Within the national HYPROB-HYBRID project, funded by the Italian Minister of Education, University and Research (MIUR), the Italian Aerospace Research Centre (CIRA) is carrying out research activities aimed to realize an on-ground demonstrator of Hybrid Rocket Engine (HRE), with the main goal to validate design methodologies and to acquire expertise on enabling technologies and manufacturing processes. This technological demonstrator, with a thrust class of 30 kN, is based on nitrous oxide (N2O) and microcrystalline paraffin wax and will have most attractive capabilities of hybrid systems compared to solid or liquid engines, e.g., throttability and re-ignition. This paper deals with a preliminary methodological assessment and optimization of paraffin grain manufacturing processes. As known, paraffin wax is intrinsically a brittle, thermoplastic material, very sensitive to the presence of surface or internal rips, flaws, voids, micro-cracks and other microstructural high radius defects. Nevertheless, during combustion, it undergoes elevated thermal (temperature of about 3300 K) and mechanical stresses (pressure chamber of 40 bar) which can initiate and propagate deleterious unstable cracks, leading to the catastrophic failure of the grain and consequent danger of combustion instability, up to explosion of the whole rocket. This undesired behaviour strongly compromises one of the most important advantages of HRE in comparison with Liquid Rocket Engines (LRE) and Solid Rocket Engines (SRE), i.e., its inherent safety due to the separate storage and phases of fuel and oxidizer. Therefore it is crucial to select an efficient and reliable manufacturing procedure able to fabricate paraffin wax solid grain free from defects of critical size.
This paper examines a new concept of adaptive tuned dynamic vibration absorbers (ATDVAs) using shape memory alloy (SMA) elements instead of spring elements. Shape memory is a class of alloys that shows a reversible change in crystalline structure. In the martensite phase, the material exhibits a low elastic modulus and yield strength. Subsequent heating of the material induces the change to austenite, with a corresponding higher elastic modulus and yield strength. The result of the phase transformation is a sizable change in the geometry, in the internal tension, a considerable deformation, and a concomitant frequency shift, as in the case where the SMA is used as the stiffness component of a tuned dynamic absorber. In this research, experimental investigations have been focused to verify the capability of an ATDVA to control vibration on a free-free aluminum panel.
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