For Project Hephaestus, a pintle injector was chosen due to its sufficient efficiency, machinability, and ability to mitigate combustion instabilities.

 One of the main challenges with using a pintle injector for a nitrous-ethanol combo is spray angle. A lower spray angle is desirable because it means the mixed propellant has to travel further to reach the wall, and so can disperse more. However, when using nitrous-ethanol as propellants, a high mixture ratio is often used. This means that the nitrous will have a lot more momentum than the ethanol, and since the pintle is ox-centered (there is basically no way to manifold the regen cooling to the center of the injector to have it be fuel centered) the spray angle will be exceedingly high. However, we intend on drilling the pintle holes at an angle to enforce a lower spray angle. 

This is the first iteration of the pintle injector, with some dimensions (i.e. number of film cooling holes or number of radial bolts not calculated yet). Also, holes at the bottom are not added yet. However, the final design will most likely closely resemble this, as it is machinable and contains no interpropellant seals.

The pintle center body/tip is made out of copper, while the baseplate is made out of steel. The interruption in the manifold is so igniter/PT ports can connect to the chamber offset from the holes. As of now, this injector stiffness is still (barely) above 15% for 25% throttle. However, we haven't accounted for pressure loss in the regen channels yet. However, since the fuel and oxidizer will be in separate tanks, the fuel pressure is not reliant on the ox pressure; we can increase the tank fuel pressure to account for the 

Future things to consider:

• Is a plate before the nitrous orifices necessary? I've seen it in a lot of designs
• What angle should the nitrous holes be at to get a good momentum ratio?
• What is the p-loss of having those holes in the middle of the manifold? 
• Should the bottom of the baseplate that is interfacing with the CC gases be made thinner? I've heard that for injector face materials with low thermal conductivity like steel, you should regeneratively cool the face, which can only be done if that thickness is VERY low. But I am unsure about whether or not a material that thin can hold up to the pressure. Also, there is increased risk of gapping (microscopic bending of the baseplate into a slightly upside-down U shape) which could compromise a seal that is not shown.