This is a technique for making a rocket propellant from potassium nitrate and sugar without the necessity of melting either of these components. Instead, they are dissolved and re-crystalized, yielding a faster-burning fuel with interesting molding properties - it can be formed by hand into almost any shape.
I gave up making traditional caramel because of a stupidity-induced accident in my adolescence - fortunate that I did not lose any parts that would not grow back. In fact, the flash-burn seemed to improve my complexion, a fresh start after a diet of Pepsi-Cola, high school, and hormones. But now, having outgrown the acne and most of the hormones, I use this method which seems much safer. I have made about a hundred batches, and have never had an accidental ignition while producing this fuel. (There have been a couple of surprises when re-heating it, as noted below.)
Perhaps the greatest safety is that I make only minor rockets, thus am content with small batches of fuel. They are mostly stick-stabilized fireworks rockets using about 10 grams of fuel to loft a handful of stars 200 - 300 feet. (Stars are more impressive at close range.) The largest rocket engine I have made used 75 grams of this stuff. With nose cone and fins, it flew to about 3000 feet, impressing my prospective girlfriend, scaring the horses, and making all the dogs bark.
100g KNO3 (potassium nitrate, saltpeter)
50g cane sugar (white sugar, cane sugar, sucrose)
20g Karo syrup (yup, that’s it. The stuff the wife uses to make pies, cakes and…candy!)
5 tablespoons water.
Mix the above in a small saucepan. Bring to a boil over medium heat. Stir until crystals are dissolved. Remove from heat and immediately divide the mixture between two 9-inch Pyrex pie-pans, pouring half into each.
Place pie pans in preheated 300 degree oven and bake until the water is (almost completely) evaporated. This will take roughly "30 minutes." But since ovens, relative humidity, barometric pressure, and phase of the moon may vary, this is only a rough estimate. The process should be observed closely and the product tested at appropriate intervals.
What to observe:
After 20 minutes or so in the oven, the mixture will start to form whitish bubbles. Then the bubbles will begin to turn into flakes, and all apparent liquid will gradually disappear. DO NOT STIR! The bubble-flakes around the edges will start turning from their initial bluish-white to a light ivory color. DO NOT STIR! When the ivory color reaches the middle of the pan it is time to start testing for texture. DO NOT STIR!*
And don't let this mixture turn brown - we are not making caramel!
(Actually, you can let it brown a bit if you want a slower-burning propellant. For larger engines this might be desirable. For the small engines I make it is not.)
* If you stir too soon the mixture will consolidate and moisture will have a hard time escaping. The flaky bubbles expose a lot more surface for evaporation, so let it remain flaky until the moisture level is correct, as determined by the following test..
Testing for texture:
When the ivory color reaches the middle of the pan, use a table-knife or laboratory spatula to take a small sample (just a pinch) about an inch from the edge of the pan. Let it cool for a few seconds, roll it into a ball with your fingers, and mash it flat on a cool, dry surface. Let it sit a moment until completely cooled, then try to bend it in half. If it folds over without breaking, the mixture is not dry enough: let it cook some more. Test again every 2 minutes or so. When a cooled sample snaps crisply, it is dry and ready for consolidation. (Let it dry much more and it will not consolidate! It needs a touch of moisture.)
NOW you can stir! Remove the pie-pans from the oven. Scrape the flakes from one pan into the other, then scrape it all up and press into a mass. It may take a bit of stirring, kneading and mashing, but the flakes should eventually consolidate into a lump with a texture resembling Silly Putty in its prime. It can be molded immediately, or stored in air-tight containers for later molding. If you reduce the oven temperature to 200 degrees, you can keep the mixture moldable for a long time without degradation.
I will often mold a few cores right away and make the rest of the fuel into slugs and put them in plastic film-cans. It will keep for years when sealed well. I recently found a film-can of this that I had made 18 years before. It was in perfect condition, softening and molding just fine and burning at the same rate and with the same sweet smoke as the day it was made.
Like traditional caramel, this mix is hygroscopic, absorbing moisture from the air. Here in Florida we have a lot of moisture hanging around just waiting for a batch of candy to expose itself. A particle left out overnight will be a puddle of strong water in the morning. Unless you live in Arizona (arid zone?) this poses a problem. I have tried rolling the hot fuel over a light dusting of sulfur, which seems to retard moisture absorption somewhat. I have considered (but have not tried) coating it with nitrocellulose lacquer or some other flammable sealant. My best results have come from placing a burst diaphragm of aluminum foil between the nozzle and the core to seal off contact with air, and have managed to keep engines functional for several days when sealed this way.
A great advantage of this mix is that it can be softened at low heat and molded by hand – assuming that your hands are a bit heat-tolerant. To re-mold, place a lump in a 200 degree oven: once heated through, it will soften nicely. I believe that the Karo syrup texturizes the mix, making it more plastic at moderate temperature – when I tried making this without the syrup, the result was grainy, incohesive, and thus hard to form. If the fuel will not soften at 200 degrees, try easing it up to 250 briefly. The fuel will darken and degrade at this temperature, so don't leave it there long.
I use a small toaster-oven for re-heating, which works well. (I tried re-heating in the microwave once. BAD IDEA – instant ignition! Fully smoked house. Nachos have never been the same. I tried an iron skillet with a heavy lid over low heat – BAD IDEA – not instant, but ignition indeed. These were the only “accidental” ignitions I have experienced. USE AN OVEN.)
The re-heated mix can be rolled into sticks (good for bottle-rockets) or other shapes, or pressed into a tube with whatever rammer or former you might want to use. Rolling is best accomplished on a slightly-rough surface - cloth works well. It tends to slide around on smooth surfaces. I use a piece of plywood covered with denim from an old pair of jeans, but as I type realize that my mouse pad has about the same texture and should work just as well. Hopefully the mouse won't mind.
Roll a stick 1/4 inch in diameter and cut it exactly 1 inch long. Take it outside with a stopwatch, light one end, and time the burn. It should burn from one end to the other in 8 to 10 seconds. This is a good burn-rate for my small rockets. If you cook it too long and it turns coffee-with-cream brown it will burn much more slowly. If it gets slower than 13 seconds/inch it may not even sustain its own combustion in the test described here. This may not be a fault in larger engines, but makes for a high failure rate in small ones like mine. Be sure to drop the stick before the flame reaches your fingers. (That was a joke.)
This mix is a bit hard to ignite, but not so diffiicult as traditional caramel – a paper match will work, if you roast the fuel for a moment. To expedite uniform ignition and reduce “coughing” I often roll a still-hot stick over a FEW grains of black powder. A speck here and there is enough – too much may cause a burst. The black powder seems to spread the flame-front quickly.
This fuel has a fast enough burn-rate to make an end-burning engine that can move itself and a small fuselage. I usually use a core-burning grain for greater thrust.
I hope you find this interesting or useful. I would love to hear from anyone who tries this process or variations on the theme. I made this up myself, but can't believe that I am the first to think of it, so if you know of any previous work along this line I would like to know about it too.
If you have questions or comments please feel free to contact me.