DYNAMIC CHARACTERIZATION OF AN INFLATABLE CONCENTRATOR FOR SOLAR THERMAL PROPULSION
An inflatable structural system that is a technology demonstrator for solar thermal propulsion and other applications is characterized for structural dynamic behavior both experimentally and computationally. The inflatable structure is a pressurized assembly developed for use in orbit to support a Fresnel lens or inflatable lenticular element for focusing sunlight into a solar thermal rocket engine. When the engine temperature reaches a pre-set level, the propellant is injected into the engine, absorbs heat from an exchanger, and is expanded through the nozzle to produce thrust. The inflatable structure is a passively adaptive system in that a regulator and relief valve are utilized to maintain pressure within design limits during the full range of orbital conditions. Modeling and test activities are complicated by the fact that the polyimide film material used for construction of the inflatable is nonlinear, with modulus varying as a function of frequency, temperature, and level of excitation. Modal vibration testing and finite element modeling are described in detail in this paper. The test database is used for validation and modification of the model. This work is highly significant because of the current interest in inflatable structures for space application, and because of the difficulty in accurately modeling such systems.
Macro-Fiber Composites for Sensing, Actuation and Power Generation
The research presented in this thesis uses the macro-fiber composite (MFC) actuator that was recently developed at the NASA Langley Research Center for two major themes, sensing and actuation for vibration control, and power harvesting. The MFC is constructed using piezofibers embedded in an epoxy matrix and coated with Kapton skin. The construction process of the MFC affords it vast advantages over the traditionally used piezoceramic material. The MFC is extremely flexible, allowing it to be bonded to structures that have curved surface without fear of accidental breakage or additional surface treatment as is the case with monolithic piezoceramic materials. Additionally the MFC uses interdigitated electrodes that capitalize on the higher d33 piezoelectric coupling coefficient, allowing it to produce higher forces and strain than typical monolithic piezoceramic materials. The research presented in this thesis investigates some potential applications for the MFC as well as additional topics in power harvesting. This first study performed was to determine if the MFC is capable of being used as a sensor for structural vibration. The MFC was incorporated into a self-sensing circuit and used to provide collocated control of an aluminum beam. It was found that the MFC makes a very accurate sensor and was able to provide the beam with over 80% vibration suppression at its second resonant frequency. Following this work, the MFC was used as both a sensor and actuator to apply multiple-input-multiple-output vibration control of an inflated satellite component. The control system used a positive position feedback (PPF) controller and two pairs of sensors and actuators in order to provide global vibration suppression of an inflated torus. The experiments
Fieldwork in geography teaching: A critical review of the literature and approaches
Abstract Virtually all lecturers in geography recognise the importance of fieldwork as a vital mode of teaching in the subject. This paper draws on material produced as part of a HEFCE review of teaching and learning in the field and assesses the implications of recent changes in higher education for field studies in geography. The literature on the development of, and recent changes in, fieldwork practice is reviewed and assumptions about appropriate forms of teaching and assessment are challenged. The need for carefully integrated preparation of project‐orientated fieldwork is stressed and the importance of debriefing and feedback after field visits is emphasised. Various suggestions for guidelines on good practice are presented. Finally, a range of future issues and problems in fieldwork is identified and discussed.