected in the vibrations. An analysis is presented that interprets the vibrations as the sudden release of stored strain energy in an unknown component of the truss. It is shown that the level of kinetic energy in the vibrations is comparable with the strain energy that would be released from a truss member undergoing comparable amplitude of deformation. This supports the notion that friction in a mechanism or a material instability led to the spontaneous vibrations, but if so, they were in a range of motion for which current theories would not expect such a release. * Graduate Research Assistant, Student Member AIAA Mark.Silver@Colorado.edu (303) 492-3576 t Associate Professor, Associate Fellow AIAA Lee.Peterson@Colorado.edu (303)492-1743 £ Research Associate, Member AIAA Lisa.Hardaway@Colorado.edu (303) 589-4565 Copyright © 2001 by Mark Silver, Lee Peterson, and Lisa Hardaway. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Structures are designed to hold a specific shape under specified load conditions. For space-based telescopes, the structural stability must be maintained to within a small fraction of a wavelength of light. Current requirements for the Next Generation Space Telescope (NGST) may require dynamic stability of the reflector panel shape to within a few nanometers. Interferometric telescopes, such as the Space Interferometry Mission (SIM), require this level of stability or better, but over a bandwidth of perhaps 1000 Hz. These requirements are necessary if the structural imperfection is to be kept compatible with the active optical elements of the telescope. Little is available in the literature about mechanics and stability at this scale of motion. Micrometer level instabilities in materials and structures have been recognized for over 100 years, yet testing at the nanometer level has been a relatively recent development. High bandwidth instruments now exist that can measure nanometer level displacements and micro gravity level accelerations. The use of these instruments has resulted in a small but growing number of experiments on the behavior of structures at these scales. The present study provides additional evidence in this regard. The objective is to measure and characterize the stability of a deployed boom under thermal loading. It is well known at the macroscopic and microscopic scales of motion that thermoelastic loading of a spacecraft structure can induce spontaneous vibrations. In the first set of HST solar arrays, for example, thermally induced bending caused a well-known pointing jitter. Analysis of the jitter problem showed that thermally induced bending in the Bi-STEM structural supports for the solar arrays caused the oscillations. The bending was, in turn, caused by temperature changes when HST passed from shadow to sunlight. The vibrations resulted when the induced loads exceeded the Coulombic friction threshold, American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization. causing a sudden loss of stiffness and a resulting release of strain energy. Such phenomena are well recognized among the spacecraft structural dynamics community. Perhaps what may be little appreciated is that the trigger of such frictional instabilities is associated in the theoretical literature with pre-slip deformations on the order of microns, and with critical velocities on the order of microns per second or more. Unless the load and load rate causes the frictional interface to approach both these critical thresholds, there should not be an induced vibration. In that case, a jointed structure should retain its stability, undergoing at most progressive microslip shearing in the friction interfaces. Said another way, if gross slip is not induced in the friction interfaces in such a way to create an unstable stiffness, there should be no sudden release of strain energy. One could simply design structures to avoid this limit. However, this expectation is based on models derived entirely from empirical evidence gathered at the macroscopic and microscopic deformation scale. Space telescopes need to be stable at the nanometer scale (perhaps below). With no theory to provide a basis, then, the first steps must be experimental. To provide evidence in this regard, the NASA Jet Propulsion Laboratory (JPL) performed flight tests on a deployed boom in the Interferometry Program Experiment (IPEX-2). The objective of IPEX-2 was to detect and characterize any spontaneous vibrations that might be induced by thermal loads in a space environment. The results of the flight test were presented in Reference 7. Perhaps the most puzzling data was collected immediately following a particular night to day transition. As the truss was heated, the temperature rose approximately 30 degrees C. Many spontaneous vibrations were recorded at this time. Following the flight, exploratory testing at JPL indicated that these low amplitude thermal snaps might have been observed on the ground during one of the post-flight experiments. A subsequent thermoelastic analysis at JPL indicated that the loads generated during flight should have remained below the Coulombic level, although there was some ambiguity as to whether the loads in the mounting hardware exceeded the stick-slip threshold. At about the same time, Reference 10 provided data on spontaneous vibrations in a different, sub-scale model truss in 1-g testing. Heating the truss slowly while suspended in a convection heat chamber apparently induced these vibrations. Accelerations were observed with amplitudes between 0.1 and 1 g's and with frequencies between 1 and 18 kHz. Radiative heating induced spontaneous vibrations between 1 and 5 g's with frequencies between 9 and 18 kHz. These vibrations seemed to be limited in extent to local regions of the structure. Although there was no analysis of the induced loads provided in these experiments, the amplitudes of motion were estimated to be at the nanometer scale, and may therefore be evidence of thermally induced vibrations in a microslip load regime, but this remained an open question. This paper presents evidence and analysis of picometer scale spontaneous vibrations induced in the IPEX boom by a heterogeneous thermal load. These experiments were able to successfully induce vibrations by heating the region of one bay of the truss near the root. Several spontaneous vibrations were captured. Moreover, the vibrations show evidence of wave transmission and dispersion down the boom away from the area of the applied thermal load. An analysis is also presented. This analysis compares the kinetic energy of the vibration to a comparable release of strain energy in an individual, but unknown, truss member. It is shown that the energy of the vibrations was comparable to the energy that would have been released by the deformation of a truss member at similar amplitudes. All of these analyses support the notion that either frictional instability in a mechanism, or material instability in the truss was responsible for the observed vibrations. This paper is organized as follows: The first section presents the experimental apparatus and procedure including the configuration, test measurements and data acquisition. The second section summarizes the results of the tests including time and frequency domain characterization. The third section presents the analysis of the data and the comparison with possible sources of released strain energy. EXPERIMENTAL APPARATUS AND PROCEDURE