Rigging with an elliptical "constraint."

I’m trying two ovoids, connected with a rod, so that you can grab the first and move it. Movement should be constrained to ZY - not too tricky, but should also be constrained so that the rod connecting the two objects is always touching each object.

Here’s a diagram:


As you can see, d3 is bigger than d2, is bigger than d1. I don’t think there’s a way to do this non manually, is there? I guess you could use driven shape keys, but then the object wouldn’t move.

Grraarrr! The whole of the blender animation system seems geared up entirely to stop this from being done!

I had an idea - I’d create a second bone for the connection between the two objects, which would be parented to the original one, but not connected. The connection object would then be parented to this new bone, so it would be able to slide in and out, relative to the rest of the armature.
My idea was to drive the movement of this bone according to the rotation of its parent bone. But lo and behold - there is no way of doing this. First I think, “I’ll use the parent bone’s rotation to drive it directly.” But no - you can’t drive a bone with its own armature, for some reason. Then I thought, I know, I’ll parent that bone to an empty, and use the rotation of the empty, but of course, rotation only shows up if it’s relative to its parent, so that didn’t work. The last idea I had was to use a constrain rot on the empty - targeting the parent bone. When I rotate the bone, the empty rotates too, but no! That doesn’t count! I can grab the empty, hit ‘R’ and, although it doesn’t rotate, it will drive the connector as it’s supposed to, but for some reason, when the empty is rotated by the constraint, it’s not allowed.

It’s seriously as if someone decided, “Yeees, let’s make it absolutely impossible for someone to do this.”

I even tried the little python expression box, but blender just says, “Invalid python expression,” for whatever I put in there, almost.

For what it’s worth, I don’t understand the question at all. It seems like you want to grab the first ovoid and move the whole structure, so you’d just parent things to the first ovoid… but clearly you’re after something a bit more complex. Is there a real world object that this is suppose to behave like, just to get more of an idea of the problem?