Samples of trinitrotoluene (TNT) and erythritol tetranitrate (ETN) were exposed to nanosecond pulses from a tunable laser source across 90 minutes. During this continued laser pulse exposure, the samples of TNT underwent heavy degradation while the samples of ETN were able to resist high energy densities without visible alteration. A degradation pathway involving a nitro-nitrite rearrangement is suggested for the TNT, which sees radical production result in carbon ‘char’ formation in the explosive, despite there being no signs of bulk or surface melting. The formation of this carbonexplosive mixture and its implications towards the safety of extended exposure laser sensing techniques is discussed.

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For our earlier study, we added metal powder to microcrystalline wax, a solid fuel, to improve the fuel regression rate. Results of those earlier combustion experiments revealed that aluminum (Al) powder increases the combustion flame temperature in the combustion chamber of a hybrid rocket engine, and that Al powder addition improves the solid fuel performance. Nevertheless, details of Al powder ignition in the combustion chamber remain unconfirmed. Therefore, numerical analysis of combustion in a hybrid rocket engine was conducted using software (ANSYS 2020 R2; Fluent Inc.). In general, solid fuels for hybrid rocket engines are made of polymeric thermosets (e.g., hydroxyl-terminated polybutadiene (HTPB)) or hydrocarbon compounds (e.g., paraffin). To use materials and methods similar to those used for earlier studies, we decided to use gaseous oxygen as the oxidizer and kerosene as the melted solid fuel for analyses. Through this numerical analysis, we reproduced the actual combustion experiment conducted at our laboratory. Based on the temperature distribution in the combustion chamber, the residence time, and the temperature distribution of the Al powder obtained from the analysis, we confirmed locations in the combustion chamber at which the Al powder melting and ignition might occur. Results of numerical analyses demonstrated that boundary layer combustion occurs in the combustion chamber and that the temperature rises sufficiently for Al powder melting and ignition. Additionally, results confirmed that the Al powder residence time is the period during which the Al powder can reach its melting point and confirmed that ignition and combustion can occur in the combustion chamber.

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Hybrid rocket engines are studied internationally because they provide many benefits. However, hybrid rocket engines are rarely put into practical use because of their low regression rate. Earlier research efforts have been aimed at improving the regression rate using paraffin or microcrystalline wax (WAX) and adding metal powder to solid fuel. For hybrid rocket solid fuels, aluminum (Al) powder is used as a metallic fuel. Nevertheless, effects of adding Al powder have not been confirmed sufficiently because Al powder is reportedly difficult to ignite in the combustion chamber of a hybrid rocket engine. Magnesium (Mg) powder ignites more easily. This study specifically assessed addition of microscale magnalium (Mg-Al) powder particles because Mg-Al can overcome these and other shortcomings. Reportedly, Mg-Al powder has good combustion completeness in solid propellant¹). This study used combustion experiments to elucidate effects of Mg-Al powder addition to WAX. Combustion experiments using a small hybrid rocket engine demonstrated that 20 mass% Mg-Al powder in WAX produces an 18.7 % increased regression rate over that of WAX alone, which is greater than adding Mg or Al. These results suggest that high combustion heat of Mg-Al powder promotes WAX combustion. Adding Mg-Al powder raises the regression rate. Additionally, residue remaining in the aft chamber after combustion was analyzed using XRD. The results of these two analyses suggest that the Mg-Al powder was ignited and burned in the aft chamber. A small combustion chamber can be used because Mg-Al powder has high density and excellent completeness of combustion. Results demonstrate that Mg-Al powder is suitable for addition to solid fuel of small hybrid rocket engines.

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It is well known that high-pressure shock waves are generated during the collapse of bubbles, on which an underwater shock wave acts. In this study, the effects of bubble gas composition on the generation of shock waves in a bubble collapse were investigated experimentally. The explosive bubbles were composed of a stoichiometric ethylene-oxygen mixture and a stoichiometric hydrogen-oxygen mixture, whereas the gas compositions of the chemically inert bubbles were air and helium. The initial volume-equivalent radius of the bubble varied from 1.0 to 3.9 mm. The nondimensional maximum pressure of the bubble-generated shock wave was inversely proportional to the non-dimensional distance from the bubble for both explosive and chemically inert bubbles. The proportionality factor for the explosive bubbles was higher than that for the chemically inert bubbles. However, the proportionality factor for the hydrogen-oxygen bubble was slightly lower than that for the ethylene-oxygen bubble. This trend occurred because the high reactivity of the hydrogen-oxygen mixture caused an earlier explosion in the shrinkage stage. Regardless of the bubble composition of the gas, the conversion ratio from the bubble energy to the shock energy increased with decreasing momentum acquired by the water around the bubble.

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A metal spark generated by grinding carbon steel has been recognized proceeding multiple bursting events, akin to the luminous branching fireworks. The mechanism of successive fragmentation observed in carbon steel spark, however, has not been elucidated yet. In the present study, we develop a new comprehensive analytical framework for estimating the droplet size of carbon steel sparks, time of flight, and time-variant temperature, combined with high-speed images of the spreading sparks, for the quantitative discussion of the timescale of bursting metal sparks. We find that the flight time to burst for a mother droplet is independent of the content of carbon, corresponding to the Fourier number for the molecular diffusion of unity. The successive fragmentation of carbon steel sparks is rate-controlled by the molecular diffusion of ambient oxygen inside the droplet. Since the measured temperature indicates that the heat is produced by the oxidation of iron, the successive fragmentation stops when the iron in the spark is fully oxidized, immediately becoming a solid particle.

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