APPARATUS FOR DETERMINING COMBUSTION RATE OF SOLID PROPELLANTS J .
P. P l C A R D
C. J . A N D E R S O N
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E. B R Y A N T
T h i s assembly is useful f o r testing materials that can be formed into strands and burned, Particularly in research studies and qualip control of solid propellants, pyrotechnic materials, and solid fuels strand-burner systems have been deNumerous veloped to investigate certain aspects of combustion for solid propellants. However, a major difficulty has been movement of the burning surface as the propellant is consumed. I n optical studies, this meant that the burning zone burned past the optical axis of the camera or spectrograph. Some workers tried to move the bomb, the optical instrumentation, or the propellant strand to maintain the burning zone fixed with respect to an optical axis. This generally resulted in a complicated array of equipment which more or less worked. Most of these systems provide for a burning period corresponding to a 6- to 10-inch strand. Rekers and Villars [Rev. Sci.Znstr. 25, 424 (May 1954)] designed one of the better systems. I n their apparatus which could accommodate strands up to 6 feet long, the propellant is coiled in a small cup at the bottom of a vertical combustion vessel. As the strand burns, it is continuously fed into the burning zone by a small motor contained inside the combustion vessel. The prime disadvantage is that only flexible strands can be studied. The Rekers and Villars principle of continuously advancing the strand rather than the optics or the entire combustion vessel is used as the basis for the apparatus described in this article. However, the feed mechanism is suitable for rigid as well as flexible propellants. The propellant system can accommodate strands up to G feet long, because the propellant is in a tube extending from the end of the combustion vessel. The drive mechanism is exterior to the combustion vessel without the usual pressure-seal problems. The optical system controlling the position of the burning surface is different. Rekers and Villars used an external light source but ours is keyed to the flame of the burning propellant. This permits the burning of strands exhibiting a wide range of luminosities.
Figure 7 . T h i s apparatus has been used in combustion studies for more than year. T h e propellants, mostly of a nitroglycerin base with metal additives such as magnesium and aluminum, burn at a rate of 0.7 to 7 inch per second
Our apparatus has been used in combustion studies for more than a year. Most of the propellants were of a nitroglycerin base with various metal additives such as magnesium and aluminum. The burning rates of these propellants ranged from 0.1 to 1 inch per second. The type of flame varied from a well-defined shape to a flare sparkler. The apparatus is capable of controlling all the propellants that were tried. I n most instances, position of the burning surface varied less than *0.005 inch. Figure 1 shows the basic components of the system. The combustion vessel has two diametrically opposite pairs of quartz windows. One set is slit windows which are used for photographic and spectrographic measurements. The other set is circular. Although there are two circular windows, only one is now used as part of the propellant feed rate control. A photocell, located above the top window, senses the light emitted from the propellant flame. The output of the cell goes to a motor control unit which drives the motor at a rate proportional to some selected light intensity value. The motor, via a pulley system, advances a magnetic pusher system which consists of a small horseshoe magnet located outside the propellant feed tube. A small cylindrical magnet is inside the feed tube. As the pusher moves forward, magnetic coupling between the two magnets causes the cylindrical magnet to move and to push the propellant strand into the burning zone at a rate equal to the burning rate. This apparatus is useful to laboratories and companies interested in materials that can be formed into VOL. 5 6
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strands and burned. It is particularly applicable to research combustion studies and quality control testing of solid propellants, pyrotechnic materials, and solid fuels. The assembly is now available from the Malaker Laboratories, High Bridge, N. J. Design Details
Combustion Vessel. The combustion vessel was designed and fabricated by Pressure Products Industries, Hatboro, Pa. The bomb was designed for a maximum working pressure of 5000 p.s.i.g. at 200' C. The bomb is square in cross section because this provides additional strength to compensate for the four windows. Two of the windows are slits measuring a/8 by 4 inches and the other two are bulls eyetype with a diameter of 11/2 inches. The windows are made of optically clear quartz. A quick disconnect dosure head providing easy access into the bomb contains four inch high pressure connections, two of whi'ch are used for the electrical ignition system, one connects the bomb to pressure gages and the remaining one is a blank. Located in the center of the
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bomb closure is a high pressure connection which accommodates the stainless steel propellant-feed tube, */I# inch in outside diameter and 6 / 1 6 inch in inside diameter. At the opposite end of the tube, a high pressure tubing cap is provided to facilitate cleaning. The inside of the tube is machined to a number 32 finish to reduce the friction between the propellant and the tube walls. Purge gas inlets are at the top and bottom sides of the bomb and one exit tube is located centrally at the back of the combustion chamber. O p t i d Control System. The function of the optical system is to sense the position of the burning propellant and signal for an increase or decrease in the rate of propellant feed. The system in its simplest terms consists of a light source and photocell with the amount of light reaching the photocell controlling the feed rate. The lenses, light source, and photocells are mounted in a 3-inch diameter black a n d i d aluminum tube. The original optical system was similar to that described by Rekers and Villars. An image of the light source filament was focused through the bottom window on the underside of the propellant strand. The size of the image was approximately one half the diameter of the propellant strand. A shadow ofthe propellant strand was produced on the photocell located above the combustion vessel. This arrangement worked very well at the low pressure ranges where a well-defined dark zone existed. Dificulties were encountered when higher pressures were used, because there was no clearly defined dark zone and the flame lumination interfered with the control. The photocells received high intensity light from the flame, thus speeding up the motor and giving erratic control. The scattered light interference was eliminated by placing a plug containing a '/8-inch tube in the well of the upper window. The tip of the tube extended to ' / a inch of the center line of the vessel. Test8 were conducted using the light from the flame instead of the control light as used by Rekers and Villars. Control was possible and in all subsequent work the auxiliary light source was eliminated. The luminosity of the flame varies; therefore, it is necessary to adjust the sensitivity of the photocell. This is done by using a '/,-megohm potentiometer as a voltage divider. Control is also improved by adding a second power supply to the motor control circuit. This power supply is adjusted to a voltage below that required by the actual burning rate. Thus, the propellant is continuously being fed into the vessel at a slow rate to prevent the
J . P . Picard is Chief of Propellant Laboratory, the Picatinny A r s m l , Dover, N . J. C . J . Andnson is OpCrntionr Manogn, R. Del Grosso, Chemist, and E. Eryant, Senior Pliysicisl with the Malaker Laboratories, High Bridge, N . J . Contributions and suggestions of E. O'Hanlon of Malakm Laboratories, and R. Rossi, C . Lenchetz, and S. Ladn of Picatinny Arsmul are acknowledged. AUTHORS
Figurc 2. The clactraic systm. A phorocdl obovc thc top Lvindow sows light nnirtcd by thc prop.dlantJ7mnc and octivatcs a control unit
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zone facilitates photographic and spectrographic techniques
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jumps that had previously occurred when the control unit went on and off. Motor Control Unit. The purpose of the motor control uuit is to regulate the applied voltage to the motor to correspond to the output of the photocell. The motor control unit employs a high input impedance operational amplifier as a sensing device for small changes in d.c. voltage level. The operational amplifier, which is drift corrected, is connected as an integrator. The time integral of the input unbalance voltage is applied to a voltage-sensitive device which derives a pulse output at an arbitrarily selected frequency. The output pulse is then shaped and applied as a drive voltage to the outer field winding of the motor. Since the output pulses frequency varies as the time integral of " the input voltage, the pulse rate and then the speed
of the motor is proportional to the voltage level of the high impedance input (Figure 2). Propellant Feed System. This part of the apparatus includes the drive motor, magnet drive, and propellant pusher. The output from the motor control unit actuates a 12-volt d.c. motor. A gear reduction of 15:l is used so that the motor runs a t its maximum torque level and yet advances the propellant a t its burning rate. Initially, a screw-type of drive was used to advance the magnet pusher. This was unsatisfactory because of vibration and the whip of the screw. The lead screw could be supported only a t the ends. A cable and pulley were found to give a much smoother drive and better propellant position control. At 18 volts, the rate of advance of the magnet pusher is 1 inch per second. This can readily be changed by changing the gear teduction ratio or the diameter of the drive pulley. The small cylindrical magnet, located inside the tube, caused a jerky feed motion because of inertia and friction. One attempted solution was to mount the magnet on a carrier of four roller bearings. This did not provide as smooth a drive as was desired. The bearings were not sealed and were susceptible to grit and dirt, causing them to bind. Finally, two rings of Teflon were machined and pressed over the magnet to provide a bearing surface. The bore of the tube was also honed to a number 8 finish to further improve its surface. A small quantity of powdered Teflon was sprayed into the tube to reduce friction to an absolute minimum. Using this technique an extremely smooth drive was achieved. The smaller diameter propellants required better propellant feed positioning. If the propellant was slightly bent or fed into the burning zone a t a slight angle, the optical system would not function properly. It was necessary to add a support that extended midway into the combustion vessel to keep the flexible strands in a fixed position with r he optical tube during
Figure 4. Excellent controI is obtoinable olm whm the angle between the burning surfoce ondside of the propcNon1 changes. This propellant, uninhibited, is burning at 500 p.s.i. Time intend bctwem photographs was less thon 30 scconds, yet the burning surface hos moucd less lhon 0.005 irah
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burning. The actual point of burning is approximately ' / 4 inch beyond the end of the support. Purge and Baffle System. Smoke removal is essential for optical measurements; therefore, a purge gas system was designed to sweep out the smoke as it was formed. The purge system consisted of a source of compressed gas, pressure reducer, flo~tmeter, and a series of valves. The gas entered through the top and bottom sides of the combustion vessel and exited through the end. The pressure and flow were controlled by means of the pressure reducer in the inlet line and a valve in the exhaust tube. A series of baffles and chimney designs were tried. The best design is shown in Figure 3. This method reduced the number of pockets in which the smoke could collect as well as the effect of the gas flow on the flame. The gas flow appeared smooth and the smoke was removed as it was formed. Experimental
Propellant Control. 4 series of 16-mm. motion pictures showed that for most propellants, position of the burning surface moved less than *0.005 inch. This was determined by projecting the image on a screen and measuring the distance from the propellant
Successive Figure 5. frames of an aluminumcontain;ing propellant, uszng a high speed movie camera. Control is pood, even 1'hough a welldejned frame area is lacking
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tip to the support. This distance was compared to the diameter of the support to convert the projected distance to the actual. Excellent control was also obtainable when the angle between the burning surface and the side of the propellant changed during combustion (Figure 4). This propellant was not inhibited and was burning at 500 p.s.i. The time interval between photographs was 30 seconds, yet the location of the midpoint of the burning surface had moved less than 0.005 inch. This propellant has a well-defined flame zone. For aluminumcontaining propellants (Figure 5), this is not the case, but good control is still possible. Automatic Data Recording. A head high meter panel (Figure 1) has been mounted on a wall near the combustion vessel. Purge-gas pressure and flow rate, time, experiment identification and remarks, and Selsyn position readouts are included on this panel. A 35-mm. electric drive sequence camera is focused on the panel and records all of the indicated information at intervals ranging from 1 to 15 seconds. Panel illumination is accomplished with N o . 47 pilot lamps to minimize scattered light effects on spectrographic measurements. Adequately dense negatives are obtained through use of Royal X pan film (ASA 1600). Burning Rate. With this apparatus, the burning rate can be obtained in several rvays which have not been previously reported. The first method has been used almost exlusively since it does not require the use of one of the viewing ports. This system utilizes a Selsyn indicator to record the distance traveled by the magnet pusher. The indicator is controlled by a 1-inch pitch circumference gear on the pusher head which meshes with a fixed rack on the pusher track. A cam-actuated micro switch advances the digital readout located just above the Selsyn indicator at the completion of each gear revolution. Fractions of inches are read off the "radio compass" to approximately 0.02 inch. Burning rate is determined, then, by reading the distance advanced between exposures and dividing by the time between exposures. The main advantage of this system is that all bomb windows are available for optical diagnostic equipments. Burning rates have also been obtained by an optical technique tvhich is keyed to the emission spectra of the flame. The second method is an optical technique which is keyed to the emission spectra of the flame. The propellant strand is seeded by drilling small holes at measured intervals and filling them with potassium perchlorate. As the propellant burns, the potassium appears in the flame from time to time. The 7698 A. line intensity is recorded, using a recording spectrometer to obtain the time interval between peaks. This and the distance between seeds give the burning rate. Optical Studies. The main advantages of t h e apparatus are the longer observation times and tlie constant position of the burning surface. Spectrograms and pictures were obtained at various pressures during one burning, because the pressure could be increased during the burning period.
