In Apollo Mission Rules and Saturn V Flight Manuals, I have seen the following abort rules:

enter image description here

I understand what most of this means -- if the launch vehicle begins turning at more than 4 degrees per second, abort is indicated, and so on.

However, in the Max Q region (50 seconds to 2 minutes after launch, so really the "high Q" region), an angle of attack of "100%" is grounds for abort. I'm used to angle of attack meaning the angle between the vehicle's long axis and the airstream, and given in degrees, not in percentage.

How should I interpret "Angle of Attack (Q α) = 100%" in this document?

  • $\begingroup$ It's quite common to mesure slopes in %. In this case it would mean for 1m traveled sideways, 1m traveled up (ie 45 degrees inclination) $\endgroup$
    – Antzi
    Jun 22, 2018 at 5:35
  • $\begingroup$ I don't think that's it @Antzi, it's not a slope, in this case 100% would mean a 90 degree angle to the relative airflow. I don't think that would be possible 50 seconds after liftoff, the spacecraft would have disintegrated before it could reach it. My guess is that 100% is actually in this case the maximum aerodynamic pressure the spacecraft could tolerate as a result of having an angle of attack, which was supposed to be 0 deg at all times. $\endgroup$
    – GdD
    Jun 22, 2018 at 7:28
  • 1
    $\begingroup$ 100% would be 45º (1:1 rise:run), not 90º, but you're right, there's no way the abort limit would be 45º at Max Q. $\endgroup$ Jun 22, 2018 at 12:42

1 Answer 1


It is not angle of attack that the rule refers to. It is Q-alpha which is the dynamic pressure times the angle of attack. The shuttle had pre-calculated Q-alpha and Q-beta structural limits; apparently Apollo did too, and this rule applied when the instantaneous value of Q-alpha reached or exceeded 100% of the pre-calculated structural limit.


This Q-alpha value was actually displayed to the crew on a meter. (On orbit the meter served as the Service Propulsion System chamber pressure meter.) The description of the value shown on the meter is:

the qa display is a pitch and yaw vector summed angle-of-attack/dynamic pressure product (qa). It is expressed in percentage of total pressure for predicted launch vehicle breakup (abort limit equals 100%).

enter image description here enter image description here


Orbiter simulator graphics for high-res closeup

Saturn V Flight Manual SA-503 for description and panel image

End Edit

Additional info:

The shuttle rules were shown as plots of Q-alpha versus Q-beta, indexed by Mach number. The plots had the strange name of "squatcheloids".

Here is a sample Shuttle squatcheloid showing the Q-alpha and Q-beta limits at a given Mach number.

enter image description here

Source of image

  • 2
    $\begingroup$ I assume the controller had a display showing the % limit reached. Abort at 100% or above. ln other words, 50% of the structural limit is ok but above 100% of the limit things are gonna break. Makes perfect sense to me but it's the world I came from. $\endgroup$ Jun 22, 2018 at 21:39
  • 3
    $\begingroup$ Great find! Interesting/terrifying that the abort limit is equal to the predicted structural failure value -- if you haven't fallen apart yet, try to keep going! $\endgroup$ Jun 23, 2018 at 21:44
  • 1
    $\begingroup$ The shuttle limits were "knocked down" for uncertainties. Hopefully Apollo's were too. $\endgroup$ Jun 23, 2018 at 21:45
  • 1
    $\begingroup$ I assume Q-beta is dynamic pressure times sideslip angle? And I assume STS had to consider Q-alpha and Q-beta separately because of the significant differences in vertical and horizontal geometry of the shuttle stack, while the Saturn V could use a single "pitch and yaw vector summed angle-of-attack" figure because it was rotationally symmetrical? $\endgroup$ Nov 15, 2019 at 2:06
  • 2
    $\begingroup$ IIRC for DOLILU on shuttle it was psf-deg. Still away from home, can confirm later. $\endgroup$ Nov 15, 2019 at 2:49

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.