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The PT is also separated a good amount from the chamber by around 6 inches of hard line; there are some graphs online that have correlations between flame temp and how far away your sensors should be. The air inside of the PT hard line will act as an insulator that will protect the PT from the hot flame in the chamber. A lot of designs Designs on the solid rocket side don't recess flame-probing sensors like this and are fine, but since our reliability and test count requirements are much more stringent demanding we think this is necessary.
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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 they come 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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