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@DavidHammen's answer to Engine failure: how to detect? mentions thruster sensors.

This intrigued me and so I thought I'd ask about how this is done.

Thrust is a critical parameter to know in real time, and when there are more than three engines you can't even unambiguously infer each engine's thrust from z-acceleration, pitch and yaw.

Instead you'd like to have a direct measurement.

Question: What are the various ways that rockets measure the instantaneous thrust of each engine? Is it as easy as slapping a standard strain gauge on each strut or mount holding the engine and feeding the numbers into a formula that corrects for geometry? Are there different or perhaps unusual ways this measurement has been made?

Organic Marble
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uhoh
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  • slightly related and as-yet unanswered question about struts: How do all the struts and crosses in the Orion Crew Capsule Service Module work together? What do each of these parts do? – uhoh May 31 '21 at 12:43
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    I mentioned thruster sensors. Thrust sensors (aka accelerometers) may not be a good idea. – David Hammen May 31 '21 at 12:50
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    No one does this that I know of in flight - attempt to directly measure thrust. It's all based on Pc. – Organic Marble May 31 '21 at 12:59
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    @OrganicMarble Mini AERCam did exactly this, and it worked, easily. A puff or two from that vehicle's orthogonal cold gas thrusters made the vehicle respond very nicely. This motivated the managers of X38 to do the same with that vehicle, and I'm sure it would have worked (that may be biased; I wrote the software). NASA/JSC had NASA/Ames look into the problem and develop an alternative frequentist approach to my Bayesian approach. So they paid for my time plus NASA/Ames personnel time plus oversight time. As expensive as chamber pressure sensors are, those sensors would have been cheaper. – David Hammen May 31 '21 at 13:10
  • @DavidHammen you're right of course. I realized after the edit time had elapsed on my comment that I should have said "Pc or acceleration". But the important point is that AFAIK no one measures thrust force directly via strain gauge or whatever. – Organic Marble May 31 '21 at 13:11
  • @OrganicMarble thanks for that; my eyes were simply not picking up the difference between the two words. – uhoh May 31 '21 at 13:26
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    @OrganicMarble for completness, please explain what "Pc" is here – Carl Witthoft Jun 01 '21 at 17:46
  • Possibly helpful discussion here: https://forum.nasaspaceflight.com/index.php?topic=52940.720 – Carl Witthoft Jun 01 '21 at 17:54
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    They right-click on the engine and look for the thrust readout. – Vikki Jun 01 '21 at 22:20
  • @CarlWitthoft Ya I had to scratch my head for a minute on that too. I don't know for sure, but I'm assuming that $P_C$ is the pressure of the combustion chamber. Since rocket motors don't have clutches or transmissions, you can't "rev them up" without producing thrust. If you have a steady chamber pressure within the range that you expect for the current throttle setting (mass flow rates of reaction products), it's hard to imagine a scenario other than that combustion is taking place and exhaust is leaving at the expected rates, and therefore thrust is as expected also. – uhoh Jun 01 '21 at 23:59
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    @CarlWitthoft But it's not going to be a direct or exact thrust determination, thus my question about an independent measurement. See instances of $P_C$ here for example: 1, 2 – uhoh Jun 01 '21 at 23:59

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