What Does This Mean? - "rigid body can't be child of a non compound rigid body"

I was rendering an older blend file I made in Blender 2.79 (a rigid body tank animation), and noticed that the rigid body cannon ball that fired from the tank’s cannon (another rigid body that the ball is supposed to be parented to) gave me this message when looking at it’s physics properties.

The animation works fine in 2.93, except that the ball remains stationary in front of the cannon after firing. The ball still activates the target (a rigid body wall held together with constraints) in spite of staying where it ends up a few feet beyond the cannon. Very weird.

Anyone know what’s going on and how to fix it? Blend file is way to big to find a place to upload for sharing.

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suffering from same so did you find any solution?

Parenting a rigid body is allowed in more recent versions, and if this is irritating to you, then I’d recommend updating Blender.

The problem here is that the rigid body is being asked to obey two masters: the parent, and the physics system.

The solution is, don’t parent the rigid body. If you want it to acquire some orientation while “animated” is enabled, give it a constraint to acquire that orientation from something. Like a copy transforms constraint, world->world, targeting an empty that is parented to something. When you want physics to take over, it may be necessary to keyframe the influence of that constraint to 0. (It is not necessary to do so in the most recent versions of Blender, but I don’t remember 2.93 stuff.)

Or, use a non-rendering rigid body, which is something that I recommend as a general practice, and acquire the positions you want on the rendering object by copying transforms instead.

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First thank you for your response
I am using the most latest version but the tutorial I am following is from 2.9 something so maybe that’s why this is happening but the other stuff you told me is beyond me as I am a beginner to rigid body physics but I will try looking for some intermediate tutorials as I have a little bit of knowledgde how rigid body works and then get back to you if I have more questions. Thank you once again

Oh, I’m sorry-- I confusing something older. The fixes I mentioned would still work.

I think it’s that Blender won’t let you parent a rigid body to another rigid body. I don’t know why, it seems arbitrary. But there is another workaround that doesn’t require learning about anything new, which is to parent both rigid bodies-- the one you want to be the parent, and the one you wanted to be the child-- to a different object, like an empty, and animate that instead.

But I think it’s well worth anyone’s time to learn about using constraints anyways.

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hahahah no worries I will try to learn constraints and will let you know how it went. Thank you :heart: