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Abstract:In this paper, based on the mature technical system of hydroxyl terminated polybutadiene（HTPB） composite solid propellants, the new energetic material have been introduced for these formulations to obtain plume with less emission and higher combustion efficiency. According to the literature, in order to achieve the low emission of the combustion plume, high-energy compounds such as hexanitrohexaazaisowurtzitane (CL-20) and polynitrogen compounds can be used to partially replace some ammonium perchlorate (AP), whereas the dinitramide Ammonium (ADN) and nitrous oxide (HNF) are expected to completely replace AP in HTPB propellants. More importantly, in order to improve the combustion efficiency of aluminum (Al) powder, aluminum hydride (AlH₃), nano-Al or various composite Al powder could be used. In general, the development of new energetic ingredients for HTPB propellants should follow the problem-oriented principle. As a result, it′s better to investigate the characteristics of such novel energetic ingredients, and thereby the practical applications of HTPB propellants could be extended, which may promote solid-power technology in both military and civilian applications.

Abstract:The 3-Nitro-1,2,4-triazole-5-one (NTO) is a high energydensity materials of keen interest for both commercial and scientific worlds owing to its reduced sensitivity, better thermal stability and high performances. It plays a significant role to replace the current energetic ingredients. In this review, we summarize various strategies involved in the synthesis of NTO as well as the existing approaches to tailor its particle morphology and sizes. The most prominent properties of NTO, such as insensitivity and performance, which are usually required to produce efficient formulations,have been concisely discussed. In addition, this overview reports on some newer forms of NTO including derivatives and co-crystals available inthe literature, which can enhance the NTO features and extend its applications. The advantages and shortcomings of various NTO forms for specific and potential use are also highlighted together with the attempts made to overcome these issues. Therefore, efforts will certainly continue to improve characteristics and performances of NTO either by chemical modification or by co-crystallization in order to produce promisingformulations for widespread applications in the near future.

Abstract:Aluminum hydride (AlH₃) has high hydrogen content capacity (mass content>10%), low molecular weight of combustion products and relatively high thermal decomposition temperature. AlH₃ can significantly improve the energy level of solid propellants by partially replacing aluminum powder, so it is considered as a very important high-energy fuel. However, there is critical dependence of the solid propellant performances on the quality of AlH₃ crystals. The choice and optimization of preparation method largely determines the quality and cost of the products. It has been shown that the unstable nature of AlH₃ could greatly affect the aging and storage performances. Moreover, the incompatibility between some components and AlH₃ decreases the safety of corresponding solid propellants. The problems mentioned above are the key technical issues and need to be solved before large-scale application of AlH₃ in solid propellant formulations. The advances in synthetic techniques for AlH₃ and its application in solid propellants over the world have been introduced in this review paper. The stabilizing strategies and post evaluation of AlH₃ in relevant research institutes are summarized as well, based on which the future research directions are proposed. The literature shows that the safety and quality control during scaling-up of AlH₃ can be solved by using ether synthetic method. The applicability of AlH₃ in solid propellant can be greatly improved by surface treatment, including coating of AlH₃.

Abstract:The cross-linked modified double base (XLDB) propellants have been prepared by an interstitial-casting technique. The effect of the temperature on the curing quality has been investigated. By observation of the dependence of micro-structure of the propellants on curing time, the possible curing mechanisms of the XLDB propellants has been proposed. The effect of temperature the curing quality of interstitial-casted XLDB propellants has been clarified. A step-curing temperature scheme has been proposed. To provide sufficient time for the smooth completion of the physical curing processes, the curing temperature of the initial stage should be lower than 45 ℃, and the curing temperature of the ending stage should be increase to 60 ℃, so as to accelerate the completion of the chemical curing process. The results show that the curing temperature is the key factor that determines the curing quality and the performances of the interstitial-casted XLDB propellants. Along with temperature rise, the increasing extent of chemical curing rate is higher than the physical curing rate, the performances of these propellants dependent largely on the ratio of the physical to chemical curing. A higher temperature at the initial stage is not preferable for the quality improvement of the propellants.

Abstract:Large amount of solid propellants would be discarded each year due to failure, and the recycling/reuse of the waste propellants is important for energy conservation and environmental protection. On the basis of thermogravimetry-differential scanning calorimetry, coupled with Fourier transform infrared spectroscopy (TG-DSC-FTIR) technique, the solid-state reaction properties of the mixtures of propellant and anthracite has been investigated with details. Propellant-anthracite blends at different ratios were heated up to 1300 ℃ at 10 K·min^-1 heating rate in air, and the FTIR spectra have been obtained synchronously. The results show that as the propellant content increases, the decomposition process moves to the low temperature zone. The ignition temperature of anthracite decreases from 560 ℃ to 383 ℃. The burn out temperature decreases from 676 ℃ to 616 ℃, and the comprehensive combustion characteristic index increases from 2.36 E-8 to 1.27 E-7. Moreover, the apparent activation energy of the fixed carbon combustion part decreases from 165.6 to 91.2 kJ·mol^-1. The FTIR spectra show that as the propellant content increases, the release of CO₂ and CO as the major gaseous products of anthracite oxidation, move to the low temperature zone. It indicates that the waste propellants can greatly enhance the oxidation process, ignition and combustion of anthracite.

Abstract:Several laboratory scale research on nitrate ester plasticized polyether (NEPE) solid propellants with and without ammonium dinitramide (ADN) and N-guanylurea-dinitramide (GUDN), featured with the same nominal composition, have been prepared and evaluated. The combustion properties (strand burn rate and flame photos) and thermogravimetry (TG) analysis of propellants with ADN and GUDN have been determined. These parameters are compared to those of reference blank compositions. It turns out that the ADN/GUDN dual oxidizer may greatly affect the combustion behavior of NEPE propellants. The addition of ADN particles to the propellant formulations can increase the burn rate and pressure exponent (n), when a fracture of AP or GUDN was replaced by ADN, the n increases from 0.52 to 0.67 and from 0.58 to 0.67, respectively. By replacing AP flakes with the same fraction of GUDN, the burning rate decrease by 18.97% at 7.0 MPa and n increased by 12.04% (1-15 MPa), in comparison to the reference propellant. The involved NEPE propellants with and without ADN/GUDN dual oxidizers at various pressure ranges show multi-flame structure, and the brightness of flame increases with the pressure increases. Addition of ADN can decrease the thermal decomposition temperature. Moreover, when part of AP was replaced by GUDN, the thermal decomposition procedure is close to that of the reference sample.

Abstract:In order to obtain the characteristics of nonlinear pressure coupling response of solid propellants, a method for measurement of the nonlinear pressure coupling response function of solid propellants was established. The nonlinear pressure coupling response functions of three formulations of aluminum propellants were obtained. The characteristics of the nonlinear pressure coupling response of propellants were also analyzed in detail. It indicates that with increase of the trigger pressure, higher-order frequency oscillations can be excited and the amplitude of nonlinear oscillation could be larged as well. By triggering at the both ends of the chamber, the oscillation is dominated by the longitudinal first-order frequency, but when triggering in the chamber middle, the oscillation is dominated by the longitudinal second-order frequency.The nonlinear pressure coupling response function is sensitive to the amplitude of the oscillation, whereas the response value near the peak frequency is the most sensitive to the change of the amplitude of the oscillation. The response value is significantly different between the second-order frequency by triggering in the chamber and the first-order frequency by triggering at end of the half-length chamber. This result just verifies the method of the linear and nonlinear methods are different.

Abstract:Due to great demands on solid rocket motor and solid ramjet for thrust control, the experimental study on the effect of high voltage electric fields on the combustion characteristics of solid fuel polymethyl methacrylate （PMMA） nwascarried out. The effect of electric fields on the flame structure of PMMA, as well as its burn rate, temperature and flame flow-field was analyzed. The results show that the deformation of PMMA flame is significant under the negative electric fields. When larger electric fields are imposed, their effects on the flame become more significant. The high-voltage electric fields can greatly change the burn rate of PMMA. The burn rate of PMMA increases with the applied positive voltage amplitude by about 34.2%. Under the negative electric fields, the burn rate of PMMA decreases firstly with the increase of the voltage amplitude, and then increases. The burn rate can be decreased by about 15.7%, and it can also be increased by about 15.4%. The maximum temperature of PMMA is decreased when it is subject to the electric fields. The PMMA flame flow increases with the amplitude increase of the voltage. The interaction of buoyancy and electric volume force may be the factors that lead to the change of flame shape, temperature and the thermal feedback of the flame to the solid phase.

Abstract:Based on uniaxial and quasi-biaxial tensile tests and microscopic damage observation experiments of three-component Hydroxyl-Terminated Polybutadience (HTPB) composite solid propellant at different thermal accelerated aging time (0, 32, 74，98 d), temperatures (-50, -40, -30, -20， 25 ℃) and strain rates (0.40, 4.00, 14.29, 42.86， 63 s^-1), the effects of loading conditions on the initial elastic modulus, strength and the corresponding strain have been analyzed. Moreover, failure criteria of the propellant under the tests have been determined. It has been indicated that HTPB propellant fails more easily due to tensile stress under dynamic uniaxial loading, and the thermal aging can further reduce this capability. Thus the strain at maximum tensile stress can be considered as the uniaxial failure criterion. In addition, tensile-compressive strength ratio can better reflect the different properties of the propellants under dynamic uniaxial loading. This value is 0.4 and 0.2-0.3 at room temperature and low temperatures, respectively. The strain at maximum tensile stress of HTPB propellant under quasi-biaxial tension is significantly lower than that in uniaxial tension. Furthermore, the extent of reduction increases with extended aging time and decreased temperature. The proportion for unaged and aged propellants is 60%-85% and 40%-60%, respectively. Finally, this strain is independent of stress state and strain rate at the lower temperature and higher strain rates. Thus the strain at maximum tensile stress under dynamic biaxial tension can be considered as the failure criterion of HTPB propellant. It can also be used to analyze the structure integrity of propellant grains for tactical missiles during ignition of solid rocket motor (SRM). Moreover, its value can be determined with the master curves and aging models.

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