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How does G force at launch differ between spacecraft when it comes to efficiency between crewed and uncrewed space flight? Does less time in the atmosphere equate to a savings in fuel for an interplanetary launch? Could the common person or seasoned pilot safely withstand more then 3Gs if the burn time is also less?

Muze
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Historically, Atlas and Titan boosters used in the Mercury and Gemini program subjected astronauts to as much as 8g on ascent for brief periods. Since the astronauts didn't need to do much during ascent, this wasn't a big problem; if they greyed out or even blacked out momentarily, the booster would continue obliviously on. These were practiced test pilots and combat pilots used to functioning through high-g maneuvers, but I believe any reasonably healthy civilian could survive a couple of short blackouts at 7g or 8g.

Later boosters purpose-built for crewed flights traded off a small amount of delta-v capability for crew comfort; Saturn-Apollo topped out at 4g and Shuttle at 3g.

The Titan II Gemini launch vehicle expended about 8950 m/s of delta-v to reach orbit; the Saturn V expended about 9200 m/s, so dropping from 8g peaks to 4g peaks incurred slightly less than a 3% ∆v efficiency penalty.

Due to the nonlinearity of the rocket equation, the difference in fuel mass is less than 3% (and depends greatly on the particular design of the launcher). For a two-stage-to-orbit launcher, all other things being equal, the fuel mass needed for an 8g peak ascent is probably about 1% more than needed for a 4g peak ascent.

Russell Borogove
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    John Stapp did survive much more than 7 or 8 g, more than 30 and 40 g. – Uwe Dec 20 '18 at 20:52
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    So they made them less efficient for the crew thats nice. How much less efficient are they? – Muze Dec 20 '18 at 20:54
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    @Uwe Stapp did ~46g for about half a second. – Russell Borogove Dec 20 '18 at 20:55
  • @Muze Added that information to the answer. – Russell Borogove Dec 20 '18 at 21:07
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    @Muze, it is not just the crew, if you halve your peak G loading you halve the thrust loads on the rocket and related equipment and can make them lighter, trading gravity losses for less structural mass/more fuel. – GremlinWranger Dec 20 '18 at 22:28
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    I think you should elaborate on why you need only 1% more fuel to get 3% more $\Delta v$ – asdfex Dec 22 '18 at 15:57
  • Re the observation about being accustomed to high-g maneuvers: It is worth mentioning that the vast majority of high-g maneuvering in fighter aircraft is performed such that the acceleration (or g) vector is pointing roughly down the pilot's spine, whereas, during a rocket launch, said vector points laterally through the chest and straight out through the back...I personally feel that the first case is much less uncomfortable... – Digger Jan 22 '19 at 17:43
  • @Digger -- My understanding is that transverse g-force is physically tolerated better and is less likely to induce G-LOC because blood isn't being drawn away from the head, and that the F-16 seat is reclined 30º to improve g tolerance. Is this a distinction between "less uncomfortable" and "less dangerous?" – Russell Borogove Jan 22 '19 at 18:18
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    @RussellBorogove Yes, I should have mentioned that the chances of G-LOC are less, perhaps much less, when considering the launch vehicle case vs. the fighter jet case. In this respect, the launch case can be considered less "dangerous." I do, however, stand by my "comfort" assertion...I assure you that, if you were able to find a car that could accelerate at 3 g's for 5 minutes, you'd find that just the act of breathing would become tiresome as you endured said acceleration event... – Digger Jan 23 '19 at 19:15
  • If you don't mind me asking, is that more comfortable g-for-g, or does being on your back at 3g in shuttle feel worse than 5-6g in an F-16? – Russell Borogove Jan 23 '19 at 19:22
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    @RussellBorogove Good question. One aspect I neglected to address was the temporal component associated with the two g loading scenarios under discussion...and I'd say that, "g-for-g," the case of being on your back is less uncomfortable. It's just that the g loading during launch is applied over a scale of minutes, while g loading in a fighter aircraft situation is applied over a scale of seconds. But, yeah, three transverse g's is tougher to endure for five straight minutes than 9 g's is for five straight seconds (even though the odds of G-LOC are much greater in the second case). – Digger Jan 26 '19 at 16:38