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Simple nozzles are made from a single piece of graphite or phenolic, heat resistant materials that can handle the exhaust. They need to be sealed to the case so exhaust only goes through the throat. The throat is important, as it factors into determining thrust, chamber pressure, and other factors. It has a step into the liner which should be a tight fit but not sealed.
The most important thing the nozzle does is have convergent and divergent sections, which chokes the flow at the throat to speed up the exhaust, then expands to match ambient pressure. The convergent section, which directs exhaust to the throat, has a half angle of 45-75 degrees. The divergent section expands the exhaust to ambient pressure and has a half angle of around 15 degrees.
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.
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Two types of igniters are shown. A bridgewire passes a lot of current through a thin wire, heating it up to lead to ignition. An e-match uses resistive heating to ignite a portion of initiator, which then ignites the fuel. We augment it using pieces of scrap solid fuel.
The ignition system is what produces the electrical signal that starts the igniter. It can be as simple as a battery and a long wire, but because long wires are annoying to deal with, we often use a wireless system, like the custom one in the photo.
The head end ignition mechanism is what we use to ignite the second stage of the rocket. Since igniting at high altitudes and the corresponding low pressure is hard, we needed to develop a robust ignition method for upper stages. This system had to be triggered by the flight computer, function at vacuum conditions, ignite motors up to “O” class, and ignites in under a second. The design of our head end ignition system, shown in the picture, is what fulfills these requirements. It is, in essence, a mini rocket motor, with a chunk of propellant grain, which is first ignited, and the exhaust flows through nozzles in the phenolic to in turn ignite the main rocket.
The head end igniter is shown firing in the picture below
The forward closure is the component that we use to prevent exhaust from leaving the forward end. It is typically made of Aluminum 6061. We use O-rings to seal against the case and liner. It is retained by the forward retention ring. Though not always required, an insulation disc is often installed onto the aft face of the Forward Closure. The forward closure is also the most common spot to install instruments into the motor, like pressure transducers and thermocouples.
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