This instructable will show you how to make a K class sorbitol rocket motor (almost an L) with a total impulse of 482 lb sec of thrust; and will show you how to attach it to a shopping cart! Warning! I am not responsible for what you do with your motor!
Step 1: Fabrication
To make this motor you will need to machine 3 parts on the lathe. I bought some aluminum tubing 2.5'' in diameter and 2.25'' ID for the motor, and some 2.25" stainless steel (304) for the nozzle. Graphite can also be used as the nozzle but wears a bit each use. I also bought some 2.5" dia aluminum for the end plugs. You will also need the appropriate snap rings and O-rings. I have drawings of the parts on CAD and have saved them as jpegs here.
Step 2: Casting the grains
The rocket fuel mixture that I'm using for this motor is potassium nitrate and sorbitol. 35 parts sorbitol to 65 parts potassium nitrate is what I use. Mix about one pound of the propellant at a time and melt it in a cooking pan on a hot plate (do this outside and do not use fire to melt the mixture!!!!). sorbitol melts around 100 degrees celcius and the propellant wont auto ignite untill it is heated to around 500 C, so there is a nice safety margin; never the less be careful and dont do this around flammable things. The grains are bates grains and are 2.24'' in diameter 4'' long with a .95'' inside diameter; and a total of five grains in the motor. I use paper as my insulation/ burn inhibitor for my grains; you can use phenolic but this propellant is much more for giving than most others. Spray silicone relase on the insides of the mold where you dont want the propellant to stick.