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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. We then unscrew the regulator to set the pressure back to a reasonable purging pressure (100-200 psi). I love self venting regulators.
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. Our main chamber pressure is 350 psi.
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