The second stage of Proton (8K-82K) rocket has two types of main engines (RD-0210 and RD-0211). Does someone knows why?
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They are the same basic engine, just the RD-0211 has a heat exchanger, that is lacking on the RD-0210.
The long history of the Proton went through three or more major revisions, and the engines were renumbered in each revision, as a pair. Each time, one with heat exchanger, one without.
There is much more detail on Wikipedia about the RD-210 Engine.