Eric John Ruggiero

Gossamer Spacecraft: Recent Trends in Design, Analysis, Experimentation, and Control

Introduction A N emerging interest in the gossamer spacecraft community is the development and design of membrane optics that meet the stringent surface quality requirement of spaceborne telescopes. Appropriately, the development of an ultralarge, multifunctional membrane optic is being tackled head on by multiple disciplines. Strides are being made in material science, engineered actuators and sensors, and modeling techniques that can handle the unique characteristics that make gossamer structures so fascinating as well as challenging. Thorough reviews of gossamer spacecraft and related issues can be found in a few key sources. In 1995, Cassapakis and Thomas1 provided a historical perspective on the development of inflated satellite technology. Their paper covers topics such as design variables for building large, inflated craft; thoughts on new deployment and rigidization techniques; multiple applications for large, inflated craft (such as satellites, space targets, decoys, and antennae); and most importantly, lessons learned from their research and areas of research most deserving of further attention. In 2001, Jenkins et al.2 assembled a bound volume for AIAA that covers many facets of gossamer technology. The volume consists of 21 chapters devoted entirely to issues important to gossamer structures, like mechanics of membrane materials, fundamentals of membrane optics, modeling of deployment and rigidization methodologies, unique materials and their properties, and conceivable applications of ultralarge, ultralightweight craft. As a follow-up to the 2001 AIAA volume, Wada and Lou3 from the Jet Propulsion Laboratory (JPL) assembled a review of the JPL’s preflight validation tests for gossamer structures. Wada and

Dynamic testing of inflatable structures using smart materials

In this paper we present experimental investigations of the vibration testing of an inflated, thin-film torus using smart materials. Lightweight, inflatable structures are very attractive in satellite applications. However, the lightweight, flexible and highly damped nature of inflated structures poses difficulties in ground vibration testing. In this study, we show that polyvinylidene fluoride (PVDF) patches and recently developed macro-fiber composite actuators may be used as sensors and actuators in identifying modal parameters. Both smart materials can be integrated unobtrusively into the skin of a torus or space device forming an attractive testing arrangement. The addition of actuators and PVDF sensors to the torus does not significantly interfere with the suspension modes of a free–free boundary condition, and can be considered an integral part of the inflated structure. The results indicate the potential of using smart materials to measure and control the dynamic response of inflated structures.

A literature review of ultra-light and inflated toroidal satellite components

Gossamer structures, also known as inflatable or membrane structures, have been a subject of renewed interest in recent years for space applications such as communication antennas, solar thermal propulsion, space solar power, and other large spacecraft applications. The major advantages of using inflatable structures in space are their extremely low weight, on-orbit deployability, and minimal stowage volume for launching. In this paper, we present a literature survey on different aspects of inflatable structures. Analytical and experimental studies of an inflated torus-the main structural support system for several inflatables-have drawn a considerable amount of attention from the vibration and control community. The inflated torus will be the main focus of this survey. First, we present an overview of gossamer spacecraft technology. Thereafter, we consider analytical studies of inflated tori and arches, and we cite several research papers on these topics. Next, we present a brief overview of research work on experimental studies of tires, inflated tori, and other types of gossamer structures, and we outline the future for ultra-flexible spacecraft technology.

Smart Materials in Inflatable Structure Applications

The focus of this work is to investigate the use of smart materials for vibration testing and control of inflated satellite components. Lightweight inflatable structures are a viable alternative in aerospace structure design. These structures, however, pose special problems in testing and in controlling vibrations due to their extremely lightweight, flexible, and high-damping properties. The smart materials offer the required flexibility with very high electromechanical coupling and, hence are logical elements for the use in the dynamics and control of inflated structures. The aim of the new concept presented in this work is to provide a refined methodology in ground testing for verifying predictive modeling efforts, and to design sensor/actuator systems to actively control unwanted vibrations of an inflated space object. Multiple sensors/actuators and modern state-space based controllers have been implemented to study the various performance of the proposed concept.

A Comparison between SISO and MIMO Modal Analysis Techniques on a Membrane Mirror Satellite

The future of space satellite technology lies in the development of ultra-large, ultra-lightweight space structures, orders of magnitude greater in size than the current satellites. Such large crafts will increase communication and imaging capabilities from orbits. Many such proposed ultra-flexible satellites are inflated structures. To get these ultra-large structures in space, they will have to be stored within the Space Shuttle cargo bay and then inflated on-orbit. However, the highly flexible and pressurized nature of these ultra-large spacecraft poses several daunting vibration and control problems. Disturbances (i.e., on-orbit maneuvering, guidance and attitude control, and the harsh environment of space) wreck havoc with the on-orbit stability, pointing accuracy, and surface resolution capability of the inflated satellite. Fortunately, recent advances in integrated smart material systems promise to provide solutions to these problems. Recent research into the use of Macro-Fiber Composite (MFC®) devices integrated into the dynamic measurement and vibration control of inflated structures has had promising results (Wilkie et al., 2000). These piezoelectric-based devices possess a superior electro-mechanical coupling coefficient making them superb actuators and decent sensors in dynamic analysis applications. Initially, research was performed on an inflated torus using single-input, single-output (SISO) testing techniques. Since then, steps have been taken to outline a new, multiple-input, multiple-output (MIMO) testing technique for these ultra-large structures. This study applies these results to an inflated torus with bonded membrane mirror to extract modal parameters, such as the damped natural frequencies, associated damping, and mode shapes within the frequency bandwidth of interest for these structures (5–200 Hz). Further, the nonlinear dynamic behavior of the inflated torus and membrane mirror is accentuated through a comparison of SISO and MIMO modal analysis techniques, and a discussion of the nonlinear results follows. The purpose of this work is to apply the results from prior works to an inflated torus with bonded membrane mirror to accomplish the following three goals: (1) to establish a baseline dynamic characterization of the test structure using SISO modal analysis techniques;(2) to perform a MIMO modal analysis of the test structure to identify natural frequencies, mode shapes, and damping ratios, and compare these MIMO results to the SISO analysis; and(3) to use the discrepancies between the two testing technique results as a platform for discussing the nonlinear nature of the test structure. In the future, the results of this work may form the premise for an autonomous, self-contained system that can both identify the vibratory characteristics of an ultra-large, inflated space craft and apply an appropriate control algorithm to suppress any unwanted vibration – all while on-orbit.

Modeling and vibration control of an active membrane mirror

The future of space satellite technology lies in ultra-large mirrors and radar apertures for significant improvements in imaging and communication bandwidths. The availability of optical-quality membranes drives a parallel effort for structural models that can capture the dominant dynamics of large, ultra-flexible satellite payloads. Unfortunately, the inherent flexibility of membrane mirrors wreaks havoc with the payloads on-orbit stability and maneuverability. One possible means of controlling these undesirable dynamics is by embedding active piezoelectric ceramics near the boundary of the membrane mirror. In doing so, active feedback control can be used to eliminate detrimental vibration, perform static shape control, and evaluate the health of the structure. The overall motivation of the present work is to design a control system using distributed bimorph actuators to eliminate any detrimental vibration of the membrane mirror. As a basis for this study, a piezoceramic wafer was attached in a bimorph configuration near the boundary of a tensioned rectangular membrane sample. A finite element model of the system was developed to capture the relevant system dynamics from 0 to 300 Hz. The finite element model was compared against experimental results, and fair agreement found. Using the validated finite element models, structural control using linear quadratic regulator control techniques was then used to numerically demonstrate effective vibration control. Typical results show that less than 12 V of actuation voltage is required to eliminate detrimental vibration of the membrane samples in less than 15 ms. The functional gains of the active system are also derived and presented. These spatially descriptive control terms dictate favorable regions within the membrane domain for placing sensors and can be used as a design guideline for structural control applications. The results of the present work demonstrate that thin plate theory is an appropriate modeling medium for capturing the relevant system dynamics of an active membrane mirror and can be used effectively to set the framework for the closed-loop vibration control architecture.

Modal analysis of an ultra-flexible, self-rigidizing toroidal satellite component

Over the past few years, much research has been performed on understanding the dynamics of an ultra-large, flexible toroidal satellite component subject to an internal pressure. However, the harsh environment of space is no place for inflated, membrane-like materials for fear of micro meteorite bombardment and subsequent puncture. Addressing this issue directly, United Applied Technologies (Huntsville, AL) has developed a novel, thin film casting approach to create a self-rigidizing torus. Once inflated, the torus structure is able to support its own shape, thus eliminating the need for any internal pressure. The self-rigidizing torus is extremely flexible, much more so than its pressurized predecessors. Such compliancy makes modal testing extremely difficult. However, through careful application of traditional modal testing techniques (shaker and accelerometer testing), the damped natural frequencies and mode shapes of the self-rigidizing torus can be discerned in the frequencies and mode shapes of the self-rigidizing torus can be discerned in the frequency bandwidth of interest, 1–12 Hz.Copyright

Multi-Input Multi-Output Modal Testing Techniques for a Gossamer Structure

Inflated space-based structures have become popular over the past three decades due to their minimal launch-mass and launch-volume. Once inflated, these space structures are subject to vibrations induced by guidance systems and space debris as well as from variable amounts of direct sunlight. Understanding the dynamic behavior of space-based structures is critical to ensuring their desired performance. Inflated materials, however, pose special problems when testing and trying to control their vibrations because of their lightweight, flexibility, and high damping. Traditional modal testing techniques, based on single-input, single-output (SISO) methods, are limited for a variety of reasons when compared to their multiple counterparts. More specifically, SISO modal testing techniques are unable to reliably distinguish between pairs of modes that are inherent to axi-symmetric structures (such as an inflated torus, a critical component of a gossamer spacecraft). Furthermore, it is questionable as to whether a single actuator could reliably excite the global modes of a true gossamer craft, such as a 25 m diameter torus. In this study, we demonstrate the feasibility of using a multiple-input multiple-output (MIMO) modal testing technique on an inflated torus. In particular, the refined modal testing methodology focuses on using Macro-Fiber Composite (MFC® ) patches (from NASA Langley Research Center) as both actuators and sensors. MFC® patches can be integrated in an unobtrusive way into the skin of the torus, and can be used to find a gossamer structure’s modal parameters. Furthermore, MFC® excitation produces less interference with suspension modes of the free-free torus than excitations from a conventional shaker. The use of multiple actuators is shown to properly excite the global modes of the structure and distinguish between pairs of modes at nearly identical resonant frequencies. Formulation of the MIMO test as well as the required postprocessing techniques are explained and successfully applied to an inflated Kapton® torus.Copyright

Application of SISO and MIMO Modal Analysis Techniques on a Membrane Mirror Satellite

The future of space satellite technology lies in the development of ultra-large, ultra-lightweight space structures orders of magnitude greater in size than current satellite technology. Such large craft will increase current communication and imaging capabilities from orbit. To get ultra-large structures in space, they will have to be stored within the Space Shuttle cargo bay and then inflated on-orbit. However, the highly flexible and pressurized nature of these ultra-large spacecraft poses several daunting vibration and control problems. Disturbances (i.e. on-orbit maneuvering, guidance and attitude control, and the harsh environment of space) wreck havoc with the on-orbit stability, pointing accuracy, and surface resolution capability of the inflated satellite. However, recent advances in integrated smart material systems promise to provide solutions to these problems. Recent research into the use of Macro-Fiber Composite (MFC®) devices integrated into the dynamic measurement and vibration control of inflated structures has had promising results. These piezoelectric-based devices possess a superior electromechanical coupling coefficient making them superb sensors and actuators in dynamic analysis applications. Initially, research was performed on an inflated torus using single-input, single-output (SISO) testing techniques. Since then, steps have been taken to outline a new, multiple-input, multiple-output (MIMO) testing technique for these ultralarge structures. Based on the matrix formulation and postprocessing techniques recently developed, the current work applies these results to an inflated torus with bonded membrane mirror to extract modal parameters, such as the damped natural frequencies, associated damping, and mode shapes within the frequency bandwidth of interest for these structures (5 – 200 Hz). MIMO modal testing techniques are ideal for large, inflated structure applications. The nature of the structure requires the use of multiple sensors and actuators for worthwhile dynamic analysis and control. Therefore, in the future, the results of this work will form the premise for an autonomous, self-contained system that can both identify the vibratory characteristics of an ultra-large, inflated space craft and apply an appropriate control algorithm to suppress any unwanted vibration—all while on-orbit.Copyright

Kevlar® Fiber Brush Seals for LNG Compressors

[Abstract] In Spring 2007, GE Global Research tested a prototype Kevlar® Fiber Brush Seal in the tertiary sealing position on each end of a full-scale LNG compressor. The goal of the test was to provide a low-leakage sealing solution capable of precluding oil migration along the rotor from the bearing housing into the dry gas seal housing. Although the test demonstrated that the seal could preclude oil migration, the start-up and operational leakage of the seals was over three times the expected leakage levels acceptable for this type of sealing technology.