Title of Invention

A METHOD AND AN APPARATUS FOR MANUFACTURING THIN WEBBED SOLID PROPELLANT GRAINS FOR PROPELLANT MOTORS

Abstract

A method of manufacturing thin webbed solid propellant grains for propellant motors is provided. A homogeneous propellant slurry containing known functionally temrinated liquid polymers and curing agents such as herein described is prepared. The slurry is subjected to a vacuum in a pressure casting chamber, and the vacuum is released and a freely moving diaphragm made of a light material is placed on the surface of the propellant slurry. A fluid pressure is applied by means of a gas which does not react with the propellant slurry and the slurry is cast into one or more propellant motor case(s) provided with a mandrel. The cast slurry is cured. An apparatus for manufacturing thin webbed solid propellant grain for propellant motors is also provided.

Full Text

The invention relates to a method and an apparatus for manufacturing thin webbed solid propellant grains for use in propellant motors. The solid propellant grains may have complicated geometries and large length to diameter ratios. Multiple pressure casting technique is used which is a viable and safe method for the production of defect free propellant grains. The invention may be used for preparing composite propellant compositions for igniter and special purpose motors. The expression "grains" used throughout in this specification refers to the solid propellant used in propellant motors and the web thickness is the cross-sectional thickness of a propellant grain measured from the propellant motor case to the port which burns through during the motor operating duration. Large composite solid propellant grains for propellant motors are generally manufactured by vacuum casting technique. Large grain segments weighing 3 to 100 tons with web thickness ranging from 200 to 1500 mm have been processed by this method. This method ensures defect free large grains, with minimum wastage of propellant slurry. Another method for making large propellant grains is the bayonet casting where the propellant slurry is pushed through bayonets from the top by the application of suitable pressure. Even though it is an easy method for manufacturing large propellant grains, it has the inherent problem of more voids in the resulting grain and more propellant wastage in comparison to vacuum casting. Methods known in the art are not suitable for making propellant grains of complicated geometries with web thickness in the range of 5 to 15 mm, especially required for special application motors like pyrogen igniter motors or control motors of launch vehicles. In systems which warrant void free complicated geometries and thin web propellent grains, these methods are not adequate. For example, pyrogen igniter motors and special purpose motors need thin webbed grains which are void free. The objective of this invention is to develop a processing technique for making thin webbed propellant grains of complicated geometries. This process is adaptable to propellant grains with various port configurations such as cylindrical, multi star lobed, wagon wheel or dog bone type with web thickness varying from 5 mm to 15 mm. The process is also adaptable to any filled and cured polymer system of similar nature. Preferably the length of the motor using such grains can be of 200 mm to 1200 mm and a diameter of 80 mm to 120 mm with propellant weight varying from 1 kg to 15 kgs. Any rigid epoxy coated tubes such as paper tubes or PVC tubes can be used as the motor case. Metallic case with rubber lining can also be used as motor case. In the motors made of such grains, the propellant adheres to the case firmly resulting in obtaining case bonded type motor. The invention provides a simple and fast method for realizing a number of defect free grains with smaller webs, complicated port configurations and large length to diameter ratios, from a single batch processing. The propellant grain geometry within acceptable deviations is realised through appropriate design of the mandrel, its minimum tapering from top to bottom and its correct positioning in the casing with minimum play. The propellant slurry viscosity build up, potlife and amenability of multiple pressure casting of 5 to 10 grains from a single batch are regulated by selecting process temperature, diisocyanate curing agent and the pressure applied during casting. The composite solid propellant slurry is prepared by mixing hydroxyl terminated polybutadiene, ammonium perchlorate, aluminium powder, ballistic modifier, curing agent and other additives to form a homogeneous slurry. This propellant slurry is subsequently vacuum cast by multiple pressure casting. The invention provides a method of manufacturing thin webbed solid propellant grains for propellant motors, said method comprising the steps of preparing a homogeneous propellant slurry containing known functionally terminated liquid polymers and curing agents such as herein described, subjecting the slurry to a vacuum of not more than 10 torr in a pressure casting chamber, releasing the vacuum and placing a freely moving diaphragm made of a light material such as wood or plastic on the surface of the propellant slurry in the pressure casting chamber, applying a fluid pressure in the range of 1 to 3 kgf/cm2 by means of a gas which does not react with the propellant slurry, pressure casting the slurry into one or more propellant motor case(s) provided with a mandrel, curing the cast slurry in the propellant motor case(s) at a temperature of 50 to 60°C for 5 to 8 days, cooling the cured propellant grain to ambient temperature and removing the mandrel and other fixtures to obtain thin webbed propellant grains in the motor casing. The invention also provides an apparatus for manufacturing thin webbed solid propellant grains for propellant motors by the method described herein above, said apparatus comprising a pressure casting • chamber having a lid provided with a safety valve, pressure guage, an inlet port and a sight glass; the said pressure casting chamber being provided with a removable freely moving diaphragm made of a light material, one or more propellant motor casings each provided with a mandrel being connected through flexible tubes and a control valve to at least one outlet provided on the pressure casting chamber. The invention will now be described with reference to the accompanying drawings. Fig. 1 : shows a schematic representation of the apparatus used for making the propellant grains according to the invention. Figs. 2 to 7 shows the various cross-sectional geometries of the propellant grains made according to the invention. The composite solid propellant used in this process comprises of a polymeric binder like hydroxyl terminated polybutadiene, inorganic oxidizer ammonium perchlorate, a metallic fuel like aluminium powder and a ballistic modifier either ferric oxide or copper chromite along with other additives such as plasticizer, curing agent, crosslinker and stabilizer in small quantities. The hydroxyl terminated polybutadiene used in the propellant may be produced by the free radical solution polymerization of butadiene. The oxidizer ammonium perchlorate may have bimodal distribution of coarse and fine grades in the weight ratio varying from 4:1 to 2 : 1. The solid loading in the propellant can be 83 to 86% inclusive of 10-20% aluminium powder. Unimodal fine grade of ammonium perchlorate with solid loading of 80-82% inclusive of 2 to 5% aluminium powder can also be used to get a high burn rate propellant Curing agents such as toluene diisocyanate (TDI), isophorone diisocyanate (IPDI) or 4,4-methylene bis (cyclohexyl isocyanate) (MCHI) could be used to achieve the required viscosity and pot life of the propellant slurry while processing. Any diol-triol mix comprising of diols such as ethylene glycol and 1,4-butane diol and triols such as glycerol and 1,1,1-trimethylol propane in the weight ratio ranging from 2 : 1 to 1 : 2 may be used to control the chain extension and crosslinking of hydroxyl terminated polybutadiene. The concentration of the ballistic modifier either ferric oxide or copper chromite ranges from 0 to 0.5 % for bimodal and 1 to 3 % for unimodal ammonium perchlorate based solid propellant formulations. Propellant mixing operations are carried out in conventional horizontal sigma kneader or vertical change can kneader for a duration of 2-3 hours. The kneader is jacketed for hot water circulation at 40 to 50°C. Castability of the slurry is monitored by measuring its viscosity at regular intervals for a duration of 3 to 5 hours. The slurry may then be subjected to vacuum casting in a pressure casting chamber (1) at a residual pressure of not more than 10 torn A quantity of 2 kgs of the slurry may also be vacuum cast into a polythene carton under the same conditions and cured for evaluation of mechanical properties and burn rate. Towards the end, vacuum is released and the vacuum casting setup is removed. A freely moving diaphragm (3) made of a chemically resistant light material such as wood or plastic is inserted in the chamber and kept on the surface of the propellant slurry. The top lid is kept in position with suitable gaskets made of nitrile rubber and tightened with bolts. The top lid has provisions for safety valve(7), pressure gauge (8), nitrogen inlet (9) and sight glass (10). The vacuum cast slurry in the casting chamber (1) may then be subjected to a gas pressure of 1 to 3 kgffcm2 and the gas is preferably nitrogen or dry air. The applied pressure of the gas pushes the slurry downwards through bent tubes (6) attached on both sides at the bottom (5) of the casting chamber (1) through a pinch valve (4) and then to a manifold (11), from which it is equally distributed to the assembled motor cases (2). The slurry flow rate is controlled suitably by adjusting the applied pressure to fill the intricate cavities between the mandrel and case. The mandrel (12) is preferably made of aluminium with teflon coating for easy removal during the decoring process. Nipples of the motor case assembly and the manifold are connected with flexible polythene hoses using ring clamps. The casting assembly at the bottom is made leak proof using suitable gaskets preferably made of teflon or nitrile rubber. The top assembly is kept open for out flow of excess slurry during pressure casting. The mandrel (12) has a tapering preferably in the range of 1 mm for a length of 500 mm to 2 mm for a length of 1200 mm. The mandrel (12) of desired configuration is positioned at the center in the bottom groove with least play and the top is centralized inside the paper tube motor chamber using suitable top and bottom fixtures using tie-rods. These precautions are necessary to maintain the web (13) with thickness within the tolerance limits of +_ 0.2 mm for grains of web thickness varying from 5 to 15 mm at either end of the grain. During multiple pressure casting, precautions are to be taken to close the two side valves (4) till the slurry attains the desired pressure of 1 to 3 kgf/cm2. After attaining the required pressure, the side valves (4) are opened. The diaphram (3) pushes the slurry which gets filled in the manifold (11) and gets distributed through the tubes (6) to the paper tube casing assembly (2). The flow of the slurry through the flexible tubes (6) can be seen from outside. The slurry finally reaches the bottom of the casting assembly, gets distributed in the landing portion within 5-10 minutes and then rises upwards almost uniformly through out the circumferential cavity. Depending on the length and geometry of the grain, the applied pressure may be varied from 1 to 3 kgffcm2. After allowing some quantity of slurry to extrude out, the valves (4) are closed, pressure in the casting chamber (1) is released and the grain assembly (2) is detached. Suitable pinches are put in the polythene tubes to prevent outflow of slurry. The pressure casting duration varies from 45 minutes to 120 minutes depending on the length and the geometry of the grain, The propellant grains (2) along with fixtures are cautiously transported to the curing oven facility and cured in hot water circulated oven at a temperature of 50 to 60°C for a duration of 5 to 8 days. After completion of curing, the grains are cooled down to ambient conditions and then the top and bottom fixtures and mandrel (12) are removed safely. The grains are trimmed to the required length and subjected to visual inspection, dimensional inspection and radiographic screening to detect voids, blow holes and debonds. Control carton propellant samples are used to evaluate physical, mechanical and burn rate properties. The propellant grains after inspection and certification are sealed at both ends using aluminium foils, • packed in wooden boxes and delivered to the user agency. The invention can be used to manufacture defect free thin webbed propellant grains of complicated geometry, in 5-10 numbers from a single batch of propellant slurry. Typical properties of the finished propellant grains obtained by this method has a tensile strength of 6-8 kgf7cm", elongation 40-40% modulus 20-40 kgtfcm2 and cured strand burn rate of 8 to 20 mm/sec at 70 kgfcm . The method according to the invention is also applicable to other propellants, composite, double base and composite modified double base, having suitable flow characteristics and pot life. The propellant grains produced by the method according to the invention can provide propellant grains with excellent mechanical properties and ballistics having web thickness varying from 5 mm to 15 mm within a tolerance of +0.2 mm and a tapering of the order of 1 to 2 mm for grain length from 500 mm to 1200 mm. WE CLAIM: 1. A method of manufacturing thin webbed solid propellant grains for propellant motors, said method comprising the steps of preparing a homogeneous propellant slurry containing known functionally terminated liquid polymers and curing agents such as herein described, subjecting the slurry to a vacuum of not more than 10 torr in a pressure casting chamber, releasing the vacuum and placing a freely moving diaphragm made of a light material such as wood or plastic on the surface of the propellant slurry in the pressure casting chamber, applying a fluid pressure in the range of 1 to 3 kgf/cm2 by means of a gas which does not react with the propellant slurry, pressure casting the slurry into one or more propellant motor case(s) provided with a mandrel, curing the cast slurry in the propellant motor case(s) at a temperature of 50 to 60°C for 5 to 8 days, cooling the cured propellant grain to ambient temperature and removing the mandrel and other fixtures to obtain thin webbed propellant grains in the motor casing. 2. The method as claimed in claim 1, wherein the pneumatic pressure is applied on the slurry using compressed nitrogen or dry air. 3. The method as claimed in claims 1 or 2, wherein the shape of the mandrel is selected to obtain the desired geometry of the web. 4. The method as claimed in any one of the claims 1 to 3, wherein the mandrels used is preferably made of aluminium with teflon coating and is provided with a tapering of at least 1 mm for a length of 500 mm. 5. The method as claimed in any one of the previous claims, wherein the propellant slurry comprises functionally terminated polybutadiene, ammonium perchlorate, aluminium powder, a ballistic modifier selected from iron oxide or copper chromite and a curing agent selected from toluene diisocyanate, 4, 4'methylene bis (cyclohexyl isocyanate) or isophorone diisocyanate. 6. The method as claimed in claim 5, wherein the said ammonium perchiorate is either a mixture of coarse and fine grades in the weight ratio of 4 : 1 to 2 : 1 or fine grade alone. 7. The method as claimed in claims 5 or 6 , wherein the amount of ballistic modifier is in the range of 0.5 to 3 % by weight. 8. The method as claimed in any one of the claims 1 to 7, wherein the mixing is carried out for a duration of 2 to 3 hours at a temperature of 40 to 50°C. 9. An apparatus for manufacturing thin webbed solid propellant grain for propellant motors by the method claimed in any one of the claims 1 to 8, said apparatus comprising a pressure casting chamber (1) having a lid provided with a safety valve (7), pressure guage (8), an inlet port (9) and a sight glass (10); the said pressure casting chamber being provided with a removable freely moving diaphragm (3) made of a 1 light material, one or more propellant motor casings (2) each provided with a mandrel (12) being connected through flexible tubes (6) and a control valve (4) to at least one outlet (5) provided on the pressure casting chamber (1). 10. The apparatus as claimed in claim 9, wherein the pressure casting chamber (1) is provided with two outlets (5) and two control valves (4) and plurality of propellant motor casings (2) are connected through flexible tubes (6) provided with a manifold (11), to the respective control valves (4). 11. The apparatus as claimed in claims 9 or 10, wherein the mandrels (12) provided in the propellant motor casings (2) have a predetermined geometry to obtain propellant grains with desired configuration. 12. The apparatus as claimed in claim 10, wherein the mandrel (12) has a cross-section with multistarred lobes. 13. The apparatus as claimed in claim 10, wherein the mandrel (12) has a cross-section with dog bone shapes. 14. The apparatus as claimed in claim 11, wherein the mandrel (12) has a cross section of a wagon wheel shape. 15. A method of manufacturing thin webbed solid propellant grains for propellant motors, substantially as hereinabove described and illustrated with reference to the accompanying drawings. 16. An apparatus for manufacturing thin webbed solid propellant grain for propellant motors, substantially as hereinabove described and illustrated with reference to the accompanying drawings.

Documents:

271-mas-2001- abstract.pdf

271-mas-2001- claims duplicate.pdf

271-mas-2001- claims original.pdf

271-mas-2001- correspondence others.pdf

271-mas-2001- correspondence po.pdf

271-mas-2001- description complete duplicate.pdf

271-mas-2001- description complete original.pdf

271-mas-2001- drawings.pdf

271-mas-2001- form 1.pdf

271-mas-2001- form 19.pdf

271-mas-2001- form 26.pdf

271-mas-2001- form 3.pdf

abs-271-mas-2001.jpg