Vacuum DegassingIt is necessary to vacuum degas the RNX-71 propellant formulation in order to expel the gas that is formed and trapped within the uncured propellant mixture. It has been found that the West System epoxy chemically reacts with the potassium nitrate (or more likely the residual moisture) which results in a small amount of gas being generated. Degassing is an important step to perform, otherwise the cured propellant will have a multitude of microscopic bubbles or voids. Such voids have a dramatic accelerating effect upon the propellant burn rate, and as such, motors prepared with non-degassed RNX-71 propellant may catastrophically fail due to overpressurization. Vacuum degassed RNX-71 propellant is deemed RNX-71V.
The RNX-57 formulation, which is made with East Systems epoxy, does not need to be degassed, as it has been found that this gas producing chemical reaction does not occur.
The vacuum degassing technique is, in fact, quite simple. The propellant is first blended and mixed, as described in the previous section. A purpose-built vacuum lid is then placed onto the mixing bowl, and a hand operated vacuum pump is employed to pump out most of the air, reducing the pressure in the bowl to a near-vacuum. I usually attain a pressure of minus 26 to minus 27 in.Hg. (650 to 680 mm Hg.). The lower the pressure that is attained, the better. The propellant is then allowed to stand for about 5 minutes, during which time most of the trapped gas will be expelled from the mixture. The vacuum is then released using the vent valve of the vacuum pump. The lid is removed, and the propellant mixture is stirred & chopped using a fork. The lid is once again put into place, and the vacuum degassing operation repeated a second time. After this, the propellant is sufficiently degassed and ready for packing into the grain mould.
Figure 1 illustrates the setup that I currently use for performing the propellant degassing. This design was inspired by the vacuum setup described in the book Experimental Composite Propellant written by Terry W.McCreary.
For larger motors such as the L-Class Liberty which requires a sizeable amount of propellant, I use an electric vacuum pump.
Figure 1 -- RNX propellant undergoing degassing
The apparatus that I designed & built for performing the vacuum degassing consists of the following:
Figure 2 -- Mixing bowl, with structurally reinforced bottom
As an alternative, a disc cut from 1/2" (13 mm) plywood may be used as the reinforcing plate. However, as plywood is porous, the surface must be sealed such that it is airtight. This can be accomplished by coating the complete surface (that which is bonded to the bowl) with a suitable layer of epoxy.
The vacuum pump that I use to draw the vacuum is a hand-operated unit normally used for automotive brake bleeding. The particular model is the Mityvac (P/N 06820) with a 1 cu.in. (16 cc) stroke, and fitted with a vacuum gauge. Typically, it takes about a hundred strokes of the handle to reduce the pressure down to minus 26 in.Hg. The air is drawn out quickly at first, with each subsequent stroke removing a slightly less volume of air. Pumping is continued until no more air can be heard being dispelled from the pump's check valve, at which time the pressure should be reduced to at least minus 26 in.Hg. (650 mm Hg.). The volume of air that is to be removed can be reduced significantly by placing a suitably sized block of styrofoam in the bowl to occupy excess free volume. Although the hand-operated pump works sufficiently well to accomplish the job, an electric vacuum pump is a superior choice and is a worthwhile investment in the long run.
The lid for the vacuum bowl was made from a 10" x 10" (25 x 25 cm.) square piece of transparent 1/2" (13 mm) plexiglas (acrylic). I decided to make a see-through lid in order to observe the propellant during the degassing operation (under close examination, it is possible to observe the formation of many very tiny "spheres" or bubbles forming on the propellant surface). A hole was drilled in the lid and a 5/16" (8 mm) brass tube was epoxied in place to serve as a fitting to which the pump hose is connected. The hole is located away from the centre of the lid where the bending stress is highest. As there is no real need to observe the propellant during degassing, the lid can be instead made from 3/4" (19 mm) plywood. As with the bottom reinforcing plate, the porous plywood needs to be sealed to make it airtight. This can be done by applying a suitable layer of epoxy on the entire surface which mates with the bowl. If the plywood is not sealed properly, it will not be possible to draw a vacuum.
Bear in mind that the lid is subjected to a very large force when the bowl is evacuated. At an internal pressure of minus 26 in.Hg., the force from air pressure acting on the lid (for a 9" bowl) is over 800 lbs. (360 kg.). Consequently, the lid is subjected to appreciable bending stresses which requires it to be made of strong material such as plexiglass, lexan, plywood, or metal of sufficient thickness. A correspondingly large force also acts on the bowl (normal to any surface). Due to the hemispherical shape, however, the bowl is inherently well-suited to handle the pressure loading without harm.
The lid requires a circular shaped, air-tight pressure seal which butts against the rim of the bowl. This may be made from any soft rubber, such as EPDM (pond or canal liner such as Firestone PondGard). However, silicone rubber is perhaps best for this application, as silicone is particularly pliable. Regular silicone sealant can be used to fabricate such a seal, The first step is to place the inverted bowl on top of the lid. Using a pencil or marker, the outline of the bowl rim is sketched onto the lid. A temporary spacer, of the same thickness as the finished seal, is prepared next. The spacer is a circular disc of smaller diameter than the seal, and can be of any material (I used thin corrugated cardboard). Thickness should be 1/16" to 3/32" (1.5 to 2.5 mm). The spacer is temporarily held in place on the lid with a dab or two of glue.
Next, a piece of polyethylene sheet is cut out to be a little larger than the lid, then placed on a completely flat, smooth surface such as a tabletop. The polyethylene sheet should be reasonably thick such that no wrinkles or creases will form when the sheet is laid flat. A few pieces of scotch tape are used to hold the sheet onto the table surface. A bead of silicone is then carefully applied to the lid using the sketched line as a guide, as shown in Figure 3. The lid is then inverted and carefully positioned onto the polyethylene sheet, and pressed down firmly. Appropriate weights should then be placed onto the lid (at centre) to keep it firmly and evenly pressed down onto the spacer. The silicone is then allowed to cure for a minimum of five days before attempting to peel away the polyethylene sheet. The finished seal should look like that illustrated in Figure 4.
Figure 3 -- Making air-tight seal for lid
Figure 4 -- Detail of seal (finished seal on right)
To commence the vacuum operation, the bowl is first placed on the floor. Using your knee, apply some weight to the centre of the lid and begin pumping. This will assist in starting to draw the vacuum. Once the air pressure within the bowl has been successfully reduced a little, the force on the lid due to air pressure will generate a positive seal, and weight needn't be further applied .
Next -- Packing and Curing
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