Condensed particles in the internal flow field of a solid rocket motor can cause serious erosion to thermal insulations under overload flight condition. Multi-walled carbon nanotubes (MWCNTs) can ...improve erosion resistance of insulations but at the cost of reduced thermal insulation performance. In this study, MWCNTs were precoated with pyrolytic carbon via chemical vapor deposition to reduce their thermal conductivity. Further, ethylene propylene diene monomer insulations reinforced with different contents of MWCNTs and precoated MWCNTs were prepared. Moreover, their ablation performances under simulated overload ablation conditions were tested and compared. Experimental results show that precoated MWCNTs can not only improve thermal insulation performance but also significantly increase particle erosion resistance of ethylene propylene diene monomer insulation materials. Charring ablation rate of the formula with 10phr precoated MWCNTs is 44.3% lower than that of basic formula and 41.6% lower than that of the formula with 3phr MWCNTs which has the same pure MWCNTs content. The char layers of MWCNTs reinforced formulas are more complete than basic formula, indicating that MWCNTs can improve char layer structure under particle erosion condition. The precoated MWCNTs can also promote the CVD reaction of pyrolysis gas in char layers during ablation, decreasing the porosity of char layers, which can increase the particle erosion resistance of insulation.
•Particle resistance of EPDM insulation was studied with the overload simulation SRM;•EPDM insulation with less quantity of precoated MWCNTs retain good particle resistances;•Precoated MWCNTs can promote CVD reaction of pyrolysis gas within char layers.
The baffle is a practical and promising passive damping method of dissipating acoustic energy and increasing acoustic losses in a solid rocket motor (SRM). In this study, numerical studies were ...conducted conjunction with the acoustic pulse response method (PRM) to evaluate the acoustic damping performance of a full-scale SRM with a baffle. The forced pulse function was imposed according to the frequency of the longitudinal acoustic mode. We comparatively evaluated the acoustic damping performance of the SRM with and without a baffle. The performances were characterized by (1) acoustic growth rate, (2) damping rate, and (3) acoustic energy. The PRM was first validated using data available in the literature. Several numerical investigations were conducted to develop geometry design criteria, which were subsequently used to ensure the effective operation of the baffle to suppress combustion-driven acoustic modes in the SRM. The effects of 1) the baffle axial location (x/L) and 2) the relative diameter (d/D) on acoustic damping performance were examined in detail. The results indicated that the baffle is effective in suppressing acoustic oscillations only when placed at 1/4⩽x/L⩽1/2. Furthermore, when the baffle was placed at x/L=1/2, a relative improvement of approximately 51% and 15.3% in the growth and damping rates, respectively, was achieved compared with those in the SRM without a baffle. In addition, an annular baffle with a smaller inner diameter was observed to have a good design. A baffle with d/D=1/2 was observed to be associated with a favorable damping effect. This research elucidates the effective design of a baffle in stabilizing combustion in a SRM.
Debonding problems along the propellant/liner/insulation interface are a critical point to the integrity and one of the major causes of structural failures of solid rocket motors. Current solutions ...are typically restricted to methods for assessing the integrity of the rocket motors structure and visually inspecting their components. In this context, this paper presents an improved algorithm to detect liner surface defects that may compromise the bonding between the solid propellant and the insulation. The use of Local Binary Patterns (LBP) provides a structural and statistical approach to texture analysis of liner sample images. Along with color information extraction, these two methods allow the representation of image pixels by feature vectors that are further processed by a Multilayer Perceptron (MLP) neural network classifier. The MLP neural network analyzes liner sample images and classifies each pixel into one of three classes: non-defect, foreign object, and defect. Several tests were executed varying different parameters to find the optimal MLP configuration, and as a result, the best classification accuracy of 99.08%, 90.66%, and 99.48% was achieved for the corresponding classes. Moreover, the defect size estimate showed that the MLP classifier correctly identified defects less than 1 mm long, with a relatively small number of training examples. Positive results indicate that the algorithm can identify liner surface defects with a performance similar to human inspectors and has the potential to assist or even automate the liner inspection process of solid rocket motors.
•An algorithm to detect defective areas on the liner surface of solid rocket motors is proposed.•A neural network model classifies image pixels based on color and texture features.•The algorithm can detect defects on the liner surface with a precision comparable to human inspectors.•The algorithm can prevent the late characterization of bonding defects.
•Thermal safety analysis of large-scale rocket motor with complex charge structure.•The cook-off response can occur in a high-temperature environment.•The temperature of the propellant sharply ...increases in the cook-off process.•The cook-off response time decreases with an increase of the external temperature.•The ignition delay time can be expressed as an exponential function.
This present work is focused to study thermal safety and heat transfer characteristics of large-scale solid rocket motor (SRM) with complex star-charged structure. Here, we have used three-dimensional unsteady heat transfer model coupled with a two-step global reaction mechanism of ammonium perchlorate/hydroxyl terminated polyether (AP/HTPE) propellant. Comparisons of the ignition delay time and temperature of computational results are done with experimental studies, and a reasonable match has been obtained in these comparisons. It is found that the heats transferred from the thermal environment and released from the exothermic reaction of the propellant jointly intensify the occurrence of the cook-off response of the SRM. The propellant at the bottom of the SRM can reach to a higher temperature over the time and thus the exothermic reaction is rapidly intensified with an increase of the propellant temperature, releasing more reaction heat simultaneously. The heat energy is gradually accumulated in the SRM and the propellant is eventually ignited under this high-heat environment. The ignition temperature of the SRM basically linearly increases with the AP content at 10.8% HTPE and the ignition temperature curve under different AP contents is closer to the quadratic curve at 7.8% HTPE. Also, the content of HTPE mainly has a certain effect on the ignition time that is 288 s and 283 s with its content of 7.8% and 10.8%, respectively. The heat transfer rate and the case temperature of the SRM are all increasing with an increase of environmental temperature, while the depth of the heat transfer is decreasing. Further, the ignition delay time can be expressed on environmental temperature as: ti = 2.073263exp11 × Te-2.89.
Hybrid rocket motors (HRMs) easily achieve variable thrust by changing oxidizer mass flow rate. This paper represents numerical and experimental studies on the throttleable HRM with aerospike nozzle, ...which can enhance the performances of the throttleable HRM theoretically. By comparing the performance of the throttleable HRM with the aerospike nozzle and that with the de Laval nozzle, the characteristics of the aerospike nozzle applied to the throttleable HRM are explored. Both numerical and experimental investigations are conducted. In the simulation part, characteristic velocity, combustion efficiency, thrust coefficient and specific impulse of the HRM with the two types of nozzles at different oxidizer mass flow rate are obtained. Comparing with the HRM with the de Laval nozzle, the HRM with the aerospike nozzle can improve specific impulse by about 8.9%∼23.4% when the working pressure ratio is lower than the design pressure ratio. The combustion efficiency of the throttleable HRM with the aerospike nozzle is 3.9%∼8.6% higher than that with the de Laval nozzle because of the core structure of the aerospike nozzle. When the oxidizer mass flow rate is 50 g/s, the thrust coefficient of the aerospike nozzle is increased by 12.3% relative to that of the de Laval nozzle which flow separation occurs in. The corresponding firing tests are performed by the lab-scale motor with 90% hydrogen peroxide (HP) and polyethylene (PE) grain. The numerical and experimental results agree well. The combination of the throttleable HRM and the aerospike nozzle has the advantages to improve the performance of HRMs.
The injection mode of oxidizer gas significantly affects the flow and heat transfer characteristics of rocket motors with separated oxidizer and fuel propellants. In the present study, the effect of ...the multi-hole injector on flow and heat transfer of a novel combined rocket motor is studied with a three-dimensional numerical model. The results show that as the number of injection holes increases from one to five, muti-jet interference effect could occur. The original single coupled combustion zone is split into multiple coupled combustion zones. Multi-peak distributions of temperature and velocity are thus observed. As the combustion heat release is gradually advanced towards the injector, the multiple injection flows form three high-flow velocity zones. The three-dimensional distribution of heat transfer coefficient is highly consistent with that of flow velocity. The average heat transfer coefficient first increases and then decreases with the increase in the number of injection holes. The 4-hole injector renders the largest average heat transfer coefficient. Due to the multi-peak distribution of flow velocity and heat transfer, a multi-peak distribution of regression rate for solid propellant is thus observed. The average regression rate increases greatly and reaches the maximum value for the 4-hole injector. The results also show that the regression rate is controlled by the combustion reaction kinetics in the proximal injector region but is controlled by heat transfer in the region far away from injector. The present study contributes to a better insight into the flow and heat transfer effects on combustion performance of the combined solid rocket motor with multi-hole injector.
•Obtained three-dimensional distribution of heat transfer coefficient of the motor.•Revealed multi-jet interference effect on mixing and heat transfer properties.•Observed multi-peak distributions of temperature and flow velocity.•Identified effects of flow and heat transfer on regression rate distribution.
Uncertainties, which exist widely in the design of aerospace systems, have a great impact on system response and may lead to design failures. Both the probability and non-probability uncertainties ...exist in real engineering cases. Thus, it is imperative to incorporate the mixed uncertainties into the aerospace systems design and optimization. Present study proposed an efficient design optimization method considering the random and interval uncertainties to conduct the uncertainty analysis and optimize the design of a hybrid rocket motor with mixed uncertainties. A polynomial chaos expansion model was selected to deal with the random uncertainties, and an interval analysis method based on the first order Taylor expansion was adopted to calculate the response bounds of interval uncertainties. A simple structure was firstly used to verify the efficiency and accuracy of the proposed method. The random-interval analysis method was very efficient in searching the optimal results, showing considerable computational cost savings. Subsequently, a design model of hybrid rocket motor was introduced and verified by a long-time firing test. In the optimization of the upper-stage hybrid rocket motor, the comparison between uncertainty analysis and traditional methods showed that the design optimization considering the uncertainty in design variables was able to achieve more robust results. Compared with the deterministic scheme, the uncertainty scheme increased the total engine mass by nearly 18%, but the probabilities of all constraints considering parameter fluctuations remained above 0.9. Capable of solving complex problems with mixed uncertainties, this method shows its potential in aerospace engineering applications.
The effects of two-phase flow and radiative heat transfer have a significant impact on the internal thermal environment and thermochemical ablation process in aluminized solid rocket motor (SRM). ...However, these two effects were not revealed systematically in previous studies. In this paper, a multiscale methodology combined radiative properties of molten alumina particles at microscale calculated by Mie theory and the two-phase flow and radiative heat transfer at macroscale calculated by Eulerian-Lagrangian method coupling with discrete ordinate method is constructed within aluminized SRM. The convection and radiation are coupled with the chemical kinetics to predict the thermochemical ablation and thermal environment. The influence of two-phase fluid flow and thermal radiation on the internal thermal environment and thermochemical ablation of a SRM nozzle is analyzed quantitatively. Comparisons are made among the methodologies that models the alumina particles as gas species, considers the effect of two-phase flow and considers the effects of two-phase flow and radiative heat transfer. The results show that the erosion rate and total heat flux at the inner wall in the convergent section increase noticeably when considering the effect of two-phase flow. Meanwhile, the erosion rate in the divergent section also increases and the thermal environment is dependent on the particle size and aluminum content. Radiative heat transfer considerably enhances the total heat flux, especially in the convergent section. On the contrary, its impact on the erosion rate is very small except for the inner wall with very low temperature.
Paraffin-based fuel is a promising and low-cost alternative for hybrid rocket motor applications due to its advantage of high regression rate. Meanwhile, paraffin fuel alone has poor mechanical ...strength that cannot meet the requirements in practical applications. Embedding skeleton polymer into paraffin-wax fuel can effectively enhance the mechanical properties and keep the advantage of high regression rate. In this paper, the mechanical and combustion characteristics of different polymer materials were comprehensively evaluated by using SEM, TG, TDG, mechanical test and flame propagation speed measurement experiments. A type of polymer reinforced paraffin-wax fuel grain has been fabricated and tested using a newly-designed optically accessible solid fuel tubular combustor. Combustion efficiency of the polymer reinforced paraffin fuel grain was further measured using a lab-scaled hybrid rocket motor. As a result, ABS is a better candidate that can be used in polymer reinforced paraffin fuel grain in terms of favorable combustion and mechanical properties. The flame propagation speed of polymers printed by SLS technique are much higher than those printed by SLA technique due to its rougher surface. Moreover, Paraffin liquid droplets and columns were clearly observed during the combustion process of the new proposed polymer reinforced paraffin fuel due to the entrainment and shearing of oxidizer flow, leading to higher regression rate and better mixing efficiency, which turns in higher combustion efficiency in fuel rich conditions compared with literature results.
Droplet entrainment offers high regression rate of HTPB/paraffin fuels, but also leads to more complicated mass and heat transfer mechanisms during combustion. The droplet entrainment of the fuels in ...a windowed two-dimensional (2D) slab burner were investigated by using a high-speed camera, and the convective heat transfer coefficient was modified. The results indicated that the entrained droplets could be easily generated below 1.29 MPa of combustion pressure (pc), which was dominant in the regression of fuels, whereas the regression rate decreased by approximately 0.9 mm when pc increased from 0.32 MPa to 1.29 MPa due to the negative correlation of droplet entrainment rate with pc. However, the regression at higher pc mainly depended on weak evaporation and thermal decomposition rather than droplet entrainment, which was insensitive to pc and decreased by only about 0.05 mm s−1 with the increase in pc from 1.85 MPa to 2.52 MPa. The carbonized layer could be easily formed on the burning surface at higher pc, which hindered the droplet entrainment. Additionally, the improvement of convective heat transfer coefficient with the presence of droplet entrainment was evaluated, and the blocking effect was weakened, especially under high oxidizer mass flux.
•The generation of entrained droplets is studied for the combustion of HTPB/paraffin fuels.•The dominant position of droplet entrainment for the high regression rate characteristics of HTPB/paraffin fuels is confirmed.•The presence of characteristic combustion chamber pressure is found for the droplet entrainment process of HTPB/paraffin fuels.•The correlation of convective heat transfer coefficient considering droplet entrainment during HTPB/paraffin fuels regression is established.
