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the question Is it possible to throttle down the thrust in a rocket engine to 1% just like in KSP? discusses the ability of liquid fuel rockets to throttle. By convention, does the “% throttle level” refer to fuel flow, or thrust? I assume there is a very non-linear relationship between the two at low fuel flow due to excessive throat area and over-expansion. But I cannot find published data on this non-linearity.

Bonus answer: at minimum throttle, do Merlins run rich and throttle only the oxidizer?

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For the Space Shuttle Main Engine, "% Throttle" (actually in shuttle vernacular, % Power Level), referred to the chamber pressure. It was the desired value of chamber pressure that was used in the control loop when a throttle command was received by the engine controller.

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(from SSME DIGITAL CONTROL DESIGN CHARACTERISTICS, emphasis mine)

This engine was certified for a throttle range of 109% to 67%. Over that range its performance was quite linear.

This graph shows vacuum thrust, oxygen inlet flow, and hydrogen inlet flow plotted as a percentage of their values at 100% power level, versus power level. To the naked eye, there is no deviation from linearity.

enter image description here

If we subtract the values plotted here from a perfect straight line, we can see there is in fact a tiny deviation from linearity.

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Data extracted from a document produced by the Johnson Space Center Booster Systems Section in June 1997. I do not find the document online, so here are images of the relevant portion.

enter image description here

enter image description here

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  • $\begingroup$ Great information. Any idea what happens when the engine is throttled beyond 67%? $\endgroup$
    – Woody
    Dec 5, 2022 at 19:00
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    $\begingroup$ @Woody there's a paper ntrs.nasa.gov/api/citations/19970028361/downloads/… where they got it down to some ridiculously low power levels (17%) It's a different (older) version of the engine than the data in my answer comes from. $\endgroup$
    – Organic Marble
    Dec 5, 2022 at 19:07
  • $\begingroup$ Great document. Figure 3 shows a run down to about 8% throttle (labeled TTB062) which Peters out before the end of the 17 second test. The text seems to imply this throttle setting failed due to fuel pump boil-out. Did I read that right? $\endgroup$
    – Woody
    Dec 5, 2022 at 19:17
  • $\begingroup$ so, is the answer to the OP "% throttle is referring to neither fuel flow nor thrust, but rather to chamber pressure." ? $\endgroup$
    – Woody
    Dec 6, 2022 at 3:22
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    $\begingroup$ @Woody Yes, but a fairly good approximation is that chamber pressure, thrust, and mass flow rate are all proportional (with some assumptions that pretty much boil down to "engine designed to be as efficient as possible at all power levels"). Chamber pressure is much easier to measure than thrust. $\endgroup$
    – TooTea
    Dec 6, 2022 at 9:08

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