This topic: What G-forces do different launchers cause? indicates that current satellite launchers are limiting peak acceleration to about 4g. I'm pretty sure the STS (Shuttle) did the same. My recollection from the Apollo days is that they threw 6g and 8g figures around for peak launch accelerations, or possibly peak re-entry forces.

  1. What are typical peak accelerations of manned launchers? Soyuz, planned SLS (Orion), planned Dragon? What about re-entry?
  2. What were values in the 60's? What was the astronaut experience then? How critical was it that we reduce the forces?
  • 1
    The Space Shuttle was limited to 3 g for all phases of flight, for payload design/carriage reasons. – Digger Sep 29 '17 at 3:52

Ascent G-Forces

The Apollo 11 AS-506 launcher flight report contains a nice graph of the G-force curve of that famous Saturn V launch:

enter image description here From this chart you can see that, off the pad, the Saturn V first stage is doing about 1.2g; this climbs rapidly as atmospheric drag falls and fuel mass is consumed. The center engine is intentionally shut down to limit acceleration, and the outboard four keep pushing to a max of about 3.9g. This is the highest acceleration in the mission until re-entry and landing.

The upper stages are less dramatic in their acceleration but follow similar increasing curves; the second stage curve steps down once for the center engine cutoff and once again when the fuel-to-oxidizer ratio is switched ("EMR Shift" on the graph, for Engine Mixture Ratio) -- this is done to optimize Isp in vacuum, with the timing dynamically chosen to ensure simultaneous depletion of fuel and oxidizer. The second stage center engine early cutoff is done to reduce longitudinal (pogo) vibrations rather than to limit acceleration; this was instituted starting with the Apollo 10 flight.

The third stage doesn't use all its fuel in this portion of the mission; most of the fuel load is for the later lunar injection burn, and that's why its acceleration curve is so flat in comparison to the others.

Mercury-Atlas missions were more dramatic: 1.35g off the pad, peaking around 7g just before the booster engines shut down and dropped away, climbing again to almost 8g before the sustainer ran out of fuel.

Here's Mercury-Atlas 7: Acceleration time series plot from 1.4g at 0:00 to 2.1g at 0:55, then in a steepening curve up to 6.8g at booster cutoff at 2:10; rising again from 1.3g to 7.8g at sustainer cutoff at 5:10

Gemini-Titan peaked above 7g on the second stage. Here's a plot from the Gemini VIII mission report:

g force time series plot, increasing from about 1.25 g at liftoff in an inverse-linear curve to booster cutoff at around 155 seconds, 5.5 g, rising again from 1.35 g at second stage ignition to nearly 7.5g at second stage cutoff at around 335 seconds

Both Atlas and Titan were designed as ICBMs, so not really optimized for human comfort.

The Space Shuttle was much more gentle in comparison; at solid booster burnout it reached the first peak of 2.5g, briefly falling a bit below 1g then slowly picked back up to 3g on the main engines; the mains were repeatedly throttled down to hold about 3g for a little over a minute.

I think Soyuz does under 4g on launch.

Other things being equal, a higher-g launch can be more fuel efficient, because less energy is lost to gravity by getting to orbit more quickly, and gravity losses normally dominate over drag losses. Keeping STS down to 3g was a challenging design goal - it's hard to build deep throttling capability into an engine, but the shuttle was designed to carry (relatively fragile) scientists rather than ex-military fighter jocks. Soyuz is a bit of a compromise there.

Falcon 9 starts at about 1.15g, and depending on payload would have a first-stage peak acceleration of around 4.5g, but it appears to throttle its engines back toward the end of the first-stage burn to maintain closer to 3.5g.

Re-entry and landing G-Forces

I haven't found a good time series graph of reentry force, but the peaks are relatively brief -- force increases as the capsule descends into denser air, but decreases as the capsule slows, so the higher the decelerating g-force, the shorter it's going to last.

Mercury astronauts took about 11g peak force on re-entry, Apollo about 6g, space shuttle about 3g.

Again, Soyuz does about 4g here, I think.

There may be a pretty good jolt at touchdown/splashdown, too. Some of the Apollos hit rising waves at the end of the ride for very brief 15g bump.

STS and Soyuz g-forces are necessarily low, again, because they carry civilian crews. In the case of the shuttle, again, it's a major design consideration: the gentle re-entry means the ship has to deal with a prolonged period of high thermal load, which requires fancy and vulnerable ceramic tiles rather than a simple ablative heat shield.

  • The center motor was not shut down to limit acceleration. It was shut down to limit Pogo-vibrations that could damage the ship. – Gunnar Øyro Aug 23 '17 at 19:51
  • The first stage center shutdown is for acceleration limiting according to the flight manual: "S-IC center engine cutoff occurs at 2 minutes 5.6 seconds after first motion, to limit the vehicle acceleration to a nominal 3.98 g." The second stage center shutdown is a pogo-control measure. history.nasa.gov/afj/ap08fj/pdf/sa503-flightmanual.pdf – Russell Borogove Aug 23 '17 at 21:34
  • "a higher-g launch can be more fuel efficient, because less energy is lost to drag and gravity by getting to orbit more quickly": Wouldn't higher-g launch mean MORE energy lost to drag? Higher acceleration means higher speed while low in the atmosphere and thus higher drag. Energy loss to drag is the same as its (negative) work, which is the integral of its force (or, rather, its projection on the velocity vector) over the path. The length of the path in the atmosphere doesn't depend on acceleration, but higher acceleration means higher force. – Litho Aug 24 '17 at 7:45
  • @Litho - whoops, good catch; I've corrected that. Note that gravity losses usually dominate over drag losses (by around 20:1 for Saturn V, for example), so the conclusion is the same. – Russell Borogove Aug 24 '17 at 16:10

To answer your second question on the astronauts' experience and how much thought went into adjusting the g-force profile of a launch, NASA published a document that contains information on the g-force survivability range of a human.

Here is the Paper, the relevant figure you want is Figure 5 which is about halfway down the page. The figure is a plot of g-forces in the y axis and time in the x axis with highlighted regions of survivability.

enter image description here
Fig. 5 - Human time-tolerance: acceleration

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