In this instructable, I'll go through the steps I took in constructing a steel model rocket engine for use in a medium-size high-power model rocket. The motor uses solid propellant casted from household materials, and the design is inspired by Richard Nakka's experience (more at nakka-rocketry.net).
Building a rocket is a fun pastime if you take the necessary safety precautions, but dealing with explosives in a confined chamber is inherently DANGEROUS and I am not responsible for any injury or harm that occurs as a result of your use of this instructable.
Step 1: Step 1: Materials
For this project, you'll need the following materials:
1-1/4 in. steel EMT conduit
1018 Low-Carbon steel bar stock
10-24 machine screws (at least 6)
Spectracide brand stump remover
Sorbitol (a type of sugar available at health supplement stores)
Metal lathe with several assorted cutting tools
Well-ventilated cooking surface
10-24 screw thread tap and drill bits around this size
Step 2: Step 2: Making the Nozzle
This is by far the most time-consuming step of the process, and once it is complete things tend to go smoothly.
On the metal lathe, position the steel bar (cut to a desired length of around 4 or 5 inches) so that it spins around its center of rotation and does not put too much shear stress on the mount. Use lathe cutting tools to work the diameter of the bar down so that it will just fit into the conduit tubing. then, come in at a 15 degree angle and make the inner part of the diverging section of the nozzle, cutting away from the center first and eventually expanding the cut into an inner cone. Then, switch the cutting side of the lathe attachment, and cut away the outside of the cone at a 15 degree angle. This doesn't help the rocket work better, but cuts down on weight and makes it look like a distinctive rocket nozzle.
Once this is finished, turn the nozzle around and cut an inner cone in the other side, this one at 30 degrees. This usually takes a while, but make sure to leave the outside of the nozzle a cylinder in this section - this is what will mount to the engine body. The two inner cones should meet at a region of minimum diameter called the "throat". This section constricts the flow of hot gases and accelerates them to mach 1 inside of the nozzle, so it is very important to make sure that this is smooth without any sharp ridges, to avoid causing acoustic shocks and disrupting the flow.
Lastly, cut a ridge near the bottom of the cylindrical section for O-ring placement. This is important as it is what seals the rocket tight, but might take some trial and error depending on the size of O-rings you can acquire.
Once the nozzle is turned, the last step is to drill holes for the machine screws. Being careful to space the holes at 120-degree cylindrical angles, use the drill press to drill 3 holes of a slightly larger diameter than the screw taps. The nozzle is now complete!
Step 3: Step 3: Pressure Bulkhead
Our last step on the lathe is to cut the pressure bulkhead. This is basically a cap to stop gases from coming out of the back of the engine. Cut a short section of steel (around half an inch) so that its diameter is close to the inner diameter of the pipe, and cut another ridge for an o-ring. Then, drill 3 holes just as in the nozzle for mounting.
Step 4: Step 4: Preparing the Casing
Using the drill press, change drill bits to a slightly smaller drill bit (ideally, the one that came with the tap when you bought it). After cutting the tube to a length of 6 inches on your favorite saw, drill holes so that they align with the ones on the nozzle and bulkhead on opposite sides of the tube. Then, carefully tap threads in each hole. You can then test the fit of everything with the screws and O-rings, making sure everything is tight and aligned linearly. In the picture, you can see the nozzle attached and sealed with high-temperature silicone sealant, because my O-rings were a little small.
Step 5: Step 5: Propellant Grain
There are several options for homemade propellant grains, but the one I used uses 60% KNO3 and 40% Sorbitol (C6H14O6) mixed and casted into a solid grain. The KNO3 (from stump remover) acts as an oxidizer, allowing the sugar molecules to combust quickly and releasing a large amount of energy. Using a pan you're not planning on cooking food on, mix 100 grams of this mixture and start it on low to medium heat. Within a few minutes, the sugar will start to caramelize, and throughout this process you'll want to move the mixture around with a spoon or spatula so that it doesn't stick and burn. Once the mixture forms a slurry with the consistency of runny peanut butter, pour the mix into your rocket with the nozzle attached and the bulkhead off. You don't want this to stick, so it is good to use some sort of lubricant on the rocket or coat the edges in paper. Also, the propellant grain needs a cylindrical hole down the middle to burn, so insert a pencil or a wooden dowel through the throat while this dries.
This type of propellant is very hygroscopic, meaning that it absorbs moisture from the environment readily. To avoid this, once the mixture dries and holds its shape, remove it from the rocket and place it in a Tupperware container until it is ready for use.
Step 6: Step 6: Ignition
Ignition can be quite challenging under the wrong conditions, and there are a few different ways to do this. In the above picture, you can see me experimenting with fuses I made by soaking yarn in a supersaturated mixture of KNO3 and sugar, then drying in an oven at 300 degrees for 20 minutes. Unfortunately, these fuses didn't work, which I expect is due to the fact that the cord was made of nylon instead of cotton. At the last minute, I was forced to buy some Estes model rocket igniters from the store and use those. For a static test of the rocket engine, I have it duct-taped to a curb far from any pedestrians or cars. This is not the best set up - it would be better and safer to use a specially-designed test stand that ensures the rocket will stay in one place. However, in this case I was using a small propellant grain and had some time constraints.
When I tested this design, I was able to ignite some different propellant grains, but unfortunately the ignition wires were shorting out on the steel nozzle, so I was forced to come in through the back and loosen the bulkhead. This ended up breaking the seal and much of the gas was directed out the back. The nozzle still worked and directed a fraction of the exhaust in the correct direction, but this model did not produce a large amount of thrust. With better igniters and further tweaking, there is no reason this wouldn't successfully provide 50-100 pounds of thrust with the right propellant grain.
The above video shows a test of the engine, with the aforementioned problem obvious. From my viewing position, you could see the plume out of the nozzle that is unfortunately obscured by smoke in the video due to the back blowing out.