Sorbitol and kn03 propellant

Here is a walk thru on how we mix and cast KNO3/Sorbitol propellant.

All of the aerospace equipment on this page came from second hand junk stores (Goodwill, etc.) or stuff we had on hand. The biggest consideration is safety, propellant fires are very dangerous to fight, most experts advise letting the propellant burn out then fighting the fires it caused. So this is something you probably don't want to do in Mom's kitchen! Click any picture to see a full size image.

Grinding the KNO3 is the first step in the process. We started out using the coffee grinder in the first picture, at on ounce a batch it quickly started taking way to long. So we retired it to iron oxide grinding duty and upgraded to the hopper type in the second picture, it will not grind as fine but is much faster. Using an old Ohaus triple beam scale we measure out 65% KNO3, 35% Sorbitol and .5% Iron Oxide.

We grind the iron oxide in the small grinder and mix it with the KNO3. It is difficult to get that small amount well mixed, so we use an old food processor, pour it in and run for about 5 minutes, works great. Combine the KNO3 mix with the sugar (WARNING FROM THIS POINT ON IT'S FLAMMABLE!) we use an old mixer, grab your self a beverage and let it run about 15 minutes. When nicely blended pour into the melt pan and turn on the heat to 240F, it takes about 10-45 minutes to melt depending on batch size. We use this time to prep and preheat the casting tools.

We use a counter top fry pan for the heat source, because it has a built in thermostat and the element is part of the pan so no surface is hotter than the mix. This is a big safety consideration, if a hot plate type burner was used and the pan moved to expose part of the burner, it will glow red hot, just waiting for that drip of propellant to make your day MUCH HOTTER! We started out using a small bread pan with rounded corners, with 2 pounds of paraffin in the fryer. It works great for small batches. For larger batches we got rid of the wax and pan and melted directly in the fryer.

As we made larger grains we started to have problems with shrinkage and voids. We were breaking up some bad grains to recast them and looking at the broken surfaces noticed many tiny bubbles throughout the grains, under magnification it almost looked like foam. So off to the second hand stores to find a pressure cooker, turned over the pressure release and hooked up a hose to a vacuum pump. Now we melt in the pressure (vacuum) cooker with the wax in the fryer to transfer the heat, when it is fully melted we pull a vacuum for 15 minutes, the propellant boils up and hits the lid. We scrape it down and vacuum it again for 5 minutes it only boils up about two inches the second time.

The casting molds are prepped by, spraying with mold release, wrapping masking tape around the top and bottom of the casting tube to make clean up easier and preheating to 275F in an oven. We use a spray mold release from the plastics industry, it makes the mandrels slide right out. We are testing different kinds, when we find the best we will post a link.

Spoon the hot propellant into the hot molds, trying not to get burned! Pour a spoon full in the mold and tap the mold on the bench to settle the propellant, repeat until it is full, set it aside to cure. Try to keep the spoon warm by leaving it in the propellant as much as possible. After the grains cure for 12-24 hours remove them from the molds and trim any drips or lumps off.

A view of one of our first 3" grains we saved it to remember how bad they were, this is not the worst one (some had voids that went most of the way to the liner) and probably would not be a problem to fly it. Adding .5% iron oxide made the mix flow much better, vacuuming made it that much better and they gained about 2 ounces per grain. Keep in mind that the grain is 2.75" OD hold a ruler up to the screen and you will see how much the picture is magnified. The red one has .5% red iron oxide and was vacuumed, the ruff pattern on the end is from the vinyl end cap it was molded in.
 

 

This is the payoff for all the hard work, the 24, 38, 54 and 75 mm loads for the Boy Scout launch on 2-14-2003. A few are plain, some with red and most with yellow iron oxide.