General NotesGrain casting moulds for the four most common grain configurations applicable to the RNX propellant are described on this web page. Each of these configurations is either neutral burning or nearly neutral burning (see Propellant Performance). These configurations, illustrated in Figure 1, are:
Figure 1 -- Grain configurations for the RNX propellant
The Epoch rocket motor utilizes the first configuration, a hollow-cylindrical grain with inhibited ends. This motor has been successfully fired, however, with both a BATES and a Pseudo-finocyl grain. The Paradigm motor utilizes the third configuration, a rod & tube assembly.
To simplify the moulding setup, the motor casing is utilized as the mould for all of these configurations.
Unrestricted Cylindrical GrainThis type of grain may be first cast as a solid cylinder and the core drilled out after the propellant has cured. Alternatively, the core may be cast-in-place using a coring tool as part of the mould. Casting as a solid cylinder is the simpler approach with regard to both mould setup and propellant packing. For both, the casing serves as the mould. Since a clearance is required to allow for flow of combustion gases that are generated on the grain outside surface, the diameter of the grain is a fair amount less than the casing inside diameter. For the Epoch motor, the clearance is 0.090 inches (2.3mm) all around. As such, a spacer made from heavy paper or light cardboard (e.g. tagboard or posterboard) is used to effectively reduce the diameter of the casing to serve as a suitable mould. I've used a posterboard paper of 0.018 inch (0.5mm) thickness, cut out to such a size that 5 layers are formed (5 x 0.018 = 0.090). To prevent the propellant mixture from bonding to the spacer, a sheet of thin plastic is bonded to the spacer at the end that will be in contact with the propellant when the spacer is rolled up. The most suitable type of plastic seems to be overhead transparency film (polyester), which has good stiffness, is heat resistant and bonds well to paper. Polyethylene sheet is also suitable, but its inherent flexibility tends to cause some difficulties, as does its poor bonding to paper. The plastic liner should be bonded to the spacer using "glue-stick" adhesive, only at one edge (approx. 1 cm), as illustrated in Figure 2.
Figure 2 -- Mould spacer and liner
Dimension "b" should be about 0.1" (2 mm) longer than the motor casing. Dimensions "a" and "c" may be estimated as follows:
The spacer is fabricated by carefully & tightly rolling the paper sheet around a mandrel of the same diameter as that of the finished grain. The mandrel may be made from any rigid tube or solid bar, and may be of wood, plastic or metal. If the diameter of a candidate mandrel is slightly less than required, it can be effectively "padded up" by wrapping thin paper sheet using enough layers such that the desired diameter is achieved.
Prior to the rolling operation, some glue-stick adhesive is applied as shown in Figure 2. Care must be taken that the paper is rolled evenly (edges remain aligned). Slight misalignment is acceptable, however, as the edges may be trimmed later if required. The rolled up spacer, with mandrel in place, is then slid into the casing. The adhesive should be allowed to fully cure prior to removing the mandrel (½ hour or so).
The casing next needs to be mounted, using a temporary adhesive bond, onto a wooden base, as illustrated in Figure 3. The base is cut square (or round) and should be ¼ inch (6 mm) larger than the casing diameter on each side, for bonding. The base should not be larger than this, otherwise the bond may become overstressed during the packing operation and may fail at a dreadfully inopertune time. The temporary bond is made by forming a generous fillet of hot-melt (polyethylene) glue around the casing-to-base joint. As it is important that this bond not fail during the packing operation, the casing should first be cleaned using isopropanol or lacquer thinner, then gently preheated using a heat gun or hairdryer. This bond will later be cut open using a boxcutter or similar sharp knife.|
The final step is to attach three or four strips of narrow masking tape to the inside of the spacer, at the top end, and folded over and attached to the casing, as shown in Figure 3. These strips of tape have an important function, retaining the spacer and preventing it from "extruding" up and out of the casing during the packing operation, owing to the significant "hydraulic" pressure that is developed (I learned this lesson the hard way).
The base shown in Figure 3 is for a grain that will have a drilled core. For the cast-in-place core method, the base should be thicker, and have a central hole drilled through for the coring tool.
The diameter of the hole should be such that the coring tool is a tight fit. A woodscrew is installed in the base to prevent the coring tool from displacing during the packing operation. A small hole is appropriately drilled into the coring tool to accept the screw tip. When applying the hot-melt glue, the casing should be adjusted such that the coring tool assembly is concentric to the casing (Click for sketch of setup).|
Unlike with the sugar-based propellants, simple lubricating of the core rod will not prevent the epoxy propellant from bonding, no matter how heavy the coating (another lesson I learned the hard way). After much trial and error, I eventually developed a very effective technique for coring tool removal. The coring tool is first wrapped with a single layer of writing paper, secured at the seam with cellophane tape. The length of the paper should be a few centimetres longer than the finished grain length. The width of the paper may be calculated as
Figure 4 -- First winding of filament tape around coring tool
The tape & paper are then trimmed off square at the bottom end such that the coring tool sits neatly into the base. After the grain has cured, the coring tool can be readily slid out of the paper sleeve. The tape is then extracted from the core by simply pulling at one end. As the filament tape has great strength (over 150 lb. tensile), removal is assured. From experience, it has been found that very little effort is required to extract the tape, as it does not bond to epoxy.
Two alternative methods of core casting have recently been suggested by my friend Roman. The first, which has been tested and found to work well, is to use a coring tool made from a metal rod covered by a vinyl tubing sleeve. Epoxy does not bond to vinyl, which is very flexible and is readily extracted from the cast grain after first pulling out the metal rod. To ease extraction, the rod is lubricated with grease. This mandrel assembly is illustrated in Figure 5.
Figure 5 -- Casting mandrel consisting of rod & vinyl tubing sleeve
Another alternative method suggested by Roman (currently untested) is to use a mandrel consisting of a metal rod that is dipped into molten paraffin wax. The wax forms a non-bonding surface on the rod. To extract the coring rod after the grain cures can conceivably be done simply by heating the grain to a temperature above the melting point of the paraffin. The rod should then slide out with little effort. Wax residue would need to be thoroughly cleaned off the surface of the propellant core to ensure ignition. This can be accomlished with mineral spirits, IPA, or similar solvent. This technique may also be suitable for more complex mandrel shapes, such as star or pseudo-finocyl.
BATES GrainThe BATES grain is similar to the hollow-cylindrical grain described above, with three key exceptions. One, the outer surface of the grain is inhibited. Two, the BATES configuration consists of two or more segments. Three, burning occurs not only at the core surface, but also on the ends of each segment. The number of segments and the corresponding segment length is typically chosen to provide a nearly-neutral Kn profile throughout the burn. A more thorough treatment of the BATES grain configuration may be found in the Rocket Motor Design Charts page for the sugar-based propellants.
As with the hollow-cylindrical grain, the motor casing may be used as the mould. However, I have found it is better to have a purpose-made mould, cut to a length equal to (or slightly longer than) that of the desired grain length. This mould is made from the same tubing as the motor casing. Instead of using a spacer/liner, an inhibitor sleeve is required which fits similarly inside the casing. The propellant will be packed into this sleeve, and when cured, will be very effectively bonded to the propellant. For ease of fabrication of the inhibitor sleeve, the mould should be no more than marginally longer than the desired grain length. The core may be either drilled after curing, or cast-in-place. Figure 6 shows the casting mould, inhibitor sleeve, and coring tool, as well as the assembled mould ready for propellant packing.
Figure 6 -- Mould components (left) and assembly (right) for casting BATES grain
The following is a suggested method of fabricating the inhibitor sleeve from a sheet of posterboard, which has a typical thickness of 0.020" (½ mm).
Figure 7 -- Rolling mandrel to expand and bond inhibitor sleeve seam
Rod & Tube GrainThe Rod & Tube grain configuration, as shown in Figure 1, consists of two separate grains, together which form a concentric assembly. There are a number of significant advantages to this configuration.
The moulding apparatus is similar to that used for the other two configurations. The coring tool, however, serves an additional purpose. It is also the mould for the Rod grain. Figure 7 illustrates the moulding apparatus made for the Paradigm J-Class rocket motor.
Figure 8 -- Components of the Rod & Tube mould assembly
The following series of photos illustrate the various components of the Rod & Tube moulding setup:
Pseudo-finocyl GrainThe Pseudo-finocyl (PFC) grain configuration, presented in Figure 1, consists of a hollow cylindrical grain, inhibited on the outer surface and having fin-like slots extending radially from a circular central core. Note that a true finocyl grain geometry is 3-dimensional. Specifically, the cross-section varies along the length of the grain, having a solely circular core at the forward end. The fins grow radially outward toward the aft end of the grain. A Pseudo-finocyl geometry is 2-dimensional, having the same cross-section throughout the length of the grain (note: "pseudo" = false).
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