Dynamics of Deployable Truss Structures
The hunt for Earth-like planets orbiting other stars is one of the primary objectives of NASAs Origins Program, which will launch a number of space-based observatories, starting early in the next decade. Due to the size constraints imposed by the payload bay of carrier spacecraft, these telescopes will undoubtedly require some form of on-orbit deployment mechanism, including joints or hinges which will introduce nonlinearity to the structure. The success of the Origins missions will hinge on whether positioning of the optical elements can be maintained to within fractions of the viewing wavelength. Consequently, any minute disturbance will pose a serious threat to the stability of the precision optical systems. Acquiring a better understanding of the effects of damping and structural nonlinearities on the submicron-level dynamics is therefore essential to the telescope design.
The overall objective of the ongoing research is to perform an experimental and analytical investigation of the microdynamics of deployable truss structures. Specifically, the main goal is to characterize the dynamic response of such nonlinear structures at sub-microstrain levels of mechanical and thermal excitation. In the case of mechanical excitation, the response will be characterized in terms of modal parameters (the natural frequency and damping ratio). The response to thermal excitation will be characterized in the time and frequency domains.
The MIT Space Systems Laboratory was also involved in a microdynamics flight experiment, the Interferometry Program Experiment (IPEX). This is a NASA Jet Propulsion Laboratory flight mission which investigates the microdynamic behavior of a representative deployable space truss, including the effects of mechanically and thermally induced disturbances. IPEX was flown on STS-85 aboard the Space Shuttle Discovery in early August, 1997.
Mechanical Excitation
Before deployable truss structures can be used for interferometry, it is necessary to gain a better understanding of the mechanisms of submicron-level vibration. The submicron dynamics of the truss impact the accuracy of precision deployment, the repeatability of the deployment mechanism, as well as the stability of the deployed structure over a range of thermal conditions. To this end, open-loop modal parameter characterization is performed on an existing deployable truss structure, the MODE Structural Test Article, at different levels of excitation. To fully observe the submicron response, levels of strain between 1 microstrain and 1 nanostrain are introduced to the structure. The small levels of strain are only measurable if noise sources are eliminated or reduced as much as possible. In order to evaluate the modal parameters of frequency and damping, a sine sweep is performed. Modal damping is computed from the transfer functions obtained during the sweep procedure.
Thermal Excitation
Space structures may be subjected to sudden changes in their thermal environment due to Earth eclipse or changes in the spacecraft orientation. Such sudden increase or decrease in the heat load may induce dynamic structural responses. The Hubble Space Telescope is a notable example where the thermal gradient in the solar array booms induced low frequency vibrations of the arrays during the orbital transitions. In addition, the Hubble experienced series of smaller disturbances throughout the orbit. These disturbances were caused by stick/slip effects in the solar array deployment mechanism due to friction and thermal stress. This stick-slip behavior is sometimes referred to as thermal creak. Thermal creak is a phenomenon where thermally induced stored elastic energy is suddenly released via a nonlinear mechanism such as friction. Such nonlinear release mechanisms can induce impulsive or high-frequency loading to the system, even in response to low frequency thermal excitations. This is a serious problem in space structures, especially in deployable and flexible structures. Nonlinear joints with deadbands, tensioning cables and pulleys, and other structural elements that depend on friction and allow relative motion are all examples of potential creak elements that are common in space structures. Simple analytical models for predicting creak events and the resulting structural response are being developed to aid in designing precision space structures.
Two experiments are currently being conducted in parallel to the analytical models to characterize thermal creak response: 1) an experiment that measures the global response of a system with creak elements and 2) an experiment that measures the local response of a creak element.
A deployable section of the truss flown in STS-48 for the Middeck 0-gravity Dynamics Experiment (MODE) is used for the dynamics test. The truss is subjected to cyclic thermal loads in a thermal chamber and the resulting high frequency dynamic response is measured. For the creak element test, an ideal slip joint is designed and built. Under cyclic thermal loads, slow stress build up and sudden stress release in the joint are mesured. Friction in the joint is varied by loosening or tightening the joint.