Jason D. Hinkle

Dimensional Repeatability of an Elastically Folded Composite Hinge for Deployed Spacecraft Optics

A new type of folded composite hinge is investigated for its use in precision deployable spacecraft structures. The hinge is an integral feature of a composite tube intended for use as a structural truss member. The design of the hinge allows the tube to be elastically folded for stowage even with tube wall thicknesses from 0.4 to 1.7 mm. Whether the large but primarily elastic folding stresses impart permanent deformations to the tube after it is deployed is experimentally assessed. The data show that any such permanent strain induces tip deformations, identified as microscopic plastic behavior, of no more than 2.5 μ axially and 9 p laterally, depending on the composite layup. This deployment repeatability is comparable to prior measurements of mechanical deployables. Moreover, stow duration and number of stows have no measurable effect, once the initial stow-deploy cycle has been completed. There is always a significant viscoelastic creep recovery following deployment that increases with stowage time. However, this viscoelastic creep is recovered. An exponential curve fit of the creep time response shows that the time constants of the viscoelastic recovery are independent of stow duration.

Submicron Mechanical Stability of a Prototype Deployable Space Telescope Support Structure

This paper describes the experimental characterization of the microdynamics of a prototype deployable telescope support structure. The experimental methods described separate thermal from mechanical microdynamics to within 0.003°C. It is shown that the intentional application of impulses to the structure, following the initial deployment, apparently stabilizes the microdynamics to within approximately 250 nanometers. This level is comparable to the background vibration of the test article. A model is presented that correlates well with the data, suggesting that this effect is due to the dynamically induced relaxation of strain energy stored by friction mechanisms within the structure.

Research on the problem of high-precision deployment for large-aperture space-based science instruments

The present paper summarizes results from an ongoing research program conducted jointly by the University of Colorado and NASA Langley Research Center since 1994. This program has resulted in general guidelines for the design of high-precision deployment mechanisms, and tests of prototype deployable structures incorporating these mechanisms have shown microdynamically stable behavior (i.e., dimensional stability to parts per million). These advancements have resulted from the identification of numerous heretofore unknown microdynamic and micromechanical response phenomena, and the development of new test techniques and instrumentation systems to interrogate these phenomena. In addition, recent tests have begun to interrogate nanomechanical response of materials and joints and have been used to develop an understanding of nonlinear nanodynamic behavior in microdynamically stable structures. The ultimate goal of these efforts is to enable nano-precision active control of micro-precision deployable structures (i.e., active control to a resolution of parts per billion).

Implications of structural design requirements for selection of future space telescope architectures

Decisions about structural architecture for future large space observatories will influence how overall optical stability scales with observatory size. This is examined using basic structural design analyses that relate overall stability requirements to telescope structural modal frequency and damping ratio. In this way, the influence of certain system level architectural choices on the performance can be assessed. In particular, trades between structural depth and optical correction requirements is examined, and compared against other design parameters such as the material specific modulus. For representative configurations and loads, the required optical correction increases with dimension to the fourth power, but reduces with the square of the structural depth and in proportion to the material specific modulus; areal density has no direct affect. This means that, unless the structural architecture improves with dimension, the optical error produced in a 6-meter telescope might increase by a factor of 123:1 for a 20-meter telescope and 77000:1 for a 100-meter telescope. If the structural depth, however, increases in proportion to telescope dimension, these requirements can be reduced by two orders of magnitude. Architectural options for achieving these benefits are discussed, with particular emphasis on considerations of the deployment or assembly scheme.

Geometric Imperfection Effects in an Elastically Deployable Isogrid Column

The experimental study and analysis of a novel gossamer structural component is described. The component is a 3-m-long thin-walled isogrid column prototype that may be elastically stowed and deployed. The column has a diameter of 0.318 m, with a linear density of 46 g/m. The static and dynamic mechanical responses of the deployed prototype are examined and compared to an idealized model. These comparisons indicate global stiffness ranging from11to28%andbucklingstrengthsfrom7to37%ofthetheoreticallyidealperformance.Initiallocalcurvatures approaching the thickness of the isogrid ribs are found to be the primary source of this performance reduction. A modeling approach based on the postbuckled response of individual isogrid ribs is proposed for predicting the global effects of local imperfections, and good agreement with the test results is found. In general, initial rib curvatures greater than 10% of the rib thickness are predicted to result in signie cant degradations of the global structural performance.

Micron accurate deployable antenna and sensor technology for new-millennium-era spacecraft

This paper summarizes recent results in a cooperative research program between the University of Colorado and NASA Langley Research Center in the deployment of precise antennas and reflectors from compact spacecraft. Advances in concepts, joints, materials, ground-test methodology, and micro-structural mechanics are presented which demonstrate the potential for passively positioning deployed components to within a few microns of their desired configuration. This technology promises to advance the state-of-the-art in deployment technology by several orders of magnitude in the next five to ten years. This means it will be possible to deploy reflectors with passive diffraction limited performance up to long wave infrared frequencies with a mass of perhaps five kilograms per square meter. Results of a ground test validation program and a planned flight validation experiment are presented to illustrate the key technical results of this research.

Modeling of Snap-Back Bending Response of Doubly Slit Cylindrical Shells

This paper presents an investigation of the bending-induced buckling response of doubly slit cylindrical shells. The goal of this work is to determine a design-relevant parametric model of the load-displacement response during stowing and deployment of folding hinges built from opposing doubly slit cylindrical shells. This relationship will assist in the design and modeling of structures built from these folding hinges for use in elastically deployed structures. Results of an investigation into computational analysis procedures for modeling the snap-back problem are presented. A review of the literature indicates that previous work has not dealt with the snap-back phenomenon that needs to be considered for the folding hinge application. The few analytical models that can be used to verify these finite element model predictions are presented, and comparisons are made to the current finite element results. A test apparatus is described which imposes controlled displacement and/or rotation at the end of the doubly slit cylindrical shells up to and through snap-back, while end moment and planar forces are measured.

Experimental Characterization of Lightweight Strain Energy Deployment Hinges

Elastic, lenticular hinges have been used for many years to provide simple, reliable, repeatable deployment of spacecraft structural components. Recent advances in high performance elastically flexible composites have opened up the application field for this useful deployment component by providing lighter, more thermally stable hinges as well as designs that can meet a much wider range of requirements. This paper describes recent advances in composite hinge performance through a combination of analysis and testing.

STRUCTURAL PERFORMANCE OF A GOSSAMER ISOGRID COLUMN WITH INITIAL GEOMETRIC IMPERFECTIONS

This paper reports on the experimental study and analysis of a novel gossamer structural component. The component is a 3 m long thin-walled isogrid column prototype which may be elastically stowed and deployed. The static and dynamic mechanical response of the deployed prototype is examined and compared to an idealized model. In general, these comparisons indicated large knockdowns in performance ranging from 11% to 28% for stiffness and 7% to 37% for buckling strengths. Initial curvatures approaching the thickness of the isogrid ribs resulted from the fabrication and test fixturing processes and are found to be the primary source of these performance knockdowns. A modeling approach based on the postbuckled response of individual isogrid ribs is proposed for predicting the global effects of local imperfections and good agreement with the test results is found. In general, initial rib curvatures greater than 10% of the rib thickness are predicted to result in significant degradations of the global structural performance.

Component Level Evaluation of a Friction Damper for Submicron Vibrations

This paper presents the component-level study of a friction damper for submicron vibrations of large optical space structures. The damping component utilizes nonconforming contact mechanics to provide high levels of dissipation at small displacement amplitudes. A novel component test configuration permitting single degree of freedom measurement along the component’s axis of symmetry is presented. Experiment results indicate that this design approach yields predictable component loss factors as great as 60% at micron scale cycle amplitudes. Modeling of the component employs both Hertzian contact mechanics and Mindlin’s analyses of microslip. Comparisons of the predicted and measured behavior indicate that the amplitude-dependent dissipation of these components are well represented by this approach. Increased compliance in the observed behavior may be indicative of contributions of surface roughness mechanics which were not included in the model.