Reading this question Why are rocket engines operating above 100% often considered nominal? leads me to a question about throttling back the SSMEs in the later stages of a Shuttle Launch to limit G forces on the astronauts. If the SSMEs were not throttled back, what would have been the anticipated G Forces on the astronauts at the end of the main SSME burn time?

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    $\begingroup$ The 3 g limit during the launch phase was in place for payload and payload berthing reasons, not to limit the forces on the crew. $\endgroup$
    – Digger
    Commented Sep 28, 2017 at 20:06
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    $\begingroup$ @Digger, Upon additional digging, I came across the following answer which backs up your assertion that the G limit was more for payload than crew accommodation, as earlier launches had significantly higher G loadings during launch to space. space.stackexchange.com/questions/7829/… $\endgroup$
    – Milwrdfan
    Commented Sep 28, 2017 at 20:44

1 Answer 1


Stealing the mass-at-MECO number from this answer, and using the 104.5% thrust numbers for 3 SSMEs from here, I get

(3 x (490,000) lbf / 308650 lbm ) = ~ 4.75 g's

Sanity check

(3 x (.65 x 490,000) lbf / 308650 lbm ) = ~ 3.1 g's

Using the approximate 65% power level at shutdown, assuming that the thrust varies linearly with throttle percent (and ignoring that they normally only throttled to 67%) gives the expected approximate 3 g's

Info on nominal throttle settings in the answer to this question: Which STS mission raised the normal engine throttle above 100%, and what change to the SSME made that possible?


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