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With Merlin-1d being able to throttle down from its full thrust potential, this post about its throttle capability does not mention much of its change in performance. On the Wikipedia page of Merlin-1d, there is a piece that says

the Merlin 1D Vacuum can throttle down to 39% of its maximum thrust, or 360 kN (81,000 lbf).

While this on the surface may seem like a "duh" definition question, especially with the quote above, but does 70% literally mean 70% of its 914 kN (vac.) 100% ability?

Thanks

UndefinedUsername
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  • @uhoh Am I anywhere close with throttle thrust? Is 70% of 914kN simply 639kN? I don't actually believe it is just that simple to be true. – UndefinedUsername Dec 30 '18 at 01:17
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    Oh -- are you just asking if throttle percentage is the same as thrust percentage? – Russell Borogove Dec 30 '18 at 01:21
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    I don't know for sure, but I would guess that the 39% and 70% are relative to whatever the thrust happens to be at the altitude where the engine happens to be. If the throttling happens at sea level, then it's 39% and 70% of 845 kN. – uhoh Dec 30 '18 at 01:22
  • @RussellBorogove I am mainly looking to find out how throttling effects the engine in terms of output. With apologies to repetition, I am just looking to see how accurate percentage relates 0-100%. – UndefinedUsername Dec 30 '18 at 01:34
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    @UndefinedUsername I think throttling figures are normally given as percentage of maximum (or nominal) thrust. Propellant flow rates (the actual thing being throttled) shouldn't be too far off the same percentage, but probably not exactly the same. – Russell Borogove Dec 30 '18 at 01:59
  • Yeah, it is all comes down to 'nitty-gritty' figures. It doesn't help that SpaceX likes to be especially secretive. – UndefinedUsername Dec 30 '18 at 02:19

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For the SSME throttle % means % of rated chamber pressure (Pc). % thrust isn't a good metric because it varies with altitude. Thus your % throttle would increase during ascent even though you changed nothing in the engine (if you based it on thrust). Vacuum thrust, flowrates, etc were roughly proportional to % Pc for the SSME.

I would be surprised if all engines didn't base it on Pc. That's what is going to be in the throttle control loop most likely - it's a lot easier to measure than thrust. Some of the SSME documentation called it the "thrust control loop" - but the parameter really measured & controlled was Pc.

Organic Marble
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  • The quote is fairly specific: "the Merlin 1D Vacuum can throttle down to 39% of its maximum thrust." Search for "%" in the quoted article. In this case do you think the article is always talking about chamber pressure and mis-using the therm "thrust" or have they perhaps done the work and gotten it right? https://en.wikipedia.org/wiki/Merlin_(rocket_engine_family) – uhoh Dec 30 '18 at 09:03
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    I can't think of a practical way for the engine control system to measure thrust. – Organic Marble Dec 30 '18 at 09:29
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    Well there could simply be some kind of strain gauges on the motor mounts (or however to say that in rocketspeak) or a look-up table with Pc and maybe flow rates based on ground tests to get it approximately, but I think the OP is just asking about how to interpret a number in Wikipedia, not how to build a rocket, but maybe I'm missing something. – uhoh Dec 30 '18 at 09:40
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    "39% of its maximum thrust." sounds like an Elon tweet, i.e. he has to skip lots of specifics to fit his message in 140 characters, and it's aimed at a layman audience. It's entirely possible "thrust" is shorthand/simplification of "chamber pressure". – Hobbes Dec 30 '18 at 11:47