The image shows an injector sub-assembly for the direct injection of fuel and oxidiser into a single FIDO combustion chamber (before bright nickel plating).
If it looks like the assembly is made from standard copper pipe brazed together, then you'd be correct - it is.
Rather than using the standard method of injection by forcing the propellant through an injector plate, this design ensures good mixing of the fuel and oxidiser
by forcing the streams into each other at multiple points within the combustion chamber itself.
The end forming the square shape resides inside the top of the combustion chamber and the correct mixing ratios are achieved at five points within this square.
The spark-plug ignition source sits at the centre.
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Injector Assembly
Photo Credit: NAs / Peter Jones
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The sizes of the injector holes themselves are crucial to ensure the correct proportions of fuel and oxisider are present in the combustion chamber for stochiometric combustion.
In the case of our propellant mixture, this is a mass ratio of 1:9.65 F/O.
Fortunately, the hole sizes can be calculated using an equation more often seen in calculating the mass flow through a de-laval rocket nozzle, where the mass-flow rate is equal to:
and solving for the cross-sectional area of the hole, A.
A confounding factor is that the hole profiles must be shaped so that they do not present a square edge to the flow, otherwise choked flow will not occur.
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