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 current iteration of the pintle injector, with some dimensions (i.e. number of film cooling holes or number of flange bolts) not calculated yet. Also, the nitrous orifices are not angled 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. We haven't accounted for pressure loss in the regen channels yet, but 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 pressure loss in the channels.
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.
- The pintle should usually be tapered and also sit on a tapered baseplate so that it's centered. If it isn't resting on a slanted surface, then the flange bolts will wobble, which will cause the pintle to translate left and right slightly, which will ruin your annular gap flow.
- Make the tip out of copper and the rest 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 the pintle will create recirculation regions below the face, I've chosen to not make the thickness of that part low. The thickness will ultimately be determined by running FEA.
Questions for future iterations:
- There seems to be a tolerance stack-up between the pintle-baseplate face seal and the taper; I want the seal to be compressed to the requirement but also want the center body taper to press down on the baseplate taper. I would prioritize the taper-taper interface, because that O-ring only serves to stop igniter exhaust from traveling radially outward through the pintle flange. If I didn't have an O-ring there, nothing would be compromised, but that feels sus...OH WAIT, pressure/gas from the chamber is going to travel upwards and leak...damn looks like I gotta fix this
- 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?
- How easy will it be to drill those angled holes?
- What is the optimal skip length (distance from annulus exit to pintle tip exit)/a rule of thumb for the skip length?
- 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?