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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?

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  • $\begingroup$ 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
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    $\begingroup$ 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

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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.

Edit:


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

References

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

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    $\begingroup$ 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
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    $\begingroup$ 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
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    $\begingroup$ The shuttle limits were "knocked down" for uncertainties. Hopefully Apollo's were too. $\endgroup$ Jun 23, 2018 at 21:45
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    $\begingroup$ 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
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    $\begingroup$ IIRC for DOLILU on shuttle it was psf-deg. Still away from home, can confirm later. $\endgroup$ Nov 15, 2019 at 2:49

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