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This page explains the casting process I use to make sugar propellant grains for my home built motors. I tend to use a 65% KNO3 and 35% Sugar mix for all my motors. It gives just about maximum performance with relatively low combustion temperatures, in fact, the combustion temperatures are low enough that no thermal insulation is required in the motor when using steel for the casing, nozzle and top closure. Stainless steel could be used, but it is an added expense not required.
To start with, the KNO3 and sugar(this is powdered sugar, under a dollar for a two pound bag) should be carefully measured by weight, to the desired 65%/35% ratio. I use a triple beam lab scale. First, weigh a plastic, sealable container (such as tupperware) on the scale. This is your tare weight, for example, let's say our plastic container weighs 11 grams, and we are making a 100 gram batch of propellant. We need to add 65 grams of KNO3 to the container so we would set the scale to 76 grams, and add KNO3 until the scale balanced at 76 grams. Now we need to add the 35 grams of sugar, so the scale is now set to 111 grams, and sugar added until the scale is again balanced. Now we have the correct ratio of fuel (sugar) to oxidizer (KNO3).
I use two different preparation techniques for mixing the sugar and KNO3.
If I am seeking maximum Isp, and a fast burn rate, I use a tumbler with leads balls to grind and mix the KNO3 and sugar. Care must be taken, as the mixture is now flammable. Keep away from heat, physical shock and static charge. I tumble the mix at a very low rpm, for about three hours. The result is a very fine, homogenous powder.
If I want a lower burn rate, and don't mind a slightly lower Isp, I will simply put the cover on the container I weighed in. And shake it for a few minutes before casting. This method also makes casting easier, as the courser particles make a thinner, less viscous propellant when melted.
Before starting the heat casting process. Make sure you have everything on hand before starting.
Heating Pot. I use a Presto Multi Cooker. It is thermostat controlled from 200 to 400 (F) and has a non stick surface. It's about $20 from any discount store. Never use a pot on a range or electric burner! Only use a heating vessel with embedded heating elements. Test the heating vessel with small amounts of propellant to identify problematic local hot spots. Always pre heat before adding the dry propellant.
Heavy Duty Plastic mixing spoon for stirring the propellant. I use one that has a flat tip, it makes scraping the bottom easy.
A couple of more smaller spoons for scraping the larger spoon.
Casting stand. I use a wood base, with a PVC support tube glued to the wood. I also drill a centering hole in the wood to insert the coring tool.
Casting tube. (if making an outside inhibited grain) This could just be a few wraps of paper lining the inside of the PVC support, a cardboard tube or even a smaller diameter PVC sleeve. I've had good luck with 2.5 to 3 layers of regular 20 pound printer paper on motors 1.5" and smaller. For larger motors I use 2 turns of tag board paper, finish up with a layer of aluminum tape over the paper or cardboard once the grains have cured. The idea is to keep the outside surface of the grain from burning, it only has to last a for a couple of seconds or less.
Wax Paper. I use a small piece of wax paper in the bottom of the support tube to keep the propellant from sticking to the wood base.
Coring tool. This is a rod to create the core in the grain. It can be most anything that's smooth and won't melt. I use hard wood dowels that I coat with paraffin for small cores, and steel bar for larger motors.
Gloves. The melted propellant will stick to you and leave painful, nasty blisters. Use gloves!
Heavy, long sleeved shirt. Again, to prevent burns.
Full face shield. You really don't want this stuff on your face. It may also save your pretty face if your propellant burns unexpectedly.
Water. Keep at least 5 gallons of cold water on hand.
Fire extinguisher. Just in case!
I also use silicone spray. Available from dive stores. I comes in a little pump bottle, spray some on the coring tool to make removal of the tool easier.
Everything ready? Now comes the fun part, sort of...
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Let your pot preheat to 325(F) then add your premixed KNO3 and sugar. This batch used prilled/granular KNO3 and was not ball milled in the tumbler. This batch weighed 450 grams (just under 1 pound). Start stirring as soon as the powder is in the pot and don't stop stirring.
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This is after about 1.5 minutes, you can see it starting to form clumps.
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This is after 2.5 minutes, there are more small clumps.
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About 3.5 minutes, don't stop stirring!
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After 5 minutes there is no powder left, it's all small clumps.
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After 6 minutes it's starting to liquefy.
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After about 7 minutes it starts looking like mashed potatoes.
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After 8 minutes it's becoming a viscous liquid. At this point I try to spread it out to cover the bottom of the pot. Rather that actually stirring, I sort of push it around with the spoon.
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Now it's truly a thick liquid, and you can stir again. At this point some bubbles will start to appear, I believe this is water boiling out.
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Keep heating and stirring until all lumps are gone, and no more bubbles come to the surface. At this point the propellant is ready to be cast. Turn the heat off and spray silicone on the coring tool.
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Update: I now add 2% of the total propellant batch weight in propylene glycol to thin the propellant. You can see some information on the propylene glycol here. Once thinned with the propylene glycol the melted propellant can, for the most part, be poured into the casting tubes when using granular KNO3.
Now scoop the propellant into the casting tube with your big spoon. You only have 2 or 3 minutes before the propellant starts to harden and thicken. Using the granular/non-milled version, the propellant is liquid enough that no packing is needed to remove air spaces. I just tap the whole thing on the counter after each big scoop to help settle the propellant. If you ball milled the mix, you may need to use a frozen steel bar to pack the propellant into the casting tube. Bubbles in the propellant can be a disaster, they increase the burn surface area, thus increasing chamber pressure to point your motor may overpressure.
Fill the casting tube not quite full, now insert your coring tool. As the tool is lowered into the propellant, displaced propellant fills the casting tube to the top. Try to keep the coring tool in the center of the casting tube, when you hit the wooden base, rotate to tool slightly to find the hole in the bottom. Push the tool into the pre drilled hole to center and hold the coring tool in place.
Set the casting stand in a cool place (not a freezer). Depending on the size of your grain, it may take anywhere from 15 to 30 minutes to firm up. I like to remove my coring tool while the propellant is still warm. Don't take it out too soon, or the propellant will slump back into the core. A little experience here will tell you when to remove the tool.
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A few notes about the propellant:
Sugar propellant is very hygroscopic, that is, it readily absorbs moisture from the air. You need to keep your grains stored in a desiccant box of some type. I use a large tupperware tub with an air tight lid. I filled the bottom with calcium chloride pellets (ice melt) and used a wire grate in the bottom to keep the grains out of the calcium chloride.
Never case bond the propellant. In other words, don't cast the propellant directly into your motor casing. The propellant is very brittle, and when the motor comes up to pressure the case will flex, the propellant won't flex, it will crack. Causing a large surface propellant area to be exposed to burning, causing a probable cato (KABOOM). Early on I tried case bonding, each and every time the motor failed.
Start with small motors and work your way up.
Some people consider sugar propellants as amateurish. Well, I think that's just fine. I can fly 60 of my J class motors for the same cost as one commercial J motor. Now, who do you think does more flying? The density of sugar propellants is high, cubic inch for cubic inch you can pack just about as much impulse in a sugar motor as you can an AP motor. The fast burning propellant also means you don't carry around the weight of unburned propellant very long, it also gives you quick acceleration for quick flight stability.
When using a paper casting tube/liner. Make your support tube removable. I use a PVC support tube and cut about 1/2" off a PVC coupler of the same size. Then glue the coupler piece you cut off to the wood base. The PVC support tube can then be easily removed. Also, with a paper liner, they tend to want to float up out of the support tube as you pack in the propellant. I started cutting the paper liners a little longer, then scored the bottom of the paper with a scissors to flare out the bottom. Then push the support tube onto the base with the flared paper caught under the support tube.
You can tape wax paper to your coring tool to keep it from sticking. The wax paper usually stays stuck to the grain but the tool comes out easy. You should clean the wax paper out of the core with a drill bit, knife or sand paper.
You can cast grains without a core at all. Once the grains are cooled you can drill them out to the desired core size. A spade (flat) bit seems to work the best. It is a waste of propellant though, and I always have a hard time drilling the core out true.
