Combustion of Propellants with Ammonium Dinitramide

This paper reports a series of experiments involving ammonium dinitramide (ADN), a new energetic oxidizer of potential use in composite solid propellants. The experiments include (a) self‐deflagration of pressed pellets of ADN; (b) combustion of sandwiches with ADN laminae on both sides of a binder lamina that is either “pure” or filled with particulate oxidizer and other additives; and, (c) combustion of propellants with a bimodal oxidizer size distribution, wherein, combustion of coarse ADN and fine AP (ammonium perchlorate) and vice versa were used, in addition to mixtures of coarse ADN and AP, fine ADN and AP, and all‐ADN or all‐AP formulations.     

Characterization and thrust measurements from electrolytic decomposition of ammonium dinitramide (ADN) based liquid monopropellant FLP-103 in MEMS thrusters

Although ammonium dinitramide (ADN) has been targeted as a potential green monopropellant in future space vehicles, its application potential in Micro-electrical-Mechanical System (MEMS) thrusters or microthrusters has been seldom reported in open literature. In this paper, electrolytic decomposition of Ammonium Dinitramide (ADN)-based liquid monopropellant FLP-103 was carried out in an open chamber and MEMS thrusters were fabricated from poly-dimethylsiloxane (PDMS) to characterize the power consumption. Two thrust measurement methods were employed to investigate the electrolytic decomposition of FLP-103 in MEMS microthrusters. The results show that the monopropellant can be successfully ignited at room temperature through 80 V, 0.1 A (8 W) using copper wire as electrodes. In the current thruster design, low thrust was obtained at FLP-103 flowrate of 40 μl·min^-1 but it generated the highest specific impulse, Isp, among all the flowrates tested. The experiments successfully demonstrated the potential application of electrolytic decomposition of FLP-103 in MEMS thrusters.     

Studies on n‐Butyl Nitroxyethylnitramine (n‐BuNENA): Synthesis,Characterization and Propellant Evaluations

The laboratory synthesis of the energetic plasticizer n‐buNENA was scaled up to one kg batch size and the compound was fully characterized by spectral data and elemental analysis. N‐buNENA was formulated with CMDB and EDB propellant compositions. The results showed an improvement in mechanical properties and burning rate over a wide range of pressure along with acceptable limits of hazard and thermal stability as compared to DEP based propellant systems.     

An Overview on the Synthetic Routes and Properties of Ammonium Dinitramide (ADN) and other Dinitramide Salts

In recent years, there has been a considerable interest in the development of novel type of high performance propellants for use in solid rocket motors. Ammonium salt of dinitramidic acid NH₄N(NO₂)₂ (ADN) has attracted wide interest as a potentially useful energetic oxidizer for rocket propellants because of its clean and environment‐friendly exhaust products during burning. ADN contains one N (NO₂)₂ group and its synthesis requires new type of N‐nitration. The present paper reviews the general synthetic methods used for the synthesis of inorganic, organic and metal dinitramide salts and their properties, with a special emphasis on ammonium dinitramide. The salient features with reference to the extent of conversion and ease of separation of the products of the various synthetic methodologies are also addressed.     

Hazard Characterization of Aluminum Nanopowder Compositions

The thermal behaviour in air of two Al nanopowders, Alss and Alsstef, a Teflon coated version of Alss, was determined using DSC, TG‐DTA and accelerating rate calorimetry (ARC). Compared to two larger Al nanopowders, for which hazards results have been reported, Alss and Alsstef are less reactive in air, possibly due to the nature of the passivating and coating layers. The stability of Alss and Alsstef in a wet environment was also investigated using ARC. Alss is very reactive with water, which could lead to a problem of aging in a humid atmosphere. The ”coating” of Alsstef significantly reduces the reactivity of Alss with water. Outgassing behaviour of mixtures of ADN, GAP and various Al powders was investigated using TG‐DTA‐FTIR‐MS. No chemical interactions were observed between ADN/Al, GAP/Al and ADN/GAP. The effect of the addition of Al nanopowders on the thermal decomposition of ADN and GAP was studied using ARC. Al nanopowders had a minor effect on the thermal stability of ADN, while the addition of Alss and Alsstef lowered the onset temperature of GAP. The electrostatic discharge (ESD), impact and friction sensitivities of Al nanopowders and their mixtures with ADN and GAP were also determined. Al nanopowders appear to sensitize ADN to ESD, impact and friction.     

Mechanical Properties of Smokeless Composite Rocket Propellants Based on Prilled Ammonium Dinitramide

Ammonium dinitramide (ADN) is a high performance solid oxidizer of interest for use in high impulse and smokeless composite rocket propellant formulations. While rocket propellants based on ADN may be both efficient, clean burning, and environmentally benign, ADN suffers from several notable disadvantages such as pronounced hygroscopicity, significant impact and friction sensitivity, moderate thermal instability, and numerous compatibility issues. Prilled ADN is now a commercially available and convenient product that addresses some of these disadvantages by lowering the specific surface area and thereby improving handling, processing, and stability. In this work, we report the preparation, friction and impact sensitivity and mechanical properties of several smokeless propellant formulations based on prilled ADN and isocyanate cured and plasticized glycidyl azide polymer (GAP) or polycaprolactone‐polyether. We found such propellants to have very poor mechanical properties in unmodified form and to display somewhat unreliable curing. However, by incorporation of octogen (HMX) and a neutral polymeric bonding agent (NPBA), the mechanical properties of such smokeless formulations were significantly improved. Impact and friction sensitivities of these propellants compare satisfactorily with conventional propellants based on ammonium perchlorate (AP) and inert binder systems.     

Triaminoguanidine Dinitramide,TAGDN: Synthesis and Characterization

Two dinitramide salts with very high heat of formation have been identified, i.e. triaminoguanidine dinitramide (TAGDN) and guanylazide dinitramide (GADN). Thermochemical calculations showed that TAGDN‐propellants have a higher impetus than the corresponding GADN‐propellants at a given flame temperature. Due to the fact that a low flame temperature has to be prefered in a gun, TAGDN seems better suited as a component in gun propellants compared to GADN. TAGDN was thus synthesized, analyzed and characterized to determine some of its properties.     

Burning Characteristics of Ammonium Nitrate‐based Composite Propellants Supplemented with Ammonium Dichromate

Ammonium nitrate (AN)‐based composite propellants have attracted much attention, primarily because of the clean burning nature of AN as an oxidizer. However, such propellants have some disadvantages such as poor ignition and low burning rate. Ammonium dichromate (ADC) is used as a burning catalyst for AN‐based propellants; however, the effect of ADC on the burning characteristics has yet to be sufficiently delineated. The burning characteristics of AN/ADC propellants prepared with various contents of AN and ADC have been investigated in this study. The theoretical performance of an AN‐based propellant is improved by the addition of ADC. The increase in the burning rate is enhanced and the pressure deflagration limit (PDL) becomes lower with increasing amount of ADC added. The increasing ratio of the burning rate with respect to the amount of ADC is independent of the AN content and the combustion pressure. The optimal amount of ADC for improving the burning characteristics has been determined.     

Isocyanate‐Free and Dual Curing of Smokeless Composite Rocket Propellants

Traditional composite rocket propellants are cured by treatment of hydroxyl‐terminated prepolymers with polyfunctional aliphatic isocyanates. For development of smokeless composite propellants containing nitramines and/or ammonium dinitramide (ADN), energetic binder systems using glycidyl azide polymer (GAP) are of particular interest. Polyfunctional alkynes are potential isocyanate‐free curing agents for GAP through thermal azide‐alkyne cycloaddition and subsequent formation of triazole crosslinkages. Propargyl succinate or closely related aliphatic derivatives have previously been reported for such isocyanate‐free curing of GAP. Herein, we present the synthesis and use of a new aromatic alkyne curing agent, the crystalline solid bisphenol A bis(propargyl ether) (BABE), as isocyanate‐free curing agent in smokeless propellants based on GAP, using either octogen (HMX) and/or prilled ADN as energetic filler materials. Thermal and mechanical properties, impact and friction sensitivity and ballistic characteristics were evaluated for these alkyne cured propellants. Improved mechanical properties could be obtained by combining isocyanate and alkyne curing agents (dual curing), a combination that imparted better mechanical properties in the cured propellants than either curing system did individually. The addition of a neutral polymeric bonding agent (NPBA) for improvement of binder‐filler interactions was also investigated using tensile testing and dynamic mechanical analysis (DMA). It was verified that the presence of isocyanates is essential for the NPBA to improve the mechanical properties of the propellants, further strengthening the attractiveness of dual cure systems.     

Characterization of ADN/GAP‐Based and ADN/Desmophen®‐Based Propellant Formulations and Comparison with AP Analogues

Several ADN‐based rocket propellant formulations containing different pre‐polymers (GAP diol/triol, Desmophen® 2200), curing agents (BPS, Desmodur® N100, Desmodur® N3400), plasticizers (BDNPA‐F, TMETN), and filler types (Al, HMX) have been manufactured. Propellant formulations were characterized by tensile tests, SEM analyses and DMA measurements. The study has focused on characterizations of the propellants in terms of evaluation of the strength and strain capability, investigation of the presence/absence of dewetting phenomena, compatibility issues and evaluation of the glass transition temperature. Ammonium perchlorate‐based propellant formulations have also been manufactured and analyzed in order to make comparisons. Aging was investigated using mass loss measurements.     

Utilization of New Non‐toxic Substances as Stabilizers for Nitrocellulose‐Based Propellants

Propellants consisting of nitrocellulose (NC) and/or other nitrate esters are inherently chemically unstable and undergo decomposition even under standard storage conditions. Decomposition of such compounds can be inhibited or nearly stopped when stabilizers are used. However, conventional stabilizers form nitrosamines that have toxic and carcinogenic effects. As a result, these conventional stabilizers should be replaced as soon as possible. A series of NC‐based propellants doped with different novel manufactured stabilizers were investigated using microcalorimetry, conventional stability tests, and sensitivity tests. The results were compared with propellants containing the conventional stabilizer Akardite II. The chemical structure of these new stabilizers and their decomposition products do not enable the formation of toxic N‐nitrosamines.     

Rheological Characterization of Metalized and Non‐Metalized Ethanol Gel Propellants

The gellation of metalized and non‐metalized ethanol with a methylcellulose gelling agent and its effect on the rheological properties (flow and dynamic study) of these gels is reported herein. The rheological study shows that increasing the shear rate reduces the apparent viscosity for a given yield stress (for a shear rate range of 1 to 12 s⁻¹) for both shear rate ranges (1 to 12 and 1 to 1000 s⁻¹) covered in present experiment. The gellant and metal particle concentrations significantly influence the gel apparent viscosity. Distinct changes in thixotropic behavior were observed, while decreasing the concentration of MC gellant and Al metal particles in the ethanol gels. The dynamic study showed that all of the linear viscoelastic regions (LVE) of the gel samples were independent of strain percentage (1 to 10). The G′ values depended on the frequency and exceeded the G′′ values, which indicated a gel‐like highly structured material. The tanδ values showed that all of the ethanol gels were elastic and weak physical gels with a high degree of cross‐linking.     

Efficient Sensitivity Reducing and Hygroscopicity Preventing of Ultra‐Fine Ammonium Perchlorate for High Burning‐Rate Propellants

In this research, several inert materials, including some functional carbon materials, paraffin wax and the well‐known insensitive energetic material 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) were selected to reduce the undesirable high sensitivity and hygroscopicity of ultra‐fine ammonium perchlorate (UF‐AP) via polymer modified coating. Structure, sensitivity, thermal and hygroscopicity performances of the UF‐AP based composites were systematically studied by scanning electron microscopy, sensitivity tests, thermal experiments, contact angle, and hygroscopicity analysis. The results showed that both the impact and friction sensitivity of UF‐AP can be remarkably reduced, respectively, with only a small amount of 2 % (in mass) desensitization agents. Meanwhile, improved thermal decomposition was gained, and the hygroscopicity can also be reduced to a large extent. Propellants containing 10 % coated UF‐AP in mass were processed and tested, the burning rate reached 45.7 mm s⁻¹, 50 % higher compared with that of normal AP, with remarkably reduced impact sensitivity from 11.5 J to 29.6 J and friction sensitivity from 76 % to 28 %.     

ADN及其固体推进剂燃烧特性的研究进展

系统介绍了二硝酰胺铵(ADN)燃烧的最新研究动态,综述了国内外近年来报道的ADN燃烧时发生的物理化学变化、ADN燃烧机理、催化剂/ADN混合物燃烧性能以及ADN基固体推进剂燃烧特性的最新研究进展。首先指出了ADN的燃烧主要受凝聚相反应控制,ADN燃烧波结构包括固相层、泡沫层(包括固-气和液-气)和气相层;其次,总结了ADN基固体推进剂燃烧特性的研究现状,对现有研究中存在的局限性进行了分析;最后,指出继续开发适用于ADN基固体推进剂的新型燃烧催化剂是今后研究的重点方向之一。另外,随着非异氰酸酯固化体系在ADN基固体推进剂中的应用,需进一步加深ADN基固体推进剂燃烧性能的研究,尤其是三唑环的引入对ADN热分解及推进剂中其他组分热分解的影响。     

火药热分解特性与燃烧稳定性间相关性的分析

火药热分解过程的复杂性与其燃烧稳定性之间,存在有某种统计性的增函数关系。火药热分解特性对其燃烧特性的影响,是通过燃烧过程中亚表面热分解表观活化能的变化来实现的,当环境温度和压力较低时,亚表面的热分解特性,与常规条件下该火药的热分析结果相近。对于热分解过程较为复杂的火药,随着环境压力和温度的升高,其亚表面的分期机理和表观分解活化能将发生改变,亚表面分解速率随压力的变化规律也要发生变化,这就导致了燃烧过程的不稳定性,而对于那些热分解过程较为简单的火药,则不会出现这种情况,此外,本文还提出,火药中某些组分的爆燃是导致一些火药燃速压力指数较高的原因。     

An Investigation on Thermal Decomposition of DNTF‐CMDB Propellants

The thermal decomposition of DNTF‐CMDB propellants was investigated by pressure differential scanning calorimetry (PDSC) and thermogravimetry (TG). The results show that there is only one decomposition peak on DSC curves, because the decomposition peak of DNTF cannot be separated from that of the NC/NG binder. The decomposition of DNTF can be obviously accelerated by the decomposition products of the NC/NG binder. The kinetic parameters of thermal decompositions for four DNTF‐CMDB propellants at 6 MPa were obtained by the Kissinger method. It is found that the reaction rate decreases with increasing content of DNTF.     

Preparation and Characterization of Metalized JP‐10 Gel Propellants with Excellent Thixotropic Performance

The preparation and characterization of a series of D‐gluconic acetal supramolecular organic gelators (Gn) which can form gel with high‐energy density hydrocarbon fuel JP‐10 are reported. Rheological measurements show G8 gel exhibits strong mechanical strength at a concentration of 2 wt %, while G18 gel has a remarkable yield stress. What's more, JP‐10 gels show thixotropy with high viscosity, especially for the G8‐based gel, which has an instantaneous recovery ratio. The CGC (critical gelator concentration) of G8 is less than 0.1 wt %, known as a super gelator. More interestingly, G18 can gel JP‐10 at room temperature, which is the first example of room temperature gel propellant formed by a supramolecular gelator. Nano accelerants (Al, B) are dispersed into the gel propellant system, which can be stable for months. The presence of 50 nm Al powder in G8‐based gel can significantly increase its mechanical strength, yield stress and viscosity, as well as thixotropy.     

Aging of ADN Rocket Propellant Formulations with Desmophen®‐Based Elastomer Binder

Desmophen® binder‐based rocket propellant formulations containing ammonium dinitramide (ADN) and different fuel filler types (Al, HMX) were manufactured and investigated. Desmophen® D2220 is a polyesterpolyol. Polyesters are seen as a binder possibility, because of the relatively low temperature of the glass transition region compared to polyether‐based prepolymers such as GAP. The analogous formulations with AP instead of ADN were also included for comparison. The aging was followed by SEM, DSC, and DMA measurements. The accelerated aging program was developed on the principle of thermal equivalent load and the generalized van’t Hoff rule with a scaling factor equal to F=2.9. The aging was performed in air (RH<10 %) at temperature values between 65 and 85 °C and aging times adjusted to a thermal equivalent load of 15 years at 25 °C. DMA measurements of the aged ADN/Desmophen®‐based propellants identified changes in the loss factor curve. In contrast to HTPB‐Al‐AP rocket propellant formulations, the loss factor curve of the ADN formulations with Desmophen®‐based elastomer binder shows only one main apparent peak. The loss factor curves were modeled with exponentially modified Gaussian functions, which have revealed the presence of a second hidden peak. It was found that the aging could be characterized by the time‐temperature dependence of the areas of the hidden peak. The area increased with aging, which is explained by scissioning of the polymer in the shell around the ADN particles. By this process the strength is reduced, which was recognized by the decrease in storage shear modulus.     

Neutral Polymeric Bonding Agents (NPBA) and Their Use in Smokeless Composite Rocket Propellants Based on HMX‐GAP‐BuNENA

Very few efficient bonding agents for use in solid rocket propellants with nitramine filler materials and energetic binder systems are currently available. In this work, we report the synthesis, detailed characterization, and use of neutral polymeric bonding agents (NPBA) in isocyanate‐cured and smokeless composite rocket propellants based on the nitramine octogen (HMX), the energetic binder glycidyl azide polymer (GAP), and the energetic plasticizer N‐butyl‐2‐nitratoethylnitramine (BuNENA). These polymeric bonding agents clearly influenced the viscosity of the uncured propellant mixtures and provided significantly enhanced mechanical properties to the cured propellants, even at low NPBA concentrations (down to 0.001 wt‐% of propellant). A modified NPBA more or less free of hydroxyl functionalities for interactions with isocyanate curing agent provided the same level of mechanical improvement as regular NPBA containing a substantial number of reactive hydroxyl groups. However, some degree of reactivity towards isocyanate is essential for function.     

Radiative Ignition of Fine‐Ammonium Perchlorate Composite Propellants

Radiative ignition of quasi‐homogeneous mixtures of ammonium perchlorate (AP) and hydroxyterminated polybutadiene (HTPB) binder has been investigated experimentally. Solid propellants consisting of fine AP (2 μm) and HTPB binder (~ 76/24% by mass) were ignited by CO₂ laser radiation. The lower boundary of a go/no‐go ignition map (minimum ignition time vs. heat flux) was obtained. Opacity was varied by adding carbon black up to 1% by mass. Ignition times ranged from 0.78 s to 0.076 s for incident fluxes ranging from 60 W/cm² to 400 W/cm². It was found that AP and HTPB are sufficiently strongly absorbing of 10.6 μm CO₂ laser radiation (absorption coefficient ≈250 cm⁻¹) so that the addition of carbon black in amounts typical of catalysts or opacitymodifying agents (up to 1%) would have only a small influence on radiative ignition times at 10.6 μm. A simple theoretical analysis indicated that the ignition time‐flux data are consistent with in‐depth absorption effects. Furthermore, this analysis showed that the assumption of surface absorption is not appropriate, even for this relatively opaque system. For broadband visible/near‐infrared radiation, such as from burning metal/oxide particle systems, the effects of in‐depth absorption would probably be even stronger.