# Purpose of Initial yaw Maneuver?

I've read the Saturn V began launch with a short yaw maneuver which lasted from t+1 through t+8.25.

The purpose mentioned for the maneuver is to avoid contact with the launch tower.

But now that I think about it, I struggle to see the logic of the maneuver.

The yaw rotation would occur about the center of mass, which starts out fairly high up. The yaw would then push the lower half of the rocket one way WRT to the tower while the upper half goes the other way.

(Imagine picking the rocket by its center and giving it weak lateral push from the bottom---the center of mass stays approximately where it is while the bottom and top fling in opposite directions.)

Because the rocket is heavy, the center of mass budges noticeably to the side only after a substantial delay of several seconds, by which point that center of mass is practically at the height of the top of the tower.

But by then the maneuver seems of little use, since most of the rocket is already clear of the tower. Nor does the maneuver seem to have been of much use before that point, since the center of mass hadn't yet deviated substantially from the length axis of the tower...

...Unless they were specifically trying to avoid contact with the very top of the tower, in which case the maneuver makes perfect sense...

The yaw rotation would get the top out of the way, and by the time the center of mass reached the top of the tower, it too would have shifted at least a few feet from the length axis... and finally when the bottom of the rocket reached the top of the tower, it too would have shifted enough to the side to avoid a collision.

Sorry for the possibly dumb question, but suddenly this seemingly logical maneuver is stumping me.

Thanks if you can clarify!

The center of mass of the Saturn V at liftoff is somewhat lower than you might expect -- 27 meters up, about a quarter of the way up the stack. This is because the upper stages are largely filled with liquid hydrogen, which is much less dense than the fuel and oxidizer in the first stage. Thus the yaw moves the upper end of the rocket much further than the lower end.

The yaw maneuver is also very small in angle -- a maximum of about a degree and a half, so the lower end of the launcher doesn't swing very far inward at all. If it turned instantaneously to 1.5º, that would be a movement of ~0.7 meters at the base; if it turns more gradually the maximum would be lower.

I've always understood that the purpose of the maneuver was to avoid the (very improbable) potential for hitting the tower if a significant wind gust or other disturbance pushed the stack towards the tower, but I'm not sure where I came up with that. My belief is that there would be no tower contact if the rocket went perfectly straight up.

• Would a yaw maneuver make sense on a rocket filled completely with RP1 and LOX, which have a similar density (roughly)on the order of 900 kg/m^3? A Falcon 9, say? I'm sure they can do without a yaw maneuver these days, but if they wanted to do it, I wonder if a higher center of mass would make the maneuver less useful?
– user36480
Jan 8, 2021 at 19:47
• Thanks for the info, BTW, Russel.
– user36480
Jan 8, 2021 at 19:49
• No, the yaw axis is towards/away from the tower. The tower is north of the rocket; yaw is north/south, pitch is east-west. Jan 8, 2021 at 20:19
• Yes, should have said "yaw rotation takes you towards/away from the tower". Thanks for linking that page, by the way -- I was about to add it to my comment after I double-checked my understanding, fell afoul of the 5 second rule, then got distracted by work. Jan 8, 2021 at 20:46
• Thank you a ton for making me realize my mistake, @RussellBorogove. I would not have known how wrong I was otherwise. So it does seem that yaw will move the tail toward the tower on launch... though clearly the yaw is small enough that the tail doesn't contact the tower. This also better matches what I think I see in video of Saturn V. Thanks again.
– user36480
Jan 8, 2021 at 22:23