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Pintle injectors at the amateur scale are oftentimes plagued by FOD due to very small annular gap sizes and tolerances. Even a +/- 0.001 tolerance on a classic pintle annulus can vary mass flow by around 10 percent, which may be unacceptable depending on your requirements. Additionally, a small annular gap means that any contamination in the system can clog it, which would lead to uneven combustion instability.
A good amount of big boy aerospace industry people will claim that collegiate teams are not capable of having a cleaning standard high enough to prevent FOD clogging of an annulus this small (and will thus advise against a pintle or suggest some weird variants of it), but we found that flushing the system with water multiple times (without it hooked up to the injector) has resulted in very clean water flows. At this point, however, we are more concerned about FOD generators (i.e. chattering check valves) but this hasn't caused a real problem for us yet.
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