| Compounding Propellants |
| One thing to keep in mind when making a propellant, the volume of fuel/binder to volume of oxidizer and additives must not be too low. If it is then the mixture will be too dry to mix well. It will also hurt the strength of the grain. You may have to cut down on the amount of oxidizer depending on the fuel you are using. For rockets weighing 1 pound and less the easiest way to make the propellant is to obtain a suitable container for mixing and put in the bottom of it the correct amount of fuel/binder. The other ingredients are added one at a time to the fuel and mixed in. One thing that really determines the success of a propellant is the particle size of the oxidizer. It should be as finely powdered as possible. Continue mixing the propellant until it is a homogeneous mixture. Now pour it or stuff it into the engine casing taking care to eliminate all air bubbles. Any mandrels needed to form the grain to shape should already be lubricated for release and in place. After waiting a suit- able time for the binder to harden, remove the mandrels and place the engine in a warm place to finish curing. Inspect the grain for any cracks or imperfections. Some large propellant grains are constructed by cementing smaller grains together. Disks of propellant can be glued and stacked to form a long grain. The disks can be drilled with a number of holes to make a progressive or regressive burning grain. The holes are lined up when the disks are stacked. If you construct a press with a number of guide rods to match the drilled holes, so much the better. The cement can be a very thin layer of the polymer used to make the grain. If you are using a PVC based grain, then dampen both mating surfaces with THF and press them together for a minute before adding the next disk. You can also load a cardboard casing with the propellant. After the propellant is cured, this cartridge is loaded into the engine. When drilling these propellants or using any power tool for shaping them, use the lowest speed while checking to make sure no heat is building up on the cutting surface. If care is used, machining propellants is safe. |