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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. There is a rule of thumb from somewhere that says your chamber inner diameter should be 3-5x your pintle diameter...I'm pretty sure this is bogus.
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.
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