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All of the resources I've managed to track down thus far about pintle injectors concern engines which only use a single, large central pintle for the entirety (or vast majority) of propellant supply to the chamber. This includes resources regarding the LMDE, Merlin, and other examples. Almost all sources agree that the single-pintle configuration can have combustion efficiency and throat erosion issues, due to the injection dynamics. Betts & Frederick (2010) indicate that this is, in part, due to recirculation zones that can form behind the injection surface.

Is there a compelling reason why the pintle injector concept has only ever been used in a single-injector configuration? It seems intuitive that multiple injector units would serve to increase the proportion of the chamber downstream of the surface, as well as potentially improve efficiency through the interaction of each propellant cone.

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Although I don't know why single pintle injector configurations are more widely used and presumably preferred, the TR-108 by TRW (Which manufactured the LMDE) used multiple pintle injectors :

https://twitter.com/dirg_rocketdyne/status/1343004145532297218?lang=gl

I'm guessing the there is added manifold complexity as well as complexity in controlling the movable sleeves.

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