Use of Dynamic Mechanical Measurements to determine the aging behavior of solid propellant

A process for determining the aging characteristics of solid rocket propellants by measuring their dynamic mechanical properties is described. Samples of rocket propellant are tested using a mechanical spectrometer at low strain levels and low frequencies. The change in the dynamic storage modulus of the propellant with aging time and temperature is used to determine the aging rate anf the likely mechanisms occuring during aging. The technique is advantageous is several respects:

• (1) because the tests are non-destructive and use small test specimens, and entire aging program can be conducted using much less propellant than is required ihn traditional aging programs (propellant requirements are reduced by more than an order of magnitude)
• (2) the technique can be applied to samples with unusual or unique geometries which cannot be tested using traditional methods;
• (3) test specimens can be obtained directly from live, operational solid rocket motors, and service life and mechanical property degradation can be determined.

     

Short-Action Solid Rocket Motors with Double-Base Propellants

Due to various reasons double-base solid propellants have been replaced increasingly by composite propellants in the past years. However, the ALARM rocket motor of the Bayern-Chemie GmbH is one exception. Applications ideal for double-base propellants are short-action rocket motors with burning times ranging from some milliseconds up to approximately 200 ms. Various rocket motors of this type were developed at Dynamit Nobel GmbH for different kinds of application. Based on multiple-tube grains improved designs and manufacturing methods have been developed to enable cost-effective solutions even at low production rates. This includes also simplified test procedures to supervise the propellant fabrication.     

Some Aspects on Safety and Environmental Impact of the German Green Gel Propulsion Technology

This article gives a short introduction into the technology of gelled propellant rocket motors and gas generators. A brief introduction outlines the specific features of the German green gelled propellant rocket motor technology and the family of monopropellants that cover a variety of requirements with respect to smoke, combustion temperature, and combustion pressure ranges. The discussion of the hazard potential comprises military insensitive munitions (IM) and civil classifications as well as a comparative assessment of the environmental and health impacts from manufacturing over use and finally to disposal. Summarizing the properties over all categories shows that gelled propellants provide a unique combination of good insensitivity and low environmental and health hazard potential compared to other liquid propellants, fuels, and oxidizers or solid rocket propellants.     

Comparative Study on Infrared Irradiance Emitted from Standard and Real Rocket Motor Plumes

The infrared irradiance signature from exhaust plume is essential for the design of solid rocket motors. To overcome the difficulty of conducting experiments using real rocket motors, experimental studies were carried out to compare standard rocket motors and real rocket motors of the same propellant. The static firing tests on standard and real rocket motors of NEPE and HTPB propellants were conducted. Despite different rocket motor size and methodology of spectro‐radiometric measurement, the spectral characteristics of the infrared irradiance signature for both rocket motors were quite similar. The standard and real rocket motors of HTPB propellant showed similar tendency of steady infrared irradiance emission throughout the combustion, whereas both rocket motors of NEPE propellant showed a rapid emission in the midstream of combustion. The total infrared irradiance of NEPE was about 55 % less than that of HTPB propellant for both standard and real rocket motor experiments. Additionally, the relative amounts of chemical products produced during propellant combustion came out to be similar for both rocket motors. The experimental results indicated that the spectral characteristics of infrared irradiance and combustion products were quite similar for different sized rocket motors of same propellant and that a correlation of infrared irradiance signature exists between small‐sized standard rocket motors and real rocket motors. Thus, the spectral characteristics of real rocket motors could be reasonably estimated from the results of standard rocket motors.     

Novel burning rate measurement technique for solid propellant by means of ultrasonics

A novel burning rate measurement technique for solid propellants using ultrasonics has been developed. Ultrasound is applied to the burning surface of the end-burning propellant grain from the bulk side, and the reflected wave, which is frequency-shifted by the Doppler effect, is analyzed with a wavelet technique. This method enables us to get the instantaneous linear burning rate and, thus, can be a strong tool for the instability study of solid propellants. The wavelet method is also favorable for the identification of the reflected signal from the burning surface even if the signal intensity becomes very low and indistinguishable from white noise with normal measuring techniques. Efforts have also been paid to eliminate the coupling material between the ultrasonic probe and the propellant grain to simulate the real situation of rocket motors. An oscillation deadener circuit has been successfully employed to reduce strong multi-deflection signals within a metallic plate between the probe and the propellant grain. With these two improvements, it becomes possible to detect and process the signals in longer propellant grains in the actual solid rocket motor situations.     

Viscosity Prediction of Composite Solid Propellant Slurry

The viscosity of composite solid propellant slurry is an important parameter in charging technical performance of solid rocket motors. It is affected not only by the liquid phase ingredients of propellants, but also by the size, content, shape and surface properties of solid fillers in propellants. This paper will present a prediction formula of viscosity of composite solid propellant and will also give a method for calculating the maximum packing volume fraction of solid in the formula. One can predict the viscosity of propellant slurry with the formula if the composition of the propellant is known.     

Effects of Relaxed Modulus on the Structure Integrity of NEPE Propellant Grains during High Temperature Aging

The nitrate ester plasticized polyether (NEPE) propellants were accelerated aged at 65, 70, and 75 °C, respectively. The dynamic mechanical properties of NEPE propellants were determined with the dynamic mechanical analysis method. The strength limit of NEPE propellants was tested by the uniaxial tension test. The relaxed modulus and strength limit both decreased. By calculating the structural integrity of NEPE grain, the effects of modulus changes on the structure integrity were analyzed. The results provide a reference for the life prediction of solid rocket motor containing an NEPE propellant.     

Rheology of HTPB propellant. I. Effect of solid loading,oxidizer particle size,and aluminum content

Composite solid propellants based on hydroxyl-terminated polybutadiene (HTPB) have become the workhorse propellants in the present-day solid rocket motors. The other major ingredients of a composite propellant are the crystalline oxidizer and metallic fuel. As the solid loading of such propellants is as high as 86–90%, their rheological behavior is very complex. The propellant slurry needs to have reasonably low viscosity and a long pot life for better casting and, hence, for a defect-free rocket motor. The primary factors affecting the solid propellant viscosity are solid content, particle size, shape, and distribution. The present study concerns the variations of solid loading from 80 to 89% at constant aluminum cotent, variation of aluminium from 0 to 22% at constant solid loading, and the coarse-to-fine ratio of the oxidizer. The plots of yield stress, consistency index, pseudoplasticity index, and thixotropic index at different time intervals are drawn for all these parametric changes. Based on these rheological studies, the optimum ratio of oxidizer coarse-to-fine ratio, aluminum content, and level of solid loading have been determined.     

Ultrasonic Investigation of Mechanical Properties of Double Base Rocket Propellants

For a series of double base rocket propellants and for poly- methylmethacrylate (PMMA) the longitudinal and transverse sound wave velocities are measured at a frequency of 0.351 MHz in t h e temperature range of −40°C to +60°C. The relations between these acoustic properties and mechanical properties are given and the principal independent elastic constants are calculated. With increasing temperature these constants of the propellants decrease gradually, which points to a softening of the material. This is important for the functioning of the propellant grains in the rocket motors.     

用临界液萃取技术处理复合固体推进剂的工艺研究

通过对火箭发动机含高氮酸铵（AP）的复合推进剂用临界液氨销毁来介绍这一工艺方法。销毁工艺由四个连续的步骤构成：第一步采用临界液氨把含AP的复合推进剂从火箭发动机中取出;第二步萃取AP并将含AP的液氨和粘结剂残渣分离开;第三步从氨水中蒸发出AP;四第步将氨蒸气冷凝并重新循环使用。     

高速动能导弹及超高速导弹用固体火箭推进剂

综述了国外几种主要动能拦截导弹、战术动能导弹和超高速导弹以及其所用固体推进剂的配方和主要性能,在此基础上分析了高速动能导弹和超高速导弹对固体推进剂在性能方面的需求。推进剂的高压燃烧稳定性问题以及采用新型含能材料组分的NEPE类配方是高速动能导弹发射推进剂研究开发的重点技术领域和发展趋势;燃气清洁、快速点火响应以及能在高过载条件下稳定工作等是当前KKV轨／姿控系统对固体推进剂性能的主要要求。尽管在实用性能上存在很多问题,但凝胶推进剂应用于轨／姿控系统具有更加灵活的能量输出控制能力,可使KKV更具智能飞行和快速机动的能力,是KKV控制用推进剂的发展方向。     

Ballistic Modification of Nitramine Propellants with Special Reference to NG‐PE‐PCP‐Based High Energy Propellants

The burning rate pressure relationship is one of the important criteria in the selection of the propellant for particular applications. The pressure exponent (η) plays a significant role in the internal ballistics of rocket motors. Nitramines are known to produce lower burning rates and higher pressure exponent (η) values. Studies on the burning rate and combustion behavior of advanced high‐energy NG/PE‐PCP/AP/Al‐ and NG/PE‐PCP/HMX/AP/Al‐based solid rocket propellants processed by a conventional slurry cast route were carried out. The objective of present study was to understand the effectiveness of various ballistic modifiers viz. iron oxide, copper chromite, lead/copper oxides, and lead salts in combination with carbon black as a catalyst on the burning rate and pressure exponent of these high‐energy propellants. A 7–9 % increase in the burning rates and almost no effect in pressure exponent values of propellant compositions without nitramine were observed. However, in case of nitramine‐based propellants as compared to propellant compositions without nitramines, slight increases of the burning rates were observed. By incorporation of ballistic modifiers, the pressure exponents can be lowered. The changes in the calorimetric values of the formulations by addition of the catalysts were also studied.     

Coupled problem of modeling the internal ballistics of nozzleless solid rocket motors

A physicomathematical formulation of the coupled gas-dynamic and geometric problem of modeling intrachamber processes and calculating the internal ballistics of nozzleless solid rocket motors is given, and a method and algorithm for solving the problem are developed. The parameters in the forward section of the motor are calculated using averaged unsteady equations of internal ballistics, and the parameters of the grain channel and the exit cone are determined using one-dimensional gas-dynamic equations in a quasi-steady formulation. The software package is verified by calculating the internal ballistics of a motor which is utilized without nozzle cluster and simulates intrachamber processes in a nozzleless solid rocket motor during the full-duration firing. The design features, motor operating parameters, and the composition characteristics influencing the energetics of propellants in nozzleless solid rocket motors are calculated. It is shown that, depending on comparison conditions (identical expansion ratios and identical profiles of the nozzle and exit cone), the specific impulse of nozzleless solid rocket motors (the main energy parameter) is slightly smaller than or nearly comparable to that of conventional solid rocket motors.     

Molecular structure of the ideal solid propellant binder

A study has been conducted to investigate the relationship between binder molecular structure and the mechanical/rheological properties of solid propellants. Beginning with the mechanical property requirements dictated by the motor grain operating conditions as well as rheological constraints imposed by available processing technology, the approach taken was to work backwards to obtain the ideal molecular structure of a solid propellant binder. Structural/processing requirements were determined from the demands of three typical rocket motor applications: space transfer, launch vehicle/ballistic missile, and tactical air-to-air. Three general formulation approaches to meet the demands of these applications were considered. These include traditional composite and nitrate ester plasticized formulation approaches, in addition to a hypothetical all-binder propellant. For each of these three formulation approaches, a variety of polymer molecular characteristics were defined in terms of molecular weight, crosslink density, solubility parameter, chain stiffness, monomeric friction coefficient, volume fraction filler, and volume fraction plasticizer. Characterization data for ten polymeric binder systems are reported to show how their molecular architecture influences the resulting propellant properties.     

Investigation of the reason for the softening behaviour of solid propellants

Composite solid propellants consist of an elastomeric polymer binder in which various types of solid particles (oxidizer, fuel, combustion and stabilizing additives, etc.) are incorported. One of the most common state of the art binders is hydroxy-terminated polybutadiene (HTPB). The mechanical properties and the damage behaviour of the solid propellant have important relevance for its application in a rocket motor. Therefore, in this work the softening of crosslinked filled polybutadienes as an effect of mechnical load, and the subsequent recovery were examined at small strains. The contributing factors to the softening are discussed in terms of macroscopic obervable phenomena and of microscopic and molecualr mechnisms which cause those phenomena. It is shown that most of the softening is recoverable because it is due to stress-relaxation at the test conditions used.     

Realization of an Eco-Friendly Solid Propellant Based on HTPB-HMX-AP System for Launch Vehicle Applications

An eco-friendly higher solid loaded HTPB propellant with improved energetics has been realized in small scale by incorporating 10–20% by weight of a high energy material namely, HMX. This propellant with 88% solid loading has given good processability, mechanical properties and reduced pressure index. Pollution due to HCl smoke in the rocket exhaust is reduced by 20 to 30% by weight. This propellant is expected to give a vacuum specific impulse of 293 seconds at an operating pressure of 70 bar and an area ratio of 10. Sub-scale motors of 2 kg size have been successfully static tested to evaluate the performance.     

Sensitivity of Solid Rocket Propellants for Card Gap Test

Card gap test, which is standardized in Japan Explosives Society, was modified in order to apply it to solid rocket propellants and carried out to evaluate sensitivities against shock stimuli. Solid propellants tested here were mainly azide polymer composite propellants, which contained ammonium nitrate (AN) as a main oxidizer. Double base propellant, composed nitroglycerin and nitrocellulose (NC), and ammonium perchlorate (AP)-based composite propellants were also evaluated in order to compare with the azide polymer propellants. It is found that the sensitivity was dominated by the oxidizer characteristics. AP-and AN-based propellant had less sensitivity and HMX-based propellant showed higher sensitivity, and the adding of NC and TMETN were contributed to worse sensitive for the card gap test. Good relationship was obtained between the card gap sensitivity and the oxygen balance of propellants tested here.     

Theoretical Estimation for Unreacted and Reacted States of Composite Propellant

Pertaining to the computer modeling for analyzing bullet or fragment impact sensitivities of solid rocket motors, the input parameters of JWL Equation to describe the unreacted and reacted states of solid propellants are inevitable. To obtain the parameters by experiments is complicated and expensive. This work proposes a theoretical method, including Rodean's theory and the additivity principle, to estimate the unreacted states of composite propellants. First, Rodean's theory and the additivity principle are appraised to be utilizable by the experimental data of homogeneous explosives and composite explosives; then they are applied to estimate the unreacted states of two composite propellants. The theoretical estimations for the unreacted states of the composite propellants are consistent with the experimental results depicted in the literature. With the Marquardt's nonlinear modeling method, the estimated states can be modeled into JWL Equation for gaining the relative parameters. In addition, the modified TIGER code is applied to evaluate the reacted states of one nitramine composite propellant and detonation properties of PBXW-115.     

Hydrogenated Hydroxy-Terminated Polyisoprene as a Fuel Binder for composite solid propellants

The synthesis and application of hydrogenated hydroxy-terminated polyisoprene (HHTPI) to a fuel binder of composite solid propellants were attempted. An HHTPI prepolymer was synthesized through the hydrogenation for the hydroxy-terminated polyisoprene (HTPI) in the presence of nickel and zirconium catalysts over kieselguhr in 2.0 MPa hydrogen and at 443 K – 453 K for 24h. A prepolymer of a number-averaged molecular weight 2500–3800, provided a viscosity level required for the use of a fuel binder from which solid propellant can be possibly made by means of direct casting method. Thermal stability and aging characteristics of HHTPI elastomer against environmental attacks are superior to those of HTPB. Some plasticizers and bonding agents can bring about the acceptable mechanical properties to the propellant grains mainly composed of HHTPI, ammonium perchlorate and aluminium powder. The linear burning rates of HHTPI-based propellants are at the same level with that of HTPB-based propellants. However, the composition that gives the maximum performance with HHTPI-based propellants, shifts to 1–2 wt% fuel-rich side from the most adequate fuel content 12 wt% in HTPB/AP/Al. The HHPTI propellants indicated the similar burning rate as HTPB-based propellants in the linear burning rates in spite of the comparatively poor ignitability. Nevertheless, the static tests of 100 mm dia. sounding rocket motors are successfully performed by an ignition operation at the pressurized condition. The ballistic performances are not inferior to those of the HTPB-based propellants.     

Rheology of HTPB Propellant: Development of generalized correlation and evaluation of pot life

Composite solid propellants based on hydroxy-terminated polybutadiene (HTPB) have become the workhorse propellants in the present-day solid rocket motors world-wide. Because of the high solid loading, the rheological behavior of such propellants is quite complex. The effect of solid loading, oxidizer particle size and aluminium content has been studied and reported. Similarly, the effect of temperature and mixer kinematics and mixing cycle time have also been studied and reported. In the present paper a generalized correlation has been developed for shear stress-shear rate based on the rheological parameters like yield stress, consistency index and pseudoplasticity index which are in turn functions of solid volume fraction, cure time, t, and temperature, and has a correlation coefficient of 0.94 with the experimental values. Also a commonly used term in propellant industries, pot life, has been defined and evaluated for the HTPB propellant system.