...
Here is a more complicated type of nozzle, with a graphite throat to handle the highest heat at the throat, phenolic insulation and expansion section, and an aluminum carrier. This design is lighter than the simpler design, has better insulation, protects the aluminum parts, and requires less graphite machining. However, phenolic isn’t much easier to machine than graphite, and this is obviously a more complex part. So deciding which part to use is a more involved decision that requires careful consideration.
When designing a nozzle, it is important to think about how to make it as minimal parts as possible while keeping it relatively easy to machine. For the past two rockets (Medusa and Pheonix), this constituted 4 pieces in total: An aluminum carrier, phenolic insert, graphite insert, phenolic top. Medusa nozzles can be viewed here. The entrance diameter inside of the motor should be the same as the outside diameter of the propellant grain to allow all the gas to flow into the nozzle. Much of the nozzle design depends on the propellant geometry designed in OpenMotor or Burnsim. Those programs allow you to design the full nozzle. We typically do a 45-60 degree convergence with a 15 degree divergence angle. It is the easiest way to achieve the maximum efficiency of the nozzle. The graphite should be taking the majority of the force and fire as it is a good insulator and hard. This means the throat should be made out of graphite. It should also have some thickness as it needs to be able to take the force of the gasses. The phenolic should make up the majority as it is lightweight. The joint between the components should have a slanted portion then flat for an effective seal using rtv. The aluminum carrier should still have enough thickness for o-rings and the bolts.
Before you have fire shooting out through the nozzles you’ve made, you need to light the fire. The only safe way to light any motor is electrically, so you need a device that converts an electric signal into heat to ignite the motor. The goal of this device is to raise the chamber pressure and temperature to a point where propellant sustains combustion. This device is nearly always installed at the forward end. The image shows a configuration for a test firing, as shown by the nozzle pointing upwards, but the idea still holds
...