A mandrel assembly for casting undercut finocyl grain and a method thereof

Abstract

Present disclosure discloses a mandrel assembly (100) for casting undercut finocyl grains (151) in a solid propellant rocket motor case (101). The mandrel assembly (100) comprises of a core mandrel (110) and a plurality of screw actuated mating wedge pairs (115, 116, 127) defined on the core mandrel. A plurality of fin mandrels (120) extends from and is releasably connected to the core mandrel (110) by the plurality of screw actuated mating wedge-pairs. Each fin mandrel of the plurality of fin mandrels (120) is defined with a compressed sandwich seal (129) and a compression spring (133) for securing the plurality of fin mandrels (120) around the core mandrel. An adjustable fin jig assembly (142, 146, 157) is mounted on the rocket motor case (101) to adjust the plurality of first fin mandrels (120) within the rocket motor case (101).

Description

[0001] A MANDREL ASSEMBLY FOR CASTING UNDERCUT FINOCYL GRAIN AND A METHOD THEREOF

[0002] TECHNICAL FIELD

[0003] Present disclosure relates to the field of undercut casting mandrels. Particularly but not exclusively, the invention describes simplified disassemble-able casting mandrel embodiments for manufacturing undercut finocyl grains of solid propellant rockets that can be tailored to deliver different types of mission specific thrust profiles.

[0004] BACKGROUND OF THE DISCLOSURE

[0005] As is known, solid rockets are self-sustaining jet propulsion devices that burn stored solid chemical reactants to produce thrust. A typical solid rocket is made of (a) thermally insulated motor case (thin walled, high-pressure vessel in the shape of a sphere or cylinder with domed ends) with a central aperture in at least one of its end domes; (b) propellant grain (shaped mass of the reactants filled inside the case); (c) igniter to start combustion of the propellant; and (d) nozzle attached to the said aperture to accelerate and eject the combustion gases to generate thrust force.

[0006] From ignition to burnout of a solid rocket, generated thrust force varies with time as a function of evolution of the propellant grain’s instantaneous burning surface area. Thrust profile is thus preprogrammed in grain geometry. Typical high performance solid rocket booster propellant grains are required to have high volumetric loading fraction as well as deliver peak thrust force soon after start when the flight vehicle is at its heaviest. These conflicting requirements are simultaneously met by configuring deep but thin slots in the grain to achieve large burning surface area with minimum free volume. The slots could be axi-symmetric about a central axial cavity. Or the slots could be longitudinal fins in a circular pattern about the central cavity - called finocyl grain configuration. Importantly, whatever the geometry, the configured grain should be safely, easily, and consistently manufacturable.

[0007] SRM manufacturing

[0008] A solid rocket is manufactured by pouring propellant slurry around a casting mandrel positioned inside a prepared case, heating the setup to help solidify the slurry into grain and finally removing the mandrel. Commonly used casting mandrels are rigid and monolith in construction. In cross section, they must be smaller than the largest aperture in the motor case to allow mandrel insertion and extraction. In advanced solid rockets using filamentwound, lightweight, monolith, composite cases, the largest aperture is usually about half the size of the motor diameter or smaller. This constrains grain design options available for achieving mission optimal thrust profiles using deep slots radiating almost up to the grain’s outer envelope. Hence, commonly used simple, rigid, monolith, pullout mandrels are not suitable for casting undercut finocyl grains.

[0009] Prior Art

[0010] To overcome the deficiency, ‘non-rigid’ undercut casting mandrels that deflate, dissolve, melt, combust or collapse after curing the propellant were developed. But they were found to have issues related to consistency in performance, shape stability, grain surface contamination, reusability, recurring cost, etc. Therefore, in recent times, ‘rigid’ undercut mandrels that disassemble into smaller parts with cross sections that can pass through the case aperture were developed. They are usually split at the core-fins interfaces into a central core and a set of detachable fins.

[0011] In one prior art (hrtosBwww. youtube. com / watch?v::::H0B»Lp£j6PkE), the undercut mandrel is assembled outside and inserted inside motor case in fully assembled condition. That is feasible in segmented rockets as in said prior art where the segment end aperture is as large as the case itself. However, after propellant casting and curing, core and fins are pulled out of the grain port one after the other. An important advantage of such ‘externally assembled undercut mandrels’ is the availability of scope for thorough inspection of all mandrel joints susceptible to propellant slurry ingress and the opportunity for taking corrective actions before inserting mandrel inside case. Absence of the same benefits is a major challenge with ‘internally assembled undercut mandrels.

[0012] In an ‘internally assembled undercut mandrel’ described in prior art (FR3090751 Al), a hollow cylindrical core is first installed inside empty case. It has circular patterned longitudinal slots on its wall for fins to enter partially. Each fin has a shoulder at its base or root. One by one, the fins are lowered inside the core and radially pushed out of its longitudinal slots till all fin root shoulders butt against inner surface of the core. Finally, to prevent fins from moving back, a central plug is inserted. While this method seems simple, the type of joint formed between fins and core makes it difficult to seal effectively and reliably. Also, the blind assembly procedure gives no room for visual inspection of the core- fins interfaces after assembly. Any leakage of propellant slurry through the joint can lead to accidental ignition due to friction during mandrel removal. There is also the risk of mandrel parts jamming during removal.

[0013] In another prior art (EPl 522711 A2), each fin has an L-shaped extended stem. The fins are first suspended inside case by temporarily fastening their stems to support structures outside case aperture. The core is a complex sub-assembly with three coaxial bodies. Its outer hollow cylinder has sets of outwardly projecting, adjustable, spring-loaded, circular patterned mushroom shaped bodies in withdrawn position. Middle body of the core is another complex sub-assembly with cam-followers positioned behind the mushroom bodies. Inner body of core is a sliding-cam whose axial motion actuates the cam-followers which in turn deploy the mushroom bodies. In another embodiment, hydraulic actuators are used instead of the cam-based mechanism. When the core is lowered inside the case between the suspended fins, the mushroom bodies in deployed position can slide down grooves provided at fin base. In withdrawn position, the mushroom bodies hold the fins close to the core by friction grip. Additionally, the fins at their stem-end are axially fastened with screws to core star-body. Overall, the mandrel has too many parts, assembly steps, unsecured threaded fasteners, and field adjustable features. During the course of assembly, some sub-assemblies and parts are assembled and disassembled repeatedly, leading to increased total duration and more chances of errors and accidents.

[0014] The present disclosure is directed to overcome one or more limitations stated above or any other limitations associated with the prior arts.

[0015] Objects of Invention

[0016] The object of this invention is to provide simple, rigid, internally assembled, undercut finocyl grain casting mandrel assembly • with reduced number of field-assembled parts that can be assembled and disassembled quickly

[0017] • with minimum or no threaded fasteners which are forbidden around explosives

[0018] • with minimum or no untethered fasteners that could be accidently dropped inside case

[0019] • that has simple and reliable mandrel parts joining mechanisms that do not need adjustments or fine-tuning during field-assembly

[0020] • whose critical seals can be inspected and verified before propellant casting

[0021] • whose propellant wetted joints are without crevices prone to ingress of propellant slurry

[0022] • without hydraulic or pneumatic actuation mechanism prone to leaks

[0023] • without repetitive steps where some parts are assembled, disassembled and reassembled

[0024] • without risk of jamming of mandrel parts especially during mandrel removal from grain

[0025] • where fin mold is supported and lifted at point(s) directly above its center of gravity

[0026] • with multi-functional mandrel assembly jig(s)

[0027] Advantages of the Disclosure

[0028] • Significant reduction in total part count

[0029] • Safe and quick handling of fins with tethered and non-threaded fasteners or no fasteners

[0030] • Simple, remotely operated, passive, mechanical, self-aligning, factory assembled mechanisms for joining fins to core mandrel inside case with minimal support tools

[0031] • Crevice- free and chamfered fin-to-core interfaces achieved using elastomeric seals

[0032] • Scope for physical verification of integrity of critical seals before propellant filling

[0033] • Suitable for mass production with quick turn-around time and reduced recurring cost

[0034] • Stable support configuration for fins during their assembly / disassembly using fin holders and fin jigs • Simple, height adjustable fin assembly jig scheme with merged functions and reduced part-count

[0035] • Can also be used to cast undercut finocyl grains in large, segmented rocket motors

[0036] • By only changing fins around the same core, grains with different geometries can be realized to deliver different thrust profile shapes

[0037] • Can also be used for casting undercut finocyl grains with blind central port or web at forward-end

[0038] SUMMARY OF THE DISCLOSURE

[0039] One or more shortcomings of existing mandrel assemblies for casting a solid propellant grain have been overcome, and additional advantages are provided through constructional aspects of the mandrel assembly as claimed in the present disclosure. Additional advantages are realized through the techniques of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered a part of the claimed disclosure.

[0040] Present disclosure discloses a mandrel assembly for casting undercut finocyl grains in a solid propellant rocket motor case. The mandrel assembly comprises of a core mandrel and a plurality of screw actuated mating wedge-pairs defined on the core mandrel, a plurality of fin mandrels extending from and releasably connected to the core mandrel by the plurality of screw actuated mating wedge-pairs, and each fin mandrel of the plurality of fin mandrels is defined with a compressed sandwich seal and a compression spring for securing the plurality of fin mandrels around the core mandrel. An adjustable fin jig assembly mounted on the rocket motor case to adjust the plurality of first fin mandrels within the rocket motor case.

[0041] In an embodiment, the plurality of screw actuated mating wedge pairs comprises at least two sets of core-wedges defined on the core mandrel and at least two sets of fin wedges defined on the plurality of fin mandrels. In an embodiment, the core mandrel comprises an externally tapered cylindrical surface with a star flange on its big end and at least two sets of core-wedges disposed on the tapered cylindrical surface of the core mandrel in a circular pattern about the mandrel axis.

[0042] In an embodiment, the star flange and the at least two sets of core wedges are integral, welded or removably fastened to the tapered cylindrical surface of the core mandrel.

[0043] In an embodiment, the star flange is defined with radial lobes having axial through-holes, equal to or in multiples of the number of fin mandrels of the plurality of first fin mandrels for fastening the core mandrel to the plurality of first fin mandrels.

[0044] In an embodiment, all the core wedges are disposed on the tapered cylindrical surface of the core mandrel and are aligned to the axial through holes of the star flange.

[0045] In an embodiment, each core-wedge is fastened to core mandrel with a dowel pins and a socket head cap screws on a local flat surface created on the tapered cylindrical surface of the core mandrel. One of the dowel pins is located near thin edge of core-wedge.

[0046] In an embodiment, the thin edges of all core-wedges face towards small end of the tapered core.

[0047] In an embodiment, one set of all the core wedges in a plane perpendicular to the mandrel axis are similar.

[0048] In an embodiment, successive sets of core-wedges are dissimilar along the mandrel axis and increase in height towards the star flange.

[0049] In an embodiment, the plurality of fin mandrels comprises of a plurality of first fin mandrels and a plurality of second fin mandrels.

[0050] In an embodiment, each fin mandrel of the plurality of first fin mandrels comprises a solid or hollow, metal, or composite fin structure, at least two sets of fin-wedges, a fin seal, and a compression spring. The fin structure has a forward region for undercut casting, an aft region for handling, a fin root for joining to the core mandrel and a fin tip for depth. In an embodiment, the aft-end of the fin structure has an axial blind tapped hole for fastening the fin mandrel to the star flange of the core mandrel, and a coaxial counterbore for partially housing the at least one compression spring and one transverse through-hole for lifting the first fin mandrel.

[0051] In an embodiment, the at least one transverse through-hole he vertically above center of gravity of the first fin mandrel.

[0052] In an embodiment, the thickness of each first fin mandrel decreases from the fin root to the fin tip.

[0053] In an embodiment, the surface of the fin root conformally interfaces with the tapered surface of the core mandrel with a uniform gap between 2 mm and 10 mm after joining the first fin mandrel to the core mandrel.

[0054] In an embodiment, the fin root has at least two stepped longitudinal slots opening at and towards aft-end of the first fin mandrel. The at least two stepped longitudinal slots are configured to accommodate equal number of submerged and similar fin-wedges matching with the core- wedges.

[0055] In an embodiment, the fin root is defined with a continuous groove running parallel to its edges to partially accommodate an elastomeric fin seal.

[0056] In an embodiment, the fin seal is ‘U’ shaped in toto and ‘L’ shaped in cross section, providing crevice-free joints with 30° to 60° chamfered corners between the first fin mandrel and the core mandrel.

[0057] In an embodiment, the fin seal is sandwiched between the fin root of the first fin mandrel and the core mandrel and compressed between 10% and 30% for maintaining leakproof joints during casting and curing of propellant slurry.

[0058] In an embodiment, the fin-wedges are integral, welded or removably fastened to the fin structure. In an embodiment, each fin-wedge has a longitudinal slot on its thin edge to receive a corewedge dowel pin during assembly with the core mandrel.

[0059] In an embodiment, the at least two sets of core-wedges and the at least two sets of fin- wedges are made of harder materials like tool steel.

[0060] In an embodiment, wedge ramp angle of the core wedges and the fin wedges is greater than wedge material friction angle.

[0061] In an embodiment, the core mandrel and the first fin mandrel are factory assembled.

[0062] In an embodiment, the aft-end of the first fin mandrel is configured for handling the first fin mandrel by fastening a fin holder at a fin lifting hole. The fin holder is supported in positioning the first fin mandrel inside the rocket motor case by a first fin jig and a second fin jig.

[0063] In an embodiment, the fin holder is made of two vertical parallel plates connected at bottom by a recessed horizontal plate, the parallel plates have two pairs of vertically spaced transverse through-holes for inserting two ball lock pins that are tethered to the fin holder. The horizontal plate has two vertical through-holes for locking and restraining the first fin mandrel.

[0064] In an embodiment, the first fin jig is a thick circular plate with holes for mounting on height adjusting studs and nuts equidistantly spaced atop harness ring, a central cutout for transiting mandrel sub-assemblies, and a plurality of circular patterned radial slots to receive equal number of the fin holders with the first fin mandrel. Each radial slot has a guide pin with a wingnut underneath. The guide pin has a transverse hole for ball lock pin.

[0065] In an embodiment, the second fin jig is mounted on first fin jig, and it has one open ended slot for receiving and sliding the fin holder with the first fin mandrel between the first fin jig and the second fin jig.

[0066] In an embodiment, the aft-end of each of the second fin mandrel comprises of two pairs of vertically spaced integral trunnion pins at its lifting holes. A forward end trunnion pins are configured to slide and suspend the second fin mandrel inside a third fin jig. An aft-end trunnion pins are provided to lift the second fin mandrel using a crane hook.

[0067] In an embodiment, the third fin jig comprises a thick circular plate with holes for mounting on height adjusting studs and nuts equidistantly spaced atop harness ring. A hollow central cylindrical section coaxially interfaces with inner face of centering cylinder using shaft seals. A plurality of circular patterned integral fin holders in the form of parallel pairs of partial trunnion bearings includes inclined bottom and horizontal top to trap the second fin mandrel trunnions during assembly and disassembly of the second fin mandrel.

[0068] In one non-limiting embodiment of the present disclosure, a method of casting undercut finocyl grains in a solid propellant rocket motor case by the mandrel assembly is disclosed.

[0069] The method comprises the steps of mounting a first fin jig on studs and nuts atop an aft- end harness ring of the rocket motor case. Mounting a second fin jig atop the first fin jig. Lowering one first fin mandrel with the fin seal and the fin holder into the rocket motor case and aligning a fin holder hole with a guide pin in a free slot of the first fin jig. Placing a fin holder on the second fin jig and sliding the fin holder radially and locking with a wingnut on the slot. Rotating the second fin jig towards a free slot and repeating above steps for a next fin mandrel. Removing the second fin jig and inserting the core mandrel between the plurality of first fin mandrels locked in circular pattern in the first fin jig and aligning entry of the core-wedges into the fin slots. Loosening the wingnuts and pushing the plurality of first fin mandrels closer to the core mandrel and placing compression springs in counterbores of the fin mandrel, and insertion of the core mandrel till the star flange rests on the compression springs. Assembling and tightening of screws fastening core mandrel to fin mandrels. Lowering of the first fin jig till the small end of the core mandrel rests on a mandrel receiver. Removing of ball lock pins, fin holder and first fin jig followed by assembling of a split centering ring and the casting accessories. Casting propellant slurry and curing it to form propellant grain. Decoring by removing of the centering ring, reassembling of first fin jig and fin holders to the first fin mandrels, reassembling of guide pins to fin holder holes using wingnuts and ball lock pins, removing of fin screws, removing of core, reassembling of second fin jig, removing of the guide pins and the wingnut of the first fin mandrel being decored, sliding of the first fin mandrel radially to center of the first fin jig and the second fin jig and then lifting it out, rotating the second fin jig towards next fin mandrel and repeating the steps till all fin mandrels are removed. Lastly, removing of the first fin jig and the second fin jig, a centering cylinder and harness rings to reveal the completely decored undercut finocyl grain in the rocket motor case.

[0070] In yet another embodiment, a method of casting undercut finocyl grains in solid propellant rocket motor cases is also disclosed. The method comprises of mounting a third fin jig on studs and nuts atop aft-end harness ring of the rocket motor case, while aligning the third fin jig with a centering cylinder. Hoisting a second fin mandrel by a crane hook about aft- end trunnions of the second fin. Lowering the second fin mandrel inside the rocket motor case and placing forward-end trunnions on integral fin holders of the third fin jig. Inserting the core mandrel between the plurality of fin mandrels, aligning entry of a core- wedges into a fin slots. Followed by insertion of the core mandrel up to a star flange. The star flange rests on the plurality of fin mandrels. Assembling and tightening of a fin screws fastening the core mandrel to the second fin mandrels Lowering of the third fin jig till the core mandrel rests on a mandrel receiver or stays suspended as per grain design. Assembling of a hopper-cum-distributer atop of the mandrel assembly and decoring by removing a fin screws, the core mandrel, and radial sliding of the second fin mandrel towards center of the third fin jig. Lastly, removing the third fin jig, centering cylinder and harness rings 104, 105) to reveal the completely decored undercut finocyl grain in the rocket motor case.

[0071] The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

[0072] BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0073] The novel features and characteristic of the disclosure are set forth in the appended claims. The disclosure itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:

[0074] Figure 1 illustrates top view and a front sectional view of a solid rocket motor case along with some accessories necessary for mandrel assembly.

[0075] Figure 2 illustrates a perspective view of a core mandrel and enlarged views of a portion of the core mandrel depicting core wedges in accordance with an embodiment of the present disclosure.

[0076] Figure 3 illustrates a perspective, top and sectional side views of a first fin mandrel of a mandrel assembly in accordance with an embodiment of the present disclosure.

[0077] Figure 4 illustrates a perspective view of a first fin jig and a second fin jig along with the assembled view of the first and second fin jigs above the rocket motor case in accordance with an embodiment of the present disclosure.

[0078] Figure 5 illustrates a perspective view of the core mandrel in partially inserted position inside the rocket motor case along with a plurality of first fin mandrels in accordance with an embodiment of the present disclosure.

[0079] Figure 6 illustrates a perspective view of the core mandrel fastened to the first fin mandrels inside the rocket motor case in accordance with an embodiment of the present disclosure.

[0080] Figure 7 illustrates a perspective view of the mandrel assembly installed with casting accessories for propellant casting in accordance with an embodiment of the present disclosure.

[0081] Figure 8 illustrates a perspective view depicting a position of the core mandrel during decoring operation while all the first fin mandrels are in restrained position in accordance with an embodiment of the present disclosure.

[0082] Figure 9 shows the perspective view depicting the position of one of the first fin mandrels during its decoring operation upon removal of the core mandrel after curing the propellant grain in accordance with an embodiment of the present disclosure. Figure 10 illustrates a top view and front sectional view of a decored undercut finocyl grain in the rocket motor case in accordance with an embodiment of the present disclosure.

[0083] Figure 11 illustrates a perspective view of a second fin mandrel and a third fin jig of the mandrel assembly in accordance with another embodiment of the present disclosure.

[0084] Figure 12 illustrates the perspective view of the mandrel assembly along with its casting accessories in accordance with another embodiment of the present disclosure.

[0085] The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the apparatus and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

[0086] DETAILED DESCRIPTION

[0087] While the embodiments in the disclosure are subject to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the figures and will be described below. It should be understood, however, that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the scope of the disclosure.

[0088] It is to be noted that a person skilled in the art would be motivated from the present disclosure and modify construction of the mandrel assembly along with a method for manufacturing a solid propellant grain using the mandrel assembly. However, such modifications should be construed within the scope of the disclosure. Accordingly, the drawings show only those specific details that are pertinent to understand the embodiments of the present disclosure, so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

[0089] The terms “comprises”, “comprising”, or any other variations thereof used in the disclosure, are intended to cover a non-exclusive inclusions, such that a device and a method that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such device and the system. In other words, one or more elements in the device or the system proceeded by “comprises... a” does not, without more constraints, preclude the existence of other elements or additional elements in the device or the system.

[0090] The present disclosure describes two embodiments of undercut mandrel assemblies for casting finocyl shaped solid propellant grains in typical filament- wound rocket motor cases with small end-apertures. Mandrel parts or sub-assemblies are sized for insertion and extraction through the end-apertures. Once fully assembled inside the motor case, the mandrel assembly is larger than said apertures. In preparation for propellant slurry casting, after the rocket motor case is sprayed with a liner coat over its internal insulation, it is to be handled very carefully to prevent any contact with foreign bodies including the mandrel itself. Therefore, all mandrel assembly activities must be carried out from outside without any need for manual intervention inside the motor case. Safety requirements mandate that mandrel removal after propellant grain casting and curing is carried out remotely. The mandrel assembly scheme in the present disclosure is accordingly devised for remote and partly blind assembly / disassembly activities. Towards the same objective, loose parts in the mandrel assembly and assembly accessories are consciously minimized or eliminated.

[0091] The following paragraphs describe the present disclosure in detail with reference to Figs. 1 to 12. In the figures, the same element or elements which have similar functions are indicated by the same reference signs.

[0092] Figure 1 shows top view and cut section of a harnessed rocket motor case (101) ready for mandrel assembly (100) followed by propellant casting. The rocket motor case (101) is a hollow, thin-walled, axi-symmetric, spherical, or cylindrical shaped, metal, or composite structure with domed ends. Each end-dome has a central, circular aperture - usually, smaller aperture (102) is in forward-end or igniter-end and bigger aperture (103) is in aft- end or nozzle-end of the case. Harness rings (104, 105) are temporarily assembled to either ends of the rocket motor case (101) for handling purposes. Atop the aft-end harness ring (105), a set of studs and nuts (106) are assembled in equi-spaced holes in a circular pattern as shown in Figure 1. On the aft-end aperture (103), a centering cylinder (107) is attached for aligning incoming casting mandrel assembly (100) to motor case axis (108). On the smaller aperture (102), a mandrel receiver (109) is attached. For further activities, the setup is mounted on flat ground with its aft-end facing up and levelled horizontal.

[0093] The basic mandrel assembly consists of a central core mandrel (110) (referred as “core (110)”, hereafter) and number of first fin mandrels (120) (referred as “first fins (120)”, hereafter) releasably connected to and extending from the core (110).

[0094] Figure 2 shows the core (110), or core sub-assembly designed with a tapered metal cylinder (111) as its basic structure. The core (110) forms the central cavity in the propellant grain (151). Big-end or aft-end of the tapered core (110) has a star shaped flange

[0095] (112) with a lifting provision (113). The star flange (112) could be integral, welded or removably fastened to the core (110). On its protruding radial lobes, the star flange (112) has axial through-holes (114) for fastening the first fins (120) to the core (110) using screws. The number of holes can be equal to or multiples of number of first fin mandrels. (120), At least two sets of core-wedges (115, 116) are defined proximate to the aft-end of the core’s curved outer surface in a circular pattern. The at least two sets of core wedges (115, 116) are in line with the axial through holes (114) of the star flange (112) to remotely attach the first fins (120) to the core (110). Provision for only three first fins are shown for explaining the concept, in the figures 1-10, however, the number of first fins (120) may be selected based on the requirement. All the core-wedges (115, 116), in one plane perpendicular to mandrel axis, are similar. Successive sets of the core-wedges (115, 116) increase in height towards the aft-end of the core (110). Thin edges of all assembled corewedges (115, 116) extend towards the small-end or forward-end (117) of the core (110). In one embodiment, shown in Figure 2, each core-wedge (115, 116) is assembled to core (110) on a local flat surface (118) created on the tapered cylindrical outer surface of the core (111) using locating dowel pins and submerged screws. In another embodiment, the core- wedge (115, 116) is integral or welded to core (110) (not shown in Figures). In yet another embodiment, shown in Figure 2, one of the dowels (119) in each core-wedge (115, 116) is located near its thin edge. Additional functions of the dowel (119) are explained in next sections. Figure 3 shows the first fin mandrel (120) or fin sub-assembly comprising the first fin (121) and its accessories. The first fin mandrel (120) is a solid or hollow structure made of metal(s), composites, plastics, elastomers, or a combination of the above. The first fin mandrel (120) structure is predominantly spread in a longitudinal plane. It has two regions a forward region (122) is shaped so as to form the required undercut fin cavity (153) in the propellant grain (151) and an aft region (123) is a stem used for lifting, hanging and fastening the first fin mandrel (120) to the star flange (112) of the core (110). When assembled to the core (110), the region adjoining the core (110) is defined as a fin root (124) and opposite end is a fin tip (125). A thickness of the fin (121) decreases from the fin root (124) to the fin tip (125). The fin root (124) has at least two stepped longitudinal slots (126) housing equal number of submerged and a similar fin-wedges (127) that are fastened, integral or welded to the fin (121). The fin root (124) slots are open at the stemend or aft-end to facilitate entry of the core-wedges (115, 116) during assembly. Further, each fin-wedge (127) has an open longitudinal slot (128) on its thin edge to engage with core-wedge dowel pin (119) for final fin-core alignment and lateral constraining during assembly.

[0096] In an embodiment, all the core-wedges (115, 116) and the fin- wedges (127) are factory assembled. As they are load bearing members, the core-wedges (115, 116) and the fin- wedges (127) may be made of harder materials like tool steel. To minimize sliding friction between mating wedges, chosen wedge angle (45°) is greater than material friction angle (for steel, 38°). Additionally, chrome plating may be done and lubricant like grease applied on mating surfaces before assembly.

[0097] As shown In Figure 3, the fin root (124) surface conformally interfaces with the curved and tapered outer surface of the core (111) with a uniform gap for sandwiching an elastomeric fin seal (129). The elastomeric fin seal (129) is partly held inside a groove (130) in the fin root (124) running parallel all along the root edges. The elastomeric fin seal (129) is ‘U’ shaped with ‘L’ cross section. When assembled, the seal edges protrude outside the fin root (124) to form a chamfer with an angle between about 30 and 60 degrees at the fin-core interfacing corner. The elastomeric fin seal (129) hardness and thickness are decided based on required compression level typically ranging between 10% and 30%. However, this cannot be construed as limiting and may be varied based on the requirement.

[0098] Further, at least one axial blind tapped hole (131) is provided at the stem-end of the first fin mandrel (120) to axially fasten fin to the star flange (112) of the core (110) at the axial through hole (114). A coaxial counterbore (132) is provided on top of the tapped hole (131) to partially house a spring (133). The spring helps with axial separation of core from fins during mandrel removal after propellant curing. In an embodiment, the spring (133) may be a compression spring.

[0099] The first fin mandrel (120) can be handled by more than one means. Two embodiments showing different methods of handling the first fin mandrel (120) are described in Figures 3 and 11. Since the first fin mandrel (120) must be vertical and stable during assembly, lifting point of the first fin mandrel (120) is located vertically above its centre of gravity. At least one through-hole (134), perpendicular to the first fin mandrel’s longitudinal plane, is provided as the lifting point near the fin’s stem-end. In case of more holes, they are vertically spaced.

[0100] The mandrel assembly (100) further comprises a fin-holder (135) for handling the first fin mandrel (120). The fin-holder (135) has two vertical parallel plates connected at bottom by a recessed horizontal plate. It also has provision (136) for tethering at least two ball-lock- pins (137). In an embodiment, the at least two ball-lock pins (127) are a type of nonthreaded, quick release fastener. The parallel plates have two pairs of vertically spaced transverse through-holes for the ball-lock-pins. The bottom holes (138) are for fastening the fin-holder (135) to the first fin mandrel (120) at its fin lifting point (134). The top holes (139) are for lifting the total fin assembly using a crane. The horizontal plate is defined with two vertical through-holes referred as a near hole (140) and a far hole (141) - for locating and restraining the first fin mandrels (120) during assembly and disassembly with the core (110).

[0101] Present disclosure also discloses a method of casting undercut finocyl grains (151) in a solid propellant rocket motor case (101) by the mandrel assembly (100). The method comprises the steps of releasably joining a core mandrel (110) to a plurality of first fin mandrels (120) disposed inside the rocket motor case (100). One by one, the first fin mandrels (120) are lowered inside the rocket motor case (101) through its aft-end aperture (103) and positioned in a circular pattern using two fin-jigs referred as a first fin jig (142) and a second fin jig (146) as shown in Figure 4. The first fin jig (142) is mounted on studs and nuts (106) atop aft-end harness ring (105). Height between the first fin jig (142) and the aft-end harness ring (105) can be altered by turning the nuts (106). The first fin-jig (142) has a central cutout to allow the first fins (120) and the core (110) through and a number of circular patterned radial slots (143) to receive equal number of fin-holders (135) with the first fins (120). On each slot (143) is assembled a guide-pin (144) with wingnut

[0102] (145) underneath for radial positioning and locking of the fist fins. The guide-pin also has a transverse hole for a ball-lock-pin (137). Next, the second fin-jig (146) is freely mounted atop the first fin-jig after aligning it with any one of the free slots (143).

[0103] Next, the first fin mandrel (120) is lifted by the fin-holder (135), aligned to second fin-jig

[0104] (146) and gently lowered inside case aperture (103). Guide-pin (144) in first fin-jig (142) is positioned to receive fin-holder’s far-hole (141). The fin-holder (135) is placed on the second fin-jig (146) and slid along slot (142) with guide-pin (143) and locked at farthest position using wingnut (145). The second fin-jig (146) is then rotated to next free slot and said steps are repeated for next fin mandrel (120). Once all the first fin mandrels (120) are in place, the second fin-jig (146) is removed. The ball-lock-pins in top holes (139) of the fin-holder (135) are removed in preparation for the core (110) insertion.

[0105] As shown in Figure 5, the core (110) is hoisted above the first fin-jig (142) using crane, aligned to the rocket motor case axis (108), and slowly lowered into the rocket motor case (101). As all the first fin mandrels are locked at their radially extreme positions in the first fin-jig (142), an annular gap of 25-5 mm is maintained between the core (110) and the first fin mandrels to ensure smooth insertion of the core (110). The core (110) may be rotated about its axis to align the forward-end core-wedges (115) entry into the fin slots (126).

[0106] Next, the wingnuts (145) are loosened, and all the first fin mandrels (120) are radially pushed closer to the core (110). The springs (133) are placed in the fin counterbores (132). When the core (110) is fully lowered, the star flange (112) rests on the springs (133). Height of the first fin-jig (142) may be adjusted accordingly.

[0107] Next, as shown in Figure 6, a plurality of fin screws (147) fastening the star flange (112) of the core (110) to the first fins (120) are assembled. At this point, the following events happen. As the screws (147) are tightened, the springs (133) of the first fins (120) are compressed. The first fin mandrels (120) are pulled up towards the star flange (112) till they make contact. Simultaneously, due to wedge action, fin-wedges (127) are radially pushed in by the core-wedges (115, 116) towards the core (110), compressing the sandwiched fin seals (129) in the process. At the same time, core- wedge dowel pins (119) enter the fin- wedge slots (128) and constrain transverse degrees of freedom of the first fins (120) with reference to the core (110). Radial and axial degrees of freedom of the first fins (120) are fully constrained by the mating wedge-pairs (115, 116, 127) and the fin-screws (147) respectively.

[0108] Next, the first fin-jig (142) along with the mandrel assembly (100) is further lowered into the rocket motor case (101) by turning the nuts (106) on aft-end harness ring (105) till the small-end of core (117) rests on the mandrel receiver (109). Using any known method, a removable, leakproof joint is made between the core (110) and the mandrel receiver (109).

[0109] The ball-lock-pins (137) connecting the fin-holders (135) to the first fin mandrels are removed. Next, all the fin holders (135) and the first fin-jig (142) are removed. In an embodiment, there is no need for rotating the first fin-jig (142), it can be simply lifted up vertically. A split centering-ring (148) is then assembled atop the centering cylinder (107) to align aft-end of the core (110) to the rocket motor case axis (108). On the forward-end, mandrel-receiver (109) makes the mandrel coaxial. Finally, a borescope may be used from outside the rocket motor case (101) to visually verify proper seating of the safety critical fin seals sandwiched between the first fins (120) and the core (110).

[0110] Now referring to Figure 7, the mandrel assembly (100) along with a propellant slurry casting accessories are illustrated. A propellant slurry distributer (149) is a hollow structure, conical at top and conformal to the core star flange (112) and fins aft- end at bottom. It is placed over aft-end of the mandrel assembly (100) to shield exposed joints from propellant wetting. A hopper (150) is mounted over the centering-ring (148). Then, propellant slurry is charged into the rocket motor case (101) using standard method. Soon after casting operation, the slurry distributer (149) and the hopper (150) may be removed. After necessary curing duration (usually, 1 -2 weeks), the setup is ready for removal of mandrel assembly (100) (referred to as decoring process) from the hardened propellant grain (151).

[0111] Decoring process is shown in Figure 8 where the undercut mandrel assembly (100) is carefully removed part by part from the cured propellant grain (151). The first fin-jig (142)s assembled atop aft-end harness ring studs and nuts (106). The fin-holders (135) are assembled to all the first fins (120) using ball-lock-pins (137). This time, guide-pins (144) are assembled to fin-holder near-hole (140) with the wingnuts (145) and ball-lock-pins (137) to restrain fins during removal of the core (110). The fin screws (147) are removed. Next, the core (110) is slowly pulled out to expose the central cylindrical cavity (152) of the finocyl grain (151). The second fin-jig (146) is assembled on the first fin-jig (142) in front of the first fin (120) to be removed. Corresponding guide- pin along with wingnut and ball-lock-pin (137) are removed to allow the first fin (120) to be slid to middle of grain cavity on the first and second fin jigs (142 ,146) as shown in Figure 9. Once the first fin (120) clears the undercut (153) it helped form, it is gently lifted out. The second fin-jig (146) is rotated to next fin (120) and the steps are repeated. After removal of all mandrel parts and accessories, resulting finocyl grain (151) in the rocket motor case (101) is shown in Figure 10. In the shown cut section, a central cavity (152) is formed by the core (110) and the undercut fin cavities (153) formed by the first fins (120) can be seen.

[0112] A unique advantage of mandrel assembly (100) is the provision to vertically lower each of the first fin (120) inside case aperture (103) and place the first fin (120) stably on the second fin-jig (146). There is no need to manoeuvre the crane to individual holding location of each fin at radial extremities of the first fin-jig (142). Instead, in fully stable configuration, the fin holder (135) can be horizontally pushed or pulled on jig rails to its holding location.

[0113] Referring to Figure 11, another embodiment of the mandrel assembly (100a) is disclosed in which a simpler method of fin handling is illustrated. A second fin mandrel (154) (hereinafter referred as second fins) has been optimized for handling with two pairs of vertically spaced integral trunnion pins (155, 156) at its lifting points. A forward-end trunnion pins (155) are used for sliding and suspending the second fin (154) into a third fin-jig (157) which is mounted on the height-adjustable studs and nuts (106) atop aft-end harness ring (105). An aft-end trunnion pins (156) are used for lifting the second fin (154) using a suitable crane hook. The third fin-jig (157) has a bottom cylindrical section (158) that coaxially interfaces with inner face of the centering cylinder (107) using shaft seals (not shown). The joint is thus self-centering and leakproof. On its top face, the third fin-jig (157) has integral fin holders (159) in the form of parallel pairs of trunnion bearings (160) in a circular pattern about case axis (108). As can be seen in Detail F of Figure 11, these partial bearings (160) have an inclined landing at bottom and horizontal top to trap the forward-end trunnions (155) and facilitate assembly / disassembly of fins and core. The third fin-jig (157) is thus a multi-functional tool that stays in place throughout the casting and curing process.

[0114] After sequential insertion of all the second fins (154), the core (110) is inserted and fastened (147) to the second fins (154), as in previous embodiment. In grain configurations where the central cavity does not extend (not shown) till forward-end dome of the rocket motor case (101), small-end of the core (117) stays unsupported, and the total mass of the mandrel assembly (100) is taken by the third fin-jig (157) through the second fin trunnions (155).

[0115] Functions of separate distributer (149) and the hopper (150) are combined into a single hopper-cum-distributer (161) that is easier to fabricate and use. In Figure 12, it (161) is shown atop the mandrel assembly (100). The decoring involves removal of fin screws (147), core (110), and fins (154), one at a time, in that order.

[0116] In mandrel assembly (100a) of another embodiment, total part count and number of field assembly steps are absolutely minimized.

[0117] Equivalents:

[0118] With respect to the use of substantially any plural and / or singular terms herein, those having skill in the art can translate from the plural to the singular and / or from the singular to the plural as is appropriate to the context and / or application. The various singular / plural permutations may be expressly set forth herein for sake of clarity.

[0119] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and / or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations.

[0120] In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and / or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and / or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B.”

[0121] While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

[0122] REFERENCE NUMERALS:

[0123]

Claims

1. Claims:

1. A mandrel assembly (100) for casting undercut finocyl grains (151) in a solid propellant rocket motor case (101), the mandrel assembly (100) comprising: a core mandrel (110) and a plurality of screw actuated mating wedge pairs (115, 116, 127) defined on the core mandrel (110); a plurality of fin mandrels (120) extending from and releasably connected to the core mandrel (110) by the plurality of screw actuated mating wedge-pairs (115, 116, 127), and each fin mandrel of the plurality of fin mandrels (120) is defined with a compressed sandwich seal (129) and a compression spring (133) for securing the plurality of fin mandrels (120) around the core mandrel (110); and an adjustable fin jig assembly (142, 146, 157) mounted on the rocket motor case (101), wherein the adjustable fin jig assembly is configured to adjust the plurality of fin mandrels (120) within the rocket motor case (101).

2. The mandrel assembly (100) as claimed in claim 1, wherein the plurality of screw actuated mating wedge pairs comprises at least two sets of core- wedges (115, 116) defined on the core mandrel (110) and at least two sets of fin wedges (127) defined on the plurality of fin mandrels (120).

3. The mandrel assembly ( 100) as claimed in claim 2, wherein the core mandrel (110) comprises an externally tapered cylindrical surface (111) with a star flange (112) on its big end and at least two sets of core- wedges (115, 116) disposed on the tapered cylindrical surface (111) of the core mandrel (110) in a circular pattern about a mandrel axis (108).

4. The mandrel assembly (100) as claimed in claim 3, wherein the star flange (112) and the at least two sets of core wedges (115, 116) are integral, welded or removably fastened to the tapered cylindrical surface (111) of the core mandrel (HO).

5. The mandrel assembly (100) as claimed in claim 3, wherein the star flange (112) is defined with radial lobes having axial through-holes (114), equal to or in multiplesof the number of fin mandrels (120) of the plurality of first fin mandrels (120) for fastening the core mandrel (110) to the plurality of first fin mandrels (120).

6. The mandrel assembly (100) as claimed in claim 3, wherein all the core wedges (115, 116) disposed on the tapered cylindrical surface (111) of the core mandrel (110) are aligned to the axial through holes (114) of the star flange (112).

7. The mandrel assembly (100) as claimed in claim 3, wherein, each core-wedge (115, 116) is fastened to core mandrel (111) with a dowel pins (119) and a socket head cap screws on a local flat surface (118) created on the tapered cylindrical surface (111) of the core mandrel (110).

8. The mandrel assembly (100) as claimed in claim 7, wherein one of the dowel pins(119) is located near thin edge of each core- wedge (115, 116)9. The mandrel assembly (100) as claimed in claim 8, wherein thin edges of all corewedges (115, 116) face towards small end of the tapered core (117)10. The mandrel assembly (100) as claimed in claim 2, wherein one set of all the core wedges (115, 116) in a plane perpendicular to the mandrel axis (108) are similar.

11. The mandrel assembly (100) as claimed in claim 2, wherein, successive sets of the core-wedges (115, 116) are dissimilar along the mandrel axis (108) and increase in height towards the star flange (112).

12. The mandrel assembly (100) as claimed in claim 1, wherein the plurality of fin mandrels (120) comprises of a plurality of first fin mandrels (120) and a plurality of second fin mandrels (154).

13. The mandrel assembly (100) as claimed in claim 12, wherein each fin mandrel of the plurality of first fin mandrels (120) comprises a solid or hollow, metal or composite fin structure (121), at least two sets of fin-wedges (127), a fin seal (129) and a compression spring (133), and the fin structure has a forward region (122) for undercut casting, an aft region (123) for handling, a fin root (124) for joining to the core mandrel (110) and a fin tip (125) for depth.

14. The mandrel assembly (100) as claimed in claim 13, wherein aft-end of the fin structure (121) has an axial blind tapped hole (131) for fastening the fin mandrel (120) to the star flange (112) of the core mandrel (110), and a coaxial counterbore (132) for partially housing the compression spring (133) and at least one transverse through-hole (134) for lifting the first fin mandrel (120).

15. The mandrel assembly (100) as claimed in claim 14, wherein the at least one transverse through-hole (134) he vertically above center of gravity of the first fin mandrel (120).

16. The mandrel assembly (100) as claimed in claim 13, wherein thickness of each first fin mandrel (120) decreases from the fin root (124) to the fin tip (125).

17. The mandrel assembly (100) as claimed in claim 13, wherein the surface of the fin root (124) conformally interfaces with the tapered surface (111) of the core mandrel (110) with a uniform gap between 2 mm and 10 mm after joining the first fin mandrel (120) to the core mandrel (110).

18. The mandrel assembly (100) as claimed in claim 13, wherein the fin root (124) has at least two stepped longitudinal slots (126) opening at and towards aft-end of the first fin mandrel (120), and configured to accommodate equal number of submerged and similar fin- wedges (127) matching with the core- wedges (115, 116)19. The mandrel assembly (100) as claimed in claim 13, wherein the fin root (124) is defined with a continuous groove (130) running parallel to its edges to partially accommodate an elastomeric fin seal (129).

20. The mandrel assembly (100) as claimed in claim 13, wherein the fin seal (129) is ‘U’ shaped in toto and ‘L’ shaped in cross section, providing crevice-free joints with 30° to 60° chamfered corners between the first fin mandrel (120) and the core mandrel (120).

21. The mandrel assembly (100) as claimed in claim 13, wherein the fin seal (129) is sandwiched between the fin root (124) of the first fin mandrel (120) and the coremandrel (120) and compressed between 10% and 30% for maintaining leakproof joints during casting and curing of propellant slurry.

22. The mandrel assembly (100) as claimed in claim 13, wherein the fin- wedges (127) are integral, welded or removably fastened to the fin structure (121)23. The mandrel assembly (100) as claimed in claim 13, wherein each fin- wedge (127) has a longitudinal slot (128) on its thin edge to receive a core-wedge dowel pin (119) during assembly with the core mandrel (110).

24. The mandrel assembly (100) as claimed in claims 2 and 13, wherein the at least two set of core-wedges and the at least two sets of fin-wedges (115, 116, 127) are made of harder materials like tool steel.

25. The mandrel assembly (100) as claimed in claims 2 and 13, wherein wedge ramp angle of the core wedges (115, 116) and the fin wedges (127) is greater than wedge material friction angle.

26. The mandrel assembly (100) as claimed in claims 2 and 13, wherein the core mandrel (110) and the first fin mandrel (120) are factory assembled.

27. The mandrel assembly (100) as claimed in claims 1 and 14, wherein the aft-end of the first fin mandrel (120) is configured for handling the first fin mandrel (120) by fastening a fin holder (135) at a fin lifting hole (134) and being supported in positioning the first fin mandrel (120) inside the rocket motor case (100) by a first fin jig (142) and a second fin jig (146).

28. The mandrel assembly (100) as claimed in claim 27, wherein the fin holder (135) is made of two vertical parallel plates connected at bottom by a recessed horizontal plate, the parallel plates have two pairs of vertically spaced transverse through- holes (138, 139) for inserting two ball lock pins (137) that are tethered (136) to the fin holder (135), wherein the horizontal plate has two vertical through- holes (140, 141) for locking and restraining the first fin mandrel (120).

29. The mandrel assembly (100) as claimed in claim 27, wherein the first fin jig (142) is a thick circular plate with holes for mounting on height adjusting studs and nuts (106) equidistantly spaced atop harness ring (105), a central cutout for transiting mandrel sub-assemblies, and a plurality of circular patterned radial slots (143) to receive equal number of the fin holders (135) with the first fin mandrel (120); and each radial slot (143) has a guide pin (144) with a wingnut (145) underneath; and the guide pin (144) has a transverse hole for a ball lock pin (137)30. The mandrel assembly (100) as claimed in claim 27, wherein the second fin jig (146) is mounted on first fin jig (142), and it has one open ended slot for receiving and sliding the fin holder (135) with the first fin mandrel (120) between the first fin jig (142) and the second fin jig (146).

31. The mandrel assembly (100) as claimed in claims 1 and 14, wherein the aft-end of each of the second fin mandrel (154) comprises: two pairs of vertically spaced integral trunnion pins (155, 156) at its lifting holes (134); forward end trunnion pins (155) configured to slide and suspend the second fin mandrel (154) inside a third fin jig (157), and aft-end trunnion pins (156) configured to lift the second fin mandrel (154) using a crane hook.

32. The mandrel assembly (100) as claimed in claim 31, wherein the third fin jig (157) comprises: a thick circular plate with holes for mounting on height adjusting studs and nuts (106) equidistantly spaced atop harness ring (105); a hollow central cylindrical section (158) that coaxially interfaces with inner face of centering cylinder (107) using shaft seals; and a plurality of circular patterned integral fin holders (159) in the form of parallel pairs of partial trunnion bearings (160) featuring inclined bottom and horizontal top to trap the second fin mandrel trunnions (155) during assembly and disassembly of the second fin mandrel (154).

33. A method of casting undercut finocyl grains in a solid propellant rocket motor case (101) by the mandrel assembly (100), the method comprising: mounting a first fin jig (142) on studs and nuts (106) atop an aft-end harness ring (105) of the rocket motor case (101); mounting a second fin jig (146) atop the first fin jig (142); lowering one first fin mandrel (120) with the fin seal (129) and the fin holder (135) into the rocket motor case (101) and aligning a fin holder hole (141) with a guide pin (144) in a free slot (143) of the first fin jig (142); placing a fin holder (135) on the second fin jig (146), and sliding the fin holder (135) radially and locking it with a wingnut (145) on the slot (143); rotating the second fin jig (146) towards a free slot (143) and repeating above steps for a next fin mandrel (120); removing the second fin jig (146) and inserting the core mandrel (110) between the plurality of first fin mandrels (120) locked in circular pattern in the first fin jig, and aligning entry of the core-wedges (115) into the fin slots (126); loosening the wingnuts (145) and pushing the plurality of first fin mandrels (120) closer to the core mandrel (110), and placing compression springs (133) in counterbores (132) of the fin mandrels (120), and inserting the core mandrel (110) till the star flange (112) rests on the compression springs (133); assembling and tightening of screws (147) fastening the core mandrel (110) to the first fin mandrels (120); lowering of the first fin jig (142) till the small end (117) of the core mandrel (117) rests on a mandrel receiver (109); removing of ball lock pins (137), fin holder (135) and first fin j ig (142) followed by assembling of a split centering ring (148) and the casting accessories (149, 150) casting propellant slurry and curing it to form the undercut finocyl propellant grain (151);decoring by removing of the centering ring (148), reassembling of first fin jig (142) and fin holders (135) to the first fin mandrels (120), reassembling of guide pins (144) to fin holder holes (140) using wingnuts (145) and ball lock pins (137), removing of fin screws (147), removing of core (110), reassembling of second fin jig (146), removing of the guide pins (144) and the wingnut (145) of the first fin mandrel (120) being decored, sliding of the first fin mandrel (120) radially to center of the first fin jig (142) and the second fin jig (146) and then lifting it out, rotating the second fin jig (146) towards next fin and repeating the steps till all fin mandrels (120) are removed; removing of the first fin jig (142) and the second fin jig (146), a centering cylinder (107) and harness rings (104, 105) to reveal the completely decored undercut finocyl grain (151) in the rocket motor case (101).

34. A method of casting undercut finocyl grains (151) in solid propellant rocket motor cases (101) as claimed in claim 33, the method comprising: mounting a third fin jig (157) on studs and nuts (106) atop aft-end harness ring of the rocket motor case (100), while aligning the third fin jig (157) with a centering cylinder (107); hoisting a second fin mandrel (154) by a crane hook by an aft-end trunnions (156) of the second fin (154); lowering the second fin mandrel (154) inside the rocket motor case (101) and placing forward-end trunnions (155) on integral fin holders (159) of the third finjig (157); inserting the core mandrel (110) between the plurality of second fin mandrels (154), aligning entry of a core- wedges (115) into a fin slots (126); inserting the core mandrel (110) till the star flange (112) rests on the plurality of second fin mandrels (154); assembling and tightening of a fin screws (147) fastening the core mandrel (110) to the second fin mandrels (154);lowering the third fin jig (157) till the core mandrel (110) rests on a mandrel receiver (109) or stays suspended as per grain design; assembling a hopper-cum-distributer (161) atop the mandrel assembly (100); casting propellant slurry and curing it to form the undercut finocyl propellant grain (151); decoring by removing the core mandrel (110) after removing the fin screws (147); radial sliding of a second fin mandrel (154) towards center of the third fin jig (157) and removing the second fin mandrel (154) using the crane hook; and removing the third fin jig (157), centering cylinder (107) and harness rings 104, 105) to reveal the completely decored undercut finocyl grain (151) in the rocket motor case (101).

35. A solid propellant rocket motor with undercut finocyl propellant grain (151) manufactured using the mandrel assembly (100) as claimed in claim 1.

Images