The Purdue University Hypergolic Propellants Lab is an interdisciplinary facility of the Maurice J. Zucrow Laboratories at Purdue University. Our research. Gelled/Neat Hypergolic Propellant Ignition and Combustion. Overview. Impinging Jets Set-up. Impinging jet apparatus featuring electromechanical actuators and. “Search for Green Hypergolic Propellants: Gas-Phase Ethanol/Nitrogen Tetroxide Reactivity”, Journal of Propulsion and Power, Vol. 21, No. 6 (), pp.

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Davis and Nadir Yilmaz. This is an open access article pfopellants under the Creative Commons Attribution Licensewhich permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A review of the literature pertaining to hypergolic fuel systems, particularly using hydrazine or its hypergoli and hydrogen peroxide, has been conducted. It has been shown that a large effort has been made towards minimizing the risks involved with the use of a toxic propellant such as the hydrazine.

Substitution of hydrazines for nontoxic propellant formulations such as the use of high purity hydrogen peroxide with various types of fuels is one of the major areas of study for future hypergolic propellants. A series of criteria for future hypergolic propellants has been recommended, including low toxicity, wide temperature range applicability, short ignition delay, high specific impulse or density specific impulse, and storability at room temperature.

In typical combustion systems an ignition source such as a spark is needed to begin the combustion reaction [ 1 hypertolic. In rocket propellants, there exists a class of materials which ignites spontaneously without the need for an ignition source.

A combination of two materials which self-ignites at room temperature is called hypergolic. Because they do not require external ignition forces compression, spark, heating, catalytic decomposition, etc. This simple mechanism for controlling combustion reduces the number of components in the ignition system which reduces the statistical chances for failure as well as the payload of the system relative to nonhypergolic systems.

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Therefore, such a system with few mechanical parts and low weight is particularly favorable to extraterrestrial craft. There are several quantifiable properties which are important in any propellant system including specific impulse the thrust per weight of propellant and adiabatic flame temperature [ 2 ].

Additionally, one of the most important quantities for a hypergolic propellant is the ignition delay, the time between fluid contact and ignition; alternatively, the chemical ignition delay may be oropellants, where the time between the onset of vaporization and ignition is utilized as defined in [ 34 ]. In [ 56 ] these delay times are used, in part, to describe the preignition behavior of hypergolic propellants. Regardless of definition the time to ignition is important to the performance of a rocket propellant where a long delay leads to combustion outside of the combustion chamber or causes hard-starts, whereas a very short delay risks damaging the injection nozzle [ 7 ].

There are two main techniques utilized to measure the ignition delay of propellwnts hypergolic mixture, the open cup or drop test and an engine test or impinging jet. In the former a pool of one component typically the oxidizer is placed below a dropper containing the other component the fuel. The measurement for the ignition delay may be taken in two different ways: In [ 6 ] a proepllants was used to set the ignition time rather than a high-speed camera, while recording the onset of boiling with a laser system as described in [ 34 ].

A diagram of the laser measurement system is depicted in Figure 1. As propdllants in Figure 1a laser is hypergooic above a pool of oxidizer. When the fuel droplet passes through this laser beam the recording bypergolic is triggered. The fluctuations in the laser signal due to the evaporation of the fluid mixture are also recorded and define the onset of measurement for chemical delay.

The ignition point is defined where the laser signal gets a boost from the production of heat and light [ 34 ]. The chemical delay time is preferred by many researchers as it eliminates the mixing factors which affect the reaction delay [ 346 ]. The chemical delay is measured in much the same way as the ignition delay, but is measured as the time between vaporization of the fuel and light ignition, thus minimizing the physical mixing aspects of preignition [ 346 ].

In [ 4 ], the chemical and physical chemical delay times were propellantss revealing that a significant amount of preignition time is involved in mixing and heating the fluids to boiling. It is also shown in [ 4 ] that for oxidizer tot fuel hypergollic between hypefgolic and four have little effect on the hypwrgolic time; outside this range mixing was worse and the delay times were less consistent.

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Propel,ants an optical system, [ 5 ] was able so show three distinct regions to the preignition of monomethylhydrazine with nitric acid in conjunction with fine-wire thermocouples. In addition to the drop test, impinging jet and engine tests may be utilized to measure the ignition delay. These tests are often preferable over the drop test because they evaluate the hypergolic ignition delay under conditions more closely resembling service in a rocket thruster [ 7 ].

In the impinging jet test, the two components are mixed as they are injected into a combustion chamber, and as before, the ignition delay is defined as the time between stream impingement and ignition as seen using high speed cameras [ 8 ]. Figure 2 is a diagram of an impinging jet apparatus.

One advantage of this test is the ability to evaluate environmental effects temperature, pressure, thermal conductivity, etc. Engine tests differ from impinging jet tests in a couple of ways. First, the ignition delay is measured using the chamber pressure [ 7 ]. According to Slocum-Wang et al. Due to this, it was recommended by Pourpoint and Anderson [ 8 ] that the drop test be utilized only as a prescreening tool for potential hypergolic propellants [ 8 ].

Due to the hypeegolic importance of ignition delay to the operation of a propellant system the effects the environment has on the system must be understood. By observing the ignition of high test peroxide HTP with Block 0 a mixture of methanol, water, manganese dioxide and acetic acid, made by dissolving manganese acetate tetrahydrate in water it was determined that the pressure dependence on ignition delay for liquid hypergols is inversely proportional to the log of the pressure [ 8 ].

However, this correlation is only slightly better than for a linear regression. The thermal conductivity and diffusion coefficient of the ambient atmosphere plays a minor role in the ignition delay; however, increasing the ambient diffusion coefficient and thermal conductivity increase the ignition delay slightly.

As expected from global reaction kinetic analysis, the higher the concentration of the reactants the faster the reaction may take place and the lower the ignition delay pro;ellants 8 ]. The examination of preignition chemistry was also explored for mixtures prropellants monomethyl hydrazine MMH and nitrogen tetroxide NTO using computer modeling techniques [ 10 ].

Molecular dynamics simulations involving Car-Parrinello Molecular Dynamics suggest that the initial bypergolic between MMH and NTO produces methyl diazene and later dimethyl tetrazane and methyl triazine. These species would be important in pulsed reactors and pulsed rockets, the buildup of which may lead to explosive events during successive firings [ 10 ]. Hydrazine-based hypergolic propellants are among the first and most widely used hypergolic propellants [ 21112 ].

When these fuels are used in combination with concentrated nitric acid or nitrogen tetroxide NTO, N 2 O 4 as oxidizers their performance is substantially better than hydrazine-based monopropellants [ 21112 ]. Hypergolic hydrazine propellants are also preferred over cryogenic propellants such as liquid oxygen LOXwhere cryogenic propellants are difficult to store long term [ 212 ]. Despite the history of hydrazine usage; the physiochemical aspects of its hypergolic activity are not fully understood.

Because hypergolic reactions are spontaneous and rapid, it is difficult to study the reaction through conventional means such as shock tube experiments. Instead, recent efforts have been made propellante explore the reactions of hydrazine and its derivatives using a combination of spectrometry, spectroscopy, high speed photography, and computer hypegolic. High speed photography studies performed by Wang and Thynell [ 5 ] and by Catoire et al.

Fourier transform infrared spectroscopy and time-of-flight mass spectrometry ToFMS suggest that when liquid MMH comes into contact with nitric acid the two begin reacting immediately forming monomethylhydrazine nitrate, methyl azide, and methyl nitrate, with small amounts of N 2 O, H 2 O and N 2. These products are formed propellwnts, leading to an increase in temperature until the liquids prope,lants vaporized and the monomethylhydrazine nitrate is aerosolized stage two.

It is in this third stage that the combustion reaction begins to produce light, and ignites [ 513 ]. This fog forms regardless of whether the system will ignite or not, and is controlled by the nypergolic and mixing ratios of the gases.

When the system ignites two different instances of light production are observed; the first light is produced over the whole gas volume and the second from a distinct location that then propagates through the rest of the gas. It has been proposed that the first instance of light production is induced by the exothermic reaction of the fog formation, because the fog itself is not chemiluminescent.

Because the second light production is more localized, it was suggested that this was the initial flame kernel [ 13 ]. Various thermodynamic and quantum chemical simulations have been performed in order to try and explain how hydrazine bipropellants prppellants and to determine the most important mechanisms responsible for self-ignition.

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However, before ppropellants valid model can be developed to explain propfllants reaction from liquid hydrazine to gaseous combustion products, a detailed understanding of the thermodynamics of hydrazine is first required.

Ina model based hpergolic the Claussius equation and the Helmholtz potential was developed in order to accurately predict the liquid-vapor phase changes of hydrazine [ 14 ]. This modified Claussius equation model correctly predicts the hydrazine liquid-vapor phase equilibrium from However, the model does not correctly predict the constant pressure heat capacity of hydrazine; this suggests changes to the state equation in later studies will be necessary [ 14 ].

Incorporation of the molecular geometry, including interaction siteselectrostatics, and Lennard-Jones potential into the model, allows accurate prediction of the density, heat capacity, viscosity and isobaric liquid-vapor phase equilibria hyergolic the neat and binary systems. The second virial coefficient in a power series approximation of pressure was correctly predicted for neat hydrazine and predictions of this coefficient for MMH and UDMH were also made, but experimental measurements for these two species were unavailable to the authors.

Another prediction made by in the Elts et al. The equation of state in this model was based on the Peng-Robinson equation with a quadratic Van der Waals, one-fluid mixing rule to incorporate experimental data.

Since many combustion reactions are studied using shock tube experiment, a detonation study of hydrazine decomposition was performed by Catoire et al. The radical N 2 H 3 is one of the most important species in this reaction [ 16 ]. Similarly, Daimon et al. It was noted by Daimon et al.

Therefore, new models are required which incorporate low-temperature reaction pathways. In two quantum chemical modeling studies by Sun and Law [ 18 ] and Zhang et al. However, the two models do not agree, with the Sun and Law [ 18 ] suggesting a faster decomposition reaction than the later Zhang et al. The discrepancy was explained by Zhang et al. It was further shown in [ 18 ] that their model matched well with low-pressure 0.

Thinking conceptually, at low pressure there are few inter-molecular collisions, and therefore the importance of radical reactions including hydrogen abstraction will be diminished, favoring instead unimolecular reactions such as N—N and C—N bond scission.

Therefore the results of both [ 1819 ] make physical sense. Shock tube experiments of MMH decomposition by Li et al.

The Cook et al. When compared to the theoretical model of Zhang et al. Other simulations of hydrazine combustion, and its derivatives, with N 2 O 4 or nitric acid have been performed, focusing on the identification of intermediate species with low activation energy formed through exothermic reactions [ 10172223 ]. In their model, Daimon et al. Similar adduct reactions occur as further hydrogen atoms are abstracted forming N 2 H 2and other radicals [ 17 ].

The hydrogen abstraction model is capable of predicting hypergolic ignition because the abstraction process has low activation energy and is highly exothermic both properties are required for hypergolic reaction.

Despite the ignition prediction capability, the Daimon et al. As described previously, methyldiazene, dimethyl tetrazene and methyltriazene have been proposed as important intermediate species during the combustion of MMH with NTO [ 10 ].

Dimethyl tetrazines are most common when excessive quantities of MMH are burned. Under cold conditions methylhydrazine, dimethylhydrazine nitrate, and methylhydrazine azide are also likely to be present [ 10 ]. In a separate study, performed by Osmont et al. Although these dimethyl tetrazines have not been studied empirically, their heats of formation were calculated to range from This same study also predicts the formation of nitroso, nitrite, nitro, and nitrate derivatives of MMH preceding decomposition [ 22 ], much like how N 2 H 3 radicals were proposed to make adducts with NO 2 in [ 17 ].

The enthalpies of formation for the nitroso, nitrite, nitro, and nitrate derivatives of MMH are between In addition to the various hydrazine and MMH derivatives predicted in [ 1722 ] the formation of isomers of N 2 O 4 is also predicted to have a pronounced effect on its reaction with the hydrazines [ 23 ]. Although molecular dynamics studies and quantum chemistry are capable of explaining, in part, the processes and chemical reactions that take place in the hypergolic reactions of hydrazines, these models are not yet capable of accurately predicting performance characteristics specific impulse, ignition delay, etc.

In developing better hypergolic propellants there are two different aspects to be explored: