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are those 4 separate engines or are they fed by common turbopump machinery?
Separate engines. The LM-2 and 3 series with boosters also use them in each booster unit with minor modifications. The 2nd stage engines of all LM-2/3/4 series uses a vacuum lit version of it too, though with verniers for attitude control & more thrust.https://en.wikipedia.org/wiki/YF-20
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Is this engine derived from Soviet tech or entirely homegrown?
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No known link to the Soviets. Remember that the two countries broke ties around 1960, some years before the Chinese started developing the DF-5 ICBM for which these rockets are based on.
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