Why can't I use Keep Upright on a Thruster?

It would really make what I’m trying to do a lot easier if I could just make a thruster that would have complete freedom of movement independent of the thing to which it is attached (achieved by ball socket centre), but always point its thrust toward the ground. So I tried to just use Keep Upright on a thruster and it seems that Keep Upright can only apply to normal props or something. Why is this? Why can’t I just keep anything upright?

Try giving the thruster some weight, I’m sure with a mass of 1 keep upright isn’t effective enough

It’s not that it’s not effective, it’s that it simply does not work at all. The action “Keep Upright” does not show up in the undo list if a thruster of any weight is clicked.

I tried welding a prop to the thruster, offset below it by a few inches, and applying Keep Upright to the prop. Even when increasing the weight of the prop to 200, it did nothing to keep the prop/thruster combo upright, and so the prop/thruster combo still spun out of control. Relative to the overall weight of the craft itself, 200 is rather considerable, so I don’t think increasing the weight even more will do any good, although I may be wrong.

The original idea was to ball socket centre an object to my frame of the craft, and then to attach a thruster to the bottom of that object. This works, somewhat. I can apply keep upright to the prop, but the thrust still causes the object to tilt and therefore the craft always drifts a little in a certain direction, rather than straight up. So then I just skipped the object entirely and ball socket centre’d the thruster to the frame, and ran into the current problem.