Jian Zheng

Expandable polystyrene (EPS) nozzle covers can be used to replace traditional metal nozzle covers due to their excellent mechanical properties, as well as being lightweight and ablatable. As an important part of the solid rocket motor, the nozzle cover needs to be designed according to the requirements of the overall system. This study lays a theoretical foundation for the engineering design and performance optimization of the EPS nozzle cover. In this paper, the method of combining test research and numerical simulation is used to explore the pressure bearing capacity of EPS nozzle covers with different thicknesses under linear load. Firstly, the quasi-static tensile, compression and shear tests of EPS materials were carried out by universal testing machine, and the key parameters such as stress-strain curve, elastic modulus and yield strength were obtained; Based on the experimental data, the constitutive model of EPS material with respect to density is fitted and modified; The VUMAT subroutine of the material was written in Fortran language, and the mechanical properties of the nozzle cover with different material model distribution schemes and different thicknesses were explored by ABAQUS finite element numerical simulation technology. The results indicate that the EPS nozzle cover design based on the two material model allocation schemes better aligns with practical conditions; when the end thickness of the EPS nozzle cover exceeds 3 mm, the opening pressure formula for the cover based on the pure shear theory of thin-walled circular plates becomes inapplicable; the EPS nozzle cover exhibits excellent pressure-bearing capacity and performance, with its pressure-bearing capacity showing a positive correlation with its end thickness, and an EPS nozzle cover with a 9 mm end thickness can withstand a pressure of 7.58 MPa (under internal pressure conditions); the pressure-bearing capacity of the EPS nozzle cover under internal pressure conditions is higher than under external pressure conditions, and when the end pressure-bearing surface thickness increases to 9 mm, the internal pressure-bearing capacity is 3.13 MPa higher than under external pressure conditions.

In order to predict the maximum elongation of composite modified double base (CMDB) propellants under different aging time, temperature and strain rate, systematic uniaxial tensile characterization was performed across six controlled temperatures (233.15 K, 253.15 K, 273.15 K, 283.15 K, 298.15 K, 323.15 K) and three decades of strain rates (3.3 × 10⁻⁴ s⁻¹, 3.3 × 10⁻³ s⁻¹ and 3.3 × 10⁻²s⁻¹) through accelerated aging protocols. The maximum elongation of CMDB propellants under different temperatures and strain rates during aging was obtained, and the aging mechanism of CMDB propellants was analyzed in combination with scanning electron microscopy experiments. The elongation master curve was constructed based on time-temperature equivalent principle, and the elongation master curve model considering aging effect was established combining with existing theories. The results show that the maximum elongation of CMDB propellant decreases monotonically with the aging time, and the microscopic damage in the tensile section gradually increases. The aging time has little effect on the temperature and rate-related properties of the material. By comparing the predicted results of the elongation master curve model considering the aging effect with the experimental results, It is shown that the elongation master curve model considering the aging effect can describe the elongation master curve under different aging time.

In the field of gun launched missile extended range rocket, the propellant grain in the rocket needs to withstand significant launch loads during their firing phase, and also bear the high pressure caused by ignition, and the impact of launch overloads and ignition shocks on the structural integrity of propellants becomes very important. So this work investigated the dynamic initiation fracture toughness of the composite modified double-base (CMDB) propellant by both experiments and numerical simulations. The dynamic mechanical properties test of the cracked straight through flattened Brazilian disc (CSTFBD) specimens were conducted using a modified Split Hopkinson pressure bar (SHPB). By comparing the results of quasi static and dynamic numerical simulations, it was found that dynamic fracture initiation toughness can be determined by time-to-fracture using the quasi-static theory. The numerical simulation results combined with the ZWT constitutive model agree well with the experimental results, indicating that ZWT constitutive model is suitable for numerical simulation calculation of propellant structural integrity under dynamic load, and provides a theoretical basis for propellant structural integrity analysis under dynamic load. During the measurement of the mechanical response, the fracture surfaces of the dynamic test specimens were observed by electron microscopy scanning. Then the evolution of the microstructure synchronously was obtained. The scanning electron microscope (SEM) results revealed that fracture modes and breakage of the ammonium perchlorate (AP) particles in the surface layer played an important role in determining the failure mechanism, which revealed the failure mechanism of the propellant under dynamic load. The result of experimental measurement showed the influence of loading rate on the dynamic fracture initiation toughness of CMDB propellant.

The increased-range solid rocket engine of new ammunition often needs to withstand extremely high acceleration overload, leading to damage to the propellant’s structural integrity. To address this issue, this paper constructs a nonlinear viscoelastic constitutive model for the CMDB propellant by using low- and high-strain rate mechanical property tests. Combined with the secondary development of explicit dynamics and numerical simulation technology, the mechanical response of a certain type of solid rocket propellant under different acceleration impacts is analyzed. The results show that under acceleration impact loads, the propellant will compress, rebound, and recover over time, continuously cycling. Its axial displacement, maximum equivalent stress, and maximum equivalent strain exhibit irregular sinusoidal wave-like periodic cycles. Looking at the time when the peaks appear, the time when the maximum equivalent stress appears always lags behind the time when the maximum axial displacement peak appears. Due to the viscous effect of the viscoelastic material of the propellant, the time when the equivalent strain peak appears will lag behind the equivalent stress. Because of the material’s damping effect, both the peak values of the maximum equivalent stress and equivalent strain decrease over time. Under continuous high acceleration impact loads, this viscous damping phenomenon continuously diminishes, and the peak value of the propellant’s axial displacement gradually increases.

To investigate the flow mechanism of feather-like rough airfoils based on swift wings, computational simulations were employed to explore their overall aerodynamic characteristics in comparison to equivalent smooth airfoils. The study focused on angles of attack ranging from 0° to 20° at low Reynolds numbers. The results reveal that the rough airfoil exhibits higher lift and lower drag compared to the smooth airfoil at moderate angles of attack ranging from 6° to 10°, resulting in significantly improved aerodynamic efficiency. Notably, at an angle of attack of 8°, the aerodynamic efficiency is increased by 19%. However, at angles of attack smaller than 6°, the increase in drag outweighs the increase in lift, leading to lower aerodynamic efficiency for the rough airfoil. Conversely, when the angle of attack exceeds 16°, both airfoils experience separated flow-dominated flow fields, resulting in comparable effective aerodynamic shapes and similar aerodynamic efficiencies. Furthermore, the study found that increasing the Reynolds number results in greater pressure differences in the flow field, leading to higher aerodynamic efficiency. These preliminary conclusions are valuable for elucidating the flight mechanisms of bird-feather-like wings and can inform the design or morphing design of bio-inspired micro aerial vehicles in the near future.

Aiming at the application of C/C composite in nozzle throat lining of solid rocket motor, the thermochemical ablation of C/C composites under CO2 or N2 different confining pressures is studied by using a continuous laser ablation test device, and some useful conclusions are obtained. In CO2 confining pressure environment, with the increase of pressure, the heat absorption effect of thermochemical reaction becomes more obvious, and the surface temperature of the sample decreases. The temperature at 1MPa and 2MPa decreases by 12% and 23% respectively, compared with the maximum temperature in N2 environment because there is no chemical reaction between N2 and C/C composites. The sample surface is carbonized after endothermic, and forms a large number of holes.

The aluminium–magnesium fuel-rich propellant is one of the classified propellant used in the solid rocket ramjet. The weapons with the rocket ramjet are under high overload by launching, the mechanical properties of the propellant should be studied under high strain rate. The uniaxial compressive mechanical curves of the aluminium–magnesium fuel-rich propellant under high strain rates have been obtained with the modified split Hopkinson pressure bar in this paper. Four groups of stress–strain curves were obtained at different strain rates of 2600, 3300, 4500 and 5100 s−1. The experimental results show that the mechanical properties of the aluminium–magnesium fuel-rich propellant are rate-dependent, and the Mg–Al fuel-rich propellant has the mechanical properties of viscoelastic material. With the increase of strain rate, the modulus of the aluminium–magnesium fuel-rich propellant obviously increases. The underlying mechanism of the significant rate sensitivity was investigated by adopting the scanning electron microscope from a micro-structure point of view.