Self-Igniting Fuel-Oxidizer Systems and Hybrid Rockets
Journal of Scientific & Industrial Research Vol. 62, April 2003, pp 293-3 10
Self-igniting Fuel-oxidizer Systems and Hybrid Rockets
S R Jain
Department of Aerospace Engineering, Indian Institute of Science, Bangalore 560 012
Fuel-oxidizer combinations capable of self-igniting simply on coming into mutual contact have been extensively used in bi-liquid rockets. In hybrid (solid fuel-liquid oxidizer) rockets too the self-igniting (hypergolic) propellant could be suited ideally. The relative non-existence of the hybrid rockets has been partly because of the paucity of suitable hypergolic solid fuels. The development of such fuelsis hampered because of the lack of understanding of the chemical reactions occurring between the fuel and the oxidizer leading to ignition. The chemistry of these highly exothermic reactions occurring in sub milliseconds in the pre-ignition stage is being studied for the past several years, in an effort to evolve suitable self-igniting systems. A major aspect of this work relates to,the solid N-N-bonded derivatives of hydrazines, which have been conceived as self-igniting fuels fo r the first time. Many of these compounds ignite readily, with short ignition delays on coming into contact with liquid oxidizers, like HN03 and N204. Polymeric resins having N-N bonds and reactive end-groups have been evolved to serve as fuel-binders for self-igniting compositions. This review narrates briefly the main highlights of the work
carried out on hypergolic systems. A report Oil the hybrid rockets and their current status is also included.
Introduction propellant rockets based on kerosene or 'alcohol Combustion of a fuel-oxidizer combination has liquid oxygen (LOX), e.g., the fuel and oxidizer are • been the main source of energy world over. By stored separately, and pumped into the combustion 'combustion' is meant a rapid oxidation reaction chamber where they are made to ignite by a separate producing heat and light. A 'fuel' in the presence of energy device, namely an igniter. Once ignited, the an 'oxidizer', when initiated by an appropriate thrust is controlled by varying the rate of pumping of impetus like, a spark, a heat source or a catalyst gets the fuel and oxidizer into the combustion chamber. ignited resulting in the onset of combustion. Once Similarly the solid propellant rockets, wherein the ignited the process is self-sustained by its own heat powder oxidizer is embedded in a matrix of release and usually lasts till one or both the polymeric fuel, and thus in constant touch with one • components (fuel and oxidizer) are exhausted, unless another, do not ignite, as such. In fact the ideal fuel the process is blown-off (extinguished) by other oxidizer combination for solid rockets must not react means. The initiation of ignition is essential to at all chemically for years, during storage. The overcome the activation energy barrier for the propellant is made to ignite by a separate hot-spot combustion reaction to take place. Fuels like wood, producing igniter incorporated in the grain-port, kerosene or gasoline for that matter do not ignite whenever required. spontaneously though they are constantly in contact There are, however, certain chemicals (fuels) with aerial oxygen. which ignite spontaneously simply on coming into Like many other energy driven devices the contact with oxidizers, and do not need a separate propulsion of the chemical rockets is based on the source for ignition. For example, liquid fuels like combustion of a fuel-oxidizer combination anhydrous hydrazine (AH), monomethylhydrazine (MMH) and unsymmetrical dimethylhydrazine • (propellant). Burning propellant gives off hot gases, which are expelled through a nozzle to produce the (UDMH) ignite on coming into immediate contact thrust that lifts the vehicle off the ground. In the with oxidizers such as, nitric acid or dinitrogen commonly used bi-liquid (liquid fuel-liquid oxidizer) tetroxide (N204), giving off hot gases. Evidently, such fuel-oxidizer combinations could be a natural choice as rocket propellants, since they simplify the engine Professor emeritus design by not using an igniter, and provide a 294 J SCI INO RES VOL 62 APRIL 2003 convenient way of achieving repeated on-and-off chemistry of these reactions is highly complex capability at no extra cost. Indeed the self-igniting, because several types of reactions occur, often also known as 'hypergolic' systems, have been simultaneously, in very short intervals. The heat widely used in bi-liquid propellant rockets. Besides evolved decomposes and/or vaporizes the solid fuel the high reliability of chemical ignition has made and liquid oxidizer, and raises the gaseous products to them the propellant of choice in the well-known ignition temperature. The understanding of these missions, like the Apollo manned lunar-landing reactions, which invariably last sub-milliseconds - expedition. Much of the credit for landing o.n the only, being poor, there is not even a rule of thumb for moon and bringing the astronauts back safely goes to predicting a priori the hypergolicity of a particular the hypergolic fuel MMH, which was used in the fuel-oxidizer system, leave aside the actual estimation - command service and lunar excursion modules in the of the ignition delay. Surprisingly, not many descent and ascent stage, respectively. systematic studies have been made to understand the Self-igniting (hypergolic) propellants are of chemistry of these reactions. special importance in hybrid rockets. In hybrid The present review is an overall summary of the systems the propellant usually comprises a solid fuel investigations carried out, mainly by the author and and liquid oxidizer, which are stored separately and his group for over the past two decades. The studies brought together only when fired. When the carried out mainly pertain to evolving an propellant is hypergolic, all that is needed is spraying understanding of the pre-ignition chemical processes of the liquid oxidizer onto the solid fuel to initiate that lead to hypergolic ignition, in an effort to evolve ignition. Since no separate igniter or fi ring solid fuels, having short ignition delays with storable mechanism is needed the system could be oxidizers like nitric acid and dinitrogen tetroxide. conveniently used for on-and-off capability, and Earl ier reports on the hypergol ic systems by J ai nl.2 modulating thrust simply by operating a singie vtilve covered only some aspects of the subject as the work controlling the oxidizer-flow. Also the hybrids being progressed. A status report on the development of inherently safer than the solid rockets, have received hybrid rockets is also presented. increasing attention in the recent past. Earlier Studies However, unlike the hypergolic bi-liquids the evolution of practical self-igniting hybrid propellants The ignition delay i.e. the elapsed time is a highly involved task. Most often the solid fuels preceding ignition after the oxidizer comes in contact do not ignite with the usual liquid oxidizers. Even with the fuel, is a complex fu nction of many physico when they do so the ignition precedes a long delay chemical processes. However, it primarily depends on after they come in contact, which leads to the 'hard' the chemical reactivity of the fuel with the oxidizer. start of the engine. On the other hand, if a highly The complexity of the pre-ignition reactions and the .\ reactive oxidizer like liquid fluorine is used, the number of other process parameters involved, has ignition delay (ID), no doubt will be reduced, but precluded the development of any theoretical model other serious problems like the corrosive nature of the to predict the 10 of any particular system. The IDs combustion products besides handling and have to be measured by experimental methods, not transportation, are encountered. Hypergolic hybrid only for screening out the promising systems but also systems compnsmg commonly used storable for determining their magnitude. While the ID should chemicals, having short igl1l!lOn delays, and be as short as possible, it largely depends upon the producing innocuous combustion products are indeed chemical nature of the fuel-oxidizer system chosen. , -- - rare. Several studies pertaining to the measurement of IDs The matn cause of the self-ignition of of hybrid systems are available in the literature. ' hypergolic systems is believed to be the rapid Oxidizers generation of heat by the exothermic chemical reactions ensued in the pre-ignition stage. In hybrid The choice of suitable liquid oxidizers for self systems, when the liquid oxidizer comes into contact igniting hybrid propellants is rather limited. Fluorine with solid fuel exothermic reactions start at the based oxidizers like, F2, CIF� and FCI03 are excluded surface of the fuel. A complex array of chemical because of the highly corrosive and toxic nature of reactions occurs before the onset of ignition. The their own and the combustion products. Other JAIN: SELF-IGNITING FUEL-OXIDIZER SYSTEMS & HYBRID ROCKETS 295
suitable oxidizers are: HN03, N204. and H202. Of aniline formaldehyde-fuming nitric acid system and these, pure nitric acid, i.e. white fuming nitric acid found these to contain the nitrated products of the (WFNA), N204 and red fuming nitric acid (RFNA), parent compound, and trinitrobenzene. These studies which is a mixture of nitric acid and dinitrogen indicate the occurrence of nitration reaction in the tetroxide, have been used most often. Highly pre-ignition stage with nitric acid oxidizer. On the concentrated H202 is tOf) risky to handle, and as a other hand, in liquid amine-nitric acid systems, a restilt it is largely avoided. Lewis-type acid-base reaction with the formation of a 8 salt occurs •9• Likewise, a pre-ignition product of the Fuels bi-liquid, hydrazine-RFNA system has been found to Unlike the hydrocarbons, used In most be hydrazinium nitrate 10. combustion driven devices including rockets, hypergolic solid fuels are rather rare. One of the early Hydrazones � I hypergolic combinations tested in hybrid motors was It is apparent that till about mid 1970s, 3 polyethylene-H202 (90 per cent) . Polymers like, approaches to develop solid fuels for hypergolic polystyrene and butyl rubber also ignite with highly hybrid rockets were based on hit and trial methods. 4 concentrated hydrogen peroxide. Bernard found Not many self-igniting solid-liquid systems were furfuramide and furfurylidene filled polymers to be reported in the open literature. A systematic study 5 hypergolic with nitric acid. Moutet and Barrere used leading to an understanding of the initial exothermic solid amines like, p-toluidene, p-phenylenediamine reactions leading to ignition had not emerged. It was and p-anisidine to make hypergolic polymeric at this stage, that the author initiated a research composites with HN03. Similarly, mixtures of amines program to examine the chemical aspects of and metal or metal hydrides powders embedded hypergolic ignition, in an attempt to evolve new solid polymeric composites have been tried as fuels for fuels for self-igniting hybrid systems at the author's hypergolic hybrid systems. Invariably, however, a Institute. powder fuel becomes non-hypergolic or its ignition delay becomes intolerably long when used as fi ller in Our approach was simple and straightforward. polymeric composites. Also, fu els like amines and An analogy to the fact that the liquid hydrazines ignite instantaneously on coming into contact with metal hydrides have to be handled and stored carefully to avoid surface oxidation. p-phenylene liquid oxidizers such as, white or red-fuming nitric acid (WFNA or RFNA) and N204, suggested that diamine, LiAIH4 etc., e.g., easily undergo aerial oxidation and could become deactivated during solid hydrazines could be considered as suitable fuels storage and lose their hypergolicity altogether. for hypergolic hybrid propellants. Indeed, solid hydrazines ignite on coming into contact with RFNA, Pre-ignition Reactions but they are rather rare and difficult to prepare. Ignition delays of several amine-based systems It is generally realized however, that the main have been measured as a function of certain variables, cause of enhanced reactivity of hydrazines towards and based on -their trends tentative conclusions have oxidizers is the presence of the nitrogen-nitrogen been drawn concerning the nature of the exothermic (N-N) bond in their structures. It was envisaged that reactions taking place in the pre-ignition stage. Only solid organic compounds containing one or more N-N in very few cases an attempt to actually identify the bond/s, like the hydrazines, might ignite on coming pre-ignition products was made using chemical into contact with liquid oxidizers. Of the simplest methods. Measurement of ignition delays of solid compounds having N-N bonds are the powdered ()- and p-phenylenediamine and furfur condensed products of hydrazines with aldehydes and aldazine with HN03 and N204 by Bernard and ketones, namely the hydrazones. The hydrazones are 6 coworkers led them to emphasize the role of surface non-hygroscopic, high melting solids, which most reactions in hypergolic ignition of liquid-solid often decompose exothelmally at higher (hybrid) systems. The study revealed that the reaction temperatures. A preliminary examination of some occurs in an adsorbed phase on fuel-surface, and the randomly selected hydrazones indeed showed that active agent responsible for promoting the hypergolic they do ignite on coming into contact with HN03 + 6 reaction with nitric acid is N02 . Munjal and (ref. 11). The ignition behavior of hydnizones, thus 7 Parvatiyar examined the pre-ignition products or beCap.le the prime focus of our investigation in 296 J SCI IND RES VOL 62 APRIL 2003 relation to developing new hybrid hypergolic less in the same order with these substituent groups systems. It may be worthwhile to mention that prior on the ring. The observed trend in the ID with the to our work no study concerning the self-igniting variation in the substituent, thus indicated the role of nature of hydrazones had been reported in the the nitration reaction in the pre-ignition stage. literature. Incidentally the presence of -N02 group on the N Our initial work was concerned with the phenyl ring also results in non-ignition; e.g., the condensed products of phenylhydrazine and UDMH various 2,4-dinitrophenylhydrazones examined - 2 with various aldehydesl . Specifically, substituted during this study were all found to be non-hypergolic with WFNA. benzaldehydes having different groups at the para position were chosen so that the products have very Further evidence for the occurrence of nitration 1 similar chemical structures but differ in chemical reaction in the pre-ignition stage was provided by a reactivity. These hydrazones could be synthesized in chemical analysis of the quenched reaction products " a single step by the following reaction I I : of benzaldehydephenylhydrazone-nitric acid system. , The isolation of p-nitrobenzoic acid and benzoic acid as reaction intermediates confirmed the occurrence of the nitration and also the oxidation reactions in the pre-ignition stage. These observations established ... (1) firmly that the hypergolicity is related to the chemical reactivity in these systems. As envisaged, several of the hydrazones in the The importance of this work was duly powder form were found to ignite readily with both recognized by the Soviets, who translated and 2 WFNA and RFNA. Some of them had very short IDs published the author's paperl in Russian in their indeed as measured by a newly-designed and journal, Rocket Techniques and Cosmonautics, as a fabricated drop-tester-type device, having an select paper of their choice, with permission of the electronic timer and a photo-sensitive transistor AIAA. circuit. In general the ignition delays of the derivatives of UDMH were shorter (35 to 165 ms) Parameters Affecting ID than those of phenylhydrazine (- 100 to 400 ms)12. It is recognized that the magnitude of ID of These hydrazones ignite with shorter IDs with WFNA powder materials depends upon physical (particle as compared to RFNA. It was also noticed that while size, compactness etc.) as well as the compositional all the dimethylhyrdrzones ignited readily with no parameters (oxidizer concentration, fuel/oxidizer particular order in ID, a comparison of the IDs ratio, additives, etc.). The effect of these parameters measured under identical set conditions of several was determined carefully using suitable systems. para- substituted benzaldehydephenylhydrazones indicated that the ignition delay perhaps depends on Particle Size and Compactness the nature of the substituent group of benzaldehyde. The ignition delay of some selected compounds Indeed, a remarkably consistent variation in the ID namely, furfuraldehyde- and p-dimethylaminobenz was observed with different substituents, and the p aldehyde-phenylhdrazones (FPH and DBPH), with nitrobenzaldehyde derivative did not ignite at all on RFNA decreases with decrease in the particle size, to coming into contact with WFNA. The ID increases some extent. Fine powders « -125 J.lm) show two with the substituent (R) in the following order: ignition delays corresponding to two flames: the first flame extinguishes soon after appearing but the second flame persists13. This strange ignition behavior of fine powders was observed for the first This order suggests that substitution by an time in our laboratory, which appears to be a general electron donating group causes a decrease in the ID, phenomenon. Powder amine fuels, benzidine and p while an electron-withdrawing group like -N02 phenylenediamine also exhibited successive two makes the system non-hypergolic. In this context, it is flames below a certain particle size. Compacting the I4 worth noting here that the velocity of nitration powder fuel results in increasing the ID . Pellets of reaction of benzene with HNOJ decreases more or two hydrazones, FPH and DBPH, made by JAIN: SELF-IGNITING FUEL-OXIDIZER SYSTEMS & HYBRID ROCKETS 297
compressing the powders of selected particle size N02+ 2 CJls-CH=N-NH-CJl5 N02 .CJl4-CH=N-NH- C�.N02 (212-106 Ilm) at diffe rent pressures (0- 100 kg/cm ) � ignited readily with nitric acid but the ID increased with pressure. o � N02,C6�-CHO + N2 + HO-C6�.N02 Oxidizer Concentration
The effect of N02 concentration in nitric acid on o the IDs of phenylhydrazones and dimethylhydrazones N02 CJ-I4-CHO � N02 .C6�-COOH powders was evaluated under carefully controlled . . .. expenmental con d'ltions 13·' 14 . A s anticIpated th e ID s N02+ are greatly influenced by the composition of the acid. HO-C6H4. N02 or C6HsOH � HO-C6H2 .(N02)3 The lowest IDs are obtained with pure anhydrous acid
� rather than RFNA. Increasing the concentration of I � N02 in the acid results in longer IDs. In some cases, a � CO2, N2, H20, etc. maximum in the ID vs N02 concentration plots was observed, with acid having around 12-16 per cent 3 OxidizerlFuel Ratio N02, giving longest IDsl . The effect of relative amounts of fuel and The concentration of water in nitric ac id has a 2 oxidizer on the ID of hydrazones with nitric acid has drastic effect on IDs.' The IDs of some typical p 3 been studied in detail.1 ,'4 Several of these hydrazones substituted benzaldehydephenylhydrazones become give minimum IDs at the mixture ratio close to the longer with an increase in the water content. At about stoichiometric ratio, although the variation is not 12 per cent water concentration, no ignition takes large. This is expected since the heat liberated by a place. It is therefore desired that the water content be fuel-oxidizer reaction should be maximum at kept as low as possible, for short IDs. The addition of stoichiometric ratio. fuming sulphuric acid in small amounts further .. increases the IDs of phenylhydrazones, but has Combustion Related Studies virtually no effect on the dimethylhydrazones. Studies relating to the combustion behavior of These results point fu rther to the nitration of the hydrazones with nitric acid include, experimental phenyl ring in the pre-ignition stage and supplement determination of the heats of combustion both with the chemical evidence viz. isolation of benzoic acid nitric acid and oxygen (calorific value), volume of the and p-nitrobenzoic acid from the pre-ignition reaction gases generated and identification of the gaseous products of benzaldehydephenylhydrazone-HN03 2 products. Apparently, both the heats of combustion system, obtained earlier. ' Based on these evidences, (with HN03) and volume of gases generated depend a scheme of reactions occurring in the pre-ignition on the substituent in the phenyl ring of the 4 s stage (as shown below) was proposed' . hydrazones' . The electron releasing groups enhance these parameters in the case of phenylhydrazones, Oxidation whereas no such effect was observed in the case of 2 HN03 � N02 +NO + H20 +02 dimethylhydrazones. The heats of formation of the hydrazones determined from the calorific values are O2 high, even positive in some cases, which is a C6HS-CH = N-NH-C6Hs � C6Hs- W-C6HS + OH- N2 + C6HsOH N = � Monothiocarbohydrazones Nitration Our discovery of the hypergolic nature of 2 HNO, � NO/ + NO,' + H20 phenyl- and dimethylhydrazones with nitric acid led 298 J SCI INO RES VOL 62 APRIL 2003 us to examine other compounds having N-N bonds in diluted with glass powder (to prevent ignition) was their structures. Realizing that the hypergolicity could reacted with nitric acid and subjected to chemical be related to the number of N-N bonds, compounds analysis. Benzoic acid, p-nitrobenzoic acid, having two N-N bonds in a molecule became the next benzaldehyde, sulphur trioxide, nitrogen dioxide and targets. Derivatives of carbohydrazide (which has two nitrogen were isolated as pre-ignition reaction N-N bonds), with several aldehydes were examined. intermediates. A scheme of reactions involving Interestingly, they were all found to be non neutralization, nitration and oxidation occurring in the - hypergolic with WFNA. It is thus apparent that not all pre-ignition stage was proposed based on the compounds having N-N bonds are hypergolic. formation of these products 19. It is likely that the Coincidentally, however, we also examined the solid neutralization (acid-base) reaction which is known to derivatives of thiocarbohydrazide (TCH) and to our be one of the fastest reactions, plays a predominant 6 surprise, they all ignited readily with nitric acidl . role and occurs in the preliminary stages in these Like the hydrazones, the monothiocarbohydrazones systems. However, unlike in the amine or hydrazine S O are non-hygroscopic solids which could be prepared nitric acid systems .I no 'salt' product could be easily by the following reaction: isolated in the present case. It is likely that the heat evolved in the neutralization process decomposes immediately the salt formed evolving more heat, and H2N-NH-C(=S)-NH-NH2 + RIR2C=O � adds to the overall exothermicity . H2N-NH-C(=S)-NH=CRI R2 + H20 ... (2) Calorimetric Values A detailed examination of the IH NMR spectra There was no suitable device available to shows that these derivatives exist in two isomeric evaluate accurately the experimental heats of structural forms: linear (as shown above) and cyclic, combustion of hypergolic system, till we commenced depending upon the type of aldehyde or ketone our studies. The method used earlier was crude and used 17. A total of eighteen mono-derivatives of gave lower than the expected values. A new thiocarbohydrzide were synthesized. Considering the calorimetric bomb device was de'veloped during the high purity of these compounds the Sad tier Research course of our investigations. The usual bomb Laboratories Inc. requested for the samples, to calorimeter was modified to release the liquid include their NMR and IR spectra in their Standard oxidizer on to the fuel (solid or liquid) in the closed Spectra supplements, which were subsequently calorimeter bomb by remote control. The technique 8. published 1 involved snapping a fuse wire connecting the two electrodes of the bomb, which, in turn, tilted a Ignition Related Studies container inside the bomb having liquid oxidizer to The monothiocarbohydrazones are extremely pour on to the fuel, by switching on electric current reactive towards anhydrous nitric acid and ignite from outside. It was an ingenious device designed and o with very short ignition delays (15 to 50 ms), much fabricated for the first time in our laborator/ . The shorter than those of the hydrazones. Here again the heats of combustion of several hypergolic systems, IDs of over a dozen of the various p-substituted including the monothiocarbohydrazones, with nitric benzaldehydethiocarbohydrazones were found to acid oxidizer, were determined. The values obtained vary more or less in the same order with the by this method matched very closely to the theoretical substituent as those of the hydrazones. Substitution heats of combustion. - by an electron releasing group, like the -N(CH3h Typical characterization data of some of the tend to decrease the ID, whereas the electron mono-thiocarbohydrazones are given in Table I. withdrawing -N02 group makes the system non hypergolic. There was no significant effect of either Performance Parameters the particle size or the OIF ratio on the IDs. In most The theoretical calculation of the rocket cases, however, the IDs were found to increase with l6 performance parameters such as, specific impulse the N02 content of the acid . (Isp), chamber temperature (Tc), characteristic velocity * In order to get an insight of the chemical (C ) and mean molecular weight of the gaseous reactions occurring in the pre-ignition stage, a products (M) requires the heats of formation data of blended mixture of benzaldehydethiocarbohydrazone the propellant ingredients, as an input parameter. The JAIN: SELF-IGNlTING FUEL-OXIDIZER SYSTEMS& HYBRID ROCKIITS 299 Table 1- Ignition delay, thermal decomposition and heats of combustion data of some mono-thiocarbohydrazones � 111iocarbohydrazone Melting/decomp. Ignition delay' Heat ofcombustion (kcaUmol) temperature (uC) (ms) LiHc (02) LiHc (HN03) Formaldehyde 177/197 64 537.6 502.7 Furfuraldehyde 182/ 192 30 964.0 870.3 Acetone 19512 14 38 856.4 792.8 Benzaldehyde 195/204 53 1239.2 925.4 ) DimethyJaminobenzaldehyde 1601 192 15 1624. 1 12D8.7 'Oxidizer : WFNA .."""t heats of formation of the thiocarbohydrazones being products, assuming homolytic cleavage of the N-N 22 unknown, these were calculated by the experimental bond as a primary step . heats of combustion determined using a calorimeter 2 in oxygen, for the fi rst time 1. The observed high Bis-Thiocabohydrazones and Thiosemicarb37.ones heats of formation of these materials indicated further The exceptional hypergolicity of the mono their usefulness as potential fuels. derivatives of thiocarbohydrazide with nitric acid led The calculation of the performance parameters us to investigate the bis-thiocarbohydrazones. Like performed by making use of the NASA-SP273 the mono-, the bis-derivatives of thiocarbohydrazide computer program was carried out under both the with aldehydes and ketones could be easily prepared frozen flow and equilibrium flow conditions at by changing the reactant ratio and altering the various OfF ratios and exit pressures. These reaction conditions suitably. Over a dozen bis parameters were derived at Pc = 30 atm and exit thiocarbohydrazones were synthesized and chara· pressure, Pe = 0.5 atm, i.e. expansion ratio, Pc / Pe = cterized by their chemical analyses, melting points, I ?' 60. It is seen that the thiocarbohydrazones-HN03 IR and H NMR spectra Thiosemicarbazones were systems have Isp in the range 240-250 s. It may be prepared by reacting thiosemicarbazide and the appropriate carbonyl compound, and characterized by mentioned that these Isp values compare well with l9 other hybrid systems, like R005-RFNA (250 s) or a their melting points • non-hypergolic hybrid, rubber-RFNA (260 s), Ignition Delays reported at expansion ratio, 68 to I atm. However, an exact comparison of the Isp and other data could not In the studies carried out so far the effect of be made because of the different chamber and exit change in the subtituent in potentially hypergolic 21 pressure conditions assumed . systems (hydrazones and mono-thiocarbohydrazones) on the ignition delay was determined to understand Thermal Decomposition the role of chemical reactions occurring in the pre Thermal analysis of monothiocarbohydrazones ignition stage. It is interesting, however, to compare by DT A/TG techniques show that they all decompose the minimum IDs of series of closely related exothermally soon after melting. The cause of compounds, to get a deeper understanding of the role exothermal decomposition is attributed to the of the basic chemical structures. presence of N-N bonds. The products of The three series of compounds selected for decomposition of benzaldehyde-thiocabohydrazone study are represented by the following chemical were analyzed on a gas chromatograph, and by other structures: methods of chemical analysis. Some of the products identified were, N2, NH3, H2S, benzonitrile, R1R2C=NNHC(=S )NH2 R1R2C=NNHC(=S)NHNH2 thiobenzaldehyde, 2,4,6-triphenyl-s-triazine and (Thiosemicarbazones) (Mono-thiocarbohydrazones) complex condensation products containing C=N linkages. A probable mechanism of decomposition R1R2C=NNHC(=S)NHN=CR1R2 was proposed based on the formation of these (B is-thiocarbohydrazones) 300 J SCI IND RES VOL 62 APRIL 2003 The average mInt mum ignition delays of these Schiff bases, azines, hydrazones, thiocarbo series of compounds (a total of 28 derivatives) were hydrazones, thiosemicarbazones, hydroxy compounds determined with nttrtC acid under identical and aromatic hydrocarbons (a total of 37 compounds) l9 experimental conditions • It was observed that the were examined. All of these excepting the derivatives of aliphatic carbonyl compounds of all the hydrocarbons, ignited readily with WFNA on mixing three series are hypergolic both with WFNA. In with Mg powder although as such many were non 24 general the IDs of the bis-derivatives are somewhat hypergolic or exhibit longer IDs. A variation of the longer than the mono-analogs. Here again, longer Ids Mg content in a typical case shows that the minimum are obtained with RFNA as compared to WFNA. The IDs obtained are around 40 per cent. The minimum IDs of the mixtures having fuellMg weight ratio, bis-derivatives of aromatic aldehydes are almost all ( non-hypergolic in contrast to the mono-analogs. The 60:40, were determined to compare their relative IDs of the thiosemicarbazones are longer than the hypergolicity. Interestingly, although the corresponding mono-thiocabohydrazones. This dif monothiocarbohydrazones ignite with WFNA with ference has been attributed to the one less number of very short IDs (- 40 ms), their mixtures exhibit basic nitrogen, and N-N bond in thiosemicarbazones. longer IDs (- 140 ms). It thus appears that the IDs However, it is interesting to note that the trend decrease generally further on mixing with Mg observed in the IDs as a result of the substitution of powder. However, highly hypergolic fuels having IDs diffe rent groups in the phenyl ring of the benzaldehyde - say, shorter than 100ms, the IDs of their mixtures derivatives is the same as in the case of the mono are increased instead. thiocarbohydrazones as well as phenylhydrazones. In a parallel study the effect of various metals Metallized Systems on ID was examined to determine if the effect of Mg was specific in some way. However, many other An interesting development in hybrid metals like, Zn, Cu, and Fe also exhibit the propellants has been the effect of mixing certain synergistic hypergolic effect similar to Mg on mixing metal powders with organic fuels resulting in with fuels. In fact, the shortest IDs are observed with enhancing the hypergolicity. The addition of metals Zn powder and not with Mg, though Zn has other to the propellant composition is preferred as they undesirable properties like low heat of combustion generally enhance the performance parameters and high molecular weight. because of high heats of combustion. However, when metal like aluminum is added it was found that the Although the complete mechanism of IDs become intolerably long. Surprisingly, some synerglsttc hypergolic ignition still remains metal powders (e.g. Mg, Cu, Zn and Fe) on mixing unresolved the effect has been attributed primarily to with certain solid fuels ignite immediately on coming the increased exothermicity of the system due to the into contact with nitric acid. Almost all hydrazones mutual contributions of the heat evolving chemical and thiocarbohydrazones are ignited readily, most reactions between the organic fuel, metal and nitric with reduced ignition delays, on mixing particularly acid. The main exothermic reactions between an with magnesium powder, both with WFNA and organic fuel and WFNA have been identified as RFNA. Also, many non-hypergolic solid hydrazones neutralization, nitration, and oxidation. These became hypergolic on mixing with metals, although reactions may lead to the fo rmation of unstable ' the metal powder by itself does not ignite with nitric intermediates that decompose exothermally, thereby acid. This strange effect of powder metals in contributing to the overall exothermicity of the ,-- hypergolic ignition was envisaged to have high system. The primary product of the reaction between potential for developing new hybrid fuel systems. metal, e.g. Mg with WFNA is magnesium nitrate. The Furthermore the chemistry of these so-called heat evolved during its formation may help in 'synergistic reactions' leading to ignition, being accelerating the rates of other reactions occurring completely unknown, it was considered worthwhile to between the organic fuel and WFNA. Magnesium investigate this phenomenon in detail. nitrate may act as a dehydrating agent, removing water from the system, which may facilitate the Synergistic Hypergolic Ignition nitration reaction between the organic fuel and In order to appreciate the scope of the effect, a WFNA. Also, the oxygen formed by the wide variety of solid organic compounds such as decomposition of magnesium nitrate may oxidize the lAIN: SELF-IGNITING FUEL-OXIDIZER SYSTEMS & HYBRID ROCKETS 301 40 organic fu el, and thereby contribute to the overall the maximum Isp for various fuels is obtained at . 24 exoth enruclt. y . per cent Mg content. These systems were found to give superior performance parameters (Js - 255 s) Heats of Combustion with WFNA p than the unmetallized systems. Heats of combustion of several self-igniting A comparison of the relevant parameters, such Mg-organic fu el-WFNA systems were determined as ignition delay, heat of combustion and specific using the modified bomb calorimetric device 20 impulse of some typical metallized systems is gi ven described earlier . The measured heats of in Table combustion of fu el mixtures having 40-50 per cent 2. Mg at different OIF ratio show a maximum around Hypergolic Ignition with N2 04 the stoichiometric ratio, as expected. Also, the heats of combustion of the metallized systems are higher Hypergolic ignition studies carried out thus far 25 than the organic fuels alone, with WFNA . were confined to nitric acid oxidizer. With dinitrogen tetroxide (N204) most of the solid organic fuels are Performance Parameters of Synergistically Igniting either rion-hypergolic or ignite with very long IDs. Systems Consequently, there was no serious study on hypergolic systems with liquid N204 as an oxidizer. The unexpected hypergolicity of metallized However, a preliminary examination showed that systems led to exploration of other relevant some of the fuels ignite readily on mixing with metal parameters pertaining to their performance. Several powders. In other words, N204 could participate in new synergistically igniting Mg-organic fue l-nitric synergistic hypergolic ignition. acid systems were chosen for evaluating their ' Schiff-base-Mg-N204 Systems theoretical rocket performance parameters (Jsp, Tc, C and M) in order to assess their viability. These The idea of using N204, a low boiling liquid parameters were calculated varying OIF ratios, metal oxidizer in synergistically igniting hybrid propellant loading and equivalence ratios at chamber pressure, systems was first conceived and exarllined in our Pc = 30 atm and expansion ratio, Pc I Pe = 60, laboratory. Fuel compositions comprising mixtures of 25 assuming equilibrium flow conditions • Interestingly Mg powder with a variety of Schiff-bases (the Table 2 - Ignition delays, heats of combustion and specificimpulse of some metallized systems. Ignition delay' �H" (HN03) Specitic impulse' Fuel composition (per cent weight) (ms) (kcal/g reactants) (s) p-Dimethylaminobenzylideneaniline (PDBA) No ignition PDBA (30) + Mg (70) 158 246 PDBA (30) + Zn (70) 70 PDBA (30) + Cu (70) 162 PDBA (30) + Fe (70) 787 PDBA (30) + AI (70) No ignition PDBA (60) + Mg (40) 118 258.1 Acetonethiocarbohydrazone (50) + Mg (50) 156 1.67 248.5 Benzylideneaniline (40) + Mg (60) 167 1.0 256.4 Furfuraldehydeplrenylhydrazone (50) + Mg (50) 138 1.19 250.2 Benzaldehydethiocarbohydrazone (50) + Mg (50) 171 1.38 246.5 'Oxidizer : WFNA 302 J SCI IND RES VOL 62 APRIL 2003 condensed products of anilines with aromatic Temperature Profiles aldehydes) were found to ignite with short IDs with A possible way of studying the pre-ignition liquid N204 (ref. 26). The ignition delays were process is to measure and record the temperature measured using a modified device. Considering the profile prior to the onset of ignition. The usual low volatility of N204 the apparatus was redesigned thermocouple methods being not suitable because of not only to cool the liquid and hold at lower pressure, their slow response, a thin metal film thermometric but also to release the oxidizer with minimum technique was devised and set up in our laboratory for = personal error. It was observed that the minimum IDs the first time. It records the temperature as sensed by are obtained at about I:I ratio (by weight) of the the thin platinum fi lm and also the ignition delay of a Schiff-base to Mg, both with N204 and WFNA. A 28 self-igniting system in a single run . The device comparison of the IDs determined under identical essentially consists of a probe cup with a micron-thin conditions of a total of 16 Schiff-bases-Mg mixtures platinum ribbon deposited on the inside surface near show that the ID values with WFNA are considerably the bottom, which acts as a quickly responding shorter than with N204. temperature sensor, and a set of sensitive photo The ignition delay was found to vary with the diodes for sensing the flame. Both the ignition delay nature of the substitution in both the benzene rings. In and the temperature profiles are recorded and fact, a linear relationship emerges when the IDs are displayed simultaneously on the two channels of a plotted against the Hammett substitution constant (cr) duel trace storage digital oscilloscope, which is (ref. 26). This constant being a measure of the interfaced to a microcomputer. reactivity of the phe nyl substituted compounds the This device was test checked, using both the linear relationship establishes unequivocally the liquid-liquid and solid-liquid systems. The recorded dependence of ID on chemical reactivity. temperature profiles of aniline-furfuryl aIcohol Furthermore the pre-ignition products of the reaction WFNA system shows synergic effect. Whereas, pure of N204 with Mg and benzylidineaniline have been aniline is non-hypergolic, it ignites readily on mixing analyzed to be magnesium nitrate, benzenediazonium with furfuryl alcohol with WFNA in as short a time salt and benzaldehyde. Based on these products, a as 0. 1 ms onl 8. probable reaction mechanism occurring in the pre l 26 The basic chemical reactions occurring between ignition stage has been proposed . organic fuels and pure nitric acid are not clearly Anilines-Mg-N104 Systems understood because of their exceedingly high rate and Like the Schiff bases, solid anilines were also exothermicilY. Most often, charred end products are found to ignite when mixed with magnesium powder, formed in sub-second periods. It was envisaged that a 2 with dinitrogen tetroxide. 7 Of the six anilines study of the temperature profiles of closely related examined, p-toluidine and p-a nisidine, and their (structurally similar) simple fuel-oxidizer systems mixtures with Mg ignite readily. Others do not iJ:;nite might be helpful in understanding the nature of these without mixing with Mg either with N204 or RFNA. reactions. A comparison of the temperature profiles ): . Interestingly, p-toluidine does not ignite either with of the reactions between mono-substituted benzenes WFNA or RFNA but ignites with very short ignition and anhydrous nitric acid, showed that the extent of delay with N204. This particular system was chosen exothermicity and the rate of heat evolution depend fo r a detailed examination of their pre-ignition on the substituent. The initial reaction was identified products, to get an insight of the exothermic reactions to be nitration, which is accompanied by other occurring prior to ignition. exothermic reactions, especially in systems leading to The quenched products of p-toluidine-N204 ignition. The profiles of the self-igniting systems, system were found to contain p-toluidinium nitrate, amine-HN03 and amine- N204, suggest that, apart I ,3-bis( p-tolyl)-triazene- I-oxide, p,p ' -dimethylazo from their individual exothermicities. the specific nature of the fuel-oxidizer reactions also plays an benzene and possibly a mixture of p-cresoles. 29 3o A probable pre-ignition reaction pathway involving important role in the onset of ignition • . nitrosonium ion and the formation of nitrosamine The temperature profiles of several hybrid intermediate was proposed, which to certain extent systems were recorded in an effort to establish a explains the synergistic ignition of amines and relationship between, temperature at ign ition point, - magnesium powder mixture with N204 (ref. 27). slope of the temperature-time (rate of heat build-up) JAIN: SELF-IGNTTING FUEL-OXIDIZER SYSTEMS & HYBRID ROCKETS 303 ' and igmtlOn delal . However, although an fuel: (a) Melt-cast technique and (b) Casting with approximate correlation emerged, a definite relation polymeric binders. The former method was adopted could not be established. Of course the IDs of hybrid in cases where the melting and decomposition systems recorded by this technique matched closely temperatures of fuel are well separated. Such fuels with that determined earlier by the drop-tester with suitable additives could be cast by melting in a method. suitable mould and subsequent cooling to room temperature. The second method was akin to casting Rapid-scan FTIR and Thermal Profiles of solid composite propellants, with fuel powder as A simultaneous rapid-scan FfIRIthermal filler and a functionally terminated polymer as binder. profiling technique was made use of for the first time after carrying out necessary modification in the cell Melt-cast Self-igniting Grains and fuel injection technique, to examine the Several highly hypergolic powder fuels exothermic reactions of some self-igniting fuel-HN03 systems. The thermal profiles show a sudden rise in including the hydrazones and thiocarbonohydrazones, temperature when the liquid oxidizer was dropped on and their mixtures with other self-igniting fuels, like to the solid fuels. Simultaneous monitoring of the gas monocyclohexanonethiocarbohydrazone (CHTCH), products of the reaction between p-phenylenediamine were melt-cast in test tubes or teflon moulds were examined as cut-pellets. Invariably the IDs of the and nitric acid revealed the formation of CO2, N02, NO and HONO, within two seconds. Immediate melt-cast pellets were longer than those determined, evolution of N02 was observed in some typical taking the composition in the powder ' form, with synergistically igniting systems comprising anilines, WFNA or RFNA. Similarly the IDs of the melt casts of synergistically igniting compositions having Mg and HN03. Thiocarbohydrazide and its acetone and benzaldehyde mono-derivatives on reacting with magnesium were found to be longer. However, minimum IDs of the order of 150 to 250 ms could be HN03 produced S02, N02 and possibly OCS. These results were explained in terms of the oxidation and obtained after optimizing the ratio in many cases. 32 Several of these melt-fuel composites had fairly good .. nitration reactions occurring in these systems . 33 34 compression strength to0 , . Hypergolic Fuel Grains The relevance of ignition studies on fuels in the Polymer-cast Self-igniting Grains powder form was to evolve a basic understanding of Highly hypergolic fuel powders when cast using the role of exothermic chemical processes and conventional binders like, carboxyl- or hydroxyl reactions occurring in the pre-ignition stage in terminated polybutadiene (CTPB or HTPB) which are hypergolic ignition. In this context several new widely used in solid propellants, either become non hypergolic hybrid systems were also discovered. hypergolic or ignite with intolerably long delay. For Studies peltaining to ignition delays of various example, a derivative of thiocarbohydrazone, systems reflect their relative hypergolicity · quite CHTCH, which ignites within 15 ms in the powder accurately. However, these measurements have little form with WFNA, the ID of its CTPB cast having 85 meaning from the application point of view, since the per cent powder loading increases to 2.74 s. It is thus fu el is used as a continuous mass cast in a particular obvious that the usual polybutadiene binders used in shape (called 'grain') in actual usage. On casting, solid propellants are unsuitable for consolidating the since the surface area available for chemical reaction hybrid fuels. Fuel grains cast using some newly with the oxidizer decreases drastically, it is only synthesized epoxy resins with ingredients comprising expected that the IDs will increase significantly. In hypergolic fuels and curing agents, and magnesium some cases, a fuel which is hypergolic as a powder, powder, however, gave promising results, having may not ignite at all in the cast form. Strangely, there minimum IDs about 200 to 400 ms. Some of these was no systematic study of IDs reported on cast fuels casts also had fairly good mechanical properties. in the open literature till we commenced our work. Increasing the amount of binder though improved the Consolidating hypergolic fuel grains having mechanical strength but retarded the hypergolicity of good mechanical strength and at the same time the grain. Here again the usual bis-phenol epoxy resin igniting with tolerable IDs is an involved task. We was found to be unsuitable as it made the fuel grain adopted two methods to give shape to the powder non-hypergolic. The epoxides based on amine and 304 J SCI IND RES VOL 62 APRIL 2003 azine moieties only were found to yield grains having found to be thermally stable at room temperature and . .. ,1 34 reasonably short IDs Wit h. mtnc aCI'd .. , . . undergo decomposition exothermally at high temperatures, as shown in DTA studies. N-N-bonded Resins as Binders Interestingly, some of the cured resins were From our ongoing study of the self-ignition found to be igniting as such, on coming into contact behavior of powder hydrazones and with Wl<"NA, although with long IDs. Solid fuel thiocarbohydrazones with mtnc . acid, it was composites processed with these resins as binders had envisaged that the polymeric resins having N-N adequate mechanical strength, and ignite smoothly bonds in their structures could have desirable ignition 3s with short IDs with WFNA. The effect of variables properties. A literature search revealed however, such as, fuel composition, OIF ratio, types of ( that polymers based on N-N bonds are relatively few, curatives, effect of solid loading, amount of metal and none suitable as binder because of the lack of powder in the composition, etc. on the ignition delay, 39 40 suitable end groups capable of curing easily without was determined , . Composites having high loading formation of volatile by-products. A comprehensive of magnesium powder exhibit synergistic ignition program of synthesizing viscous resins having N-N with nitric acid, and ignite with short IDs. In general bonds in their structures and suitable end groups, to the epoxy composites of bis-carbohydrazones have cast hypergolic propellant grains was therefore shorter ID� than those based on bis-thiocarbo undertaken in our laboratory. Our initial attempts to hydrazone resins. This is surprising, considering the prepare viscous N-N-bonded polymers were " fact that the bis-hydrazones are nonhypergolic as unsuccessful . Most often the polymers obtained powders, while the bis-thiocarbohydrazones are '924 were solid products which were unsuitable for casting highly hypergolic with WFNA , . The cast grain powder compositions. composites are stable in air with no evidence of oxidation on storage. These studies clearly show that Resins based on bis-Carbohydrazones and bis-Th iocarbohydrazones the N-N bonded binders have several desirable characteristics for processing hypergolic fuel grains .. Eventually, however, the epoxidation of bis for hybrid propellant systems. carbohydrzones and bis-thiocarbohydrzones yielded resins having adequate viscosity for high solid Dicarboxy l Bis(hydrazones) and their Epoxides loading. The resins could be cured easily with amines Yet another class of N-N bonded compounds like, diaminodiphenylmethane (DDM). This was were synthesized with a view to prepare new epoxy indeed a turning point in our research. Subsequently, resins having varying amount of chain flexibility to we synthesized and characterized a series of epoxy provide improved mechanical properties to the grain. resins by various, chemical, spectroscopic and 36 38 These were based on N, N'-aliphatic dicarboxyl thermoanalytical techniques . . The di-epoxy resins bis(hydrazones), the aldehyde/ketone derivatives of had about two end groups per molecule. The tetra dihydrazides of various aliphatic dicarboxylic acids. epoxides could also be prepared when the parent It was anticipated that the linear hydrocarbon spacer .)c.. ' compound had additional hydroxyl groups, like in the groups, -(CH2)n- present in the parent hydrazone case of bis-vanillin-thiocarbohydrazone. Apparently, would impart flexibility to the resin backbone. These the epoxidation takes place both on the -NH and -OH bis(hydrazones) could be prepared easily by protons in these systems. The structure of a typical di converting the dicarboxylic acids like, malonic, epoxy resin based on bis-aldehyde-thiocarbohy adipic or sebacic acid to their methyl esters, reacting drazone could be represented as follows: them with hydrazine hydrate and subsequently 4' condensing with a aldehyde or ketone . The structure CH2 - CH - CH2 - N - C (=S) - N - CH2 - CH - CH2 of a typical dicarboxyl bis(hydrazone) can be "0/ � � V represented as follows: H � R H � R Several such resins with various substituent (R) Over a dozen bis(hydrazones) were prepared groups were synthesized by epoxidizing the parent with different substituents (R,and R2) and methylene bis-thio- or bis-carbohydrazones. The resins were spacer groups (n = 1,4 and 8). These compounds are JAIN: SELF-IGNITING FUEL-OXIDIZER SYSTEMS & HYBRID ROCKETS 305 non-hygroscopic, stable in air and decompose after e.g., has been the work horse binder for solid melting at high temperatures. propellant compositions. Solid propellant grains based on HTPB not only have superior mechanical Interestingly, many of these compounds ignite properties but also give higher specific impUlse. Since spontaneously on corning into contact with nitric the use of HTPB binder for hypergolic powder acid, often with very short ignition delays. The ID 33 composition results in non-ignition of the fuel grain , increases with increase in the number of methylene it was decided to synthesize new resins having N-N spacer groups. Simultaneou:i DTAffG study show bonds in the structure but tenninated with hydroxyl that typically the bis(hydrazones) that decompose groups. However, synthesis of a viscous resin having exothermically usually ignite with WFNA; also, those 2 terminal -OH groups turned out to be an involved decomposing soon after melting have short IDS4 . The task. Ultimately, we were successful in synthesizing hydrazones which do not ignite on coming into resins that contained a carbonyl or thiocarbonyl contact with WFNA, show synergistic ignition on moiety; N-N bonds; ethereal linkage; and terminal mixing with magnesium powder. hydroxyl groups. The epoxidation of the bis(hydrazones) resulted The resins were synthesized by condensing in the expected formation of their di-epoxides having bisvani1lincarbono- and bisvani1linthiocarbono convenient viscosity. Several bis(hydrazones) based 9 hydrazone, respectively, with J,4-dibromobutane4 . resins were synthesized and characterized by various 41 4-S The resins were characteri7..ed by various chemical tec h·mques - - _ When used as b'tn ders for powder and spectroscopic techniques, and other properties compositions these resins are capable of taking high relevant to their use propellant binders, such as solid loading. In fact, these could be conveniently a<; calorific values, thermal analysis, viscosity and used as energetic binders for solid propellants ba<;ed ignition behavior. They have convenient viscosity, on ammonium perchlorate. The propellant grains indicating their suitability as binders for processing processed with these resins not only have good high solid loading, and could be cured easily with mechanical strength but also significantly high 4<>47 toluene diisocyanate and glycerol. These resins ignite burning rates, and specific impulse . Detailed immediately on coming in contact with pure nitric studies on these and other N-N-bonded resins have 50 acid indicating their hypergolic nature • It thus seems been made considering their potential use as binders 48 that these resins could be highly suited as binders for in solid propellants . It is anticipated that these resins hypergolic hybrid systems. would act as good binders for powder fuel composition for hybrid systems too. Typical characterization data of some of the N N-bonded resins are given in Table 3. Hydroxyl Terminated N-N-bonded Resins Findings in-Brief Hydroxyl terminated pre-polymers are preferred binders as they could be converted to polyurethanes The hypergolic ignition is an area, which was on reacting with isocyanates and cured easily with largely unexplored. The earlier reports aimed triols. Hydroxyl ternlinated polybutadiene (HTPB), primarily at developing workable hypergolic hybrid Table 3 Characterization data of some N-N bonded epoxyresins ' " Resin Calorific value Viscosity at 3S C GlilSstransition! Density (glcc) (kcaJlg) (Poise) Decomposition temp' ('C) DEFCH 7, 185 404.7 300 dec 1.37 DEMFH 5.32 350 551215 1.27 DESFH 6,45 100 -5/220 1.22 DEABuH 6.64 157 30/205 1.15 DESBuH 7. 13 75 -131205 1.12 J- DVCBE 6.29 150 220 dec 1.18 • DEF�H, diepoxide of ?is-furfuraldehydecarbohydrazone; DEMFH, diepoxide of malonyl bis-furfuraJdehydehydrazone: DESFH, dtepoxtde of sebacoyl bts-furfuradehyde hydrazone; DEABuH, diepoxide of adipoylbis-butanonehydrazone' ' DESBuH , d'tepoxt 'd eo f .. . . se b acoy I b·tS- . b uanone h y d razone; DVCBE, dt(bts-vantlltncarbohydrazone-4'---oxy) butane 1,4 diyldiether. 'Curedresins 306 J SCI INO RES VOL 62 APRIL 2003 systems without putting much of an emphasis on the grain surface, makes the oxidizer-free grain evolving an understanding. The basic chemistry of . insensitive to cracks, bond line separation and the self-ignition process remained unexamined. The joint openings - factors often responsible for present scheme of work spread over two decades catastrophic failures in solid motors. aimed at arriving a chemical basis fo r understanding (d) High Isp - The theoretical vacuum specific self-ignition of N-N-bonded compounds, which were impulse of hybrids is considerably higher than considered as potential candidates was attempted for the solid rockets, and the density impulse is the first time. As it happens usually in experimental higher than the bi-liquid rockets. studies the query raised by the previous experiment (e) Aging - Unlike the solid propellants, the hy became the question of the next. The group brids have no serious aging problems since the substitution approach led to an understanding of ( fuel and the oxidizer are not In physical condensed phase exothermic reactions, primarily contact prior to use. responsible for ignition. It is conceivable that a judicious combination of reactive groups in the fuel (f) Cost - The cost of fabrication, handling, trans molecule could lead to the required ignition porting, and even that of the propellant used is characteristics. The effect of the number of N-N much lower than either the solid or liquid bonds in a molecule on hypergolicity led to other rockets. series of compounds. Finally, the evolution of N-N Among the major disadvantages of the hybrids bonded viscous resins/binders was a major step in may be mentioned: casting large solid hypergolic fuel grains. Thus, a (a) Slow Regression Rate - The regression rates of beginning has been made to study and evolve new the existing solid fu els are relatively low - self-igniting systems. A broad understanding of the about 1110 of solid propellants. self-ignition phenomenon is at hand. It may also be mentioned that the systems explored so far comprise (b) Lower Combustion Efficiency - Hybrid rockets off-the-self, easily available chemicals. It is possible typically have 93 to 95 per cent efficiencies, that solid fuels having high potential for self-ignition slightly lower than the other types. with liquid oxidizers may be specifically synthesized, (c) Lower Density Impulse - In comparison to solid utilizing the information available in these studies. propellant rockets the hybrids have lower density impulse, thereby resulting in larger Progress in Hybrid Rockets volume. Why Hybrids ? However the problems associated with the hybrids are not inherent and could be overcome by It is recognized that hybrid rockets combine the addressing them appropriately. technical advantages of both solid and bi-liquid engines. Several distinct advantages associated with Hybrid rockets are well suited for missions like hybrid rockets have deservedly made them center of putting satellites into low earth orbit, and raising the attraction for several years. Some of these are: orbit of a satellite from near earth to geo-stationary orbit - a process that needs thrust control as well as (a) Safety - Since the fu el and oxidizer are stored high-energy propellants. They could be well used for in diffe rent chambers the hybrids provide sounding rockets, maneuvering spacecraft during safety during storage, fabrication, trans ascend and descent fl ights on other planets, as portation and operation without any possibility payload carriers to the International Space Station, of accidental explosion. and even in missiles where thrust control is required. (b) Control -The hybrids offer stop-start-and In spite of many advantages and wide restart capability like the bi-liquid hypergolic applicability, however, the hybrids are hardly in use systems, and yet are simple to operate more or in any major mission. This is largely because the less like the solid motors which have no thrust technology of both the bi-liquid and solid rockets is variation control. well established. The two types work well and singly (c) Insensitive Combustion - In hybrids the regres or jointly they cover the mission requ irements of all sion of the fu el grain during combustion being types. It is considered wasteful to spend money on normal to the flow of the oxidizer and not to perfecting another technology fo r the same purpose. JAIN: SELF-IGNITING FUEL-OXIDIZER SYSTEMS & HYBRID ROCKETS 307 There appears to be reluctance in supporting a new polymethymethacrylate (PMMA) as fuel, was used in system by the monopoly manufacturers of solid or Sandpiper target drone by United Technology liquid propellant rockets although the hybrids may Chemical Systems Division (UT-CSD) in 1967. prove beneficial in the long run. Throttleable engines based on FLOX and LiILiH in Another major reason could be the non HTPB were demonstrated by UT-CSD in the late availability of suitable self-igniting solid fuels that 1960s. Aero Jet General conducted test firings of have short ignition delays in the cast form, with liquid hybrid motors having polymeric fuels with metal oxidizer like HNO). Since no serious efforts were additives using OF2 oxidizer. The Reaction Motors of made to understand the pre-ignition process, it is not Thiokol tried polyurethane-based fuels with oxidizers surprising that there is a paucity of non-hazardous like, FLOX, RFNA and N204• Lockheed Propulsion efficient hypergolic hybrid systems. As a result, all Company (LPC) developed a tactical hybrid missile recent hybrid systems use non-hypergolic propellants. for the US Army that had a rubber-based fuel loaded The studies have virtually converged to HTPB-LOX with boron and CIF) oxidizer in 1969. system only. Needless to say that additional ignition Hypergolic hybrids based on p-touidine/ device or injection of a pyrophoric material is needed amninophenol (30170) fuel and WFNA oxidizer, were to initiate ignition of such a system. Obviously, the investigated in Germany in 1974 for their use in system becomes complicated and a major advantage sounding rockets. The DFVLR in Germany also of the hybrids i.e. the 'simplicity' is lost. It is in this developed space engines based on polyethylene, Li & context that our studies become relevant. LiH / FLOX propellant. Around this time, a feasi bility of hybrid systems was made at the author's A Brief History and Present Status Institute. Several hybrid motors having organic fuels The concept of hybrid rockets is hardly new. (undisclosed) hypergolic with RFNA were tested by 51 � The Russians are reported to have conceived the idea conducting static-bed firing . as early as 1937. In the US too the initial attempt to There was virtually no activity in hybrids in the design a workable hybrid device comprising wood period between 1975 to 1985. It appeared as if the and LOX, was made by California Rocket Society hybrids had lost out to the bi-liquid and solid (CRS) in the late 1930s. The CRS was also propellant rockets. A study conducted by Martin responsible for the first test fl ight of a hybrid rocket Marietta in 1982, however, revealed that of the having rubberlLOX propellant, that reached a height twenty technologies investigated, hybrid propulsion of 9 km in 195 J. offered the greatest return cost-wise. Interest in the The first use of self-igniting propellants in hybrids rejuvenated after the space shuttle Challenger hybrids was made in France and Sweden and USA disaster in 1986, caused by failure of one of the solid almost simultaneously in the mid to late I 960s. boosters. Another study carried out by the Hybrid The French hybrid engine (LEX-02) meant for Industry Propulsion Action Group compared a hybrid • � launching sounding rockets was based on an amine rocket booster sized for space shuttle to the existing fuel, m-toluenediamine (96 per cent) in nylon (4 per solid booster and concluded that in propellant cost cent) having a coating of phosphorus tri-N alone the hybrid booster could yield a saving of $ 5.7 methyl imide (to initiate quick ignition) and RFNA millionlflight. Realizing the potential of the hybrid oxidizer. Swedish workers developed their sounding systems, NASA in 1989, awarded four contracts for rockets (HR-4 and SR-I) based on a self-igniting fuel the evaluation of hybrid rockets. All four studies named Tegaform, which comprised an amine having concluded that hybrid merited further development based on the potential advantages in 'safety, additives like lithium aluminum hydride (to reduce ,52 the ignition delay) with RFNA oxidizer. In the US reliability and economy . much emphasis was placed on using fluorine based By 1985 the emphasis on propellants shifted � oxidizers for hypergolic systems. Mixtures of fluorine from hypergolic to non-hypergolic systems. and liquid oxygen (FLOX), OF2, CIF) etc., were tried. American Rocket Company (AMROC), USA, a Also, the mixtures of oxides of nitrogen (NO and commercial space company that has been engaged in liquid N204) were developed as oxidizers. A hybrid rocket development based its studies on a non propellant having mixed oxides of nitrogen (MON), hypergolic HTPB-LOX system. During 1986 to 1992 as oxidizer, and magnesium powder embedded in the company built 94 hybrid rocket booster engines 308 J SCI IND RES VOL 62 APRIL 2003 having polybutadieneJLOX propellant. Ignition was uses a low energy HTPB/ nitrous oxide propellant initiated by injecting triethylaluminum, a pyrophoric that has an Isp of about 225 s only. The 1.1 MN thrust liquid. In early 1991, AMROC proposed an all-hybrid turbo-pump fed HTPBILOX hybrid motor (13.7 m vehicle for maintaining Motorola's Iridium long and 1.78 m diam) was tested at NASA-Stennis 53 satellites . The 4-stage 'Aquila' rocket built by this in 1999. Apparently, they used non-pyrotechnic company was capable of boosting a 1500 Ib payload ignition - possibly electric operated torch igniter. into a 300-mile polar orbit for half the cost of then Hybrids based on hydrogen peroxide alternatives. AMROC also developed H-500 and H- (HP)/polyethylene (PE) have been proposed, and 1800 hybrid motors capable of giving 75,000 and compared with those having HTPBILOX propellant. 250,000 lb thrust, respectively, using HTPBILOX According to this study, HP is comparable in both propellant. A new 'wagon wheel' configuration was safety and handling risks to LOX, and suitable for 56 used to cast the fuel. It is evident that a fair amount of auxiliary monopropellant usage • Since mono expertise in hybrids was developed by this company, propellants, like H202 and N2H4 ignite simply on but due to lack of co rcial support it stopped mme passing through a catalyst bed, these have been operating by the mid 1990s. considered for using in auxiliary ignition devices with A loint GovemmentlIndustry Independent non-hypergolic hybrids. In this dual mode operation Research and Development Program (JIRAD) was the hot exhaust gases of the monopropellant motor established in the US during 199 I -94 to develop ignite the hybrid motor based on say, HTPBILOX. hybrid rocket technology. The participants were Thus not only the repeating on-and off ignition Thiokol, UT-CSD, AMROC, Martin Marietta and capability of the hybrid is achieved easily but the NASA-Marshall. The main purpose of this group was monopropellant motor can also be operated to acquire data that will eventually allow application independently when only small thrust variation is 54 of hybrids to boost propUlsion fo r space launches required. Dual mode hybrids have been studied in Many test firings of hybrid motors were conducted Israel in 1996 (ref. 57). using the Thiokol, and UT-CSD's (UTF-2990I) solid An interesting hybrid combination based on fuels with LOX or gaseous oxygen (GOX) oxidizers, hydroxyl ammonium nitrate (HAN) as oxidizer and under the lIRAD program. HTPB as fuel has been proposed. HAN is highly A Hybrid Technology Option Project (HyTOP) miscible with water and is safe to handle. It is was initiated in 1994 to bring large-scale hybrid reported to give slightly higher Isp with metallized 58 motors to flight status with funding from Defense HTPB compared to HP or nitrous oxide • A new type Advanced Research Projects Agency (DAPRA). The of staged combustion hybrid rocket has been project members included AMROC, Lockheed proposed recently which is believed to be safe, Martin, UT-CSD and USAF's Phillips Lab, and environmentally tender and inexpensive. Its industry participants. The AMROC's H-250k motor combustion mode resembles the end burning grain of (F = 1.1 MN) that used polybutadiene based solid the solid rocket since the fuel bed bums from the ): 59 fuelILOX propellant, was fired successfully in 1993 bottom end • However, an auxiliary igniter device is and again in 1994. AMROC however, failed to raise needed to achieve on-off capability in these systems. its share of HyTOP funding and its leadership was replaced by Lockheed-Martin. Conclusions The Hybrid Propulsion Demonstration Program It is apparent that a reliable hybrid rocket (HPDP) was established in 1995. This was a system is yet to be evolved, in spite of all its restructured HyTOP, comprising NASA-NSFC, conceptual advantages. One of the main advantages APRA and Industrial Consortium of Lockheed of the hybrids has been their simplicity of operation. Martin, UT-CSD, Thiokol, Boeing-North Ami This major plus-point is marginalized once the Rocketdyne, AlliedSignal and Environmental propellant chosen is non-hypergolic. The installation A Aeroscience Corp. (EAC). It aimed at a flight test of of an additional ignition device for achieving start 1.1 MN booster in 1999. A single-stage Hyperion restart capability adds to the complexity of the rocket built by the EAC fo r the sounding-rocket flight system, and makes it less competitive to the bi-liquid demonstration phase of the program was test-fired to as well as solid systems. However, an understanding an altitude of 33.5 km in 1997 (ref. 55). The Hyperion of the self-ignition process is slowly emerging. 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