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Every fitting connecting to the ASI is a copper crush except the fitting that connects it to the main injector, which is an ORB fitting. We chose an ORB fitting here mostly due to the fact that it'd make installation and clocking way easier, and also because we already had an ORB porting tool of the right size lol. However, we recently de-integrated the ASI and observed some O-ring melting. Currently our potential solutions are buying FFKM O-rings which are rated to 600 F (previous high temp silicone O-rings are rated to 450 F) or just burning for less time. However, we are concerned that the FFKM can decompose into toxic chemicals (HF) if it still melts. I'm honestly kind of surprised this is an issue; one of the reasons why I chose steel as the material is because I reasoned that it would not conduct heat as quickly to the O-ring on the outside. But I guess that's not good enough.
Nozzle
add cross sections
The "nozzle" is not really a traditional rocket nozzle, as it does not have a diverging section. Its converging angle is also not optimized because we wanted it to be easy to manufacture (just drill with two different sizes to get a converging geometry). There isn't really a reason why the igniter flame needs to be supersonic – if it comes in sonic that's fine. So, we didn't see a need to drill on the other side for a diverging section.
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The ASI initially did not have purging capability, but we realized that in the event of a failed ignition, there would be some residual methagox mixture left in the chamber, which is a hazard if approached. To make the system safer, we added purging capability to the ASI by having some tee fittings that inletted nitrogen from a separate nitrogen cylinder (not the same cylinder that feeds nitrogen to the rest of the engine). There are manual valves and check valves to prevent backflow into the lines during ASI operation; additionally, right before firing the ASI, we set the nitrogen reg to be at a higher pressure than the ASI operating pressures so that it is physically impossible for the gas to backflow into the nitrogen lines unless there was some significant over pressurization event in the chamber.
Feed System Pressures
The feed system pressures in the ASI were chosen to be a "good bit" higher than the chamber pressure to mitigate backflow of gas in the main combustion chamber into the ASI lines. This is almost a non issue because the solenoids are almost guaranteed to close after the ASI burn time, but it's a good practice to have. Our oxidizer reg pressure is currently at 400 psi, and our fuel reg pressure is currently at 550 psi.
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