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We demonstrate a co-crystal-based strategy to create new solid hypergols, that is, materials exhibiting spontaneous ignition when in contact with an oxidant, from typically non-hypergolic fuel molecules. In these materials, the energy content and density can be changed without affecting the ignition delay. The use of an imidazole-substituted decaborane as a hypergolic "trigger" component in combination with energy-rich but non-hypergolic nitrobenzene or pyrazine yielded hypergolic co-crystals that combine improved combustion properties with ultrashort ignition delays as low as 1 ms.

We demonstrate a cocrystal-based strategy to create new solid hypergols, i.e. materials exhibiting spontaneous ignition in contact with an oxidant, in which the energy content and density of the material can be changed without affecting the ignition delay. Using an imidazole-substituted decaborane as a hypergolic "trigger" component, in combination with energy-rich but non-hypergolic nitrobenzene, provides a hypergolic cocrystal with an ultrashort ignition delay, composed of hypergolic and fuel-containing domains.

The design and synthesis of new hypergolic ionic liquids (HILs) as replacements for toxic hydrazine derivatives have been the focus of current academic research in the field of liquid bipropellant fuels. In most cases, however, the requirements of excellent ignition performances, good hydrolytic stabilities, and low synthetic costs are often contradictory, which makes the development of high-performance HILs an enormous challenge. Here, we show how a fuel-rich boranophosphate ion was rationally designed and used to synthesize a series of high-performance HILs with excellent comprehensive properties. In the design strategy, we introduced the {BH3 } moiety into the boranophosphate ion for improving the self-ignition property, whereas the complexation of boron and phosphite was used to improve the hydrolytic activity of the borohydride species. As a result, these boranophosphate HILs exhibited wide liquid operating ranges (>220 °C), high densities (1.00-1.10 g cm-3 ), good hydrolytic stabilities, and short ignition delay times (2.3-9.7 milliseconds) with white fuming nitric acid (WFNA) as the oxidizer. More importantly, these boranophosphate HILs could be readily prepared in high yields from commercial phosphite esters, avoiding complex and time-consuming synthetic routes. This work offers an effective strategy of designing boranophosphate HILs towards safer and greener hypergolic fuels for liquid bipropellant applications.

A novel family of hydrophobic triaminocyclopropenium cation based ionic liquids have been synthesized, and their structures and physicochemical properties characterized by NMR and IR spectroscopy, elemental analysis, differential scanning calorimetry, and hypergolic tests. The experimental results showed that all of these ionic liquids exhibited the expected hypergolic reactivity with the oxidizer white fuming nitric acid. Among them, the hypergolic ionic liquid based on the cyanoimidazolylborohydride anion showed excellent integrated properties, including high decomposition temperature (194 °C), high density (0.95 g cm-3 ), moderate viscosity (44 MPa s), ultrafast ignition delay time (6 ms), and high specific impulse (301.9 s); this demonstrates its potential as an environmentally friendly alternative to toxic hydrazine derivatives.

Hypergolic ionic liquids (HILs) as potential replacements for hydrazine derivatives have attracted increasing interest over the last decade. Previous studies on HILs are mostly concentrated on the anionic innovations of ionic liquids to shorten the ignition delay (ID) time, but few attentions have been paid to the cationic modifications and their structure-property relationships. In this work, we presented a new strategy of cationic functionalization by introducing the energetic nitrato group into the cationic units of HILs. Interestingly, the introduction of oxygen-rich nitrato groups into the cationic structure has obviously improved the combustion performance of HILs with larger flame diameters and duration times. The density-specific impulse (ρIsp) of these novel HILs are all above 279.0 s*g*cm-3, which are much higher than that of UDMH (215.7 s*g*cm-3). In addition, the densities of these HILs are in range of 1.22-1.39 g*cm-3, which is much higher than that of UDMH (0.79 g*cm-3), showing their higher loading capacity than hydrazine-derived fuels in propellant tank. This promising strategy of introducing nitrato group into the cationic structures has provided a new platform for developing high-performing HILs with improved combustion properties.

1,5-Diazabicyclo[3.1.0] hexane (DABH) was found a potential hypergolic liquid propellant. The physical and energetic properties of DABH, 2-(dimethylamino) ethyl azide (DMAZ), and monomethyl hydrazine (MMH) were compared. The ignition delay time of DABH with nitrogen tetroxide was 1 ms, which was shorter than DMAZ and similar with MMH. The toxicology experiment showed that half lethal dose (LD50) of DABH was 621.0 mg kg−1, which suggested that DABH was promising to be used as low-toxic liquid propellant. Thermal decomposition experiments showed that the apparent activation energy (E) was about 66.3 kJ mol−1. The thermal decomposition calculated results from Madhusudanan-Krishnan-Ninan integration, Satava-Sestak integration and Achar differential methods were compared and the pre-exponential factor were obtained.

Hypergolic fuel has unique importance in missiles and satellite launch vehicles. Hydrazine and its derivatives (Monomethyl hydrazine, MMH and Unsymmetrical Dimethyl Hydrazine, UDMH) are the continuous choices of fuel in bipropellant though it is carcinogenic and toxic. Development of new, green and cost-effective fuel blends based on trust-worthy, ionic liquid and biofuel in different volume proportion have been reported. The blends exhibit reasonably good stability (chemical and thermal), low viscosity (η 1 g/cm3), low ignition delay and high performance in comparison with UDMH - a proven liquid fuel presently being used as promising fuels for liquid rocket application. The reported blends appear to be a technologically promising, eco-friendly and affordable fuel blend to replace currently used liquid toxic fuels.

Currently, toxic and volatile hydrazine derivatives are still the main fuel choices for liquid bipropellants, especially in some traditional rocket propulsion systems. Therefore, the search for safer hypergolic fuels as replacements for hydrazine derivatives has been one of the most challenging tasks. In this study, six imidazolylidene-borane compounds with zwitterionic structure have been synthesized and characterized, and their hypergolic reactivity has been studied. As expected, these compounds exhibited fast spontaneous combustion upon contact with white fuming nitric acid (WFNA). Among them, compound 5 showed excellent integrated properties including wide liquid operating range (-70-160 °C), superior loading density (0.99 g cm(-3) ), ultrafast ignition delay times with WFNA (15 ms), and high specific impulse (303.5 s), suggesting promising application potential as safer hypergolic fuels in liquid bipropellant formulations.

A series of asymmetric monoimidazolium dihydroboronium-based ionic liquids (ILs) were synthesized from amine-boranes. All the resulting ILs were fully characterized by 1H and 13C NMR, IR spectroscopy, elemental analysis or high resolution mass spectrum. Compared with the symmetric bisimidazolium dihydroboronium-based ILs, these new ILs exhibited improved properties with shorter ignition delay times (IDs), higher densities, and lower phase transition temperature showing the promising application potential as green propellants.

Hydrazine and its derivatives are used as fuels in rocket propellant systems; however, due to high vapor pressure, toxicity, and carcinogenicity, handling of such compounds is extremely hazardous. Hypergolic ionic liquids have shown great promise to become viable replacements for hydrazines as fuels. Borohydride-containing ionic liquids have now been synthesized using a more efficient synthetic pathway that does not require liquid ammonia and halide precursors. Among the eight new compounds, 1-allyl-3-n-butyl-imidazolium borohydride (1) and 1, 3-diallylimidazolium borohydride (5) exhibit very short ignition-delay times (ID) of 8 and 3 ms, respectively. The hydrolytic stability of borohydride compounds has been greatly improved by attaching long-chain alkyl substituents to the imidazole ring. 1,3-Di-(n-octyl)-imidazolium borohydride (3) is a water stable borohydride-containing ionic liquid. 1,3-Di-(n-butyl)-imidazolium borohydride (2) is a unique example of a borohydride liquid crystal. These ionic liquids have some unusual advantages, including negligible vapor pressures, good ignition delay (ID) times, and reduced synthetic and storage costs, thereby showing good application potential as environmentally friendly fuels in bipropellant formulations. In addition, they also have potential applications in the form of reducing agents and hydrogen storage materials.

Currently, monomethyl hydrazine is the most widely used hypergolic rocket fuel. However, due to its high vapor toxicity, there is a need to develop low-toxicity hypergolic fuels. Ionic liquids are one such potential category of fuels, since they are consistently characterized by ultra-low vapor pressures, but designing ionic liquid propellants with ignition delay times that are comparable to that of monomethyl hydrazine is a challenge. This is because a fundamental understanding of the hypergolic nature of ionic liquids is far from clear. Quantitative structure property relationship (QSPR) represents a simplified design approach for quantitatively predicting the ignition delay times based on linear correlations using a set of descriptors, which, in this work, define electrostatics, hydrogen bonding, and other structural features of the ionic liquids. Experimental ignition delay times for a set of 41 ionic liquids were collected for QSPR development. Experimental measurements of the ignition delay times were then correlated to theoretical descriptors determined from quantum mechanical calculations. A number of multi-descriptor linear equations were analyzed by regression of the ignition delay data, showing reasonable success. The ignition delay values were observed to spread over a wide range, in large part due to the presence of oxygen in the fuel molecule. The training data was thus split into two sets and refitted to a set of linear equations, showing a systematic improvement in the correlation coefficients. The success of the present QSPR results is encouraging, which should motivate further efforts to enlarge the training set to include experimentally measured properties of ionic liquids for developing highly predictive capabilities.

Complexation of amines with borane converts them to hypergols or decreases their ignition delays (IDs) multifold (with white fuming nitric acid as the oxidant). With consistently low IDs, amine-boranes represent a class of compounds that can be promising alternatives to toxic hydrazine and its derivatives as propellants. A structure-hypergolicity relationship study reveals the necessary features for the low ID.

In propellant systems, the most common bipropellants are composed of two chemicals, a fuel (or reducer) and an oxidizer. Currently, the choices for propellant fuels rely mainly on hydrazine and its methylated derivatives, even though they are extremely toxic, highly volatile, sensitive to adiabatic compression (risk of detonation), and, therefore, difficult to handle. With this background, the search for alternative green propellant fuels has been an urgent goal of space science. In this study, a new family of cyanoborohydride-based ionic liquids (ILs) with properties and performances comparable to hydrazine derivatives were designed and synthesized. These new ILs as bipropellant fuels, have some unique advantages including negligible vapor pressure, ultra-short ignition delay (ID) time, and reduced synthetic and storage costs, thereby showing great application potential as environmentally friendly fuels in bipropellant formulations.

In propellant systems, fuels of choice continue to be hydrazine and its derivatives, even though they comprise a class of acutely carcinogenic and toxic substances which exhibit rather high vapor pressures and require expensive handling procedures and costly safety precautions. Hypergolic ionic liquids tend to have low volatility and high thermal and chemical stability, and often exhibit wide liquid ranges, which could allow the use of these substances as bipropellant fuels under a variety of conditions. A new family of borohydride ionic liquids and borane-ionic-liquid solutions is described which meets nearly all of the desired important criteria for well-performing fuels. They exhibit ignition-delay times that are superior to that of any known hypergolic ionic liquid and may thus be legitimate replacements for hydrazine and its derivatives.

Use of green propellants is a trend for future space propulsion. Hypergolic ionic liquid propellants, which are environmentally-benign while exhibiting energetic performances comparable to hydrazine, have shown great potential to meet the requirements of developing nontoxic high-performance propellant formulations for space propulsion applications. This Concept article presents a review of recent advances in the field of ionic liquid propellants.

This paper investigates the reaction between pool chlorinators and gasoline. In particular, the propensity for self-ignition and the resulting chemical products were studied. An organic pool chlorinator was combined with gasoline in varying proportions in an attempt to form a hypergolic mixture. None of the combinations resulted in self-ignition, but larger quantities of chlorinator produced vigorous light-colored smoke and a solid mass containing isocyanuric acid and copper chloride. Additionally, the chlorinating abilities of different commercially available pool chlorinators were explored. When Ca(ClO)(2) and sodium dichloro-s-triazinetrione-based chlorinators were used, the presence of gasoline was still visible after 10 days, despite limited chlorination. The trichloro-s-triazinetrione-based chlorinator, however, caused efficient chlorination of the C(2)- and C(3)-alkylbenzenes, making gasoline no longer identifiable.

A number of fuel blends based on 3-carene, norbornadiene, furfuryl alcohol, ethylidene norbornene, and kerosene in different weight proportions have been developed as rocket fuels which exhibit synergistic hypergolic ignition with red fuming nitric acid (RFNA) as oxidizer with low ignition delays (IDs

One way being considered to destroy trinitrotoluene (TNT) land or surf mines is to exploit its reactivity using darts containing chemicals, which, upon contact with TNT, cause instantaneous decomposition, but not detonation. To determine the best candidates to fill the darts, liquids, specifically amines, which react in a hypergolic fashion with TNT were examined for both the rate of reaction and amount of energy released. Micro-calorimetry was used to measure heat release while spectroscopy and conventional peak intensity monitoring by chromatography were used to examine the rate of reaction. Calorimetry measurements showed little variation between different amines reacting with TNT (about 110–130 kJ mol−1 TNT). TNT reaction with hydride actually produced more heat than with amines. Further, dinitrotoluene (DNT), which generates substantial heat, did not undergo a hypergolic reaction with amines suggesting that heat release is not the controlling factor for the hypergolic reactions. Rate constants, determined for the loss of TNT in dilute acetonitrile solution, clearly showed distinctions among the amines. The more primary amine functionalities in the amine compound, the faster it destroyed TNT. Hydrides or amine mixtures spiked with hydride decomposed substantially faster than the amines alone. However, a direct correlation between reaction rate and time-to-ignition was not observed.

Monomethylhydrazine (MMH) is currently the most widely used hypergolic propellant, due to its high performance and low ignition delay. However, its toxicity is a major concern. The present work aims at developing high-performance hypergolic fuels that are based on tertiary amine azide functionality. A number of potential amine azide candidates have been proposed, and some of their physical and chemical properties have been computed using state-of-the-art molecular modeling techniques. Gas-phase heats of formation have been calculated using the CBS-QB3 method, and the first-principle COSMO-RS method has been used to compute heats of vaporization and vapor pressures. A density correlation, based on molecular-volume calculation, has been established to predict the densities of the candidate molecules. Finally, the liquid-phase heats of formation and densities have been used to predict the specific and density impulses of the proposed candidate molecules. The results show that many of the molecules proposed here have much higher density impulse than that of MMH, and may be considered for experimental studies.

Novel resins containing carbonyl or thiocarbonyl moiety; N-N bonds; ethereal linkage; and terminal hydroxyl groups, that is, bis(bisvanillincarbonohydrazone-4-oxy)butane 1,4-diyldiether and bis(bisvanillinthiocarbonohydrazone-4-oxy)butane 1,4-diyldiether, have been synthesized by condensing bisvanillincarbono- and bisvanillinthiocarbono-hydrazone, respectively, with 1,4-dibromobutane. The resins were characterized by elemental and hydroxyl equivalent analyses, infrared and $^1H$- and $^{13}C-NMR$ spectra, and other properties relevant to their use as propellant binders, such as calorific value, thermal analysis, viscosity, and burning rate measurements. Both these resins have convenient viscosity, indicating their suitability as binders for processing high solid loading, at slightly above room temperature. The resins could be cured easily with toluene diisocyanate and glycerol. The DTA-TG data show that the cured samples start to decompose exothermically at relatively low temperatures, in the range around 160°C. The resins ignite immediately on coming in contact with pure nitric acid, indicating their hypergolic nature. The burning rates of the ammonium perchlorate-based propellant compositions having these resins as binders are much higher than those processed with a conventional butadiene binder.

Crosslinked chitosan membranes were synthesized using glutaraldehyde and characterized by infra-red (IR) and wide-angle X-ray diffraction (WAXD) spectroscopic methods. The membranes were applied for the pervaporation-based dehydration of the highly hazardous and hypergolic unsymmetrical dimethylhydrazine liquid propellant. The characterization techniques were an efficient tool in identifying polymer–liquid interaction sites and the separation mechanisms involved. The crosslinked polymer was found to have good potential for the separation of the aqueous azeotrope of the propellant (20 wt%) and its enrichment to >90% purity. An equilibrium sorption study examined the preferential affinity of the membrane amongst the two penetrating liquids. The pervaporation performance of the membrane was evaluated by varying the experimental parameters of feed composition, membrane thickness and permeate pressure, and found to be promising. © 2001 Society of Chemical Industry

Use of green propellants is a trend for future space propulsion. Hypergolic ionic liquid propellants, which are environmentally-benign while exhibiting energetic performances comparable to hydrazine, have shown great potential to meet the requirements of developing nontoxic high-performance propellant formulations for space propulsion applications. This Concept article presents a review of recent advances in the field of ionic liquid propellants.

In propellant systems, fuels of choice continue to be hydrazine and its derivatives, even though they comprise a class of acutely carcinogenic and toxic substances which exhibit rather high vapor pressures and require expensive handling procedures and costly safety precautions. Only recently (2008), ionic liquids (salts with melting points less than 100 °C) with the dicyanamide anion were shown to exhibit hypergolic properties (instantaneous ignition when contacted with oxidizers (100 % nitric acid, WFNA)). Such liquids tend to have low volatilities, and high thermal and chemical stabilities, and often exhibit long liquid ranges which could allow utilization of these substances as bipropellant fuels over a variety of conditions. A new family of dicyanoborates is presented, which can be synthesized in water, with substituted N-acyclic, N-cyclic, and azolium cations has met nearly all of the desired important criteria needed for well-performing fuels.

Space-qualified lubricants: Graphene and graphene oxide (r-GO) can strongly improve the low-temperature performance of hypergolic ionic liquids by reduction of viscosity. Key to success is to match the graphene type to the specific ionic-liquid functionality.

The effect of relative amounts of fuel and oxidizer, composition of the oxidizer and compactness of the fuel on ignition delays of hydrazones with nitric acid has been studied. Several of the hydrazones give minimum ignition delays at the mixture ratio close to the stoichiometric ratio. Ignition delays of phenyl- and dimethylhydrazones are greatly influenced by the acid composition. Longer ignition delays are observed with the acids having higher concentrations of $NO_2$. A reaction mechanism leading to hypergolic ignition has been proposed for phenylhydrazones/nitric acid systems. Longer ignition delays are observed when the fuels are taken in compacted forms.

Hypergolic or self ignition delays of unsymmetrical dimethylhydrazine (UDMH) and several amine fuels, mixed with three fuming nitric acid oxidizers, have been determined, at room temperature, in a highly sensitive “Cup Test” apparatus. Ignition delay (ID) variations have been studied with respect to the chemical structure of fuel, oxidizer composition, and oxidizer-to-fuel (O/F) ratio. Probable preignition reactions and structure-hypergolicity correlations have been suggested. Some non-hypergolic hydrocarbons and petroleum fractions have been hypergolized by addition of UDMH, and ID variations have been studied with respect to UDMH-content in fuel and catalytic additives (ammonium metavanadate, ammonium dichromate, and cuprous oxide) in the red fuming nitric acid oxidizer (RFNA). Increment in UDMH-content improves the hypergolicity of fuels towards RFNA. For example, kerosene + UDMH 60:40 blend ignites with RFNA with a remarkably low ID of 6 ms. However, the catalytic effect of the additive in RFNA varies widely with the fuel-blend composition.