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Sugar and potassium nitrate model rocket fuel : Casting fuel propellant grains at home.

Making your own model rocket fuel propellant grains at home out of sugar and potassium nitrate is illegal in the UK. However, that doesn't stop me telling you how it can be done. This is very dangerous, but so is crossing the road. If you want to buy model rocket propellant grains buy them from a company like Estes Rockets or Apogee Rockets.

First you need some household sugar, and some potassium nitrate. Various online retailers sell potassium nitrate on line and sugar is easy to get.

The principle here is to melt the sugar in a bowl over a boil of hot oil. The potassium nitrate will not melt, but the sugar will, making a running sludge of molten sugar is tiny grains of potassium nitrate floating in it. The correct mix is 65% potassium nitrate and 35% sugar by weight. Mill the potassium nitrate into a fine dust in a coffee grinder, and mix with the sugar.

Put the mixture in a pyrex bowl and stir with a plastic or wooden spoon. This process takes quite a while. The oil should be about 180C. It takes ten or twenty minutes for the water content of the sugar to be driven off, and slowly the mixture becomes sticky. Keep going and it will get more and more running. The smaller the grains of potassium nitrate, the runnier it gets. Eventually, you get a light brown/yellow sludge that that runs off the back of a spoon.

Pour the mixture into a cast - the cast should be metal, and heated in the oven at 150C and coat the inside with a roll of paper soaked in silicone grease. Once cooled the grain will harden into a solid propellant grain.

This method, although difficult, is vastly superior to the normal DIY rocket propellant method of mixing the two chemicals in water and letting the water evaporate. The casting process removes all the water content, and gives you an accurate shape and much better burning.

The most difficult part is pouring the hot molten slurry into the cast - you'll have to experiment to get this right. Practice with salt instead of potassium nitrate - this will give a slurry of similar properties. Also, drying the sugar a bit first makes the process fast - put it in the oven on low or the microwave. Do not microwave potassium nitrate.

Another obvious question: What happens if I put the sugar in the oven at 180C ? I do not know, but I think you'll just end up with a bowl of caramel.

IT is also better to use an un enamelled cast iron pot to make the fuel in, because it holds the heat better.