I've been wondering how exactly it is that actuators are joined to the structure of a thrust vector control system. The reason I'm confused may be illustrated by the following image of a Vernier engine.

Vernier Engine

It seems as if the joints upon which the two actuators are fixed allow movement only upon a single axis, and the two actuators are both fixed to the same rigid structure. The Rutherford engine, too, seems to have this structure. The two actuators are fixed $90^\circ$ apart from one another. If one actuator, then, extends to push the engine along its axis, how does the actuator fixed perpendicular to it allow for this motion?

I have tried to draw a diagram to make this more clear. enter image description here

The blue shape in the middle represents the thrust chamber, and the orange rectangles represent the actuators. The green rectangles represent joints with a single degree of freedom, which is the case in the above image so far as I can tell. The problem is that if one actuator were to push the thrust chamber, the other actuator which faces the perpendicular axis would have to twist on an angle to follow the movement of the thrust chamber. I don't see how this is possible if the joints all allow only one degree of freedom.

  • 1
    $\begingroup$ The two actuators arent fixed to rigid structure, they have 1-dof joint, if you see closely. That would allow the motion of 2-dof to engine bell $\endgroup$
    – zephyr0110
    Jun 21, 2021 at 10:52
  • $\begingroup$ Can you add a diagram to the question where you show "the actuator fixed perpendicular to it" doesn't "allow for this motion"? The actual joints have some limited flexibility to allow for movement. If you can draw a diagram, we can point out where this flexible part is located. $\endgroup$
    – AJN
    Jun 21, 2021 at 12:51
  • $\begingroup$ @Prakhar I did say that the joints upon which the actuators allowed motion upon a single axis, meaning 1-dof. The problem is that each of the two actuators can only move upon a single axis, and on both ends the actuators are joined to the same structures. If one actuator pushes, the other actuator has to move perpendicularly to its degree of freedom to allow for this, which it cannot as far as I can tell. That is why I am confused. $\endgroup$
    – lucasp
    Jun 21, 2021 at 13:03

1 Answer 1


edit : Apparently it is called an "rod end bearing"

edit 2 : As mentioned in the comments it could even be a Clevis joint.

The place where the green rectangle in your diagram touches the blue rectangle has a ball and socket like joint. This prevents one actuator from blocking the movement due to the other actuator.

free hanging actuator

Above picture from blog.nasa.gov shows one of the two ends of an actuator where details are not obscured by the brackets on the nozzle.

Notice the ball and socket like arrangement at the bottom end of the actuator. There would be a similar arrangement on the so called "thrust take off side" also.

This ball and socket joint allows free movement in two directions. In one direction is the actuators own motion. The other direction movement is for use when the other actuator is moving.

See the images from this post. I think you can see a gap between the bracket to which the actuator attaches. A hint of the ball and socket joint can be inferred from the picture.

I have crudely marked the location on the image from the question as well as the above linked question.

marked actuator

  • $\begingroup$ I did think that a ball-and-socket type joint would be necessary, so if this is the case on the engine in the image you have attached, it does make sense to me. The problem is just that I can't seem to find where such a joint exists on the image of the Vernier engine I have attached, it does not appear that there is anything like a ball and socket at the joints of the actuators at all. $\endgroup$
    – lucasp
    Jun 21, 2021 at 13:19
  • $\begingroup$ I still find the image of the Vernier engine ambiguous, but I think I understand the idea of the mechanism well enough now. Thank you for your help. $\endgroup$
    – lucasp
    Jun 21, 2021 at 13:53
  • $\begingroup$ I will try to update this post if I can find a higher resolution photo. $\endgroup$
    – AJN
    Jun 21, 2021 at 14:23
  • $\begingroup$ To me, the joint in the first image looks more like a universal joint rather than a ball and socket, and the joint in the second image looks more like a clevis joint. Some kind or joint that provides one or more rotational degree of freedom is needed on each end of the actuator. From googling patents, it appears a wide variety of joint types have been proposed (and used; some of these patents are for real thrusters as opposed to imaginary ones). It does not have to be a ball and socket. $\endgroup$ Jun 21, 2021 at 21:44
  • $\begingroup$ @DavidHammen I initially web searched both terms. But the term universal joint gave too many results for other joints including the joint which allows the engine to swivel (e.g. the linked NASA blog post). So I edited it out of my answer. $\endgroup$
    – AJN
    Jun 22, 2021 at 1:20

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