For Project Hephaestus, a pintle injector was chosen due to its sufficient efficiency and machinability. It has also been shown to mitigate combustion instabilities because of its spray characteristics, but we intend on remaining at or above .15 dP/Pc regardless.

This is the current iteration of the pintle injector. The pintle tip is made out of copper, while the baseplate and centerbody are made out of steel. The interruption in the fuel manifold is so igniter/PT ports can connect to the chamber offset from the radial holes.

Lessons learned from previous iterations:

• Radial holes in the baseplate are actually good because they reduce the flow velocity into the annulus. Just make the total area of the holes ~4 times the area of the annulus to make the velocity lowest.
• Make the tip out of copper and the center body out of steel or aluminum if you think it'll survive so that if the tip gets scorched slightly after a burn you won't have to make the whole center body again.
• For injector face materials with low thermal conductivity like steel, a good practice is to regeneratively cool the face exposed to the combustion chamber gases, which can only be done if that thickness is very low. However, since this engine is operating at a mixture ratio far from stoichiometric, and since the center body is steel, I've chosen to not make the thickness of that part low. The thickness will ultimately be determined by running FEA.
• In pintle injector design, the reason why there's a taper in the centerbody is so it can sit against a similar taper on the baseplate. Since the bolts connecting the centerbody to the baseplate are axial, the centerbody can translate horizontally when the engine is running due to vibrations. Although this may seem counterintuitive because the centerbody sits between two walls in the baseplate, remember that these walls aren't touching as there needs to be a diameteral clearance for the O-ring. Having a taper, however, eliminates this issue.
• If you're concerned about angle tolerances here, know that angularity doesn't matter so long as it's close enough. Surface runout does, but typically this is a small fraction of the total assembly runout. A taper fit essentially trades a slightly worse runout on the piloting surfaces for much better (0) assembly runout, whereas two cylindrical surfaces with a clearance fit can have a slightly better runout on the surfaces but the clearance on assembly creates a significant assembly runout. At the end of the day the concentricity error won't be such a big deal but considering a tapered fit is much more tolerant to assembly runout without being that much harder to make, and that pintles already suffer from flow maldistribution without even considering concentricity, it's probably the way to go.
• If using a taper-taper design, however, it is discouraged to use face seals for the centerbody-baseplate connection. It is unlikely that you can tolerance the part well enough to have sufficient O-ring compression and full contact with the tapers. We had this issue previously, and fixed it by making all seals between the baseplate and centerbody radial seals.
• As said by CommanderSpork (halfcat god) about nonhypergolic pintle injectors: "Pintles are fundamentally shear injectors. They are not supposed to be impingers. The whole idea is to have a thin film of fuel passing by high-mass flow streams of oxidizer. As fuel passes between these big jets of ox, it is sheared off and carried away, getting much better mixing than impingement. It then slams into the wall, creating high turbulence and the well-known recirculating regions which help improve residence time. The blockage ratio parameter, which is the amount of circumference occupied by oxidizer slot frontal area, is important because it describes the relative amount of area for that shear mixing to happen. It's a highly empirical science; too much blockage ratio and it becomes more impinger, with not a lot of slot length for shear to happen. Too little blockage ratio and, while you get plenty of slot length to shear fuel, the fuel may be too far away from the streams to effectively shear. Test campaigns and CFD go into figuring this stuff out on an engine-by-engine basis, though for injectors of similar design parameters you can scale it up. The primary slots do all the heavy lifting in efficiency; my understanding is that secondaries only exist to catch any fuel that made it past, and that's why they're usually very small. The wall temperature also has a non-trivial effect on pintle efficiency. Consider that the Lunar Module Descent Engine was ablative (and hypergolic) and what effect that might have." This statement motivated us to switch from radial holes on the pintle tip to radial slots. They are currently 10 thou in thickness. 

Questions for future iterations:

• What is a good momentum ratio for Nitrous/IPA? 
• Will the top of the chamber wall melt if it's being sprayed by nitrous exiting radially out of the pintle tip?
• What is the optimal skip length (distance from annulus exit to pintle tip exit)/a rule of thumb for the skip length? How will we enforce that length if we're screwing the pintle tip in?
• For some pintles, I've seen the bottom face of the baseplate be slanted – is there any advantage to this? Perhaps if the taper is parallel to the spray angle it will prevent hotspot accumulation on the sides?
• Does combustion happen right at the face of the pintle tip, where the two fluids collide? If it does, is it a concern?
• How will pressure of the nitrous manifold be measured? Is it okay to just have a PT right before the nitrous flows into the center body?