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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.
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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 photoare 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.
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