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Hybrid rocket propulsion can contribute to reduce launch costs by simplifying engine design and operation. Hypergolic propellants, i.e. igniting spontaneously and immediately upon contact between fuel and oxidizer, further simplify system integration by removing the need for an ignition system. Such hybrid engines could also replace currently popular hypergolic propulsion approaches based on extremely toxic and carcinogenic hydrazines. Here we present the first demonstration for the use of hypergolic metal-organic frameworks (HMOFs) as additives to trigger hypergolic ignition in conventional paraffin-based hybrid engine fuels. HMOFS are a recently introduced class of stable and safe hypergolic materials, used here as a platform to bring readily tunable ignition and combustion properties to hydrocarbon fuels. We present an experimental investigation of the ignition delay (ID, the time from first contact with an oxidizer to ignition) of blends of HMOFs with paraffin, using White Fuming Nitric Acid (WFNA) as the oxidizer. The majority of measured IDs are under 10 ms, significantly below the upper limit of 50 ms required for functional hypergolic propellant, and within the ultrafast ignition range. A theoretical analysis of the performance of HMOFs-containing fuels in a hybrid launcher engine scenario also reveals the effect of the HMOF mass fraction on the specific impulse (Isp) and density impulse (ρIsp). The use of HMOFs to produce paraffin-based hypergolic fuels results in a slight decrease of the Isp and ρIsp compared to that of pure paraffin, similar to the effect observed with Ammonia Borane (AB), a popular hypergolic additive. HMOFs however have a much higher thermal stability, allowing for convenient mixing with hot liquid paraffin, making the manufacturing processes simpler and safer compared to other hypergolic additives such as AB.

Task-specific dicyanamide anion ([DCA]−), cyanoborohydride anion ([CBH]−) and 1-ethyl or 1-allyl-3-methylimidazolium cations ([EMIM]+ or [AMIM]+) were mixed in different ratios to form hypergolic ionic mixtures. Their viscosities, densities, decomposition temperatures, specific impulses were determined and calculated. Ignition delay time was obtained by a drop test recorded by a high-speed camera. Theoretical calculations and molecular dynamics simulations were applied for exploring their energy and microstructure. The results indicated that ionic nature and ionic ratio in mixtures were two decisive factors for properties of ionic mixtures. Specially, mixture [EMIM][CBH]0.7[DCA]0.3 possessed the shortest ignition delay time of 3 ms with white fuming nitric acid (WFNA) as oxidizer, which was shorter than that of pure ionic liquid [EMIM][DCA] (32 ms) or [EMIM][CBH] (8 ms). However, with a reverse ratio of anions, mixture [EMIM][CBH]0.3[DCA]0.7 possessed a very long ignition delay time of 373 ms and showed a different phenomenon in ignition process. Thermogravimetric analysis indicated that mixture [EMIM][CBH]0.7[DCA]0.3 decomposed faster than mixture [EMIM][CBH]0.3[DCA]0.7. The radial distribution functions (RDFs) suggested that the H-bond interactions in mixtures obviously were influenced by ionic ratios. As a new design strategy, hypergolic ionic mixtures can not only combine properties of different ions, but also show more prominent performances in a specific ionic ratio than each parent ionic liquid.

The effect of an N-alkyl substituent R on the characteristics of ignition of hypergolic ionic liquids (ILs) upon contact with white fuming nitric acid (WFNA) is examined by a modified drop test method that simultaneously combines high-speed shadow imaging with high-speed two-color pyrometry. The number of carbon atoms in the synthesized N-alkylated propargyl-imidazolium dicyanamide with alkyl substituents varied from 1 to 7. The ILs under study can be grouped in two categories depending on the initial location of the flame zone. When R has 1 to 3 carbon atoms, ignition is visualized as a glow inside a bubble (formed after droplet fall) and above the liquid surface. When the number n of carbon atoms in R is 4 to 6, no glow is recorded inside the bubble. In the case of a heptyl substituent, ignition does not occur since no flame is observed during approximately one minute. It is shown that the value of ignition delay can be interpreted as an Arrhenius-type function where the exponent depends linearly on n. An analysis of high-speed video records shows that the evaporation starting time of WFNA measured from the first IL–WFNA contact is also an Arrhenius-type function of n. Furthermore, it is found that the time for a bubble to reach its maximum diameter is independent of n. A possible mechanism of reaction between ILs of this class and WFNA is suggested and discussed.

A range of ionic liquids (ILs) have been synthesized and modeled to better understand the role of the cation in the ignition of hypergolic ionic liquids. Vogelhuber et al. have shown by density functional theory methods that the addition of sodium cations to an ionic liquid promotes ignition with white fuming nitric acid (WFNA) by lowering energy barriers. To validate this prediction, solid sodium dicyanamide (Na+DCA–) was added at various weight percents to 1-butyl-3-methylimidazolium dicyanamide (BMIM+DCA–). The ignition delay was measured for each mixture with WFNA. Overall, it was found that the Na+DCA– lowered the ignition delay by 11 ms at 7 wt %. The calculations done by Vogelhuber et al. appear to be consistent with this observation. The sodium cation may play a role by orienting the anion with the WFNA resulting in the favorable reaction energetics observed.

In this study, a hypergolic linker (dicyanamide, DCA) and a high-energy nitrogen-rich ligand (1,5-diaminotetrazole, DAT) were applied to construct high-energy metal-organic frameworks (HEMOFs) with hypergolic property. Three novel metal-organic frameworks (MOFs) were synthesized via a mild method with fascinating 2D polymeric architectures, and they could ignite spontaneously upon contact with white fuming nitric acid (WFNA). The gravimetric energy densities of the three HEMOFs all exceeded 26.2 kJ·g-1. The cupric MOF exhibits the highest gravimetric and volumetric energy density of 27.5 kJ·g-1 and 51.3 kJ·cm-3, respectively. By adjusting the metal cations, high-energy ligands and hypergolic linkers can improve the performance of hypergolic MOFs. This work provides a strategy for manufacturing MOFs as potential high-energy hypergolic fuels.

Binary collision between a smaller N, N, N′, N′- tetramethylethylenediamine (TMEDA) droplet and a larger white fuming nitric acid (WFNA) droplet was investigated experimentally and computationally for understanding the influence of off-center collision on the hypergolic ignitability of the system, which is controlled by the mass interminglement and mixing subsequent to the droplet coalescence. The ignition delay time was experimentally found to non-monotonically vary with the impact parameter, which measures the degree of off-center collisions. This phenomenon was hypothetically attributed to the non-monotonicity of mass interminglement of colliding droplets with increasing the impact parameter—the increased droplet stretching by slightly off-center collision promotes mass interminglement, but the stretching separation by significantly off-center collision reduces mass interminglement. This hypothesis was computationally verified by a validated volume-of-fluid (VOF) simulation of a simplified problem, in which the transport phenomena and chemical reactions are neglected and the controlling physics of droplet mass interminglement is emphasized. Furthermore, a parametric study for wide ranges of controlling non-dimensional parameters, such as the collision Weber number of 20–140 and the droplet size ratio of 1.3–2.0, further confirms that the non-monotonicity of ignition delay time with the impact parameter is a general characteristic of the present hypergolic system.

In this work, the hypergolic ignition process of 8 recently synthesized ionic liquids (I.L.s) with [BH3(CN)BH2(CN)]− anions with white fuming nitric acid is experimentally investigated by using droplet test method and long distance microscope-high speed photography techniques. Results show that the hypergolic ignition process of the present I.L.s is of a completely different three stage nature and ultra-fast hypergolic ignition is observed for this family of I.L.s ( 1). Subsequently in the second stage, liquid spikes that are ejected into the oxidizer pool during CEDT significantly increase the reactive surface area underneath the liquid surface. As a consequence, local temperature increases, gas phase intermediate product and oxidizer/fuel vapor accumulate due to the continuous liquid phase reaction underneath the surface. When the local pressure overcomes the surface tension, disintegration of the surface in terms of larger ligaments and secondary droplets ejection was observed, together with shooting out of the vapor/smoke. Finally, in the third stage when the local temperature and the vapor concentration increase sufficiently, further gas phase reaction leads to ignition. Fine and stable flame with strong bright and green luminescence continuously evolve from the initial mixing layer. Some red and dark smokes are gradually generated above the burning bright and green flame until finally the flame dies out. The ignition delay time (IDT) decreases with the increase of unsaturation index of the heterocyclic core in the cation, and also the decrease of CEDT. The enthalpy of formation of the I.L.s for different cation structures are correlated with IDT, which represents both the fuel structure chemistry and the ‘coulomb explosion’ enhances mixing effect on the overall reactivity of this diffusive system.

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.

Bipropellant liquid rocket engines are typically fueled by hydrazines that exhibit extreme toxicity and carcinogenity. Recently hypergolic ionic liquids (HILs) have been emerging as environmentally friendly propellant that holds the potential for hydrazine replacement due to their low volatility, easy and safe storage and handling. The HIL is a novel concept both for chemistry community and for combustion community, and plenty of research waits to be initiated to promote practical propulsion application, especially for comprehensive understanding on their fundamental combustion. In this work, hypergolic ignition of two HILs (1-ethyl-3-methylimidazolium dicyanamide([EMIm][DCA]) and 1-butyl-3-methyl-imidazolium dicyanamide ([BMIm][DCA]) were tested with three oxidizers (white fuming nitric acid (WFNA), red fuming nitric acid(RFNA), and nitrogen tetroxide). A drop test system was designed to control the collision event of a single droplet of fuel with an oxidizer pool. Two independent high speed cameras were spontaneously employed to capture the self-luminous images of the whole hypergolic ignition process as well as the initial impact process. The results showed that hypergolic ignition process of [EMIm][DCA]-WFNA is quite similar to that of monomethyl hydrazine-WFNA. Combined with thermo physical properties of HILs, hypergolic ignition process of [EMIm][DCA]-WFNA is proposed. To qualitatively demonstrate the overall hypergolic ignition and reactivity, the explosion delay time and the ignition delay times of various combinations of the fuel-oxidizer pairs were determined. The effects of the Weber number on the explosion delay time and the ignition delay time of [EMIm][DCA]-WFNA are quantitatively investigated. Hypergolicity of different propellant combinations is tested to investigate the influence of oxidizer’s oxidative capacity and HILs’ cation structure. Additionally, the propulsive performance of the two dicyanamide-based IL fuels was evaluated and compared with traditional hydrazine-based fuels, based on a steady rocket model under the same initial conditions. Results show that using WFNA as the oxidizer, BMIM DCA and EMIM DCA presented a comparable propulsive performance to that of UDMH.

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.

New-generation green bipropellants based on ionic liquids for aircraft have attracted the attention of researchers. However, the major defects of ionic liquid propellants are their low specific impulse and incomplete combustion, which limits their practical application. In this work, a series of novel hypergolic fluids were prepared by mixing oxygen-rich nitroglycerin (NG) with fuel-rich dicyanamide-based ionic liquids. The complete dissolution of NG in ionic liquids makes these hypergolic fluids became self-supplying oxygen composite fuels, which generate much less solid residual products than pure hypergolic ionic liquids (HILs). Moreover, the addition of NG inhibited the micro-explosion between fuel and white fuming nitric acid (WFNA), extended the contact time between fuel and oxidizer. And a proper amount of NG in the fuel could suppress the secondary combustion. These composite fuels have high density (>1 g cm−3), low viscosity (as low as 13.49 cP), wide liquid operating ranges and acceptable ignition delay time. Additionally, the specific impulses of the composite fuels were higher than pure HILs. For the first time, this work provides a simple and reliable method to control the ignition properties of HIL by adding oxygen-enriched additives.

Dimethyl amino ethyl azide (DMAZ), as a non-carcinogen liquid fuel, is a good fuel in space programs. However, it suffers from high ignition delay (ID) time with liquid oxidizer white fuming nitric acid (WFNA). DMAZ were blended with amines in order to reduce the ID time of bipropellant DMAZ-WFNA. A drop-on-pool impingement set-up, coupled with a high-speed camera, was applied to investigate the ID time at ambient pressure and temperature. It was found that using of amines had dissimilar effects on the ID time of the bipropellant DMAZ-WFNA. Results showed that among the amines, pyrrole, tert-butylamine, n-octylamine reduced the ID time significantly. This study identified pyrrole as the most efficient “additive” for decreasing ID time (from 90 milliseconds to 20 milliseconds). Finally, a quantum molecular dynamics simulation was conducted to probe how the additive affects on the ID time.

Non-toxic hypergolic propellants have considerably generated recent research interest in the field of green propulsion technology because they can replace highly toxic hypergolic combinations currently used. In this experimental research, sodium borohydride-based non-toxic hypergolic fuel was prepared by blending sodium borohydride in the mixture of energetic hydrocarbon solvents. In a drop test, sodium borohydride as an ignition source enabled the hydrocarbon mixture to initiate hypergolic interactions with H 2 O 2 oxidizer. Two different heating methods were utilized to analyze the characteristics of autoignition and combustion of the hypergolic fuel. As a reference fuel, a non-hypergolic fuel which has the identical chemical compositions to the hypergolic fuel except for sodium borohydride was tested and compared. As a one of the heating methods, thermogravimetric analysis was not suitable for evaluating the inherent thermophysical properties of the hypergolic fuel. In a droplet combustion chamber test, the autoignition and combustion of the hypergolic fuel droplets occurred exposed to elevated temperatures (in a range of 400–800 °C) at atmospheric pressure (1 bar), whereas the non-hypergolic fuel droplet was automatically ignited only at 800 °C. The ignition delay and total combustion time of the hypergolic fuel droplet were lower than those of the non-hypergolic fuel droplet. According to the temporal histories of the droplet size, sodium borohydride-based hypergolic fuel droplets did not obey the d 2 -law of diffusion-controlled combustion, which indicates the droplet evaporation rate was not a controlling factor in the combustion process. Consequently, the addition of sodium borohydride into the hydrocarbon mixture expedited the autoignition and combustion process of the fuel at elevated temperatures.

It is critical for hypergolic fuels to be dense so as to enhance the performance of propellants and improve the loading of rockets. Considering that dense elements and bicyclic rings contribute to high density, two series of ionic liquids were prepared with 1-aza-bicyclo[2.2.2]octane-/1,4-diazabicyclo[2.2.2]octane-based cations and the dicyanamide anion. Their key properties were measured or calculated as decomposition temperature (208–322 °C), density (1.06–1.31 g cm−3), specific impulse (260.5 to 266.2 s) and ignition delay time (14–1053 ms). Compared with 1-aza-bicyclo[2.2.2]octane-based ionic liquids, the corresponding 1,4-diazabicyclo[2.2.2]octane-based ionic liquids exhibit a lower decomposition temperature, higher density and bigger specific impulse. As expected, 1-methyl-1-azonia-bicyclo[2.2.2]octane and 1-(prop-2-ynyl)-4-aza-1-azonia-bicyclo[2.2.2]octane dicyanamide possess higher densities (1.20 and 1.19 g cm−3) than the corresponding pyrrolidinium and imidazolium-based isomers (1.05 and 1.07 g cm−3), besides hypergolicity with white fuming nitric acid.

Hypergolic ionic liquids (HILs) have gained significant importance in recent years as green space propellants. It is mainly due to their attractive physicochemical properties, such as high density, negligible vapor pressure and wide liquid range. More importantly, they show low ignition delay with storable oxidizers viz. white fuming nitric acid (WFNA), red fuming nitric acid (RFNA), dinitrogen tetroxide (N2O4) and hydrogen peroxide (H2O2). Therefore, investigation into the performance parameters of HILs, such as specific impulse (Isp), density and vacuum-specific impulse (ρIsp & Ivac), and characteristics velocity (C*), has received enormous attention. In the present study, 14 HILs have been selected for evaluation of their performance parameters using the NASA-CEC-71 program. Amongst them, three HILs have been developed in the laboratory by the authors. The effects of oxidizers, oxidizer to fuel ratio, heat of formation, hydrogen content of the HIL, and the chamber pressure parameters were studied in detail. The performance of HILs was also compared with that of a proven fuel, UDMH. The HILs 1-ethyl-3-methyl imidazolium cyanoborohydride, [EMIM][BH3CN], and 1,3-diallyl imidazolium cyanoborohydride, [diAIM][BH3CN], were particularly promising with all the oxidizers. The H2O2 seems to be a good potential oxidizer as it exhibited a higher value of ρIsp for most of the HILs than the UDMH. A propellant based on high-density HIL as a fuel and H2O2 as an oxidizer can be classified as a green propellant. Thus, the investigation helps in designing or selecting the best combination of HIL and oxidizer for rocket applications.

In bipropulsion systems, hypergolic ionic liquids with B–H groups, as promising alternatives to toxic hydrazine-based fuels, can improve propulsion performance. A series of borohydride cluster-based salts with excellent hydrolytic and thermal stabilities (Td > 200 °C), and the highest density (1.18 g cm−3) of any known borohydride hypergolic ionic liquids were prepared. Upon contact with white fuming nitric acid or N2O4, these new salts have ultra-fast ignition delay times (as low as 1 ms), which is a superior ignition performance. Theoretical studies were employed in order to understand their high stability and good ignition performance.

A series of hypergolic ionic liquids (HILs) based on the cyano (1H-1,2,3-triazole-1-yl) dihydroborate anion were synthesized by introducing the {BH2-CN} moiety into the 1,2,3-triazole anion. This introduction allowed us to improve the self-ignition property and decrease the viscosity of ionic fuels. The synthesized series of HILs exhibited wide liquid operating ranges (>220 °C), high densities (>1 g cm−3), low viscosities (as low as 22.48 cP), and ignition delay (ID) times as short as 5 ms by using white fuming nitric acid (WFNA) as the oxidizer. Furthermore, these HILs can be ignited upon contact with a 90 wt% H2O2 oxidizer in the presence of iodine.

Hypergolic ignition by the head-on collision of a smaller N,N,N',N' tetramethylethylenediamine (TMEDA) droplet and a larger white fuming nitric acid (WFNA) droplet was experimentally investigated by using a droplet collision experimental apparatus equipped with a time-resolved shadowgraph, a photodetector and an infrared detector. The investigation was focused on understanding the influence of droplet collision and mixing, which vary with the collisional Weber number (We = 20-220) and the droplet size ratio (Delta = 1.2-2.9) while have a fixed Ohnesorge number (Oh = 2.5x10(-3)), on the hypergolic ignitability and the ignition delay times. The hypergolic ignition was found to critically rely on the heat release from of the liquid-phase reaction of TMEDA and nitric acid, which is subsequent to and enhanced by the effective mixing of the droplets of proper size ratios. Consequently, the ignitability regime nomogram in the We-A space shows that the hypergolic ignition favors small Delta s and large Wes; the ignition delay times tend to decrease with either decreasing Delta, or increasing We, or both. A non-monotonic variation of the ignition delay times with We was observed and attributed to the non-monotonic emergence of jet-like mixing patterns that enhance the droplet mixing and hence the liquid-phase reaction. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Protonation of the anion in an ionic liquid plays a key role in the hypergolic reaction between ionic liquids and oxidizers such as white fuming nitric acid. To investigate the influence of the cation on the protonation reaction, the deprotonation energy of a set of cations has been calculated at the MP2 level of theory. Specifically, guanidinium, dimethyltriazanium, triethylamine, N-ethyl-N-methyl-pyrrolidinium, N-ethyl-pyridinium, 1,4-dimethyl-1,2,4-triazolium, 1-ethyl-4-methyl-1,2,4-triazolium, and 1-butyl-4-methyl-1,2,4-triazolium were studied. In addition, the net proton transfer energies from the cations to a set of previously studied anions was calculated, demonstrating an inverse correlation between the net proton transfer energy and the likelihood that the cation/anion combination will react hypergolically with white fuming nitric acid. It is suggested that this correlation occurs due to a balance between the energy released by the proton transfer and the rate of proton transfer as determined by the ionicity of the ionic liquid.

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 family of hydrophobic borohydride-rich ionic liquids was developed, which exhibited the shortest ignition delay times of 1.7 milliseconds and the lowest viscosity (10 mPa s) of hypergolic ionic fluids, demonstrating their great potential as faster-igniting rocket fuels to replace toxic hydrazine derivatives in liquid bipropellant formulations.

An experimental study is conducted to evaluate amine–borane materials in solid fuels that create hypergolic combustion with nitric acid. Ignition delays as short as 3.3 ms have been measured for fuels containing amine–boranes in an epoxy binder: one of the fastest ignition delays ever recorded for a hypergolic hybrid fuel. The heats of formation of the amine–boranes were measured and theoretical (ideal) performance calculated. Some amine–boranes exhibit a double peak performance allowing for a range of oxidizer/fuel combinations resulting in high performance. Amine–borane/fuel binder combinations can produce a 3.8 and 8.2% increase in Isp and ρIsp, respectively, over monomethylhydrazine and a 4–11% increase in Isp and a 17% decrease to 5% increase in ρIsp over other hypergolic hybrid rockets that use similar oxidizers. Amine–boranes also exhibit trends of being less toxic and more air stable than their base amines. In addition to the hypergolic properties, performance alone motivates the use of these mate...

A boron-based catalytic and energetic fuel additive is explored as a mechanism to enhance performance of hybrid motors using nitric-acid-based oxidizers. Ethylenediamine bisborane is highly hypergolic with nitric acid, thereby eliminating the need for a separate ignition system and providing the potential for restart. Ignition delays of 2.9±0.3 ms for ethylenediamine bisborane powder and 31.7±19.6 ms for ethylenediamine bisborane/hydroxyl-terminated polybutadiene combinations have been measured using white fuming nitric acid as the oxidizer. Theoretical specific impulse values higher than any other nitric-acid-based hypergolic hybrid combination tested are obtained; in addition, ethylenediamine bisborane is air stable and not toxic. Also presented is a method of manufacturing large quantities of high-purity ethylenediamine bisborane.

A series of hypergolic cyanotetrazolylborohydride (CTB) anion-based ionic liquids have been synthesized by a straightforward N-hydroboration of tetrazole followed by a salt metathesis reaction, which exhibited remarkably low viscosity ( 1.1 g cm−3), and ultra-short ignition delay time (as short as 1.4 ms) upon contact with white fuming nitric acid (WFNA).

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.

Recently extensive research has focused on replacing toxic hydrazine, monomethylhydrazine, and unsymmetrical dimethylhydrazine as liquid propellant fuels. 2-Azido-N,N-dimethylethylamine (1) is a good candidate to replace hydrazine derivatives in certain hypergolic fuel applications. Energetic ionic liquids that contain the 2-azido-N,N,N-trimethylethylammonium cation with nitrocyanamide, dicyanamide, dinitramide, or azide anion have been successfully synthesized in good yields by metathesis reactions. Ionic liquids have received considerable attention as energetic materials. The replacement of hydrazine with tertiary ammonium salts is especially attractive since many ionic liquids are models for green chemistry. In this work, new azide-functionalized ionic liquids are demonstrated to exhibit hypergolic activity with such oxidizers as 100% nitric acid or nitrogen tetraoxide (NTO).

A class of room-temperature ionic liquids (RTILs) that exhibit hypergolic activity toward fuming nitric acid is reported. Fast ignition of dicyanamide ionic liquids when mixed with nitric acid is contrasted with the reactivity of the ionic liquid azides, which show high reactivity with nitric acid, but do not ignite. The reactivity of other potential salt fuels is assessed here. Rapid-scan, Fourier transform infrared (FTIR) spectroscopy of the preignition phase indicates the evolution of N 2O from both the dicyanamide and azide RTILs. Evidence for the evolution of CO 2 and isocyanic acid (HNCO) with similar temporal behavior to N 2O from reaction of the dicyanamide ionic liquids with nitric acid is presented. Evolution of HN 3 is detected from the azides. No evolution of HCN from the dicyanamide reactions was detected. From the FTIR observations, biuret reaction tests, and initial ab initio calculations, a mechanism is proposed for the formation of N 2O, CO 2, and HNCO from the dicyanamide reactions during preignition.

Rocket exhaust products from the combustion of conventional liquid propellant combinations such as hydrazine and its derivatives with nitrogen tetroxide have been studied extensively. However, little work has been done on the characterization of exhaust products resulting from the ignition of less-toxic green liquid hypergolic propellants. In this work, we experimentally investigate the gaseous combustion products of a green hypergolic combination of triethylamine borane (TEAB) and white fuming nitric acid (WFNA). We use a drop test experiment to generate these gases, and thereafter route them into a gas cell for the collection of a spectrum using a FTIR spectrometer. To perform the analysis, we divide the spectrum obtained into the fingerprint (1000 cm to 1500 cm) and the functional group (1500 cm to 4000 cm) regions, and resolve the peaks into various functional group vibration modes as well as propose the presence of various species in the gas sample. Using spectral libraries and spectral data available in the literature, we identify the chemical signatures of O-H and C-H stretching vibrations, N-H bond vibrations, as well as absorbance peaks corresponding to water vapor (H2O), methane (CH4), carbon monoxide (CO), carbon dioxide (CO2), and the vapors of triethylamine borane (C6H18BN) and nitric acid (HNO3). In addition, we observe the condensation of the gaseous products from the decrease in absorbance of the different spectral peaks as a function of time.

The ignition delay of turpentine is not affected by the proportion of oxidizing agent added. The delay is reduced by addition to the nitric acid of oleum up to a certain concentration or by addition of catalysts in the order Fe Cl3 > NH4VO3 > Cu Cl2, but is increased by dilution with petroleum.

A new method to measure and examine ignition delays for hypergolic acid-organic reactants is described, Methods of previous investigations will be critically compared by mixing techniques, sensing devices, repeatability, and chemical composition for the influence of these parameters on ignition delay measurements, The time delay for hypergolics to react from initial contact to the appearance of flame is classically termed the ignition delay. This technique. as a result of its resolution, is the first to measure the chemical reaction time just prior to ignition, This new chemical performance measurement defines the time during which free radicals generated by the reaction should be analyzed by spectroscopic techniques, Unsymmetrical dimethylhydrazine, hydrazine, and furfuryl alcohol were individually reacted with various nitric acid mixtures. For the furfuryl alcohol-white fuming nitric acid system, this technique is the first to show that an initiation-propagation step is required for complete hypergolic ...

The focus of this research work was to characterize ignition and combustion of TMEDA / DMAZ fuels and white fuming nitric acid oxidizer, using a single-element injector to provide characteristics on hypergolic propellant reactants in terms of ignition delays under atmospheric pressure and ambient temperature conditions. This study assists in determining whether TMEDA and DMAZ are viable replacements for highly toxic fuels such as hydrazine. A series of experiments was conducted to determine the ignition delay as a function of: volumetric flow rate, momentum ratio, L/D ratio, and equivalence ratio. Highspeed images of the ignition transient were ascertained. As the total reactant volumetric flow rate increases, the ignition delay was reduced for  = 1 and MTMEDA/MWFNA = 1. The ignition time delay decreased from nearly 17 ms down to approximately 5 ms when the total propellant flow rate was increased by a factor of six. The jet velocity of the fuel stream, VTMEDA, ranged from 8 to 66 m/s (ReD from ~ 0.3x10 4 to 3x10 4 ), while the jet velocity in the oxidizer stream, VWFNA, ranged from 2 to 60 m/s (ReD from ~ 0.4x10 4 to 4x10 4 ) in order to change the momentum ratio. The effect of equivalence ratio on ignition delay shows that at very fuel rich conditions ( = 5.8) the ignition delay was approximately 3 times longer. However, for cases at  = 3.2 and below, it was evident that the ignition delay is not significantly affected by stoichiometry. This can be interpreted, that for ignition, stoichiometry is not as nearly as important as for combustion. Blends of TMEDA and DMAZ fuels were examined. It was found for 70%TMEDA/30%DMAZ (by wt%), the ignition delay decreased nearly 38%. To simulate the reactants being introduced into a rocket engine, a curved glass surface was applied tangent to the injector exit to study the effect of wall impingement. Data show that when the curved glass surface is present, up to an 8 ms increase in ignition delay is observed.

Two series of boronium-cation-based ionic liquids were prepared and fully characterized by (1)H, (13)C, and (11)B NMR and infrared spectroscopy, differential scanning calorimetry (DSC), and elemental analysis. The structure of bis(1-methyl-1H-imidazole-3-yl)dihydroboronium dicyanoborohydride (5 a) was determined by single-crystal X-ray diffraction. The densities of these ionic liquids range from 1.05 to 1.28 g cm(-3), and the heats of formation, predicted on the basis of Gaussian 03 calculations, fall between -164.6 and 430.5 kJ mol(-1). Compound 5 b, bis(1-allyl-1H-imidazole-3-yl)dihydroboronium dicyanoborohydride, exhibits the lowest viscosity (35 mPa s) and shortest ignition-delay time (14 ms) in combination with 100 % HNO(3).

Ionic liquids are candidates for replacing hydrazine and hydrazine derivatives systems. A detailed chemical kinetics model has been built to examine the gas-phase chemistry between isocyanic acid (HNCO), white fuming nitric acid (WFNA), N2O, CO2, and water, especially at low temperatures (ambient to 423 K). This kinetics model is able to explain the gas-phase ignition observed during hypergolic ignition of the ionic liquid 1-butyl-3-methyl-imidazolium dicyanamide (BMIM-dca) with WFNA. Sensitivity analyses have been performed to examine the reaction pathways for ignition. Ignition is predicted to occur via an exothermic reaction between isocyanic acid (HNCO) and nitric acid (HONO2), and subsequent HONO2 thermal decomposition that has NO2 and OH radicals as the primary chain carriers. A detailed understanding of the initiation processes in the liquid phase is needed, as the BMIM-dca and WFNA begin to react to produce the above preignition species for the proposed chemical kinetics model to describe the ignition behavior of the system.

Ionic liquids are being explored in the search for advanced propellants that provide higher performance and lower environmental impact than traditional propellant systems. Because of the flexibility in designing ionic liquids, both mono-propellants and bi-propellants are being pursued. A family of bipropellant ionic liquids that react hypergolically with white fuming nitric acid was recently synthesized. The overall goal of this research program was to contribute to the fundamental understanding of the ignition of energetic ionic liquids by determining the effects of cation and anion structure on initiation reactions, and subsequent reactions leading to ignition, for representative compounds in three classes: amino-1,2,4 triazoles, amino-1,2,3-triazoles, and amino-tetrazoles. In order to achieve this objective, experiments were performed to determine the decomposition species and their temporal evolution over a range of temperatures. To gain further insight into the mechanism of decomposition, simplified kinetic models were formulated and overall kinetic parameters were derived. The second major research objective was to contribute to the understanding of the hypergolic reaction mechanism for the dicyanamide-based ionic liquids. Experimental results are reported for the species produced during hypergolic ignition as well as for the detection of a postulated key intermediate, dinitrobiuret.

Nitrocyanamide ionic liquids with substituted imidazolium, guanidinium, and tetrazolium cations have been synthesized and fully characterized. Aminoguanidinium nitrocyanamide (7) crystallizes in the triclinic system P1. The results obtained from theoretical calculations based on 7 are consistent with the single-crystal structure data. These ionic liquids exhibit desirable physicochemical properties, such as low melting points and good thermal stabilities. Furthermore, they are all impact insensitive materials. Their energetic performances, including heats of formation, detonation pressures, and detonation velocities, were studied by a combination of theoretical and empirical calculations. The ionic liquids 1-4 have large liquid ranges and low viscosities. They were shown to be promising candidates as hypergolic ionic liquids through the combustion tests with 100% HNO(3).

N,N-Dimethylhydrazinium dicyanamide and nitrocyanamide ionic liquids (ILs) were prepared by quaterization of N,N-dimethylhydrazine with alkyl halides followed by metathesis reactions with silver dicyanamide or silver nitrocyanamide. The key physicochemical properties, such as melting point and decomposition temperatures, density, viscosity, heat of formation, detonation pressure and velocity, and specific impulse were measured/calculated. The impact of anions and alkyl-substituted cations on these properties is demonstrated. Droplet tests with white-fuming nitric acid (WFNA) as an oxidizer were utilized to show that the 14 new N,N-dimethylhydrazinium salts are hypergolic with ignition delay (ID) times ranging from 22 to 1642 ms, thereby suggesting that some may have potential as bipropellants.

No flame, no gain: A hypergolic mixture is composed of stable species that readily react/ignite on molecular contact. Both the anion and the cation in an ionic liquid play prominent roles in determining hypergolic properties as well as ignition delay times. With the 2,2-dialkyltriazanium cation, salts with nitrate, chloride, nitrocyanamide, and dicyanamide anions are hypergolic.

Several imidazolium-based ionic liquid azides with saturated and unsaturated side chains were prepared, and their physical and structural properties were investigated. The reactivity of these new as well as some previously reported ionic liquid azides with strong oxidizers, N 2O 4 and inhibited, red-fuming-nitric acid (IRFNA), was studied.

Energetic salts offer many advantages over conventional energetic molecular compounds. The use of nitrogen containing anions and cations contributes to high heats of formations and high densities. Their low carbon and hydrogen content gives rise to a good oxygen balance. The decomposition of these compounds is predominantly through the generation of dinitrogen which makes them very promising candidates for highly energetic materials for industrial or military applications.

The aim of this paper is to report on hypergolicity investigations of other fuel-rich anions, especially the dicyanamide anion. With the discovery of these ionic liquid hypergols, a new path for transitioning these materials into bipropellant applications seems clear. It is expected that these systems can now be fine-tuned for energy content, performance, and desirable physical properties and that they could replace state-of-art; highly toxic hydrazine and its derivatives

Borane compounds which are hydrogen storage materials are hypergolic with white fuming nitric acid as oxidizer. When dissolved in ionic liquids, the borane solutions exhibit ignition delay times which are superior to any known hypergolic ionic liquids. These borane–ionic liquid solutions appear to be the brightest hope so far to replace hydrazine and its derivatives as fuels in hypergolic propellant systems.