From an article about the Orion launch abort system*:

Should they need to abort, they'll be subjected to extreme forces. The launch abort motor provides 400,000 pounds of thrust, enough to accelerate the capsule from zero to 800 kilometers per hour in three seconds—and that's on top of whatever speed the launch vehicle is already going. The published numbers on how many Gs this produces varies, but it's somewhere north of 11.

Let's see, now.

Assuming constant acceleration, that's 800000 m/hour, divided by 3600 s/hour, equals 222.22 m/s added by the LAS (once it disconnects from the booster, the booster no longer contributes any acceleration and can safely left out of this *particular* calculation**). Divide by the 3 seconds it takes to reach that added speed: 74.07 m/s/s. Divide by 9.8 m/s/s: 7.56 G.

So where does the "somewhere north of 11" come from? Is the thrust wildly nonuniform? Is the "three seconds" actually much shorter? If the mass of the capsule were handily provided, dividing the thrust thereby ought to give a proper number for acceleration, but it's not mentioned in the article and I'm too lazy to go digging.

(Oh, if they're heading straight up during this process, add 1G, for a total of 8.56, obviously. Still well south of 11.)

* Which does not involve setting off a string of little nukes.

** As can a live dragon.

The 3 second average may be 8.5, but the peak could be over 11.

Posted by: Jeff Bell | Tuesday, 21 October 2014 at 15:39

True enough; that'd be the "wildly nonuniform" thing. I suppose it might make sense to design the motor for extra thrust right at the beginning, for a quick getaway.

Or... hm. We know the Δv, and may guess that the ISP is around that for APCP (call it 220 "seconds"; I hate that unit). That means, for a 222 m/s total boost, the mass of the capsule + rocket system will decrease by roughly 10% during the burn, so with constant thrust the acceleration will indeed vary significantly, peaking right at the end.

Posted by: Eric Wilner | Sunday, 26 October 2014 at 08:57