High Power Rocketry ||
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
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.
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 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
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
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.
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.
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).
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.
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.
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.
High Power Rocketry ||