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Fuel comes up from the regen channels and feeds into the injector manifold, and then through the annulus. A small part of the fuel (~15% total fuel mdot) is used as film cooling, and is discharged through the ring of 10 0.5mm holes.
Face Seal
In the assembly, there is one face seal, which interfaces with the combustion chamber. We were a bit concerned that there would be plastic deformation here when torquing the bolts, as we sized the injector such that the face with the face seal on it contacts the chamber before the injector flange contacts the chamber flange. However, we were able to validate that the lip with the face seal on it would not snap or plastically deform with +/- 0.002" tolerances.
Centerbody
The centerbody inlets N2O at around 600 psi with an ORB fitting (it's an NPT in the photo, but we switched to ORB because ORB superiority). Really though, you should try to minimize the number of NPT fittings going into your injector, as you're probably going to run into a lot of integration issues that will make you uninstall and reinstall injector fittings multiple times.
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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 a 10 thou gap annulus (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, we are more concerned about FOD generators (i.e. chattering check valves) but this hasn't caused a real problem for us yet.
Annular Gap Trades
Although I say that small annular gaps are fine, there is, of course, a limit to that. I've seen people do gaps as small as 0.003" successfully, but we tried our absolute hardest to make our gap as large as possible. To do that, we had to trade annular gap with pintle diameter, oxidizer manifold pressure, throttling, and stiffness. Our requirement is to stay above 15% dP/Pc for both manifolds, which becomes a challenge as you throttle down because the "dP" decreases faster than the "Pc" as you throttle. However, lower stiffness also means a larger annular gap for the same mdot, as your "A" term needs to increase to compensate for a lower "dP" term. As for pintle diameter, we scaled it down as much as we reasonably could. If we had scaled it down to be even smaller than it currently is, we would have gotten a huge pressure drop into the pintle tip, and the radial holes around the tip probably would not have fit.
Face Seal
In the assembly, there is one face seal, which interfaces with the combustion chamber. We were a bit concerned that there would be plastic deformation here when torquing the bolts, as we sized the injector such that the face with the face seal on it contacts the chamber before the injector flange contacts the chamber flange. However, we were able to validate that the lip with the face seal on it would not snap or plastically deform with +/- 0.002" tolerances.