The injector is responsible for taking in propellant and injecting it into the combustion chamber. It needs to thoroughly mix and atomize the propellant, while also withstanding high pressure and thermal loads. For our engine design, we chose a fuel to oxidizer ratio of 1:4.5, which posed a challenge for injector geometry selection. Ultimately, an unlike impinging triplet geometry was chosen, with each triplet composed of two 1/8 oxidizer holes and one 5/64 fuel hole. Originally, the element pattern was F-O-O, but we changed this to an O-F-O because the mixing of an F-O-O configuration was not optimal. That is, an F-O-O configuration results in unevenly mixed propellant, as the outer side of the resultant spray would be fuel rich while the inner side would be ox-rich.
To calculate mass flow rates for each propellant, we can simply multiply the total mass flow by the mixture ratio fractions. For the nitrous oxide mass flow rate m_ox, we get 1.18 * (4.5 / 5.5) = .965 kg/s. For the fuel mass flow rate, we get 1.18 * (1 / 5.5) = .215 kg/s.
Some other necessary parameters that we need are the discharge coefficient, which was calculated to be .44 using the tank model used for the engine (it's low, I know). We also need the densities of the fluids, which are rho_nitrous = 817 kg/m^3, rho_ethanol = 789 kg/m^3. The pressure drop across the injector should be 15-20% of the chamber pressure; for our engine, we chose an average pressure drop of 96 psi, which is enforced by orifice area. The reason for this has to do with the feared combustion instability, which will be covered later on this page. Finally, we can calculate the velocities of the nitrous and ethanol propellant at the outlet using Bernoulli's equation. We get v_nitrous = 40.463 m/s, and v_ethanol = 41.175 m/s.
For a triplet design, in order to maximize mixing, the center orifice must be vertical, and the side orifices must be at the same angle from the vertical. For the oxidizer angles, we chose an angle of 30, as that would entail an included angle of 60. Studies have shown that triplet impingement angles of 60 and 90 degrees perform better than most impinging designs, with the 90 degree element slightly outperforming the 60 degree element. However, due to spacing constraints, going for a 90 degree impingement angle was not possible.
The other item to consider is impingement distance, or how far below the face plate the elements collide. Generally, smaller impingement distances correlate to increased performance, but that also means that combustion happens closer to the faceplate, which increases risk of faceplate melting. Impingement distance is also constrained by the geometry of the injector. An impingement distance that is 5 times the length of the average diameter of the orifices is recommended by the literature, and fortunately we were able to come close to this factor with an impingement distance of INSERT VALUE.
The other length that we need to consider is the thickness of the injector faceplate. The length of an orifice generally should range between 3-10 times its diameter; for this design we chose 5 for the oxidizer orifices and 4 for the fuel orifices (the only reason why they are different is due to spacing constraints. Using simple trig, this gives us a faceplate thickness of INSERT VALUE inches for the oxidizer, and INSERT VALUE inches for the fuel.
Now that we have our angles and our injector plate thickness, we need to start thinking about how to design a manifold. This is especially tricky since there can only be one inlet for oxidizer due to spacing constraints between the chamber assembly and tank (remember that this needs to fit inside of a rocket) so having two separate annular regions separated by a fuel annulus in the middle would be impossible. Since the length of the faceplate for fuel is smaller than the length of the faceplate for oxidizer, a clever way to design the manifold is to actually make it two parts. The height of the fuel manifold can be designed such that it creates an even surface if you place it inside a groove in the faceplate. Then, you can place the oxidizer manifold above the faceplate and fuel manifold so that the oxidizer circulates above the fuel annulus. This is really hard to explain with words, so here's a photo:
Here, you see that the region in which the fuel circulates (the annulus) is positioned below the region in which the oxidizer circulates. The cross-sectional area of a circulation flow region is optimized at 4 times the area of the orifices contained within that region, which we calculated to be TBD and TBD. The reason why the oxidizer annulus height is so small is because its x distance is very large.
Additionally, you will see that there are screws that screw in the oxidizer and fuel annuli into the faceplate. This is a point of uncertainty; there needs to be cylinders that protrude out of the fuel manifold that contain a screw hole and an O-ring around it to ensure to fluid leaks into the screw threads. Ideally, there is no obstruction in an annulus, so we are currently working on a way to get rid of these cylinders. We will probably just have the screws go through the fuel manifold instead of the ox manifold so that only the screw head would be obstructing flow, not an entire cylinder. However, there also needs to be a sealant to prevent fluid from leaking through the screw; we think that gaskets will do the trick. We are also using gaskets to prevent fluid from leaking from the fuel annulus; originally we had O-rings there, but due to spacing constraints we decided a gasket would be better. Although the screws in the oxidizer annulus don't secure the oxidizer manifold to the faceplate, we can ultimately just place a bunch more bolts radially around the edge of the oxidizer manifold if needed. The screws are also offset from the orifices because they run into each other if not.
Yet another thing you will notice is the placement of the igniter hole. It was quite difficult (if not impossible) to fit O-rings and screws along the faceplate surface to prevent leakage from the igniter exhaust and nitrous. It was discovered that a better way to do this was to extend the faceplate and oxidizer annulus upwards and have the O-rings be radial seals and bolts be radial bolts.
That's pretty much the design – there are many seals because it's important that fuel, oxidizer, and igniter exhaust do NOT mix prior to combustion. There are also O-rings on the bottom of the face plate that prevent combustion gases from leaking through the mating surface between the phenolic and injector; by the way, the outer sides of the injector sit on top of the combustion chamber, but the injector is secured to the chamber by radial bolts (shown in cross section 2 (ADD THIS)). Lastly, the injector face was filleted to prevent stress concentrations.
Here is the Jupyter Notebook used to calculate many of the parameters of the injector.
