Responsible Engineer: Bayanni Rivera , MIT AeroAstro '27
Injector Subteam members: Jordan Bergmann, MIT AeroAstro '28; Eddy Chen, MIT AeroAstro '28; Ethan Lai, MIT AeroAstro '28; Joey Liu, MIT AeroAstro '28
Current injector for Polaris. Type: Unlike impinging triplet
The injector is responsible for taking in propellant and injecting it into the combustion chamber. It must 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 5 triplet elements positioned radially around the injector faceplate. Each triplet is composed of two 1/8 oxidizer holes and one 5/64 fuel hole. This configuration was obtained by iterating the nitrous drain tank model towards the injector orifice area that would result in a mixture ratio and mass flow rate close to our target. 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.
Polaris Injector Pattern: 5 Groups of Impinging Triplets. I like to call it the star injector!
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 are multiple photos:
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 an annulus is optimized at 4 times the area of the orifices contained within that region, which we calculated to be INSERT VALUE and INSERT VALUE. Assuming the propellant is incompressible, changing this flow area only changes circulation velocity (how fast the propellant travels radially around the annulus). A high circulation velocity should be avoided, as it increases the risk of propellant traveling unevenly through the orifices. That is, if circulation velocity is high, the propellant will have a lot of inertia, and as a result it might become pinned to one side of the orifice as it travels through it, impeding atomization and mixing.
Additionally, you will see that there are screws that screw in the oxidizer and fuel annuli into the faceplate. As expanded on later in the bolt calcs section, we determined that 16 bolts were necessary to withstand the pressure from the oxidizer manifold, and 10 bolts were necessary to withstand the pressure from the fuel manifold. Well actually, that's a lie – the 10 bolt configuration in the fuel manifold was more so necessary from the geometry of the injector itself; the screws must be offset from the orifices, otherwise these holes will run into each other. Therefore, since there are 5 groups of triplets, there must also be 5 pairs of screws offset from these triplets. The number of triplets also influence the number of radial bolts screwing into the faceplate, as if they are not a multiple of 5, they will run into the orifice. Since 5 does not provide the desired factor of safety, 10 radial bolts were chosen as the number of bolts securing the injector to the combustion chamber.
There are two aspects about this design that seem to be suboptimal. However, we do not know the extent to which these "aspects" will negatively impact our combustion performance. First, there are cylindrical screw heads in the oxidizer manifold region. Now, assuming the propellant is incompressible (which it may not be due to issues like cavitation, which will be discussed in another section), the presence of these screw heads will slightly increase the speed of the oxidizer as it flows around the heads; however, after the screw heads, the oxidizer will return to its original speed. This means that, assuming the fluid is incompressible, the flow of the oxidizer will be the same at each oxidizer orifice cross section, so the mixing and atomization of each oxidizer group should be the same, right? Well, yes, if not for the huge blue cylinder in the middle of the oxidizer manifold. This cylinder just needs to be here – it's unavoidable. It is the fuel inlet, or where the fuel passes through to enter the fuel manifold. What this cylinder entails is likely a bit of discrepancy between injector sprays at each element, but it is impossible to compute the extent of this discrepancy without high-fidelity modeling software. Thus, this is something we have to test during cold-flow testing.
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 instead of face seals.
That's pretty much the design! A couple more miscellaneous things are our O-ring selections – we are using a #00 series O-ring for the fuel downcomer line, a #100 series O-ring for the radial oxidizer-faceplate seal, and three #200 series O-rings for our phenolic-faceplate, chamber-faceplate, and manifold-faceplate seals.
Here is the Jupyter Notebook used to calculate many of the parameters of the injector.
