Skillet Rcandy Large Batch made in Small Skillet

Click Here for a video of this process. Large File Warning!  70 megs, .wmv format 9 minutes of video.

After years of making propellant in the oven, and hearing from so many folks who wanted to try my method but couldn't do it in the kitchen, I was finally convinced to try some alternatives.  The Sugar Syrup method worked, but required milling the KNO3 and yielded an inferior propellant.  The toaster oven method worked for tiny batches, if done very carefully.  The skillet method made somewhat larger batches, with considerably less trouble.  At first, I was careful to make a very small amount so that only a thin layer of propellant solution was cooking, thin enough that it could form flaky bubbles characteristic of the oven method and dry out undisturbed.  

On a whim, I tried stirring the propellant just at the point the bubbles were forming, and found that the propellant "made" in just a few minutes rather than the customary hour.

Intrigued, I tried it with a somewhat larger batch, and found that it worked well.

To date, the largest batch I have made in this 10 inch skillet was 900 grams.  This was a lot of work, so I have backed off to a still-substantial 650 gram batch.

This will be illustrated below.

Here is all my stuff, out on the picnic table.  No doubt I will need something else before it is done, but this is all I could think of at the moment.

 

I've measured out 400 grams food-grade KNO3, 200 grams of food-grade sucrose, and 80 grams of food-grade Karo Syrup.  400ml of food-grade water is poured over it....

   

If you can't get corn syrup, or would like to have better control of the sugar content, is to use a mixture of sucrose, dextrose, and fructose.  This provides a pretty good approximation of corn syrup, creating a propellant with very nice texture.   The proportions are:  KNO3 - 64%, Sucrose - 28%, Dextrose (monohydrate) 6%, Fructose - 2% This batch is a bit larger, starting with 500g KNO3, 220g sucrose, 50g dextrose, and 7.5g fructose.  This batch should make enough propellant to fully load a 4-grain 54mm motor casing.

Water is added equal to the mass of the KNO3.  So for a batch calling for 500g KNO3, I add 500 ml of water.  The exact amount is not critical, as long as there is enough to dissolve the solids completely. I will be illustrating this KN/SU/FR/DX batch from here on out.

Temperature controller is set to 300 degrees, and the solids start  to dissolve as the mixture heats up.

A quick stir gets the last bits dissolved as the mixture heats up.  I recommend a heavy-duty wooden spatula for stirring.  Wood is non-sparking, non-scratching, and a sturdy one will be required for later stirring, mashing, and kneading.  I have broken lesser spatulae while making this propellant.

Clock starts at boiling.  Minute Zero.  The mixture does start to color a bit at this point, but not as much as the picture shows.  The green color is mostly a reflection of the lush new oak leaves above.  It's springtime!

8 minutes into the boil, bubbles are more persistent and crystals are beginning to form in solution

Time to put on the silly stuff!   I like protective gear.  Do not attempt this procedure without a good face mask, or at least protective goggles.  Singed eyebrows smell really bad, make it hard to get a date.  At my tender age, I don't need further impediments. This stuff looks innocent enough in the pan.  But if you have ever seen a quantity this size burn you will have more respect for it.  This batch could send a tongue of flame 20 feet in the air and engulf the person stirring it, if only briefly.  Burns to the body would likely be second degree, and I am willing to tolerate that level of risk.  But eyeballs don't recover from this kind of trauma easily.

After 11 minutes of boiling, the liquid starts to turns opaque.  This is the beginning of the "crystal mush" stage. Note that my times are approximate.  One can't make this propellant by simply "boil for 11 minutes, then..." as different pans will cook at different temperatures, different kinds of KNO3 and sugar may cook differently, and other factors such as relative humidity will affect evaporation rates.  So there may be some differences between your batches and mine.  There are differences in cook-time among my batches, even when everything is "the same."  So look for the clues and landmarks, and do the tests to determine when your batch is ready to move to the next stage.

 

Popping crystal mush. Sometimes the pan wants to hurl droplets out at this point.  A spatterguard helps keep them in.  Be sure to pour your rinse water out in the garden!  It is superb fertilizer.

  

After 33 minutes of cooking, the first evidence of caramelization appears.  Little fumaroles bubble up golden fluid from below.  Time to stir!

  

Two minutes later, I stir it again.   Stirring creates a porrigelike mush.  The propellant is still very moist - when stirring one can see it boiling on the surface of the pan.

Two minutes after that, it is stirred once more.   Does this seem a bit repetitive?  Yes it is.  I must stir repeatedly.  Lots of stirring is the key to getting a large batch done in a small pan, done quickly, and without excessive caramelization. I stir once every 2 minutes or so, letting the propellant rest in between.

Actually, letting me rest in between. Here I am using Florida for what it is good for and doing some heavy reading between stirs: One page of Sport Rocketry is about right between stirs, or ten pages of Sutton.   Since I don't understand Sutton, I can read it much more quickly.

I believe that the more often the mix is stirred, the more quickly it dries out.  But there is a point of diminishing returns.  I have not determined where that point is, but seems to be stirring every 2 minutes is on the lazy side of optimal.  Stirring every minute is on the industrious side.   44 minutes after first boil, 14 minutes after first stir and 7 stirrings later, the propellant is beginning to assume a "scrambled egg" form.  It is cooking well, but still not dry.  If these were eggs, it would be breakfast time.  But this stuff needs to be dry so we must overcook the eggs a bit.

It may be hard to see in the photo, but after 51 minutes of cooking AFB (after first boil) and several stirrings, the propellant has lost some of its "sheen" and takes on more of a "peanut butter" texture.   It is getting dry.   Time to take a sample and test it.

A small sample is taken from the pan and spread on a cool, dry, smooth surface.  In this case it is a marble tile destined one day to be part of the bathroom counter.  Contact with the tile will cool the sample to working temperature quickly.   After a few seconds, the sample is scraped up and rolled into a rod.  This allows a quick test of its dryness and texture.  If too wet, it will seem sticky and flaccid.   If you have a hard time letting go of it, it is too wet.  Throw the sample back in the pot, stir it again, and let it cook a little more.

At some point, the sample attains a dryish texture, and can be rolled into a rod with the fingers. We are getting close.    If/when the rod seems a bit dry and stiff, then pinch off a little chunk, roll it into a pea, and mash it flat on the cool surface.

Pinch off a little chunk maybe 1/4th inch.  Roll it into a pea, and mash it flat on the cool, dry surface.

When the sample is cooled, bend it in half.   If it bends with some resistance, then we are getting there.  But if it bends like this without breaking, it is not quite done.  Cook it a little more. When a fully-cooled sample snaps crisply, the batch is done.   Click Here for a short video of a successful snap test. (1 meg .wmv file) Your sample does not need to snap quite that crisply.  If it breaks in half after being bent some, consider it done.  Remember that the propellant has been cooking for a minute or two while the sample cools. Reduce temperature to 250 degrees immediately!  Stir it again, while you are at it.

Time for a burn-rate test.  Finally, we get to make some smoke!  And test the propellant for its 1 atm burn rate.   A strand is rolled precisely 1 inch long and roughly 1/4 inch in diameter.  The diameter is not critical, as you will see when it is burned.  Another snap-test sample has been made, just for the heck of it.

Adjusting the texture

The Taffy Pull

But wait.... the stirring isn't over yet!  This propellant has a curious property for which I don't know the correct term.  It seems to have two phases of solidification upon cooling.  Remember, right now it is cooking at 300 degrees.  It can't be left there for long or the sugar will begin to degrade.  It already has, but to a trivial degree - that's the brown color in the mix.  Cook it too much and it can become a darker brown, which can seriously decrease its performance.  So we turn it down to 200 degrees.

Now if you just let it sit and cool, the mixture will harden to a crunchy, praline-like texture.  That is not good.  It must be "disturbed" as it cools to attain a smooth, pliable texture.  

So I turn the heat down to 250 degrees and wait for it to cool.  After 10 minutes, I stir again.  The propellant starts to crack and crumble, but after a bit of stirring and kneading with the spatula a small miracle happens - it becomes pliable putty again, this time with an even nicer texture.  Heat is then reduced to 225 degrees and the process repeated.  

Just recently, I recalled that my family used to do this.  Dad would buy a couple of bottles of cane syrup, mom would cook it on the stove until it was reduced to a thick goo, then poured it on a greased baking pan.  After it had cooled awhile it would turn almost solid.  Dad would cut it into squares - I recall they were about 1 inch square and 1/4 inch thick.  Then we would all take a piece and pull it.  Stretch it out, fold it back on itself, and pull it again.  If the cooked syrup cooled completely without being pulled, it would become a hard candy.  Pulling it changed the sugar lump to a soft, chewy candy.  References I have seen suggest that the mechanism is air inclusion, but I am not sure that is the only factor.  Cane syrup is sucrose with other sugars and impurities in it.  I suspect there is some form of polymerization at work to create this stretchy texture.  And I believe it works in recrystallized propellant too, but it must be "pulled" by repeated stirrings at lower temperatures to bring it about.  Just a hunch.

In any event, it is necessary to stir, knead, and work this propellant at successively lower temperatures in order to obtain the best texture.  If it is re-heated and becomes "chunky," stir it again.  

Why fixate on texture?  Well, this stuff is a "moldable" propellant.  It can't be poured, it's way too thick.  It must be formed like modeling clay, and if it has poor texture there will be two problems.

1.  It will be hard to handle.  With proper texture, this propellant can be warmed to 200 degrees F (oven) or 225 degrees (skillet) to return it to a soft, pliable form.  At this temperature, chunks can be pulled off the pile and molded by hand into whatever shape.  Most often, it will be packed into a mold of some sort.  With poor texture, the propellant might need to be heated to an uncomfortably high temperature to get it to mold.  It is well worth the effort needed to achieve a good texture, as it makes the molding process much easier and more satisfying.

2.  Very poor texture may cause a weak, crumbly grain.  This can be dangerous.  Crumbly grains can... well, crumble when exposed to high pressure.  This could turn "candy propellant" into "candy explosive."  Crumbly propellant can burst strong motors.  Good texture creates a rock-like grain, with just a little viscosity.

Jimmy Yawn
jyawn@sfcc.net
rev. 4/22/06
Recrystallized Rocketry