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The second stage of Proton (8K-82K) rocket has two types of main engines (RD-0210 and RD-0211). Does someone knows why?

Proton rockets - the first version, 1965, and the recent M version.

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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.

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    $\begingroup$ The heat exchanger is used to pressurize the propellant tanks; presumably only one is needed, so leaving it off 3 of the 4 engines is weight-saving? $\endgroup$
    – Russell Borogove
    Jun 17, 2015 at 19:22
  • $\begingroup$ At least on SSME the heat exchanger was a scary thing; best to leave it off if you can. $\endgroup$
    – Organic Marble
    Jun 18, 2015 at 1:33
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    $\begingroup$ what makes me wonder is why just one engine has a heat exchanger. I would expect 2 engines. $\endgroup$
    – Junior Miranda
    Jun 18, 2015 at 14:23
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    $\begingroup$ @JuniorMiranda: This design decision suggests the proton upper stage can't tolerate an engine out anyway. $\endgroup$
    – Joshua
    Jan 23, 2019 at 17:09

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