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It appears during most launches that the only engines working are the main first stage engines. It was my understanding that these engines are fixed in place and cannot turn the rocket, but I must be mistaken. What I am looking for is the mechanical system in place that slightly shift the engine direction so that the rocket can turn.

When a rocket launches and it begins to tilt into its orbit, how does it control this? Are the first stage engines able to move slightly to move the vehicle down range, or is there some other type of control that keeps the launch vehicle from heading directly upwards?

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Most rockets use gimbaled engines. They generally turn a few degrees and redirect their thrust. Not unlike driving a front wheeled car backwards.

The Space Shuttle is a good example of this. You can even see the gimbal test in the count down.

http://en.wikipedia.org/wiki/Gimbal#Rocket_engines

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  • $\begingroup$ One exception for a non-gimballed vectored thrust (well, sorta) was Merlin 1a that used turbopump exhaust for roll control. $\endgroup$
    – Eren Güven
    Commented Mar 19, 2016 at 20:03
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Note that even in the absence of any need to turn, there must be some method to stop the rocket from turning. The thrust of the rocket is directed along a specific line from aft to forward. This line must pass through the center of mass of the rocket; otherwise, the thrust will exert a torque around the CofM, and the rocket will tumble.

As the fuel and oxidizer are consumed, the CofM moves from side to side and the rocket thrust no longer acts through the center of mass, and the rocket begins to tumble out of control. The rocket nozzle twists slightly in just the right direction to stop the rotation, rotate back to eliminate the accumulated rotation, and then stop to direct the thrust through the new CofM. Repeat multiple times per second.

Picture balancing a looong broom stick on your finger tip. Now imagine that the broomstick is alive, hurricane winds are blowing, and an earthquake is occurring...

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  • $\begingroup$ I love that broom analogy. How can they adjust the engines so quickly? Hydraulics? $\endgroup$
    – Stu
    Commented Nov 19, 2013 at 14:57
  • $\begingroup$ I think computer controlled hydraulics moving only the nozzle... $\endgroup$
    – DJohnM
    Commented Nov 19, 2013 at 20:06
  • $\begingroup$ The broomstick is a bad analogy. It's a variant of the Pendulum fallacy. With the broomstick, the thrust from your finger is always vertical, so any deviation of the broomstick from vertical will create an increasing moment due to the horizontally projected distance from center of mass to thrust point. Not so for a rocket with the thrust axis fixed in the body frame, which is neutrally stable. $\endgroup$
    – pericynthion
    Commented Feb 25, 2015 at 1:48
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    $\begingroup$ Most often, the gimbal system uses hydraulics to pivot the entire thrust chamber, which is rigidly fixed to the nozzle. Typically the turbopump assembly moves with it as well. The tail end of the fuel lines going to the engine are flexible. $\endgroup$
    – Russell Borogove
    Commented Jul 6, 2018 at 18:17
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In the ideal case of launch in a vacuum, all that is needed is an infinitesimal nudge to start a gravity turn.

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    $\begingroup$ Yes, I am aware of that. How do they create the "nudge"? $\endgroup$
    – Stu
    Commented Nov 19, 2013 at 0:16
  • $\begingroup$ If we depart from our ideal case ever so slightly, it is actually impossible to avoid that initial nudge. $\endgroup$
    – Erik
    Commented Nov 19, 2013 at 0:55
  • $\begingroup$ You can maintain a zero angle of attack and reach orbit successfully in an ideal gravity turn. $\endgroup$
    – Erik
    Commented Nov 19, 2013 at 18:27

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