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
The injector is responsible for taking in propellant and injecting it into the combustion chamber. It needs to 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 each triplet 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.
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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 an annulus 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.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
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.
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