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Blue Origin says the BE-3PM engine in its New Shepard vehicle can throttle down to 18% max thrust.

Compare to 57% for the Merlin 1D engine in stage 1 of SpaceX's Falcon 9 (and to 39% for the Merlin Vacuum engine in stage 2, though stage 2 doesn't have to land, and its on landing approach that low throttle would most help).

So I'm wondering: What are the challenges in increasing throttle range, and what keeps SpaceX from matching Blue Origin's 18%?

The BE-3 runs on liquid hydrogen while the Merlin runs on RP-1. Does the choice of fuel account for the difference? And could it be the big reason why Blue Origin chose that difficult fuel for New Shepard?


There is some background information available in answer(s) to Do Blue Origin's BE-3 engines need to run for 7 seconds to "warm up"?

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    $\begingroup$ SSME: LH2, min throttle 65% (later upped to 67%). $\endgroup$ May 8, 2021 at 3:08
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    $\begingroup$ Oh wow. That really puts Blue Origin's 18% in perspective. And I guess the choice of hydrogen by itself wouldn't magically increase your throttle range. $\endgroup$
    – user39728
    May 8, 2021 at 3:20
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    $\begingroup$ Another example, the RS-68 is a gas-generator cycle (like the Merlin) burning liquid hydrogen fuel, with a minimum throttle of 58%. $\endgroup$ May 8, 2021 at 4:09
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    $\begingroup$ Could it be the physical scale(size) of the engine? I'm thinking of Reynolds number type effects in the exhaust flow but I am no expert on that. $\endgroup$ May 8, 2021 at 15:00
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    $\begingroup$ Apparently the engine can throttled using a combination of all three valves. $\endgroup$
    – uhoh
    May 9, 2021 at 3:58

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