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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
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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 an annulus is optimized at 4 times the area of the orifices contained within that region, which we calculated to be TBD INSERT VALUE and TBDINSERT 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 This is a point of uncertainty; there needs to be cylinders that protrude out of the fuel manifold that 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 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. instead of face seals.
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. ! 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.
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