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Carmack's latest on his throatless rocket engine includes a comment:
The fuel stream can only penetrate a short distance, probably under an inch, before it is strongly swept downstream. In a 2" diameter motor this gives decent mixing, but as we get larger and larger, it leaves a huge hole in the center that is basically just pure oxygen with no fuel mixed with it, and an excessively rich mixture near the walls. In the video last month looking down the throat of the 6" ID engine you can actually see a ring of combustion fire around the outside edge and a dark center.
You can see the ring of fire in this video just before the axial camera gets blown away by the rocket. I think I'd try putting the oxygen injectors on the outside and using swirl injectors for better mixing. It just takes milling the injector holes at a different angle. The heavier gasses are uncombusted so they get centrifuged to the outside where the ring of fire is. Of course if he's reliant on reductive coolling putting the oxygen on the outside won't work.
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