Nonsteady Combustion Mechanisms of Advanced Solid Propellants

Nitrate ester plasticized polyether (NEPE) propellants, are high energy solid propellants that are used widely in solid rocket motor. It is necessary to study the mechanical properties of NEPE propellants in order to ensure an accurate prediction of the performance of solid rocket motors and ensure the safety of launch platforms.We studied the viscoelastic hyperelastic constitutive model of NEPE propellants. The speckle defocusing method and digital image correlation(DIC) method are used to obtain the strain of solid propellant NEPE in three directions during unidirectional stretching, and the volume change rate, Poisson's ratio and true strain curves of NEPE under different stretching rates are obtained. With the increase of loading rate, the maximum stress that NEPE can encounter increases, and the modulus at the initial elastic stage also increases. At the initial stage of stretching, the volume change rate is close to 1, the Poisson's ratio fluctuates within a range close to 0.5. When the engineering principal strain in the tensile direction reaches a certain value, the volume change rate begins to increase and the Poisson's ratio begins to decrease. The strain of solid propellant NEPE in three directions during uniaxial tension was successfully measured by speckle defocus method and digital image correlation (DIC) method. NEPE is close to being a super elastic material at the initial stage of stretching. The change trend of the volume change rate curve of different loading rates is consistent, and the stretching rate has no effect on the volume change rate of NEPE.

Solid rocket motor (SRM) is a core component of rockets. Considering the high cost of solid rocket motors and the high demand for transportation and storage, there is an urgent need to predict the performance of solid rocket motors before the ground ignition test to lower the cost of tests. Using data mining technology to predict various parameters of rocket motors with historical data has become a new development direction. In order to improve the data fitting effect and prediction accuracy, a novel prediction model was proposed, which tries to combine the Whale Optimization Algorithm (WOA) with the Error Correction Gated Recurrent Unit neural network (ECGRU) to correct the basic prediction performance of the Long Short-Term Memory neural network model (LSTM). Compared with traditional methods, experiments showed that the novel model was beneficial in improving the reliability of the prediction of the thrust and specific impulse.

Curing activity in the preparation of solid composite propellants determines the performance of solid rocket motors in operation. Limited by the lack of effective monitoring tools, the complete curing behavior and thermal-induced curing kinetics are rarely disclosed. It is still a challenge to monitor in situ and in real-time the physical and chemical cross-linking reaction during the curing of propellant. Herein, we demonstrate a promising approach based on optical fiber capable of being implanted inside the propellant to monitor the internal stress evolution during the curing process, by taking hydroxyl-terminated polybutadiene propellant as an example. Attributed to the strain and temperature sensitivity of a pair of optical fiber gratings, the thermal-assisted physico-chemical cross-linking states of curing process have been demonstrated in detail. By tracking the stress-induced wavelength shifts of fiber gratings and calculating the curing mechanism function, the complete curing roadmap, including the viscous flow stage, gel stage, hardening stage can be clearly revealed, and the curing completion times are obtained as 154, 81, and 40 h, at the curing temperatures of 60, 70, and 80 °C, respectively. The apparent activation energy of this curing system obtained by calculation is 73.88 kJ/mol. This flexible fiber-based sensor provides an effective tool for unraveling the cure kinetic mechanism, and paves a universal pathway to guide the preparation and applications of versatile composite materials for solid rocket motors.

Polymeric composites are widely used in aerospace. One such important use of polymeric composites is the filament-wound composite case for the Solid Rocket Motor (SRM). Environmental conditions expected during flight and other phases of the SRM’s life cycle affect its performance. The performance of SRM, in turn, depends primarily on the composite case and propellant grain. In this study, the performance evaluation of SRM with the composite case is carried out for different environmental conditions. These cases are designed to sustain very high pressure and temperature environments generated due to the combustion of propellant grains and are intended to perform adequately at any time during their service life. The performance evaluation test matrix is formulated considering various expected dynamic and thermal conditions. The small SRM with a composite case is subjected to planned tests as per the matrix, followed by static tests. The pressure versus time data is plotted for all test conditions. The ballistic parameters result of the virgin (without subjecting it to any test) small SRM is considered as a reference for comparative studies with the rest of all other test conditions results. All the ballistic performance test results are also analysed with respect to the pressure-time curve for mission bounds. The analysis of experimental test results reveals that various environmental dynamic conditions like random vibration, acceleration and shock, do not affect the composite case and the ballistic performance of a small SRM. The effect of grain temperature on ballistic performance is also found as expected.

The performance of solid rocket motors is limited by finite-rate chemical reactions occurring in the nozzle expansion. In the present work, a numerical approach is adopted in order to investigate the properties of the reactive flow and their impact on the motor specific impulse. Numerical simulations are performed on two different test cases, the second stage of the Minuteman III and the third stage of the Vega launcher, comparing the impact that different chemical mechanisms have on flow evolution and specific impulse estimate. The outcomes of the analysis suggest that CFD simulations represent a viable and robust method to provide a satisfactory prediction of the chemical non-equilibrium efficiency, outperforming semi-empirical laws when it comes to dealing with new solid motors of latest generation.

During the ignition process of a solid rocket motor, the pressure changes dramatically and the ignition process is very complex as it includes multiple reactions. Successful completion of the ignition process is essential for the proper operation of solid rocket motors. However, the measurement of pressure becomes extremely challenging due to several issues such as the enormity and high cost of conducting tests on solid rocket motors. Therefore, it needs to be investigated using numerical calculations and other methods. Currently, the fundamental theories concerning the ignition process have not been fully developed. In addition, numerical simulations require significant simplifications. To address these issues, this study proposes a solid rocket motor pressure prediction method based on bidirectional long short-term memory (CBiLSTM) combined with adaptive Gaussian noise (AGN). The method utilizes experimental pressure data and simulated pressure data as inputs for co-training to predict pressure data under new operating conditions. By comparison, the AGN-CBiLSTM method has a higher prediction accuracy with a percentage error of 3.27% between the predicted and actual data. This method provides an effective way to evaluate the performance of solid rocket motors and has a wide range of applications in the aerospace field.

A lightweight rubber foaming insulation reinforced by carbon nanotubes and carbon fibers for solid rocket motors

Due to the influence of internal factors and external environmental factors, the internal ballistic performance of solid rocket motors (SRM) shows certain deviation during its production and working. In order to describe the internal ballistic performance more accurately, a Monte Carlo method-based internal ballistic prediction and analysis method is proposed, and compared with the traditional limit deviation method. First, a simulation model of internal ballistics was established. Then according to the probability distribution law of the input factors, the simulation was carried out. The results show that under the premise of the same input deviation boundary, the probabilistic method can effectively reduce the dispersion of the internal ballistic performance parameters. Finally, according to the established internal ballistic probabilistic model, the sensitivity of the specific impulse to the influencing parameters was analyzed, and the degree of influence of each parameter to the specific impulse was obtained.

A number of options for Solid Rocket Motor (SRM) performance improvement were studied and evaluated by NASA with respect to payload improvement potential, technical risk, schedule, cost, and impact on Shuttle system performance. It was found that the SRM delivered specific impulse could be improved by increasing the nozzle expansion ratio from 7.16 to 7.72. This was achieved by decreasing the throat diameter of the nozzle exit cone, while staying within tooling, facility, and ICD limitations. A simple modification of the grain inhibiting pattern was made to reshape the thrust-time history and thereby obtain more delivered impulse during the early portion of motor operation. These performance options, initiated in October 1980, provide a 3,000 lb payload increase. Longer range options are the use of a filament wound composite case, a HTPB propellant, and further increases in the nozzle expansion ratio. These improvements have a potential of 10,000 to 11,000 lb payload increase.

Aging effects can have a significant consequence on the mission readiness of solid propellant rocket systems. A potential end result of grain cracking caused by aging is the catastrophic failure of the rocket motor. Propellant changes leading to grain cracking are an increase in cross -link density and/or induced dam age. The detection of this crack generating degradation within the solid propellant is critical. Initial manufacturing variability creates rocket motor assets with varying vulnerability to grain cracking. This significantly raises maintenance requirement s, reduces operational life times and threatens mission readiness. Several experimental methods are currently used to determine the cross link density of solid propellant. Unfortunately, these cross -link density measurement methods are all destructive te st determinations meaning the solid rocket propellant material must be dissected out of the main rocket asset for analysis. Dissection work is extremely expensive in terms of manpower, materials and money. Current non -destructive evaluation (NDE) and non -destructive inspection (NDI) techniques offer limited capability for the determination of cross -link density and/or induced damage. Photon Induced Positron Annihilation (PIPA) has demonstrated that non -destructive results on strain buildup and grain crac king in solid rocket propellant can be determined using the PIPA technique and that propellant damage could be detected using this technique through the rocket case of actual rocket systems, which could result in accurate and reliable assessments of compon ent readiness either at the manufacturing level or during field inspections. This NDI process will provide reliable material damage characterization including the buildup of defects and changes in the atomic structure of the propellant without disassembly and at detection levels below current NDI methods.

ABSTRACTNitrocellulose (NC) and nitroglycerin (NG) are conventional energetic ingredients of solid double-base gun propellants. NC and NG are aliphatic nitrate ester compounds that are manufactured by nitrating through immersion in acid. Solid propellants are used in a wide range of both civilian and military applications, for example, as gas generators in airbags, as fuels in rocket motors or to shoot projectiles in guns. In all illustrations, the function of solid propellant is to generate gas, which is then employed to do mechanical work. The maximum pressure and the projectile velocity are important parameters of the interior ballistic process. The goal of the interior ballistic design of the gun is to attain a higher muzzle velocity under the smaller maximum chamber pressure. The burning rate of the propellant directly affects the formation rate of burning gases, which affects the ballistic parameters, either by diminishing the ballistic effectiveness or raising the safety hazard to the operator during firing. Note that the effect of the change of burning rate is not always negative; the fact is that we could change/control the burning rate to get better ballistic performances, includin ballistic efficiency as well as launching safety. Nevertheless, nitrate ester propellants are subjected to physical and chemical degradation during storage and this can modify the burning rate. Here we describe the effect of aging on the burning rate of spherical double-base propellants both qualitatively and quantitatively. We experimented unaged propellant, and propellant that was artificially aged at 71°C and 80ºC. The durations of heating at 71ºC were 10, 20, 30, 37, 48 and 62.5 days, respectively. The durations of heating at 80ºC were 1.8, 3.63, 5.41, 7.22, 9.02 and 10.83 days, respectively. The burning rate of investigating propellant was obtained by using closed-vessel experiments. The results show that towards low pressures, the burning rate increased with aging. Towards high pressure, the burning rate decreased as a function of aging. The effect of aging was quantified by the determination of defined aging sensitivity coefficients.

The application of optical diagnostic techniques, predominantly Planar Laser Induced Fluorescence (PLIF) and UV-Visible absorption, to the study of solid propellant flame chemistry, structure, and kinetics is described. Measurements outlined include time resolved 2D images of species concentrations and flame structure during transient ignition of solid propellants, calibrated ID and 2D species profiles during deflagration of solid propellant energetic ingredients, ID and 2D temperature profiles in solid propellant flames, multi-pass absorption measurements through solid propellant flames, and 2D images of species concentrations and diffusion flame structure during ignition and deflagration of sandwiches of solid propellant oxidizers and binders. Species measured under some or all of these conditions include CN, CH, C2, OH, NH, NO, NO2, NCO, and H2CO. Temperature has been obtained from fitting rotational and vibrational population distributions for OH, CN, and NO. These studies are a useful adjunct to the overall task of understanding the mechanism of solid propellant combustion and developing a computer code for the a-priori prediction of propellant ballistic properties.

Regression rate and combustion performance investigation on hybrid rocket motor with head-end swirl injection under high geometric swirl number

Erosive burning rate studies of composite propellants under high cross flow velocities

The addition of carbon nanotubes throughout the binder phase of energetic particulate composites is investigated in an effort to develop real-time embedded sensing networks for detection of small-scale damage in polymer bonded explosives undergoing mechanical load. The experimental effort herein focuses on the exploration of multi-walled carbon nanotube (MWCNT) concentration in energetic composite material; the fabrication of such specimen include the substitution of sugar as a mock energetic for the crystal particulate Ammonium Perchlorate (AP), an oxidizer most often used in solid rocket propellants. Further explored was the particulate embedded into a binder of PDMS, a polymeric silicone, as well as the addition of aluminum powder, a common combustive fuel in solid propellants, in the particulate. Electrical and mechanical properties of neat (no MWCNTs) energetics and MWCNT hybrid energetics were quantitatively evaluated under monotonic compression, and tension to failure. Noteworthy electro-mechanical response was obtained for these MWCNT AP hybrid energetics, justifying further study of CNT binder network formation for real-time electro-mechanical sensing in an effort for structural health monitoring (SHM) in energetics.