CHEMICAL PROPELL Mononitromethane FREDERICK BELLINGER1.H. 6 .F R I E D M A N ? ,W. H. BAUER".
J. W. E A S T E S J ,A N D J. H. GROSSa CHEMICAL CORPS. ARMY CHEMICAL CENTER. M D
M o n o n i t r o m e t h a n e was investigated as a possible source of powerfor launching buzz bombs. When used i n combination w i t h oxygen and a n igniter i t could be made t o give pressure-time relationships of t h e type necessary for such a n operation. T h e sensitivity t o detonation of mononitromethane under certain conditions made t h e design of apparatus for injecting i t i n t o a reaction chamber a m a j o r problem.
conjunction with the portable flanie thi O M ('1 112-2, lor t h r ignition of mononitromethane was a fortunatr choice. It proved to be entirely hatisfactory and dependahlc. This flamr throm er igniter consists of a plastic body about t he size and shape of the cylinder of a 45-calibei revolver. There is a 0.75-inch hole in the center of the plastic cylinder and evenly spaced in the body around this hole are five small holes, each containing a n individual igniter complete with its firing pin. For use with the thrust motor these five shot cylinders were sawed into five parts, each one containing an individual igniter. One ignitei was used for each thrust motor run by inserting it into an adaptcr on the thrust motor. The adapter was such that the firing pin of the igniter could be struck by a plunger which was driven by the movable core of a solenoid. The solenoid was controlled by the safety firing circuit ( I ) . The adapter directed the &inch flame of the igniter into the thiust motor reaction chamber. (Each igniter gives a flame for about 10 seconds.) The igniter Mas always fired before the entrance of mononitromethane into the thrust motor. The thrust motors used in this work rvete t h r same ones used for studies on hydrogen peroxide ( I ) ; the only change was the installation, in the threaded taps of the desciibed igniter adapter, of inlet nozzles for nitromethane and oxygen and the capacitor piessure gage in place of inlet nozzles for peroxide and permanganate. h schematic diagram of one assembly used is shoJ9i.n in Figure 1. I n this assembly the oxygen flow is controlled manually whereas in some expcriinentr the oxygen flow a a s controlled by valves operated by the safely firing circuit. Concomitant u ith the pioblem of injecting and igniting monoiiitroniethanc and oxygen to obtain suitable pressure-time relatioiishipq n a s the ever-prcsent danger of detonation of thrb moiion~tronicthiiriein thc bloxv raw, line%,or a t a nozzle ( 2 ) I n aiiangment oi the iiijectioii iystem t o avoid this mas as much of a prohlem a3Ihe obtaining of \nti-factoix presswe-timc curves
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ONOKITROMETHATE has been demonstrated t o be a powerful explosive capable of being detonated under certain conditions ( 2 ) . It was pointed out t h a t these conditions for the detonation of mononitromethane were present in the injector system used by the Germans for launching buzz bombs with peroxide and permanganate and t h a t such a n injector system would have to be redesigned to ensure safe handling of mononitrome t hane. The purpose of the work desciibed in this paper was t o devisi. means of reacting mononitiomethane to obtain power development of a suitable chaiacter for use in launching buzz bombs. The reaction of mononitromethane for this purpose 1% as studied through the operation of a thrust motor. A similar procedure, used for hydrogen peioxide and permanganate, is described and the power requirements for launching buzz bombs are given in a previous paper ( I ) . K o r k a t the Guggenheim heionautics Laboratory of the. California Institute of Technologv (6)showed that t o use mononitromethane successfully for sustained thrust motor operation, it was necessary to use oxygen or its equivalent and also to have a means (as a spark) of igniting the mixture in the reaction chamber of the thrust motor. The use of a coinbustioii catalyst 'r\ ab fouiid to be advantageous. (The oxygen requirement prevents inononitroniethane from being classified as a true nionopr o p ~ l l a n.t) Any delay lin ignition of the niononitromethan? oxygen mixture after its entry into the reaction chamber is likely to result in an explosion JT hoqe ( violence is in proportion t o the amount of monoAIR nitromethane accumulated in the ieaction chamber prior to ignition. Therefoie, a prerequisite to the use of mononitromethane for launching buzz bombs was a positive and infallible means of ignition. h y delay mould result in the explosion or detonation of about 20 gallons of mononitromethane because niaximuin breech piessure on the launcher must be attained in 0.1 second or less; the whole operation inust be completed in 0.7 second or Iess. The selection of the flame thrower igniter ( 3 ) , developed by the Chemical Warfare Service in 1 Present address, Georgia School of Technology, Atlanta, Ga. 2 Present addiess, Zep lIanufactuiing Company, Atlanta, Ga. 8 Present addiess, Rensselarr Poiytechnic Institute, Tioy, N. Y. 4 Present addiess. Ameiican Cyananrid Company, Bound Brook, S J.
OR N r
SAFETY FIRING
CHI NO. BLOW CASE
Figure 1.
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Schematic Diagram of T h r u s t M o t o r and Accessory Equipment
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The pressure-time curves obtained in the first experiments were not consistently the type* drsiied. For convenience in discussing and analyzing them, the pressure-time curves were divided into the types shown schematically in Figure 2. They may be described as follows: Type A, no pressure oscillations; B, pressure oscillation in first part of run only; C, pressure ostillation a t each end of run but not in the middle; D, pressure oscillation a t end of run only; E, pressure oscillation during entire run; *, explosion that blew exhaust nozzle off motor by shearing the stud bolts; and 0, no ignition. Any of the above types may b(3 further qualified by the addition of the following symbols: X, irregular variation in pressure, or Z, pressure vibration during run. The addition of a number, as B-70, t o a run type signifies that the condition (B) persisted for t h a t portion (70%) of the run. The presence of the many factors t h a t could cause undesirable results made the work with mononitromethane difficult t o interpret i n terms of the effect of a single variable. For this reason it is, better t o consider the data in its entirety rather than in portions. All of the data are given in Table I. RESULTS OF T H R U S T M O T O R EXPERIMENTS
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The nitromethane used in this work was commercial material, 96% mononitromethane, obtained from the Commercial Solvents Corporation. The properties of this and other materials used were described in a preceding paper (2). The first 47 runs were carried out using a quick-opening valve between the motor inlet nozzle and the blow case containing the mononitromethane which was under pressure (nitrogen or air). This valve was opened by the safety firing circuit after tlie flame thrower igniter had been fired. With the valve in this position faster pressure buildup occurred than in later experiments (47 t o 200), in which the valve was situated ahead of the blow case so t h a t i t suddenly released high pressure gas t o the blow case instead of releasing the mononitromethane itself. None of the first seven runs was successful primarily because oxygen was not used to help start or maintain the reaction. I n these runs either the reaction never started or else there was a n explosion; the latter resulted from the reaction starting after excess mononitromethane had accumulated in the motor. This showed t h a t although the flame thrower igniter could start the reaction, i t could not be depended upon to give satisfactory results alone. Lack .of knowledge in the selection of proper nozzle sizes also added t o the lack of success in some of the first seven runs. Beginning with run 8, oxygen was injected into the motor along with the mononitromethane. I n runs 8 to 46, inclusive, oxygen was supplied in the following manner: A tank of oxygen (2000 pounds per square inch) was connected t o a nozzle on the motor by means of a needle valve and steel tubing. A pressure gage was in the line between the needle valve and nozzle. The needle valve was opened so t h a t the gage showed a given pressure; the run was made, and then the flow of oxygen was shut off. This procedure provided a n atmosphere of oxy en in the motor at the beginning of the run, but i t is believed t f a t the flow of oxygen through the nozzle was diminished, and perhaps stopped during the run, by the pressure in the motor due to reaction. No means was provided for reading the pressure on the tank side of the oxygen inlet nozzle during the run. The desirability of having oxygen present was demonstrated by the seventeen runs of type -4;all of these had exactly the time and pressure relations sought. I n this series of runs (11 to 40), however, there were three violent explosions in the motor and eight cases where ignition of the mononitromethane did not occur. Later experiments (128 to 200) showed t h a t these difficulties were directly attributable to a n inadequate and uncertain supply of oxygen. The three explosions occurred only when air was used t o pressurize the blow case. I n view of previously reported data (2) on the detonation of mononitromethane,
Figure 2.
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Classification of Pressure-Time Curves
i t was believed t h a t the cause of these explosions uvs the long exposure of the mononitromethane t o high pressure air. Runs 41 .to 46 illustrate the adverse effects produced when a large diameter exhaust nozzle was used in place of a smaller one. I n all six of these runs the reaction was explosive in that continuous pressure was not developed; only high pressure peaks (1000 pounds per square inch) occurred. This reaction, as substantiated by later experiments, was caused by the large exhaust nozzle which did not provide sufficient oxygen pressure in the motor. I n runs 47 t o 123 the oxygen was supplied to the motor by a different arrangement than previously used in order to obtain higher oxygen pressure in the motor. I n thenew arrangement the full pressure of, a n oxygen cylinder was connected to the oxygen inlet nozzle through a quick opening and closing valve. The valve was controlled by the safety firing circuit so that it opened 0.14 second before the entry of the mononitromethane, and closed after the conclusion of the run. In addition to this change, the method of introducing mononitromethane was changed beginning with run 47. The blow case was placed on a level below t h a t of the motor and its bottom outlet was connected directly t o the motor (inlet nozzle) by a length of 0.5-inch stainless steel tubing. The mononitromethane was forced into the motor by opening a valve between the blow case and drive gas so t h a t the full pressure (1000 pounds per square inch) of the drive gas was suddenly applied to the mononitromethane in the blow case. This eliminated extended contact of the drive gas and mononitromethane. The first valves used in the new arrangement slowly leaked gas and forced some mononitromethane into the motor before the run was started. This caused the explosions in runs 47 and 48. I n the other runs (49 to 59) using these valves, this premature forcing of mononitromethane into the motor was prevented by having a vent (a slightly open needle valve) on the blow case a t all times. I n spite of the difficulties with the leaking valves in runs 49 t o 59, most of the runs including those in which spraytype nozzles were used for injection of the mononitromethane had good pressure-time characteristics. Beginning with run 60 the leaking valve was replaced with a valve taken from a German asvisted take-off unit. The valve, which was relatively simple in construction and operation, proved very satisfactory and was used thereafter. Because this type of valve was not known t o be made in this country, it was taken apart and a scale drawing made and included in the final report of this project (4). When this va1i.e was in the closed position, i t vented the blow case and thus allowed oxygen from the motor t o blow back through the blow-case if oxygen pressure developed in the motor. Oxygen pressure caused by too large a n oxygen inlet nozzle developed during runs 62, 63, and 64; in run 64 the mononitro-
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methane blew up in the blow case when sudden air pressure was applied. I n succeeding runs, tests were made t o ensure t h a t the combination of oxygen inlet and exhaust nozzle would not allow excess oxygen pressure t o build u p in the motor. Beginning with run 65 the following general policy was adopted: Blow case explosion was minimized by the use of nitrogen a s the drive gas and by precaution against the development of excess oxygen pressure in the motor. Data were collected on exhaust nozzle area, mononitromethane inlet nozzle area, and internal volume of the motor (assuming t h a t these were the three important variables other than oxygen flow) t o determine their relation for satisfactory operation. T o find a relation between these three variables, the following dimensionless constants were defined and calculated for all runs:
the ratio of the two-thirds power of the volume of the motor t o the area of the exhaust nozzle. [This parameter is very similar t o t h a t used by workers at the Guggenheim Aeronautics Laboratory of the California Institute of Technology (6). Their parameter was not dimensionless.]
the ratio of the area of the exhaust nozzle t o the area of the nitromethane inlet nozzle. The volume of the thrust motor without B nozzle was 37.4 cubic inches [previous paper (2) shows diagram of thrust motor]. The volume with the exhaust nozzle was 4.69 cubic inches. The volume of the motor could be increesed by t h e usc of two extensions, 3 and 6 inches in length with the same bore as the motor (5.94 cubic inches per inch of length). The extensions were equipped with flanges so t h a t they could be fitted to the motor, to each other, and t o exhaust nozzles. Thus, with t h e extensions, the volume of the motor with nozzle a t tached could be set a t 42.1, 59.9, 77.8, and 95.6 cubic inches. Runs 65 to 71 were all alike (V* = 231, A* = 5.05). There was much pressure oscillation. Changing the size of mononitromethane inlet nozzle (runs 73 t o 78) so t h a t V * = 231, A* = 10.5, smoothed out the pressure curve just as did a sufficient change in the motor volume so that V* = 400, A* = 5.05 (runs 85 and 86). Runs 86 t o 97 showed t h a t the combustion catalyst, chromic acetyl acetonate, was not necessary for the reaction t o take place (V* = 400, A* = 5.05). Runs 100 t o 105, w i t h a n e w exhaust nozzle (Vu = 119, A* = 9.18), showed pressure oscillation in the first p a r t of the runs only; this was improved (runs 106 t o 110) by use of a' larger motor volume, V* = 205, A * = 9.8, or a smaller mononitromethane inlet nozzle, V* = 119, A* = 20.4 (runs 111and 112). Runs 111 t o 118 (V* = 119, A* = 20.4 to 6.6) showed the increase in roughness of the pressure-time curve, due to the increased size of the mononitromethane inlet nozzle, up t o the point where explosions took place. R u n 114 failed to ignite because no oxygen was used in the run. Runs 98 and 99 showed t h a t air could not be substituted for oxygen. I n runs 119 and 120, a 0.060-inch dia.mekr mononitromethane inlet nozzle was used with the result t h a t the mononitromethane exploded a t the nozzle, bursting the 0.5-inch stainless steel tubing for a few inches. This w a s due to the gas hammer effect reported
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in A pirvious paper ( 2 ) . I n the next LR'O runs (121 aiid 122) a -lightly largrr n o z ~ l e (0.070-inch) was used LT ith descnsitizcd monoriitioniethane (2Vc cliioinic acetrl acetonatr and 47" pasoline). This prevented explosive rupturc of the apparatus but 111 each run there nriv only a couple of high prc-suic peaks 1800 p01111d~p r squttrc' ~ iiich) inhtexd , I f a regulatr prcvxiw-tirn~~
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flow. The rneaiurenienis riiade M itli a n ater-fillrd manoineter attached to t h r niotot and observation of the height of thc \vatpi column cauerd b j var s 0x1 tlw ouvgt~iiinkt i~ozzle. It FT a i iound that a n urr of about I o inch( h ot \+ a t r r in the motor R ould f ach I hi.ouyh f h(>morionitromethane (200 in1 ) in th Figure 3 shoas the 1 elation bet\?een oxygen inlet nomlc p i msure and motor pressure due to o 11. All of the data givcri iri Figure 3 a l e for a single s c t 01 nozile conditions: the variation \%asdue to t h e oxygen inlet rio~zlrnhich \?as so inial1 that il could not he kept completely open all of the time. 1 gicat deal of tiouble N as cawed b j pal tial clogging of t h e nozilv. HOA ever, 1 he reaultaiit data gave coniidrt ablo iiiformatiori 011 tlir. r o l f , 01 oxygen in the reaction. 5
Run 128 nas caiiied oul u h g what wah t h u g l i t 1 0 bt) a iittr oxygen pressure--9 inches (of nater) on the motor -but thta mononitromethane blew up in the inlet riozzle; this ihowed that some oxvgen had pushed back into the lines. The run was repeated (129) with wccess using desensitized mononitromethane From these two runs it was decided t h a t 9 inches (watw) prrssure mas too much for safety. I n all runs the oxygen, flame, and mononitrometliane inlets nere situated on a diameter of the bore of the thrust motor. Tn runs 1 to 127 the flame and oxygen inlet were 45" from one another and the mononitromethane inlet was 180" froin the oxygen inlet. This had the disadvantagc of allowing oxvgeii to get back into the nitromethane lines it the oxygen T+as turned on befoie the start of the run. Con>equcxntlyiri 1 un 128 and all succcrtfing runs the flame inlet and rnononitrornethane inlei n c i c 46" apart and the oxygen and flame irilct M PIC)180" apart. Runs 130 and 131 show that 1.6 inches of oxygen pressuie o i i the motor insufficient. Runs 132 t o 142 show t h a t 5 t o 6 inches oxygen pressure on the niotor is sufficient to obtain consistently good operation. Beginning with run 132 Lhe piessure in the oxygen tank, the pressure placed on the oxygen inlet nozzle, and the maximum pressure shown on the oxygen gage duiing a run were recorded. Thus in Table I, 2000/500/800 signifies an oxygen tank pressure of 2000 pounds per square inch, 500 pounds per square inch behind the oxygen inlet nozzle and 800 pounds per square inch maximum gage pressure during the run. In all cases (132 to 142) n.heie there was reaction, the pressuie shown on the oxygen gage during a run was higher than the original oxygen pressure placed on the noLzle. Runs 141 to 145 show that the critical factor is the piessure 01 the oxygen in the motor rather than the amount flowing through the motcr. The same amount of oxygen was flowing through the motor in run 142 as in i u n 143, yet no ignition occuried in 143. In run 143 a larger exhaust nozzle n a s used; hence, theie was a lower pressure of oxygen in the motor. Runs 142 t o 152 show t h a t sustained ignition can be obtained using large exhaust nozzles if sufficient oxygen pressure is used. Runs 1
