Emil V. Ardelean

Dynamic Modeling and Experimental Validation of a Cable- Loaded Panel.

Power and signal cable harnesses on spacecraft are often at 10% of the total mass and can be as much as 30%. These cable harnesses can impact the structural dynamics of spacecraft significantly, specifically by damping the response. Past efforts have lookedat how to calculate cable properties and the validation of these cablemodels on onedimensional beam structures with uniform cable lengths. This paper looks at how to extend that process to twodimensional spacecraftlike panels with nonuniform cable lengths. A shear beam model is used for cable properties. Twomethods of calculating the tiedown stiffness are compared.Of particular interest is whether or not handbooks of cable properties can be created ahead of time and appliedwith confidence. There are three frequency bands in which cable effects can be described. Before any cables become resonant, the cable effects are dominated bymass and static stiffness. After all the cables become resonant, the effect is dominated by increased damping in the structure. In between these two frequency cutoff points, there is a transition zone. Thedynamic cablemodelingmethod is validated as a distinct improvement over the lumped-mass characterization of cables commonly used today.

Modeling and Experimental Validation of Space Structures with Wiring Harnesses

Powerand data-handling cables, which can account for up to 30% of a satellite’s dry mass, couple with the spacecraft structure and impact dynamic response. Structural dynamicmeasurements suggest that amore complete representation of cable effects is needed to improvemodel predictive accuracy. To that end, a studywas performed to characterize cable harness impacts on dynamic response. From this study, a finite element modeling method supported by empirically determined cable properties and structural behaviorwas developed. Themodelingmethod was validatedwith a considerable amount ofmodel simulation and experimental data for a variety of cables attached to a free–free beam. At low frequencies, the cable effect was dominated bymass and stiffness, changing the apparent stiffness; damping was a secondary effect. At higher frequencies, where the cables themselves were resonant, the cable effect was dissipative, increasing the apparent damping in addition to affecting the overall frequency response. Tiedown stiffness was found to be an important, but difficult to measure, parameter. Finite element models of a cabled beamwere shown to be valid for all cable families studied.As a result, thefinite elementmodelingmethod itself was validated.

Experimental Techniques and Structural Parameter Estimation Studies of Spacecraft Cables

Signal and electrical power cables pose unique challenges to spacecraft structural design and are often poorly modeled or even neglected. The objective of this research was to develop test methods and analysis techniques to accurately model cable-loaded spacecraft, using linear finite element models. Test methods were developed to characterize cable extensional and bending properties when subjected to low-level lateral dynamic loads. Timoshenko beam theory, including shear and bending, was used to model cable lateral dynamics, and the model formulation applicability was validated through experiment. An algorithm was developed to estimate cable area moment of inertia and shear area factor, shear modulus product, from a single driving point mobility function. Test methods and the parameter estimation algorithm were validated, using metallic rod test specimens. Experiments were performed on cables of differing constructions and spans, to develop a database for finite element modeling validation experiments.

Chamber Core Structures for Fairing Acoustic Mitigation

Abstract : The U.S. Air Force Research Laboratory is pursuing an innovative composite structure design called chamber core for constructing launch vehicle payload fairings. A composite chamber core fairing consists of many axial tubes sandwiched between face sheets, tubes that can be used as acoustic dampers to reduce low-frequency interior noise with virtually no added mass. This paper presents the results of experimental studies of noise transmission through a 1.51 m diameter x 1.42 m tall chamber core cylinder. It was tested in a semireverberant acoustics laboratory using band-limited random noise at sound pressure levels up to 110 dB. The bare cylinder provided approximately 12.7 dB of attenuation over the 0-500 Hz bandwidth and 15.3 dB over 0-2000 Hz. The noise reduction increased to over 18 dB for both bandwidths with the axial tubes acting as acoustic dampers. Narrowband reductions in excess of 15 dB were measured around specific acoustic resonances. This was accomplished with virtually no added mass to the composite cylinder. Results were compared with the performance provided by a 2.5 cm acoustic blanket treatment. The acoustic dampers were as effective as the acoustic blanket at low frequency, but not at higher frequencies. The acoustic dampers were better able to couple with and damp the low-frequency acoustic modes. Together, the acoustic blanket and dampers provided over 10 dB more noise reduction over the 2000 Hz bandwidth than the bare cylinder.

Cable Effects on The Dynamics of Large Precision Structures

A top level overview of the effect cables have on the dynamic response of precision structures is presented. The focus of this paper is on precision, low-damping, low-first modal frequency space structures where cables are not implicitly designed to be in the load path. The paper presents the top-level, Phase I results which include pathfinder tests, an industry/government/academia survey, modeling and testing of individual cable bundles, and modeling and testing of cables on a simple structure. The end goal is to discover a set of practical approaches for updating well defined dynamical models of cableless structures. Knowledge of the cable type, position and tie-down method is assumed to be known. Simulation sensitivity analysis of the effect cables have on a precision structure has also been completed. Each section of the paper will focus on the details of each area.

Cable Effects Study: Tangents, Rabbit Holes, Dead Ends, and Valuable Results

Lessons learned during a study on the effects that electrical power and signal wiring harness cables introduce on the dynamic response of precision spacecraft is presented, along with the most significant results. The study was a three year effort to discover a set of practical approaches for updating well-defined dynamic models of harness-free structures where knowledge of the cable type, position, and tie-down method are known. Although cables are found on every satellite, the focus was on precision, low damping, and very flexible structures. Obstacles encountered, classified as tangents, rabbit holes, and dead ends, offer practical lessons for structural dynamics research. The paper traces the historical, experiential progression of the project, describing how the obstacles affected the project. First, methods were developed to estimate cable properties. Problems were encountered because of the flexible, highly damped nature of cables. A beam was used as a test article to validate experimentally derive...

Dynamics of Cable Harnesses on Large Precision Structures

This paper presents experimental results and modeling aspects for electrical power and signal cable harnesses used for space applications. Dynamics of large precision structures can be significantly influenced by subsystems such as electrical cables and harnesses as the structural mass of those structures tends to become smaller, and the quantity of attached cables continues to increase largely due to the ever increasing complexity of such structures. Contributions of cables to structural dynamic responses were observed but never studied, except for a low scale research effort conducted at the Air Force Research Laboratory, Space Vehicles Directorate (AFRL/VSSV). General observations were that at low frequencies cables have a mass loading effect while at higher frequencies they have a dissipative effect. The cables studied here adhere to space industry practices, identified through an extensive industry survey. Experimental procedures for extracting structural properties of the cables were developed. The structural properties of the cables extracted from the extensive experimental database that is being created can be used for numerical modeling of cabled structures. Explicit methods for analytical modeling of electrical cables attached to a structure in general are yet to be developed and the goal of this effort is to advance the state of the art in modeling cable harnesses mounted on lightweight spacecraft structures.

Parameter Estimation and Structural Dynamic Modeling of Electrical Cable Harnesses on Precision Structures

Signal and power electrical harnesses on precision spacecraft have the potential to change structural dynamics not included in high bandwidth plan models, thereby degrading mission performance and destabilizing structural control loops. Non-structural mass is commonly used to model of cables on such structures. This approach cannot predict cabled-structure dynamics because cables are dynamic systems that can couple strongly with the spacecraft structure. The Air Force Research Laboratory is leading a test and analysis program to increase structural modeling fidelity and develop tools to decrease program risk on precision spacecraft flight programs by developing technology to include cables in structural models. Details of innovative cable parameter estimation algorithms are included in this paper that is part of this research effort.

Creep Effects and Deployment Characterization of Rollable Composite Shell Reflectors

This paper presents a comparison of composite laminates fabricated into spherically curved radio-frequency reflective surfaces. Prior work revealed the design challenges of this shell reflector approach: stress relaxation effects due to long-term stowage, roll-stowage handling due to high strain energy, and the potential impulse loads generated during a freerelease deployment. The goal of this work is to investigate laminates with low creep, manageable folding, low stored strain energy, and high stiffness for ease of ground-based testing. Carbon fiber reinforced epoxy materials are preferred due to the combination of high strength, low mass, and high electrical conductivity. Glass fibers are also used but only in combination with carbon. A series of subscale shells were constructed to study the behavior of many different candidate laminates. The two most promising were manufactured to the full-size diameter, 1-meter. Stress relaxation effects were measured after several days of storage. High speed videogrammetry techniques were used to measure the deformation motions of these shells during un-restrained free-release deployment.

Structural determinancy and design implications for tensioned precision deployable structures

Tensioned precision structures has been shown to have potentially greater passive stability, more deterministic dynamics, and simpler metrology requirements than those of the traditional truss structure, where structural stiffness is derived from the nonlinear geometric or stress stiffening of the tensioned structure, rather than from the mechanics of materials. With the absence of structural depth, the traditional design issues with thermal deformations are reduced to two-dimensions, and metrology and shape control may be simplified into the load management and planarity control of discrete tensioning points. The presented structure is based on a discretized membrane approach, composed of effectively rigid in-plane panels and tuned, complaint hinges, which are especially complaint in shear. This research investigates and reports the effect of shear compliance on the reduction of buckling or wrinkling within a tensioned structure, as well as, the implications of membrane discretization density on efficient and effective structural design. Generalized back-of-the-envelope models are used to identify critical design trades and limits in design feasibility.