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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?

Russell Borogove
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  • 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) – Antzi Jun 22 '18 at 05:35
  • 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. – GdD Jun 22 '18 at 07:28
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    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. – Russell Borogove Jun 22 '18 at 12:42

1 Answers1

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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

Organic Marble
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  • Okay, but "this rule applied when 100% of the limit had been reached" seems tautological -- the mission rule says the limit is 100% of the limit. I assume the actual Q-alpha information is in some other doc somewhere, but this still seems weird. – Russell Borogove Jun 22 '18 at 21:36
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    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. – Organic Marble Jun 22 '18 at 21:39
  • Will try a rewrite. – Organic Marble Jun 22 '18 at 21:47
  • That helps a bit, thanks. I guess if the only direct application of Q-alpha is the abort limit it makes sense to track it a %age of the limit since it otherwise has somewhat odd units. – Russell Borogove Jun 22 '18 at 22:01
  • For shuttle these limits were primarily used in testing trajectory designs prelaunch, not for aborts, I was surprised to learn that Apollo used them that way. – Organic Marble Jun 22 '18 at 22:29
  • @RussellBorogove I found support for my speculations in the AS-503 flight manual, edited answer. – Organic Marble Jun 23 '18 at 20:39
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    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! – Russell Borogove Jun 23 '18 at 21:44
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    The shuttle limits were "knocked down" for uncertainties. Hopefully Apollo's were too. – Organic Marble Jun 23 '18 at 21:45
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    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? – Russell Borogove Nov 15 '19 at 02:06
  • Right about q-beta. I don't know any more about Apollo limits than what I wrote here. – Organic Marble Nov 15 '19 at 02:29
  • Any units given for those squatcheloid plots? – Russell Borogove Nov 15 '19 at 02:33
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    IIRC for DOLILU on shuttle it was psf-deg. Still away from home, can confirm later. – Organic Marble Nov 15 '19 at 02:49
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    @RussellBorogove back home and checked my logs from the STS-114 launch. The units for shuttle were indeed pounds-force per square foot times degrees. – Organic Marble Nov 15 '19 at 12:10