US2976678A

US2976678A - Restricted solid propellant

Info

Publication number: US2976678A
Application number: US553767A
Authority: US
Inventors: Don E Kennedy
Original assignee: Standard Oil Co
Current assignee: Standard Oil Co
Priority date: 1955-12-19
Filing date: 1955-12-19
Publication date: 1961-03-28
Anticipated expiration: 1978-03-28

Description

March 28, 1961 2,976,678

D. E. KENNEDY RESTRICTED SOLID PROPELLANT Filed Dec. 19, 1955 3 Sheets-Sheet 1 E Q 4% Fig. 1

Dan 5. Kennedy V INVENTOR. 27, 2 W

ATTORNEY March 28, 1961 D. E. KENNEDY 2,976,678

RESTRICTED SOLID PROPELLANT Filed Dec. 19, 1955 3 Sheets-Sheet 2 Fig. 3

1 i a I i Fig. 4 I 45 Fig. 5 fi'wiffii.

r BY V6 2 D ATTORNEY March 28, 1961 D. E. KENNEDY 2,976,678

RESTRICTED SOLID PROPELLANT Filed Dec. 19, 1955 3 Sheets-Sheet 3 nnsrnrcrnn SOLID PROPELLANT Don E. Kennedy, Park Forest, 11]., assignor to Standard Oil Company, Chicago, 11]., a corporation of Indiana Filed Dec. 19, 1955, Ser. No. 553,767

1 Claim. (Cl. 650-356) This invention relates to an article of manufacture for the generation of a gas at high pressure. More particularly the invention concerns a solid propellant grain which is restricted with respect to burning surface whereby the gas pressure generated during ignition and initial burning of the grain is controlled.

The gas produced in the combination of grains made from propellants such as double base powders (mixtures of nitrocellulose and nitroglycerine) ammonium perchlorate or ammonium nitrate may be used for the propulsion of rockets, the assisted-take-oif of airplanes and for the operation of motor starters. Normally only a part of the surface of the propellant grain is permitted to be a burning surface. In order to obtain an equilibrium pressure in the combustion chamber a restrictor coating is placed on a predetermined portion (S) of the surface of the propellant grain which restrictor coating remains afiixed to some part of the surface of the grain over essentially the entire burning time of the grain. The restrictor coating is relatively non-fiarnmable as compared to the flammability of the propellant portion of the grain. By a combination of surface restrictor coating and grain configuration a particular propellant composition can be burned to provide gas in a given chamber at a predetermined pressure i.e. following the ignition period, until the propellant grain is essentially completely burned. Many materials have been used for restrictor coating such as cellulose acetate, polyvinyl acetate, polyvinyl chloride, asphalt, asbestos tape, etc.

In the ignition ofsolid propellant charge under conditions usually encountered in rocket, assist-take-otf and gas generator service in general, it is necessary to generate and transfer heat energy to the propellant grain surface until the ignition temperature is reached and combustion of the propellant charge is sustained. This heat energy is provided by an igniter which may consist of any form of combustion exposive powders such as black powder or cannon powder.

Igniters function by releasing a quantity of high temperature gases and hot particles which fiow over and give up part of their energy to the propellant grain surface. The igniter is designed to have expended itself shortly after ignition of the propellant grain becomes self sustaining. During the ignition phase of the combustion of the propellant grain the pressure level in the gas generator chamber, which encompasses the igniter and propellant grain, is determined jointly by the rate of gas generation from the igniter, the rate of combustion of the gasproducing propellant grain, and the size of nozzle orifice for escape of gas from the chamber. Upon the establishment of ignition of the gas-forming propellant grain and the dissipation of igniter gases, an equilibrium gas pressure is obtained which is determined by the nature and burning characteristics of the propellant material of the grain, the burning surface area of the grain and the throat area of the nozzle orifice.

For a given propellant charge in the form of a gasforming grain and associated chamber and nozzle, a

restrictor.

, 2,976,78 Patented Mar. 28, 1961 specific minimum igniter loading is required to obtain self sustaining combustion of the solid propellant grain and thereby attain a flow of gas at equilibrium pressure. The minimum loading varies with the temperature of the grain at the time of firing, being larger for lower temperatures than for high temperatures. It is not practicable to use variable amounts of igniter loading to meet the ignition requirements of extremely low or extremely high temperatures. Thus over ignition may result when igniters suitable for low temperature firing are used for firing the same grains at high temperature conditions; as a result ignition pressure peaks may occur in the pressure-time trace of the firing of the grain.

The starting of turbojet engines is a difiicult feat because of the tremendous mass of metal composing the larger turbojet engines. One of the techniques now used for the starting of such engines involves the use of a solid propellant driven turbine operated starter. The starter is aflixed to the frame of the engine and the turbine of the starter is connected by clutch to the rotor of the turbojet engine. A solid propellant grain mounted in a combustion chamber of the starter produces gas which spins the turbine of the starter and turns over the rotor of the turbojet engine to the necessary minimum rotational speed prior to introduction of the fuel to the main combustors. One of the serious problems associated with the use of such starters lies in the peak torque produced immediately after the igniter loading has been actuated in the starter. This momentary torque results in excessively high maintenance on the bearings and other moving parts of the starter.

An object of the invention is a solid propellant grain the ignition and sustained burning of which is accomplished by the use of a gas-producing igniter without producing an initial pressure during the burn-out period of the igniter loading and the establishment of sustained burning of the solid propellant which is greater than the equilibrium pressure maintained in said combustion chamber during the burning of the solid propellant grain. Another object of this invention is a solid propellant grain which attains relatively slowly the equilibrium pressure producable by the solid propellant grain. Still another object of the invention is a solid propellant grain wherein the initial pressure is relatively lower than the equilibrium'pressure produced by the burning of the solid propellant grain. A particular object of the invention is a gas-generating device utilizing a solid propellant grain and a gas-producing igniter which device does not sustain aninital pressure upon the firing of said igniter which is higher than the equilibrium pressure of the burning of the solid propellant grain. Other objects will become apparent in the course of the description of the invention.

Figure l is a longitudinal view of an assist-take-ofr' unit with solid propellant grain.

Figure 2 is a cross-sectional view of said assist-take-oif unit taken along 22.

Figure 3 is an elevation view of a cylindrical gasproducing grain showing short-term restriction of the grain.

Figure 4 is a cross-sectional view of Figure 3.

Figure 5 is a segmentation of Figure 3 with short-term restrictor shown in detail.

Figure 6 is a pressure-time trace illustrating the firin and burning of a gas-propellant grain without short-term Figure 7 is a pressure-time trace of the firing and burn- .ing of a grain with one short-term restrictor.

polyglycols and polyglycols. ticizers for the synthetic resins are ethylene glycol diglycolate, the 'acetins (mono, di and tri), triethyleneglycol 3 applied to a part of said body portion which coating is adapted to restrain burning of the body material beneath said coating for a period of time at least substantially equal to the burn-out time of an igniter adapted to establish the sustained'burning of said body portion and not more than about 2 seconds inexcess of said burn-out time.

The burn-out time of the igniter is defined herein as the time interval between the actuation of the element which activates the burning of the igniter loading and completion ofthe burning of said .igniter loading. In general burn-out times will range from about 20 mils to as much as 1 second. A mil is defined herein as onethousandth of a second.

. The period of time that the short-term restrictor coating remains on the surface of the body. portion of the grain, designated as the residence time, corresponds to a period at least substantially equal to the burn-out time of the igniter and an added period up to about 2 seconds following said burn-out time. The residence time of the short-term restrictor coating depends on several factors. Some of these factors are combustibility, heat cnductivity and thickness of the coating as affecting the overall rate of heat transfer through the coating material. Other factors are ignitibility of the body material beneath the short-term restrictor coating as Well as the temperature of the grain at the time of firing. The material is preferably combustible under the temperature conditions existent in the combustion chamber, that is the motor, during the burn-out time of the igniter.

. Materials which may be used for short-term restriction are adhesive tapes consisting of paper, cloth or plastic provided with adhesive to insure intimate contacting of the tape with the body portion. Thin coatings of asphalt and coatings of liquid self-setting plastics may be applied in the form of solutions thereof or a solution of polyisobutylene may be applied as a short-term restrictor coating. f

The area subjected to short-term restriction determines the effective remaining burning surface of the grain dur- "ing the ignition period thereof and hence controls the of the short-term restriction time was about 1 seconds The short-term restrictor coatings of this invention. are particularly adaptable to ammonium nitrate based gasforming propellant grains. These grains usually comprise at least about 67% ammonium nitrate, from about 10% to about 25% binder material and from 1% to about 10%,. combustion catalyst. The binder material is thermoplastic and consists of about 18% to about 50% of a plasticizable synthetic polymeric material, thatis, a

thermoplastic synthetic resinand from about 50% to about 82% of at least one plasticizer for the synthetic resin. Examples of synthetic resins which may be used are cellulose acetate, cellulose acetate butyrate,-polyvinyl' acetate and polyvinyl chloride. The plasticizers are oxidizable andpreferably oxygen-containing and maybe" classifiedbroadly'as polymeric esters, esters of polyhydric alcohols, ethers of nitrophenols, nitromonocyclic aromatics, esters of polycarboxylic acids, alkyl ethers of Specific examples of plasdi-Z-ethylbutyrate, triethylene glycol. .di-2-ethylhexoate,

polyethylene glycoldi-Z-ethylhxoate, triethyl citrate, acetyl triethyl citrate, dimethyl ph'thalate, dioctyl phthalate', ,nitromethylpropanedi'ol diacetate, dinitrophenyl thickness.

awasrs propyl ,ether, dinitrophenyl allyl ether, nitrodiphenyl ethers, dinitrophenoxyethanol, bis(dinitrophenoxy)ethane, dinitrotoluene, triethylene glycol, and polyethylene glycol. Combustion catalysts which may be used in the ammonium nitrate based grains are the Prussian blues (soluble and insoluble), the alkali metal chromates, alkali metal dichromates and ammonium chromate or ammonium dichromate or organic combustion catalysts such as the mono sodium salt of barbituric acid, which is particularly suitable as catalyst-for grains used in gas turbine starting service. Other components such as asphalt and carbon may be added to the ammonium nitrate based grain to improve cold temperature ignition of the grain. Amines may be added to chemically stabilize the grain against decomposition in hot storage.

The ammonium nitrate based gas-producing propellant compositons may be molded or extruded into grains. The molds may be provided with insets to provide longitudinal apertures in the grains. These may be cruciform, circular, star-shaped or any other desirable shape in cross section to provide for internal burning of the grain.

An illustrative double base gas-forming propellant which can be cast into grains and restricted with shortterm restrictors comprises, on a weight basis, about to of a mixture of nitrocellulose and nitroglycerine, about 10% to 15% of a mixture of triacetin and dioctyl phthalate, about 3% of leadsoap combustion rate modifier and about 1 to 2% of an amine such as 2-nitrodi phenylamine. Likewise the short-term restrictor coating can be applied to ammonium perchlorate based grains for use in assist-take-otfservice and for rocket propulsion. A representative composition of such body material comprises on a weight basis about 70% to 75% ammonium perchlorate, about 25% of a binder material consisting of a polyester resinstyrene copolymer and less than 0.5% of copper chromite catalyst.

Figure 3 is an illustration of a cylindrical gas-producing grain having body portion 41 which has centrally located longitudinal cylindrical aperture 42 extending therethrough. The annular ends of body portion 41, are restricted with

restrictors

43 and 44. The permanent 'restrictors may consist of plates of synthetic resin such burned. A

Body portion 41 is also provided with short-term restrictor coating 45 which may consist of a pressure sensitive tapewhich suitably'may be of about 0.006 inch Short-term restrictor 45 encircles the exterior cylindrical surface of body portion 41 and adheres thereto adjacent permanent restrictor coating 44 for a predetermined period, such as 2 seconds, following the ignition of the igniter charge thereby delayingignition and combustion of the part of thecylindrical surface beneath tape coating 45 .over that period when gases from the ignited igniter are effective in producing over-pressure and over the period requiredjto establish sustained burning of the body material. 7 Y

3 Figure. 4. is a cross section "of the grain of Figures showing internal aperture .42, permanent restrictor coatings14 3 and-44 and short-term restrictor 45 and :Figure 5, is a segmentation of the grain of Figure ,3 showing. permanent restrictorxcoating 44' and 'short term restrictor coating 45.

. 1a Figure. 1, the fbody of an arc unit is meanest atubuiar member 11' which is closed at one end and which is provided with threads at the open end. Member 11 is provided with two loops, 12 and 13. These loops are used to hang the unit from a carrier, not shown, which is attached to the wing of the aircraft. This carrier makes it possible to jettison the unit after take-off. A funnel shaped member 14 is attached to member 11 by engagement of the threads at the large open end of member 14 with the threads of member 11. Member 14 is provided with a nozzle 16 through which the decomposition products pass. The size of nozzle 16 determines in part the pressure maintained inside the chamber formed by members 11 and 14.

Solid propellant fills the cylindrical portion of member 11. The solid propellant of this illustration consists of seven tubular grains, 17, 18, 19, 20, 21, 22 and 23; each having an CD. of about 3 inches and having a centrally located cylindrical opening 1 inch in diameter the full length of the grains which are approximately 30 inches long. The grains consist essentially of combustion catalyst, oxidizable binder and ammonium nitrate.

Each grain has the

annular end areas

24, 25, 26, 24a, 25a and 26a restricted against burning with permanent restrictor coating consisting of asphalt or other suitable material such as cellulose acetate in order to limit burning to the cylindrical surfaces. Each grain is restricted with

combustible tape

27, 28 and 29 applied to the external cylindrical surfaces near the ends of the grains most remote from the igniter charge, that is, the firing end of the chamber. The thickness of this tape restrictor may be varied to obtain the desired time of short-term restriction and the width of the tape restrictor may be varied to obtain the desired amount of pressure reduction over the period of ignition and to establish sustained burning of the grain.

Although a multiplicity of separate tubular grains encased in the single combustion chamber are illustrated herein, the invention is not limited to such shaped grains or to use of multiple grains in the combustion chamber. Any particular shape may be utilized. Examples of other shapes are cylinder, cruciform, triform, hexoform, octaform and slab. Where perforated grains are employed the longitudinal perforation may be circular or star-shaped with various numbers of points in the star. Furthermore, a single cylindrical grain having a single longitudinal perforation or multiple longitudinal perforations may b used in the combustion chamber.

The grains are held in position and prevented from sliding back and forth in the chamber by means of a wire grid 30. Wire grid 30 consists of a ring cut to fit the threads of member 14 and is provided with a grid of metal wires which resist the high temperature existing in the combustion chamber.

An igniter means is positioned within member 14 so as to close off the nozzle 16. The igniter means consists of a container 31 filled with black powder or cannon powder or cannon powder or other easily ignited material, which upon ignition and burning produces a large volume of gas at elevated pressure.

Mixtures of the different grades of black powder may be used to adjust the overall burning rate of the powder. The igniter for commercial size assist-take-olf grains may consists of the C grade cannon powder granules or mixture of the C grade with black powder having an average diameter less than the C grade. The amount of igniter is determined by the surface area to be ignited, freevolume space in the combustion chamber, the ease of igniting as determined by the surface and composition of the grain and temperature of the grain when fired. A squib 32 for igniting the powder, is attached to the container 31 in communication with the powder contained therein. Electrical wires 33 connect a wire in the squib to the electrical system of the aircraft and a switch therein.

On one side of the conical portion of member 14 a safety venting means 34 is provided for the combustion chamber. Venting means 34 comprises a tubular member fastened to member 14, which tubular member has full access to the combustion chamber and is provided with a rupture disc, not shown. The rupture disc is of such construction that excess pressure in the combustion chamber will blow out the disc whereby damage to the unit resulting from over-pressure in the combustion chamber is avoided.

The assist-take-off unit is assembled as follows: Grains are inserted into member 11. Venting means 34 is attached to member 14. Igniter 31 is inserted through the large open end and fitted so as to close the nozzle, wires 33 having first been passed through nozzle 16. Wire grid 30 is screwed into the large open end of member 14 and the assembled nozzle portion is then screwed onto member 11. The assembled unit is then attached to the wing of the aircraft by loops 12 and 13 and wires 33 are connected to the electrical operating assembly in the aircraft. When the pilot desires to obtain the assist-take-off, he throws the switch which causes a current in wire 33 and heats the firing wire in squib 32 which in turn ignites the powder in container 31.

Container 31 is of suflicient strength to withstand the initial pressure generated by the gases from the powder. However, the hot igniter gas raises the pressure and causes the pressure to rise to a point which cannot be resisted by container 31. Container 31 disintegrates and the fragments are discharged through nozzle 16.

As the gases pass out of the nozzle the reaction acts on the aircraft and adds to the thrust to assist the aircraft propellers and a marked increase in forward speed results. A shorter space of time for take-off and/ or a heavier load can become airborne than when propellers only are used.

Tests A series of ammonium nitrate based turbojet auxiliary turbine starter grains was prepared. These grains were in the form of cylinders approximately 4 inches in length and 5 inches OD. and were provided with centrally located longitudinal cylindrical apertures having a diameter of 1.5 inches. Both annular ends of the grains were restricted with permanent restrictor disc plates about 4 mm. in thickness. These disc plates consisted of plasticized cellulose acetate. The composition, method of preparing the composition and method of shaping the composition into permanently restricted propellant grains is described on pages 11 and 12 of the copending application of William G. Stanley entitled Restricted Solid Propellant Grain, Serial No. 549,275, filed November 28, 1955.

To one of the prepared grains was applied a single layer coating of short-term restrictor consisting of pressure-sensitive fiat-back paper tape having a width of inch and a thickness of 0.006 inch, the tape being positioned at one end of the grain immediately adjacent the periphery of the permanent restrictor disc around the external cylindrical surface of the grain. A second grain of the above series of grains was provided with short-term restriction I by applying two layers of the above paper tape adjacent the permanent restrictor disc thus providing this grain with a shortterm restrictor of inch width and approximately 0.012 inch thickness around the external cylindrical surface. A third grain of the above series was used as a control grain, that is, the grain was provided only with permanent annular end restrictor discs and not with short-term restrictor tape coating.

The above grains were fired separately at 70 F. grain temperature in a test motor having a gas eiilux diameter of 0.236 inch using the same amounts of the same igniter charge to ignite the grains. The grains, provided with short-term restrictor, were positioned in the motor with the tape-restricted section being at the end thereof most.

V is usually the time interval between about 0.2 and 0.3 secgrain provided with a single layer of the paper backed tape and Figure 8 is the trace of the grain having double thickness of the tape applied thereto. Pressures developed are indicated on the vertical axes and time intervals are indicated on the horizontal axes. The legend at the top of Figure 6 indicates the time interval of 3 seconds for 15 divisions, that is 0.2 second for each division which scale is the same for Figures 7 and 8.

The trace of Figure 6 shows a maximum initial pressure of about 1360 p.s.i.g. with pressure of about 1320 p.s.i.g. in critical period of 0.2-0.3 second after firing of the igniter charge... 1

Comparing the trace of the firing of single thickness tape restricted grain, Figure 7, with the trace of Figure 6, the maximum pressure occurring in the 0.2-0.3 second critcial period was 1000 p.s.i.g. The residence time of the short-term restrictor in the firing of the grain was about one second.

The effect of increasingthe thickness of the short-term restrictor is shown in Figure 8. The maximum pressure over the 0.2-0.3 second critical period was about 1080 p.s.i.g. Doubling the thickness of short-term restrictor applied increased the residence time to about 1.6 seconds,

3 Having thus described my invention, I claim:

An apparatus adapted for the production of high pressure gas from the burning of a solid propellant which comprises a chamber provided with a gasdischarge'opening and positioned within said chamber, an igniter assembly containing a gas-producing material the burning of which affords a sustained burning of a solid propellant grain positioned within said chamber, said assembly containing an igniter means for actuating the burning of said gas-producing material, wherein said solid propellant com- I prises a tubular body portion comprising ammonium nitrate, oxidizable thermoplastic binder and a combustion catalyst, permanent restrictor coatings applied to the annular ends of said tubular body portion which remain on the unburned surface of the grain until that surface is substantially burned away and a short-term restrictor coating applied to the external cylindrical surface of said body portion immediately adjacent the periphery of one of said permanent restrictor coating which short-term restrictor coating is adapted to restrain burning of the body material beneath said short-term coating for a time at least substantially equal to the burn-out time of said g'asproducing material and not more than 2 seconds in excess of said burn-out time and wherein said short-te1m restrictor coating consists of a combustible pressure sensitive tape having a thickness of about 0.006 inch.

References Cited in the file of this patent UNITED STATES PATENTS 2,366,165; Whitworth et a1 Jan. 2, 1945 2,464,181 -Lauritsen Mar. 8, 1949 2,484,355- -Parsons Oct. 11, 1949 2,539,404 Crutchfield Jan. 30, 1951 2,574,479 Hickman ..'Nov. 13, 1951 2,643,611 Ball June 30, 1953 FOREIGN PATENTS Great Britain Aug. 15, 1949