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 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.