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When the Space Shuttle launched, it would perform a roll and then pitch to establish its ascent trajectory. There is fairly straightforward rationale for this. The vehicle's aerodynamic shape, the benefit to keep the horizon visible through the flight deck's upper windows being the most obvious.

Flying long before the Shuttle, the Apollo launch profile has been described in a very similar way - a roll to orient the vehicle so that a pitch maneuver puts it onto trajectory.

My question is this: With no large-scale external features to break the vehicle's apparent radial symmetry, why was it necessary/preferred for Apollo to roll first then pitch instead of forgoing the roll and simply yaw or yaw/pitch? Since the net thrust and drag forces are aligned with the vehicle's long axis, would it make any difference which way the vehicle is oriented in roll? Were the considerations similar to Space Shuttle (despite the capsule windows being covered until escape tower jettison)? Did Mercury and/or Gemini do the same?

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Regarding pitch, are you referring to a gravity turn? Regarding a roll, a few WAGs may be to possibly to reduce crew discomfort, or change alignment from the launch alignment to orbital alignment. I'd love to hear a definite answer. –  user5826 Jul 27 at 18:49
    
I am referring to the initial maneuver off the vertical... as I understand it, the Apollo vehicle launched plumb vertical, cleared the tower, performed a roll while presumably still vertically above the pad, then began to arc its trajectory off the vertical via rotation about its pitch axis. As I understand it, a gravity turn would happen later in the ascent, and would presumably amount to further rotation about the same axis. –  Anthony X Jul 27 at 19:17
    
To my understanding, the initial slight pitch downrange is done prior to getting any significant airspeed/airflow, and then a more significant pitch is done after the atmosphere is somewhat thinner. Another possibility for a roll might be for antenna alignment/improve antenna performance. –  user5826 Jul 27 at 19:24
    
The shuttle rolled and pitched on ascent primarily for antenna exposure to ground stations. There was a project to switch to "heads up" ascent later in the program, but it was never completed. –  Erik Jul 28 at 16:36

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You essentially asked the same question a week ago, Anthony X. The initial orientation of the Saturn V / Apollo stack on the launchpad was fixed. The launchpad had a north-south / east-west orientation, as did the launch vehicle + umbilical tower.

That launchpad orientation was close to but not quite the optimal orientation of the stack for going to the Moon. The optimal orientation would have placed the vehicle with its axes oriented such that all that was needed post-launch was a gradual pitch down to place the vehicle on a gravity turn. That was not possible. The easiest way to achieve that was to have the stack oriented reasonable close to that optimal orientation at launch and then roll shortly after to place the vehicle in that launch-time-specific optimal orientation.

Your problem is your view that the vehicle was apparently axially symmetric. That's just not true, both from an automated guidance perspective and from a human factors perspective. The latter was very important. NASA didn't quite trust their automation in the 1960s. The human crew were an integral part of the failure detection process. The commander had ultimate authority to trigger an abort. He needed to see the horizon. Orienting the vehicle so that the horizon was visible and nominally flat was essential to making he commander's job easier.

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My previous quesion was focused on an attempt to visualize what's going on when watching launch footage; this one is about understanding why it's going on. That Apollo crews had as much need of reference to a visual horizon as Space Shuttle I think is the answer I was seeking. –  Anthony X Jul 27 at 21:33
    
There is also the very simple practical fact that, while a rocket may be symmetric, the layout of the thrusters is not. There is only one pitch axis and it is orthogonal to the direction that the pitch thruster is pointing. Without rolling first, the manoeuvre would require a careful mix of both yaw and pitch. While it may not be obvious, controlling two axes at once is much more difficult than controlling only one. Doing it in a 1->2 step is much simpler and far less prone to error than trying to strike the exact balance of two independent axes at the same time. –  J... Jul 28 at 10:36
    
@J... OK... but wind gusts don't respect the thruster gimbal axes... correcting for a gust would presumably require a mixture of thruster corrections; from a feedback control system point of view, is maintaining a uniform trajectory against upset forces all that different to following through a trajectory adjustment? –  Anthony X Jul 29 at 1:19

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