Propellant and Grain Preparation

 

 

 

 

 

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Setup Grain Mold and Supplies

If you use a double boiler with oil or wax, turn it on and let it start heating up while you get everything else ready.

You should set everything up before you start your propellant because when it is ready to cast, you want to hurry up and get it cast before it starts caramelizing.

You should have already made your grain casting fixture with your grain molds and coring rods and also your inhibitor sleeves.  Layout some newspaper or polyethylene sheeting on a bench top or table where you will be doing your casting to catch your excess propellant, propellant splatters, and especially the slugs from your propellant when you push the coring tool through the grains.  It should be close to where you make your propellant.  If you make your coring rods from steel, put them in a freezer or in a bowl of water with ice in it.  This will help so the rods won't stick to your propellant (everything wants to stick to your propellant).  You should have some wooden spoons handy, some bamboo skewer sticks are nice or even some popsicle sticks (sometimes called craft sticks), a high temperature pancake turner, a a roll of paper towels, a bowl of water for washing utensils and your hands and for washing off the splattered propellant from your bare skin in a big hurry.  A a can of water for throwing in excess propellant for disposal, a fine point felt tip pen for writing on the propellant, and a wet rag for cleaning up and washing off that splattered propellant again, and a dry cloth towel if you want to use that instead of the paper towels sometimes.  You also need some WD-40 or silicone axle grease for greasing the coring tools (I like the axle grease best).  Just before you are ready to cast your first grain, take the coring tools out of the freezer or bowl of ice water and in the later case, dry them off.  If you use WD-40, spray them away from everything else -- you don't want WD-40 on the rest of your supplies and equipment.  If you use grease, put a very thin layer on -- just enough to coat the tool.  You should just be able to see the sheen, not streaks of black grease.

Cut strips of aluminum foil a little larger than the diameter of the grain mold or hole they go into on the fixture.  I just tear them into squares by hand then.  Lay them over the top of each hole before and then push the square into the hole in the fixture with the grain mold (piece of PVC pipe the length of the grain).   Do this for each mold in your fixture and then put an inhibitor sleeve into the mold.  You are now ready for casting.

Propellant Preparation

Read the page "Carmel candy" Propellant first.  This section assumes you will use the propellant dissolved and heated method.  You can use an electric skillet, deep fryer directly, or deep fryer double boiler as described on that page.

Weigh Your Ingredients

Weigh out your ingredients. Let's say you are going to make a 250 gram batch.  Put in about a cup of water, 162.5g of KNO3 (lumpy is okay, it will dissolve just fine), and 87.5g of sucrose, dextrose or sorbitol.  Let's use sucrose.  You don't need to use powdered sugar, though that is fine.  Use the cheaper granulated sugar.  It also doesn't make as big a mess if you slop a little.  Turn your cooker on to 300 F.  You can actually leave it at that temperature the whole time.  Again, read the page  "Carmel candy" Propellant, especially the section on the advantages over the dry heated method.

Stir the Mix

Put all the weighed ingredients into your deep fryer or fry pan.  First stir the mix so that both the KNO3 and sugar dissolve completely.  I use a wooden spoon for that because a metal spoon will get very hot very fast as the heat is transferred up the handle.  Wooden spoons are cheap.  After it is dissolved, you can just wait for it to start boiling.  When it starts boiling, it will kind of "spit" and pop and you will end up with some splatter up on the sides of the heating vessel.  You can use a pancake turner or spatula or your wooden spoon to wipe down the sides if you want.  You don't really need to stir it a lot at the first stage.

Blurping Stage

As the boiling continues and the water is being boiled away, the mixture will start to get thicker and then as the steam tries to come off the bottom of the heating vessel where it is the hottest and rise through the thickening slurry, the steam bubble will pop at the surface and spray the top layer of the slurry all over.  This is what I call the blurping stage. As you see the slurry approaching this stage, you definitely want to start stirring it and stir it steadily.  Actually, you can stir it before this stage if you want to.  The more it is stirred, the faster it will be ready.  At this stage, a high temperature resistant pancake turner is best.  Work the slurry from the bottom to the top in a folding method.  You want to keep moving the cooler slurry on top which has the most water, down to the bottom where the most heat is and you do this by turning and folding the mixture.

Mashed Potato Stage

Soon the blurping will mostly stop because the majority of the water has been driven off.  You still need to continue to fold and mix the mixture a little bit more because there is still moisture.  Actually, the mix will get fairly thick before it gets thin again.  The mixture will have the majority of the water driven off before it gets to the temperature where the sugar is melting.  After most of the water is driven off, the temperature will still be just above boiling, 212 F and at that temperature, the mixture is very thick.  You need to continue to heat it (and stir it) until the temperature of the mixture gets closer to the melting temperature of the sucrose.  As the temperature increases, it will start getting thinner again.  It never gets pourable, but does get to a point where it is more like frosting than mashed potatoes.

Casting Stage

When it gets to the frosting consistency, or thick ketchup or mustard, then it is ready for casting.  Remember that this is sugar and hot sugar is sticky.  If you get it on you, it will stick to your skin long enough and it is hot enough that it can blister your skin if you don't have water to wash it off instantly.

Cast grains

When is it ready?

When it is 280 F to 300 F is usually enough. You can use a candy thermometer or if you want to get fancy, an infrared digital thermometer gun isn't all that expensive anymore -- cheaper ones available for a little less than $50.  A very little experience and you will just know when it is ready by the consistency.  If it is starting to turn brown as you turn it, you either have your heating vessel thermostat too high or you have waited considerably too long to start.  If you are making a lot of grains in one batch, each two or three grains will get progressively darker.  I find eight or ten grains is about all I want to make if I want them to stay fairly light, though I have made as many as 24 or 28 small (1/2" or 3/4") grains in a batch.  That was definitely too many.  As the propellant gets cooler, it gets thicker and more difficult to cast.

The Casting Process

Make sure to keep mixing and stirring your propellant left in the heating vessel in between each grain cast.

Casting is not that hard.  Just take some propellant from your heating vessel using your wooden spoon.  Use the bamboo skewer to take the propellant off the spoon and to stuff down into the grain mold.  Kind of poke it down to the bottom and make sure you don't have any voids or bubbles in it, but don't poke a hole through the aluminum foil.  If you didn't get enough on the spoon to fill up the grain mold, just get another dab on the spoon and finish pushing it into the mold.  Poke it down with the skewer but don't poke holes in down into the propellant.  You want to put small dabs in at a time and mash each dab down rather than trying to put a large bunch in all at once because doing that can cause voids in your grain which will cause your rocket motor to explode (burst).

Coring

After you have a mold full, put a cap on it.  Take your coring tool which should be cold and greased, and push it through the cap, down through the propellant, and through the tinfoil and clearance hole at the bottom of your fixture.  make sure you push it far enough through so none of the tapered section is still in the propellant.  You want to just leave it there for a short time to allow the propellant to set up some.  if you pull it out too soon, the inside will sag and bulge out into the core hole.  If this happens, sometimes you can push the coring tool back through again but if you wait too long, you won't be able to get it back through because it will be hardened.  You can wait until it hardens more and then try using a twist drill to bore it out to a straight hole again, or if it is just very slight bulging, use it anyway.  If you can't do any, just destroy it in a bucket of water.

You should have enough time to cast a second grain and when you have inserted the coring rod in that one, it should be about time to take out the coring rod of the previous grain.  When you take out the coring rod, use a twisting motion and work it up and out.  The grain will probably come right out of the mold (it should) and then you will just hold onto the grain with one hand and twist and pull the coring tool with the other hand.  Clean off any excess propellant left on the coring tool and put it back into the freezer or ice water bath.  Don't forget to wipe off water and re-grease the coring rod before each use.  As you pull out the coring rod, you can also take the grain out of the mold and then put another inhibitor sleeve in it and it is ready for another grain.  If they don't come right out, you can take them out of the fixture and leave the grain in the mold until they are cooler.  Sometimes the grains come out of the molds easier after they have cooled.

Can you just drill the grains after they are cast?  Some propellants can be but sucrose propellant becomes very hard and very brittle.  The grain will usually break before you get it drilled and if it didn't, it could have cracks that aren't visible but can still CATO your motor.

Finished Grains

The finished grains should have smooth ends and the inhibitor sleeve should go all the way to both ends without bare propellant showing on the sides.  A grain slightly longer than the inhibitor sleeve won't hurt anything but your goal should be for the grain and the inhibitor sleeve to be exactly the same length.  The aluminum foil should peel right off the grains after they have cooled.  If there are some slight wrinkle marks from the aluminum foil wrinkling or slight unevenness on the other side from trimming off what comes through the cap sides, that is also okay or you can scrape it or sand it with coarse sandpaper a little if you want to.

Desiccator -- storage

It is a good idea to put your grains into a desiccator box for 48 hours before loading a motor to insure all the moisture is out.  Use a plastic food storage container with a good fitting, sealing, lid.  Put some calcium chloride or other desiccant in the bottom of the box or in another open container in the box.  You want as much surface area exposed to the desiccant as possible which is why it is better to spread it over the entire bottom of the box.  You can use a small cooling rack inside it if you can find a combination that will fit.  You want to put your grains in the box and seal it but you don't want your grains to come in contact with the desiccant.  Initially it won't hurt but after a while, the desiccant can become moist and then if the grains are touching it, they will reabsorb moisture from the desiccant.  If your desiccant looks moist, it is past time to throw it out anyway.  Use your imagination to keep your grains up off the desiccant and still not prevent air flow between the two.  Any winter ice melter mix should work and it is very cheap.  I keep my grains in the desiccator box for long term storage.  You can also seal them in freezer bags and put them in the freezer for long term storage.  Jimmy Yawn wraps his up very tightly and securely in aluminum tape and says they last for years if properly sealed with it.

 

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