# How much less fuel do rockets burn with higher G force?

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?

• Define "efficiency". What's your hurry? – Russell Borogove Dec 20 '18 at 20:39
• @RussellBorogove is this better and nice answer. – Muze the good Troll. Dec 20 '18 at 20:48
• – Russell Borogove Dec 20 '18 at 21:10

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.

• John Stapp did survive much more than 7 or 8 g, more than 30 and 40 g. – Uwe Dec 20 '18 at 20:52
• So they made them less efficient for the crew thats nice. How much less efficient are they? – Muze the good Troll. Dec 20 '18 at 20:54
• @Uwe Stapp did ~46g for about half a second. – Russell Borogove Dec 20 '18 at 20:55
• @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
• 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