It sounds like you want a revolute joint between the turret and the main chassis, as per the Box2D manual
. I'm reasonably sure that Fizzy actually doesn't surface this, but I'm sure JBox2D does. I'm not sure, but I suspect you'll need to configure the turret and the tank explicitly to not collide with each other. So...unfortunately, this does mean you need to do a little bit of leg work to modify Fizzy. If you'd be so kind, you can do this by forking my repository, here: https://github.com/nanodeath/fizzy
If you find the joints system of JBox2D to be really rough around the edges, feel free to design your own joint abstraction layer on top of JBox2D. Otherwise, just create doppelganger classes that mimic JBox2D without tightly coupling application code to JBox2D. Then send me a pull request when you're finished and I can try to incorporate it into the "main" Fizzy engine.