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A smoothly running rocket engine will have a center of thrust axis. If the engine is gimbaled, that axis should pass through each of the gimbal axes (if it's a classical gimbal), or if it is a ball joint, the gimbal point.

I am sure engines are fabricated carefully to be symmetrical to high tolerances, but nothing is perfect. Any misalignment would result in a static torque, which would have to then be maintained by whatever mechanical system was actuating the gimbal motion, and if there is a mechanical disadvantage such as the one in the way these images suggest, the job is even worse.

So do they test fire engines and monitor the position of its center of thrust with respect to the gimbaling joints, and shim or adjust the engine mounting or nozzle in order to zero out any major asymmetries, or do manufacturing processes result in engines that are naturally very very well centered?

Here is some text from previous comments that help further define why centering might be important:

If the thrust axis does not pass through the center of rotation of the gimbaling motion, the engine would have a tendency to "auto-slam" to one side as soon as it starts, so the actuators would have to deliver substantial force quickly, and track the moment-by-moment increase in thrust as the engine starts. This could potentially be much faster than the rotation of the entire spacecraft since it involves only the moment of inertia of the individual engine, not the spacecraft.

The actuator would have to continue to deliver this extra force against the engine's thrust for the entire time the engine is running, (perhaps more of a problem for hot, battery-draining electric actuators than for hydraulic actuators) then track the thrust of the engine back to zero as it shuts down. From a dynamics and control system point of view, is a different animal than steering the spacecraft.

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  • $\begingroup$ Nothing is perfect, the rocket is not perfectly vertical at the launch pad, the center of mass is not exactly were it should be, there may be wind with gusts at launch, the liquid propellants in the tanks may swash. But the attitude control of the rocket will detect and compensate any deviation. $\endgroup$
    – Uwe
    May 23, 2017 at 20:11
  • $\begingroup$ @Uwe this is not related to attitude. The engine could be on a test stand by itself. $\endgroup$
    – uhoh
    May 23, 2017 at 20:42

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They are certainly centered for obtaining the exact thrust profile, to be able to control the rocket, but they absolutely don't need to be perfectly centered with the gimbal - the gimbal(s) may be way off-center, and carry a significant torque if other concerns prioritize freeing up the middle - say, you gimbal just the nozzle, and need the throat obstruction-free. Besides, if you have two 1-axis gimbals instead of one 2-axis, at different heights, you're not getting the thrust through the center of the second one.

In that case you simply use actuators strong enough. They must be pretty strong anyway, because the engine is quite heavy and the gimbal must be moving fast - that's a considerable acceleration needed, and as result, considerable force. Making them strong enough to carry some extra torque from off-center thrust is not an excessive demand.

Fast, strong actuators used in general engineering often provide different force in different direction - for example, piston-based actuators are usually stronger in the "extending" direction than "retracting" due to higher area of piston exposed to one of the chambers (part of it on the "retracting" side occupied by the piston shaft). In that case a biased load is welcome - you only regulate the level of reaction force without ever needing to change its sign (e.g. only regulate pressure in the piston, without ever toggling the valve to force retraction). Gimbals based on such actuators would welcome a semi-constant torque/force exerted by the engine, since they'd need to provide power only in one direction, just yielding to the engine's torque/force in the opposite direction.

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  • $\begingroup$ So engines destined for gimbaled operation generally can not be manufactured with the thrust centered well enough, and they usually do require a test firing to center them? Yes or no? Which is more common? Also I don't follow the throat obstruction point, what would obstruct the throat of a gimbaled engine? $\endgroup$
    – uhoh
    May 23, 2017 at 9:49
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    $\begingroup$ @uhoh: No. They are usually manufactured with thrust centered, but the reason is making the engine play well with the whole rocket and payload, not aligning it with the gimbal. If they are centered with the gimbal as result, that's just a minor boon / side effect. Aligning engine thrust for the gimbal is like "astronauts on spacewalks need to wear EVA suits to cover their nudity not to offend audiences watching the live stream broadcasted to Earth." $\endgroup$
    – SF.
    May 23, 2017 at 10:31
  • $\begingroup$ and for the throat obstruction, that's for gimbaled nozzle, not the whole engine. $\endgroup$
    – SF.
    May 23, 2017 at 10:33
  • $\begingroup$ OK I understand a little more. I still don't know what the thing is that would obstruct the throat, but that's OK, I'm mostly asking about the situation where most of the engine gimbals as a unit. I've fine-tuned the title to match more closely the explanation in the body. It's only the centering of the point of gimbaling on the thrust axis that I'm interested in. $\endgroup$
    – uhoh
    May 23, 2017 at 10:50
  • $\begingroup$ @uhoh: If the point (axis) of rotation is not, like in place of rotating whole engine, somewhere above the engine, but - like when rotating the nozzle - within the volume of the engine, you won't get away with just a single centralized gimbal/hinge/bearing, simply because that room is occupied by something else. In that case the gimbal will be actually a set of actuators around the circumference, sharing the load between them. $\endgroup$
    – SF.
    May 23, 2017 at 11:57

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