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Throughout its burn, a rocket gets lighter as propellant is consumed. As a result, acceleration (and g-force) increase. In the shuttle, the SSMEs were throttled back towards the end of the burn according to What G-forces do different launchers cause?

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A: liftoff to throttle-down

B: throttle-down for max dynamic pressure

C: throttle-up to start of SRB burnout

D: SRB burnout and sep

E: constant main engine throttle setting with ET prop burning off

F: main engines throttling back to maintain 3G design limit

G: main engine cutoff and ET sep

According to What are the G forces leaving Earth orbit? Saturn-V first stages were "throttled back" by shutting off the center engine

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Is this throttle-down typical for crewed flights? Uncrewed? To limit airframe loads? I couldn't find verifiable data for Space-X flights.

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  • $\begingroup$ Anecdotally (I can't provide documentation, which is why this is not submitted as an answer), we were told that, at least for ascent, the 3-g limit during the Shuttle days was there for payload considerations. Lots of relatively fragile stuff was hauled in those payload bays over the years... $\endgroup$
    – Digger
    Nov 16 at 3:55

2 Answers 2

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Partial answer submitted as a counterexample for "throttling down is done for humans onboard"

The uncrewed final flight of the Saturn V, the launch of the Skylab, also "throttled down" by shutting down the first stage center engine at 140 seconds.

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Source: SATURN V LAUNCH VEHICLE FLIGHT EVALUATION REPORT -SA-513 SKYLAB 1

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    $\begingroup$ That could just be holdover from the sequencing from the crewed Saturn V missions. $\endgroup$ Nov 15 at 23:31
  • $\begingroup$ Yeah, that's what I'm thinking. They may have never actually developed a non-manned sequence and you don't really want to monkey with that for the one time you aren't going to have crew aboard. $\endgroup$ Nov 17 at 15:18
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I expect the throttle-back was to limit the G-forces on the crew more than anything else. 3g is already pretty uncomfortable for a human, and anything higher could make the crew unable to respond in an emergency. I don't know if the Space Shuttle was mechanically able to launch at higher g-loadings -- the shuttle was designed from the start to always be manned, so once they chose a maximum g-loading for the crew, that would functionally be the maximum for the rest of the system as well (plus a nice safety margin, of course).

That said, unmanned rockets routinely run at 6g or higher (unless the payload demands more gentle handling, which may impact the rocket's performance).

For example, the Falcon User's Guide provided by SpaceX shows the envelope of forces that might be applied to the cargo of a Falcon 9 in section 4.3.1: Graph of accelerations

For a normal payload (black outline), the axial acceleration can go as high as 6g, and for a "light" payload of less than two tons (red outline), it could hit 8.5g.

As another example, the ULA Atlas V user's guide ascent profiles (section 2.2.1) all share similar language:

Near the end of the booster phase, the RD-180 engine is continuously throttled so that axial acceleration levels are not exceeded. These g-levels may be a function of payload weight and do not exceed 5.0 g steady state.

That is to say, a very large load might mean they launch slower, but 5.0g is the maximum sustained acceleration given no other limiting factors, which suggests that's a structural limit.

And I can't find original documents right now, but if I remember correctly, the Delta IV (not Heavy) launched at a sustained 6g, but I'm sure that depends on which specific variant you're looking at, as there were several.

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